Welding is a processing process and connection method that combines atoms between two workpieces by heating, pressurizing, or both. Welding is widely used for both metals and non-metals.

Development history of welding technology

Forge welding technology appeared in Egypt in 3000 BC.

In 2000 BC, the Yin Dynasty of China used casting and welding to make weapons.

1801 - H.Davy of England discovers electric arc.

1836 - Edmund Davy discovers acetylene gas.

1856 - James Joule, an English physicist, discovers the principle of resistance welding.

1959 - Deville and Debray invent hydrogen - oxygen welding.

1881: Frenchman De Meritens invents the earliest carbon arc welding machine.

1881: Dr. R. H. Thurston of the United States spent six years to complete all the experiments on the strength and extensibility of a full range of copper-zinc alloy brazing materials.

1882 - The austenitic manganese steel invented by Robert A. Hadfield of England and named after him is patented.

In 1885, Elihu Thompson, an American, patented a resistance welding machine.

1885 - Russian Benardos Olszewski develops carbon arc welding technology.

1888: Russian H. l. C. L.

1889-1890: C. L. Coffin, an American, performed the first arc welding using a light wire as an electrode.

In 1890; The American C. L. Coffin proposed the concept of welding in an oxidizing medium.

1890 - British man Brown makes the first attempt to rob a bank using oxygen and gas cutting.

1895 - Bavarian Konrad Roentgen observes X-rays produced by a stream of electrons passing through a vacuum tube.

1895 - Frenchman Le Chatelier receives a certificate for inventing the oxyacetylene flame.

1898: German Goldschmidt invented thermite welding.

1898: The German Klein. Schmidt invented arc welding of copper electrodes.

1900: Strohmyer invented the thin-coated electrode.

In 1900, the French Fouch and Picard made the first oxy-acetylene cutting torch.

1901 - German Menne invents oxygen spear cutting.

1904: The Swede Oscar. Kjellberg established the world's first electrode factory - ESAB's OK Electrode factory.

In 1904, Avery invented the portable steel cylinder.

1907: When demolishing the old Central railway station in New York, more than 20% of the engineering cost was saved due to the use of oxy-acetylene cutting.

1911 - Philadelphia & Suburban Gas Company builds the first 11-mile line to be welded using oxygen solvent gas welding.

1912: The first oxy-acetylene gas welded steel pipe was put on the market.

1912 - The Edward G. Budd Company in Philadelphia produces the first all-steel automobile body welded with resistance spot welding.

Circa 1912: In order to produce the famous Model T car, the Ford Motor Company in the United States completed the modern welding process in the laboratory of its factory.

1913 - Avery and Fisher perfect acetylene cylinders in Indianapolis, USA.

1916: Ansel. The first is the invention of X-ray nondestructive testing in the welding zone.

1917: During World War I, 109 ship engines captured from Germany were repaired using arc welding, and half a million American soldiers were transported to France using these repaired ships.

1917: Webster & Southbridge Electric Company in Massachusetts used arc welding equipment to weld 11 miles of pipeline with a diameter of 3 inches.

1919: Comfort A. dams forms the American Welding Society (AWS).

1919 - C.J.Halslag invents AC welding.

1920 - Gerdien discovers the heat flux effect.

1920 - Fulagar, the first steamship with an all-welded hull, is launched in England.

Circa 1920: Began using arc welding to repair some valuable equipment.

Circa 1920: The Johnson Process for welding steel pipes using resistance welding is patented.

Circa 1920: The Poughkeepsie Socony, the first oil tanker built using the welding method, is launched in the United States.

Circa 1920: flux-cored wire is used for wear-resistant surfacing.

1922 - Prairie Pipeline Company successfully completes the laying of an 8-inch diameter, 140-mile crude oil pipeline from Mexico to Texas using oxy-acetylene welding technology.

1923: Stody invents surfacing welding.

1923: The world's first floating roof storage tank (used to store gasoline or other chemicals) is built; Its advantage is that the tank can be raised or lowered like a telescope by a welded floating roof and tank wall, so that the volume of the tank can be easily changed.

1924: Magnolia Gas Company builds 14 miles of all-welded natural gas line using oxy-acetylene welding technology.

1924: H.H. ester was the first in the United States to use X-ray photography to test the quality of castings to be installed at a steam pressure of 8.3Mpa for the Boston Edison power plant.

1926: American Langmuir invented atomic hydrogen welding.

1926: Alexandre invented the principle of CO2 gas shielded welding.

1926: A.O.Smih company in the United States took the lead in introducing the production method of applying a protective solid coating (that is, manual arc welding electrode) on the metal electrode for arc welding.

1926: Chromium-tungsten-cobalt alloy receives the first patent for flux-cored wire.

1926: Americans M. Hoart and P.K. evers obtain a patent for the use of helium as an arc protection gas.

1927: Lindberg successfully flew the Ryan monoplane over the Atlantic Ocean, with a fuselage made of welded steel tubes.

1928: The first structural steel welding code, Code for Fusion Welding and Gas Cutting in Building Structures, is published by the American Welding Society, which is the predecessor of today's D1.1 Structural Steel Welding Code.

1930: The Georgia Railroad Center uses a continuous welding method to lay the railroad in two tunnels. The welded track was put into use two years later when the line was through.

1930: The former Soviet Union Robinov invented submerged arc welding.

1931 - The Empire State Building is built with a welded all-steel structure.

1933: The first joint welded using the arc welding process was laid with a long transmission line with no liner construction.

1933 - San Francisco's Golden Gate Bridge, then the highest suspension bridge in the world, opens to traffic, made of 87,750 tons of welded steel.

1934 - Barton Welding Institute is established.

Barton Institute founder Yevkin Oskalovich Barton

The largest welded iron bridge over the River Deniebe in Europe - Barton Bridge

1934: The Unheated pressure Vessel Code is published by the API - ASME collaboration.

1935: Linde Air Products of the United States perfected the submerged arc welding technology.

1936: Wasserman invented low temperature brazing.

1939: American Reinecke invented the ion flow spray gun.

1940 - Exchequer, the first all-welded ship, is launched at Ingalls Shipyard in the United States.

1941: American Meredith invented tungsten inert gas shielded arc welding (helium arc welding).

1941: During World War II, ships, aircraft, tanks and various heavy weapons were manufactured using a large number of welding techniques.

1943: Behl invented ultrasonic welding.

1943: Aircraft builders first welded the hollow blades of aircraft steel propellers using atomic hydrogen welding, submerged arc welding, and MIG welding.

1944: British Carl invented explosive welding.

1947: The invention of electroslag welding in the former Soviet Union by Bopo Noebech (Voroshevich).

1949 - The first FORD car with an all-welded structure made using arc and resistance welding processes rolls off the assembly line.

In 1950, Americans Muller, Gibson and Anderson obtained the first patent for the excessive welding of MIG.

1950 - German F. B. uhorn discovers plasma arcs.

Circa 1950: Electroslag welding is first used in production in the former Soviet Union.

1953: Hunt invented cold pressure welding.

1953: The former Soviet Union Lyupovsky, Japan Sekiguchi and others invented CO2 gas shielded arc welding.

1954: Self-protecting flux-cored wire was put into production at Lincoln Electric Company in the United States.

1954 - The Nautilus, the first welded nuclear submarine, enters service with the U.S. Navy.

1954: Benard invents the tubular electrode.

1955: Thom, USA. Claverd invents high-frequency induction welding.

1956: Harbin Welding Research Institute was established in China.

1956: The former Soviet Union Chudikov invented friction welding technology.

1957: The invention of electron beam welding by Schgill in France.

1957: The former Soviet Union Kazakov invented diffusion welding.

1957: "Welding" is published, which is the first professional welding magazine in China.

Circa 1957: The United States, the United Kingdom, and the former Soviet Union all used CO2 as a protective gas in the process of short circuit welding.

1960: Airco of the United States introduced the metal pulse gas welding process.

1962: The patent for gas welding was granted to the Belgian Arcos.

1962 - Electron beam welding is first used on supersonic aircraft and B-70 bombers.

1964: The patent for the hot wire welding method and the coordinated control of the MIG welding method is granted to the American Manz.

1965 - The welded Appllo 10 spacecraft successfully landed on the moon.

1967: Arada invented continuous laser welding.

In 1967, the world's first submarine pipeline was successfully laid in the Gulf of Mexico, which was manufactured by the Krank Pilia company of the United States using the thermal thread process and welding process.

1968: Welded 22 floors above the John Hancock Center in Chicago to create the world's tallest sharp-angled steel structure at 1,107 feet.

1969: Linde Company of the United States proposed the hot wire plasma arc spraying process.

1970: Thyristor inverter welding machine came out.

1976: Arada invented series electron beam welding.

Around 1980: Semiconductor circuits and computer circuits are widely used to control welding and cutting processes.

Circa 1980: Use steam brazing to weld printed circuit boards.

1983: The circular top of the 160-foot diameter flap structure on the space shuttle was welded using the submerged arc and shielded welding method and inspected using a radiographic flaw detector.

1988: Welding robots began to be widely used in automobile production lines.

Around 1990: Inverter technology has been greatly developed, and the result is a reduction in the weight and size of welding equipment.

1991: The British Welding Institute invented friction stir welding and successfully welded aluminum alloy plates.

1993: The United States Army Abrams main battle Tank was successfully welded using a robot-controlled CO2 laser.

1996: A research group of more than 30 people headed by B.K.Lebegev, academician of the Barton Welding Institute in Ukraine, researched and developed the welding technology of human tissue.

2001: Human tissue welding was successfully applied in clinic.

2002: Welding of the Three Gorges Turbine is completed, the largest turbine in the world that has been built and is currently under construction.

1985:Huarui was established

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How to judge the size of the current listen to a welder who has been engaged in welding for more than 10 years, beginners can correctly weld the current when welding will be very important to the quality of welding, because the welding current has a great impact on the molding of the weld.

The welding current is too small, which will cause the phenomenon of non-penetration, non-fusion and sticking electrode. The current is too large, will cause welding penetration, edge bite, and welding nodules and other quality defects.

How should the welder choose the right welding current when welding?

The master said that the correct choice of welding current needs to be selected according to a variety of parameters. Like the thickness of the weldment, the diameter of the electrode and the welding position.

For manual arc welding, the size of the welding current depends on the diameter and size of the electrode, which is usually the diameter of the electrode multiplied by the empirical coefficient.

The experience coefficient of the common 2-4 mm electrode is 30-40, and if the diameter of the electrode is 4-6, the experience coefficient is 40-60. It can be adjusted appropriately according to the specific situation of the site.

In the field, the welder can intuitively judge according to the actual situation, the arc is difficult, the welding rod is easy to stick, the forming is not smooth, and the welding rod is difficult to melt. On the contrary, if the welding spatter is too large, the weld pool is excessive, and the welding rod is red, the current is too large.

The size of the welding current selection also depends on the technical level and operating habits of a welder. Some novices need to adjust the current size according to the ammeter on the welding machine at the beginning, and experienced welders can judge by experience and the sound emitted when the welding rod burns.

According to the master, the sound emitted by the appropriate current is very uniform and smooth, and the shape and sound of the molten pool can be adjusted in a timely manner.

The old master said that each welder has its own skills and experience, and the current selection varies from person to person.

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In the industrial manufacturing of nuclear power, construction, oil and gas pipelines, when the position of structural parts is difficult to adjust, welding in non-planar positions is inevitable. Overhead welding is the most difficult of the four basic welding positions. Therefore, it is of great significance to improve the weld forming quality and process adaptability of overhead welding. In the process of overhead welding, when the heat input is too small, it is difficult to penetrate the base metal, and when the heat input is too large, it will lead to an increase in the volume of the weld pool. In this case, the tail of the molten pool, which has lost the support of the arc force, will flow downward under the action of gravity and complete the solidification. In addition, even if the base metal can be completely penetrated, it is difficult to form a high enough additive solid on the back weld, and even a concave defect will be formed, significantly reducing the shape and mechanical properties of the welded joint.

 This paper presents a new overhead welding technique, which uses a laser-arc hybrid heat source and adds a copper liner (COL) on the back of the weld. The weld morphology, mechanical properties and microstructure of the joint under three forming methods of copper gasket, ceramic gasket (CEL) and no gasket (NOL) were compared in order to improve the welding quality and mechanical properties of the joint. In addition, the process window and process adaptability of the process were also studied.

Figure 1- Test methods: (a) schematic diagram of overhead welding, (b) backbend test method, (c) process adaptability test (dislocation) assembly method, (d) process adaptability test (butt gap) assembly method

Figure 2- The macro morphology of the weld and the mechanical properties of the welded joint under different welding parameters and forming methods

Figure 3- Forming mechanism of the back side when COL is used in overhead welding

Figure 4- Microstructure of the top center of the back weld under different forming processes and parameters: (a) NOL-140A, (b) CEL-140A, (c) COL-140A, (d) COL-150A, (e) COL-160A, (f) COL-170A

Figure 5- Hardness distribution of weld section: (a)NOL-140A, (b) CEL-140A, (c) COL-140A, (d) COL-150A, (e) COL-160A, (f) COL170A

Figure 6- Detection results of copper content in the weld: (a) NOL-140A, (b) CEL-140A, (c) COL-140A, (d) COL-150A, (e) COL-160A, (f) COL-170A

Figure 7- Macroscopic morphology of weld back and cross section under different butt gap and dislocation: (a) cross-sectional topography of 0-1.00mm dislocation, (b), (c), (d), (e) cross-sectional topography of 0.25, 0.50, 0.75 and 1.00mm dislocation, (f) cross-sectional topography of 0.50-2.00mm butt gap, (g), (h), (i), (j) 0.50, 1.00, 1.50 and 2.00 mm butt clearance

 The main conclusions are as follows:

In this paper, the macroscopic morphology and mechanical properties of the downward welding joint with three forming methods are analyzed. By adding COL to the back weld, the weld appearance quality and process adaptability of LASer-arc hybrid overhead welding were improved.

(1) In the process of overhead welding, the use of COL can eliminate the concave defects of the back weld, and significantly improve the appearance and quality of the weld. COL can be used to form a stable backing and solid under different misalignment and butt clearance. The backing plus solid and its width are not less than 0.71 and 3.66 mm respectively.

(2) The use of COL can expand the process window of laser-arc hybrid overhead welding and improve the process adaptability. When the welding current is 140-170A, the butt gap is 0.5-2.0mm, or the dislocation is 0-1.0mm, a good weld can be obtained.

(3) When COL is used, the cooling rate at the back weld strengthening is accelerated, the grain is refined, and the hardness is increased. The concave defect of the back weld does not significantly reduce the tensile strength of the welded joint, but the specimen with backing plus solid can withstand greater loads in the back bending test.

(4) The test results of copper content in welds show that the use of COL does not significantly affect the content and distribution of copper in welds.

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                                                Methods to prevent welding cracks

1. Select materials correctly

Choose alkaline low-hydrogen electrode and flux to reduce the content of diffused hydrogen in weld metal; Do a good job in the selection of base material and welding material matching; If the technical conditions permit, materials with good toughness (such as welding materials with a lower strength level) can be selected, or "soft" covers can be implemented to reduce the residual stress on the surface; If necessary, chemical analysis, mechanical properties, weldability and crack sensitivity tests are carried out on the base material and welding material before manufacturing.

2. Welding operation is carried out in strict accordance with the correct process specifications obtained by the test

Mainly includes: strictly according to the specification of welding rod drying; Select appropriate welding specifications and wire energy, reasonable current, voltage, welding speed, interlayer temperature and correct welding sequence; Check and treat spot welding; Clean the root of double-sided welding; Carefully clean the groove and welding wire to remove oil, rust and moisture.

3. Select a reasonable welding structure to avoid excessive restraint stress; Correct groove form and welding sequence; Reduce the peak of welding residual stress.

4. Preheating before welding, slow cooling after welding, controlling interlayer temperature and post-welding heat treatment are effective methods to prevent cold cracks for high-strength steel with poor weldability and unavoidable high-restraint structural forms. Preheating and slow cooling can slow down the cooling rate (prolong the residence time of △t 800~500℃), improve the microstructure of the joint, reduce the hardening tendency, and reduce the microstructure stress; Post-welding heat treatment can eliminate welding residual stress and reduce the content of diffused hydrogen in the weld. In most cases, stress relief heat treatment should be carried out immediately after welding.

5. Hammering immediately after welding to disperse the residual stress and avoid causing high stress areas is one of the effective ways to prevent cold cracks during local repair welding.

6. In the weld root and the weld surface where the stress is relatively concentrated (the restraint stress of the heat affected zone is low), the welding rod with lower strength level is often used to achieve good results under high restraint.

7. The use of inert gas shielded welding can maximize the control of the hydrogen content of the weld and reduce the sensitivity of cold cracks, so TIG and MIG welding should be vigorously promoted.

                                                                       Method of preventing welding hot crack

1. Limit the content of elements and harmful impurities that are prone to segregation in steel and welding materials, especially the content of S, P, and C, because they not only form low melting point eutectic, but also promote segregation. C≤0.10% thermal crack sensitivity can be greatly reduced. If necessary, chemical analysis of the material, low power inspection (such as sulfur printing, etc.).

2. Adjust the chemical composition of weld metal, improve the organization, refine the grain, improve plasticity, change the shape and distribution of harmful impurities, reduce segregation, such as the use of austenite plus less than 6% of the ferrite biphase structure.

3. Improve the basicity of the electrode and flux to reduce the content of impurities in the weld and improve the degree of segregation.

4. Select a reasonable groove form, weld forming coefficient ψ=b/h > 1, avoid narrow and deep "pear-shaped" weld, (welding current is too large will form a "pear-shaped" weld), prevent the cylindrical crystal in the center of the weld, resulting in center segregation to form a brittle section; Multi-layer and multi-pass welding is adopted to disrupt segregation aggregation.

5. The use of small (appropriate) welding line energy, for austenitic (nickel-based) stainless steel should try to use a small welding line energy (no preheating, no swing or less swing, fast welding, small current), strict control of the interlayer temperature, in order to shorten the residence time of weld metal in the high temperature zone;

6. Pay attention to the protection of arc retraction, arc retraction should be slow and fill the arc pit to prevent the thermal crack caused by arc pit segregation;

7. Try to avoid multiple repairs to prevent lattice defects from gathering and producing polygonal hot cracks;

8. Take measures to minimize the joint stress, avoid stress concentration, and reduce the stiffness near the weld, properly arrange the welding order, and try to make most of the weld welding under a small stiffness, so that it has room for shrinkage.

                     Methods to prevent reheat cracking

1. When selecting materials, attention should be paid to the carbide forming elements that can cause precipitation, especially the content of V. When high V steel must be used, special attention should be paid to welding and heat treatment.

2. Avoid the reheat sensitive area during heat treatment, which can reduce the possibility of reheat crack, and do heat treatment process test before heat treatment if necessary.

3. Minimize residual stress and stress concentration, reduce residual height, eliminate edge bite, incomplete penetration and other defects, and polish residual height and weld toe smoothly if necessary; Increase preheating temperature, slow cooling after welding, reduce residual stress.

4. Appropriate line energy to prevent heat affected zone from overheating and coarse grains.

5. Under the premise of meeting the design requirements, choose a lower strength grade of the electrode to release a part of the stress eliminated by the heat treatment process (let the stress relax in the weld), which is good for reducing reheat cracks.

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                           First stop：EXPOMAFE 2023 BRAZIL@9-13th,May 2023

When we first arrived at the airport, the weather was not very good.But we are so happy to be in this beautiful country again.

The show was held at the Sao Paulo Expo-Exhibition & Convention Center. We packed our bags and checked into the hotel to wait for the show to start the next day.

Looking forward to meeting our customers

     Our booth No.I132,We displayed and tested our products on site.

    We were very happy to meet many old and new partners, and hope that we can cooperate happily in the future

                          Second stop:FABTECH 2023 MEXICO @16-18th,May 2023

     Mexico is a very beautiful country, we were very happy to be here for the exhibition.

     In Mexico, our company also has many customers who have cooperated with us for a long time to come to our exhibition hall, and we are also very happy to meet new friends

        Our booth is NO.3039

     We brought a lot of the company's best-selling products and new products to show

           Last stop:BEIJING ESSEN FAIR @27-30th,June 2023@Shenzhen,Booth No.15006

       We are waiting for your...

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Welding quality testing refers to the testing of welding results, with the purpose of ensuring the integrity, reliability, safety and serviceability of welding structure. In addition to the requirements of welding technology and process, welding quality inspection is also an important part of welding structure quality management.

This time we will talk about the welding quality test method: tightness test.

So how to test the tightness of welded joints?

In general, the following methods can be used for detection:

1. Sinking test

Used for small vessels or pipes subjected to low internal pressure. Before inspection, the container or pipe should be filled with compressed air at a certain pressure (0.4-0.5MPa), and then submerged to check the tightness, such as right leakage; Bubbles must occur in the water. This is also a common part of the bicycle tube to check for air leaks2. Water holding test

The static pressure generated by water dead weight is used to check whether the structure has leakage phenomenon. Based on visual inspection, it is suitable for general welding structure which is not compressed but requires sealing.

3. Ammonia leakage test

It is used in the same way as coal drainage leakage test, and its sensitivity is higher than kerosene leakage test. Before the test, paste a white strip or bandage soaked with 5% HgNO3, aqueous solution or phenolphthalein reagent on the easy observation side of the weld, and then fill the container with ammonia or add compressed air with 1% nitrogen.

 If there is leakage, it will stain the white paper strip or bandage. The solution of 5%HgNO3 was black spot, and the solution of phenolphthalein was erythema.

 4. Kerosene leakage test

It is used for welding structure with small internal pressure and certain sealing requirement. Kerosene has strong permeability and is very suitable for sealing inspection of welds. Before inspection, brush lime water on the side of the weld for observation, and brush kerosene on the other side of the weld after drying. If there are penetration defects, the lime layer will spill coal oil spots or kerosene belts. The observation time was 15-30min.

5. Helium mass spectrometry test

Helium mass spectrometry test is the most effective means of sealing test at present. Helium mass spectrometer is very sensitive and can detect helium with volume fraction of 10-6. The container was filled with helium before the test, and the leak was detected outside the weld of the container. The disadvantage is the high price of helium and the long inspection period.

Although helium gas is highly permeable, it takes a long time to penetrate very small gaps that cannot be detected by other means, often tens of hours in some thick-walled vessels. Proper heating can speed up leak detection.

6, air tightness test

Air tightness test is a conventional test means for boiler, pressure vessel and other important welded structures requiring air tightness. The medium is clean air, and the test pressure is generally equal to the design pressure. The pressure should be increased step by step during the test.

After reaching the design pressure, apply soapy water on the outside of the weld or sealing surface and check whether the soapy water is bubbling. Because of the risk of explosion in the air tightness test, it should be carried out after the water pressure test is qualified.

Air tightness test is different from pressure test:

1, its purpose is different, air tightness test is to test the tightness of the pressure vessel, pressure test is to test the pressure strength of the pressure vessel. Secondly, the test pressure is different. The air tightness test pressure is the design pressure of the container, and the air pressure test pressure is 1.15 times of the design pressure.

Air pressure test is mainly to test the strength and tightness of the equipment, air tightness test is mainly to test the tightness of the equipment, especially the small penetration defects; The air tightness test focuses more on whether the equipment has small leakage, and the air pressure test focuses on the overall strength of the equipment.

 2, the use of media

 Air is generally used in the actual operation of the air pressure test. In addition to the air tightness test, ammonia, halogen or helium is used if the medium is highly toxic and leakage or easy penetration is not allowed

 3. Safety accessories

Air pressure test, do not need to install safety accessories on the equipment; Air tightness test is generally carried out after the installation of safety accessories (tolerance gauge).

 4. Order

The air tightness test should be carried out after the pressure or hydraulic test is completed.

 5. Test the pressure

The pressure test pressure is 1.15 times the design pressure, and the internal pressure equipment needs to be multiplied by the temperature dressing coefficient; Air tightness test medium for air test pressure for design pressure, such as the use of other media, should also be adjusted according to the medium situation.

 6. Use occasion

Pressure test: hydraulic test is preferred. If the hydraulic test cannot be used due to the equipment structure or support, or the equipment volume is larger, the pressure test is generally used. Air tightness test: medium for high or extremely harmful medium, or do not allow leakage.

 Air pressure test belongs to pressure test, in order to check the pressure strength of equipment. Air tightness test belongs to the compact test, in order to test the sealing performance of the equipment.

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Dehydrogenation treatment, also called dehydrogenation heat treatment, or called post-welding heat treatment.

The purpose of post-heat treatment of the weld area immediately after welding is to reduce the hardness of the weld area or eliminate hydrogen and other harmful substances in the welding area. In this point, the role of post-heat treatment and post-welding is partly the same.

After welding, the cooling rate of weld and welding joint is reduced by heat, which promotes hydrogen escape and avoids hardness appreciation.

(1) The post heating for the purpose of improving the performance of the welded joint and reducing its hardness can be effective only when the welding zone is still at a high temperature after welding.

(2) The post heating in order to prevent low temperature cracks is mainly to promote hydrogen energy to be fully excluded in the welding zone.

 The removal of hydrogen depends on the temperature of the afterheat and the holding time. The temperature for the main purpose of dehydrogenation is generally 200 ~ 300 degrees, and the afterheat time is 0.5 ~ 1 hour.

For welds under the following conditions, post-heat dehydrogenation treatment should be carried out immediately after welding (4 points) :

(1) The thickness is greater than 32mm and the standard tensile strength σb is greater than 540MPa;

(2) low alloy steel material with thickness greater than 38mm;

(3) Butt weld between the embedded nozzle and the pressure vessel;

(4) Welding process evaluation to determine the need for hydrogen elimination treatment.

 The value of afterheat temperature is usually expressed by the following formula:

Tp = 455.5[Ceq] p-111.4

 mode, Tp -- afterheat temperature ℃;

 [Ceq]p -- carbon equivalent formula.

 [Ceq]p＝C+0.2033Mn+0.0473Cr+0.1228Mo+0.0292Ni+0.0359Cu+0.0792Si－1.595P+1.692S+0.844V

To reduce the hydrogen content in the welding zone is one of the important effects of post heat treatment. It has been reported that hydrogen diffuses outward from the mild steel weld in 1.5 to 2 months at 298K.

 Increasing the temperature to 320K shortens the process to two to three days and nights, while heating to 470K takes 10 to 15h.

 The main function of post-heat and dehydrogenation treatment is to prevent the formation of cold cracks in weld metal or heat-affected zone.

 When the preheating is not enough to prevent the formation of cold cracks, such as in the welding of high-restraint joints and hard-to-weld steel, the post-heating process must be used to reliably prevent the formation of cold cracks.

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The operating procedures are as follows:

1, argon arc welding must be operated by special personnel switch.

 2. Check whether the equipment and tools are good before work.

 3. Check the welding power supply, whether the control system has ground wire, and add lubricating oil to the transmission part. For normal rotation, argon and water must be unblocked. In case of water leakage, call for immediate repair.

 4. Check whether the welding gun is normal and the ground wire is reliable.

 5. Check whether the high-frequency arc starting system and welding system are normal, whether the wire and cable joints are reliable, and whether the adjusting mechanism and wire feeding mechanism are in good condition for automatic electrode argon arc welding.

6, according to the material of the workpiece to choose polarity, good welding circuit, general material with DC positive connection, aluminum and aluminum alloy with reverse connection or AC power supply.

7, check whether the welding groove is qualified, the surface of the groove shall not have oil, rust, etc., on both sides of the weld within 200mm to remove oil and rust.

8, for the use of fetal gear to check its reliability, the welding parts need to preheat also check the preheating equipment, temperature measuring instrument.

9. The control button of argon arc welding shall not be far away from the arc, so that it can be closed at any time in case of failure.

10, the use of high frequency arc must often check whether there is leakage.

11, equipment failure should be power off maintenance, operators shall not repair themselves.

12, in the vicinity of the arc is not allowed to naked and naked storm other parts, not allowed to smoke and eat near the arc, so as to avoid ozone, smoke inhalation in the body.

13. Wear mask and gloves when grinding thorium tungsten electrode, and abide by the operating rules of grinder. The best choice is cerium tungsten pole (less radioactive). The grinder must be equipped with a suction device.

14. Operators should wear electrostatic dust masks at all times. Minimize the high frequency electrical action time during operation. Continuous work shall not exceed 6 hours.

15, argon arc welding work site must be air circulation. Ventilation and detoxification equipment should be activated during work. When the ventilation device fails, it should stop working.

16. Argon gas bottle is not allowed to be smashed, and must be placed with a support, and away from the open flame more than 3 meters.

17. When argon arc welding is carried out inside the container, a special mask should be worn to reduce the inhalation of harmful smoke. The vessel should be monitored and coordinated by a person outside.

18. Thorium tungsten rods should be stored in lead boxes to avoid injury due to the concentration of a large number of thorium tungsten rods, the radioactive dose beyond the safety regulations.

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Fillet weld is the most common weld splicing form in the welding process of steel structure, so many welders will encounter different styles of fillet weld, but also have different standards, so how to weld good fillet weld, and to meet the requirements?

   1.Selection of base material

 we take the base material Q355, size of 200x125x12mm test plate as an example for a simple introduction.

   2.Selection of welding materials

Welding material: E5015 (J507) electrode is selected, the drying temperature is 350℃, and the heat preservation is 1 hour. The welding wire meets the national standard ER50-6 (American standard ER70S-6).

              3.Pre-weld cleaning

 For fillet welds, both sides of the weld should be cleaned within 15~20mm. The oxide film, rust, oil and water on the surface of the plate are removed by grinding wheel or chemical treatment.

   4. Assembly point fixation

Generally, fillet welds require as little weld clearance as possible. Point fixing is generally carried out on the back of the plate, point fixing length is about 10mm. Generally, two or three points can be fixed, as shown in the figure:

Bottom welding

1.1 Angle of electrode and method of transporting electrode.

      The Angle of the electrode for flat fillet welding at the root is shown in the figure: right welding method is adopted, and arc is initiated on the left side of the test plate for short arc welding. Adopt straight line transport, welding to the right, arc aligned with the top Angle of the root, low arc, ensure that the top Angle and the two sides of the plate fusion. When the bottom welding swing method adopts the straight line, no swing fast welding, so that the required penetration can be achieved.

At the beginning and end of the weld, it is easy to appear magnetic bias phenomenon, which affects the quality of the weld. At this time, the Angle of the electrode should be properly adjusted, and the arc is generally directed to the weld pool to control magnetic bias. See the picture.

1.2 Pass joint

The joint starts the arc 10mm before the arc pit, and when the elongated arc moves quickly to the arc pit, fill the arc pit along the shape of the arc pit, and then weld normally.

Cover welding

Before welding the cover, remove the slag and spatter from the root pass to prevent slag inclusion defects.

     Cover welding welding two, first welding the lower weld, and then welding the upper weld. When welding the bottom pass, the arc should be aligned with the bottom of the root pass, the straight line, the Angle of the electrode should be greater than 45°; When welding the upper pass, the arc is aligned with the root pass along, and the straight rod can also swing laterally. The Angle of the electrode should be < 45°, as shown in the figure:

Theoretical throat: The vertical distance from the base of the joint to the hypotenuse in the maximum intangential right triangle drawn in the fillet weld profile.

 Effective weld throat: Minimum distance from fillet weld surface to weld root minus raised part.

 Actual welding throat: the shortest distance between fillet weld surface and weld root. For concave fillet welds, since there is no raised part, the effective welding throat is equal to the actual welding throat.

 Foot size: Distance from joint root to fillet weld toe.

 Toe: The point at which the surface of the weld meets the base material.

When determining the fillet weld size, it must first be clear whether the fillet weld is convex or concave. Convex refers to the slight convex surface caused by the uplift of the weld, which is related to the height of the uplift. The raised height of fillet weld is the same as the strengthening height of groove weld.

     If the weld is dented, it indicates that the surface of the weld is concave.

     For these two shapes, the weld size of a fillet weld with equal foot height is expressed as "the straight edge of the largest isosceles right triangle obtained in the fillet profile (two feet of equal length)."

    Therefore, for a convex fillet weld, the weld foot is equal to the weld size, but for a concave fillet weld, the weld size is slightly less than the weld foot length.

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In the process of welding, the shape of tungsten needle end of tungsten electrode has a very important effect on the quality of welding

Good end forming can ensure the stability of arc, prolong the service life of tungsten needle and improve the welding quality, otherwise it will cause unstable arc drift, easy to form welding defects..

Many of our welder brothers polish tungsten needle on the grinder, manual operation is not only difficult to ensure the end shape, and tungsten is a heavy metal, inhalation causes internal radiation!

Then is there a simple operation convenient to carry, and harm to the human body small sharpening machine?

Changzhou huarui welding&cutting Machinery Co., Ltd. according to the demand of the market and the majority of welder friends to launch a handheld tungsten needle sharpening machine!

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Gas welding machine is a machine, electricity, gas trinity equipment, in the use of the process, for its problems should be understood, analyzed and solved from three aspects. Generally speaking, can not welding for circuit fault, bad welding for mechanical fault, welding is not good for process problems or protection gas impurity, gas path problems and other reasons. This is a reflection of experience, and the latter two account for 90% of the questions.

Users in the encounter of arc instability, welding effect is not good, air holes and other abnormal phenomenon, do not prematurely conclude that the failure of CO2 welding machine, the occurrence of the above problems or abnormal phenomenon, often have the following factors: Such as blown fuse, loose fastening part, forget switch, improper parameter adjustment, cable break, gas hose crack leakage, CO2 welding gun damage, these faults and abnormal phenomena can be eliminated by the user. If the circuit fault cannot be removed, you can go to the local sales outlets for after-sales treatment.

Next, we will explain to you what problems are easy to encounter in the process of gas welding, what defects are easy to produce in the weld and what preventive measures.

                                       Welding pore

Cause

1. Gas regulator flowmeter is damaged or frozen and blocked;

2. The set value of gas flow does not match;

3. Failure of the welding gun body;

4, solenoid valve failure;

5, the surface of the base material has oil, dirt, rust, paint film or welding wire extended too long;

6. Rust or other quality defects on the surface of the welding wire.

Preventive measures

1. Replace the flowmeter or heat the gas flowmeter;

2, adjust to the appropriate airflow value;

3. Replace the welding gun;

4. Replace the solenoid valve;

5. Clean the surface of the base metal, and adjust the length of the welding wire to the appropriate length;

6. Replace the welding wire.

                                          The welding current is unstable

cause

1, the input and output cables are too long and too fine to cause current instability;

2, the wire feeding mechanism has damage and wear phenomenon;

3. Aging of wire hose;

4, welding gun head wear, damage phenomenon;

5, conductive nozzle wear serious (or aperture wrong).

Preventive measures

1. If the input and output cables are too long, use cables with larger diameters;

2. Replace the new wire feeding mechanism;

3. Replace the new wire hose;

4. Replace the welding gun head;

5. Replace the new conductive nozzle.

                                          The wire feed is unstable

cause

1. Large resistance of wire hose;

2. Improper adjustment of wire feeding pressure;

3, poor welding wire, welding wire cross, uneven diameter or hard bending;

4, the size of the conductive nozzle is wrong or the inner diameter is too small;

5. There is dirt in the wire feeding wheel;

6. The bending radius of the welding torch cable is small, and the bending radius should be greater than 300mm.

Preventive measures

1. Clean the wire hose with compressed air or replace the wire hose;

2, adjust the pressure arm, select the appropriate wire feeding pressure;

3, choose good quality welding wire;

4. Select a conductive nozzle matching the diameter of the welding wire;

5. Clean or replace the wire feeding wheel in time;

6. Make the torch cable straight as far as possible during welding.

                                                                    Arc instability

cause

1. The output voltage is unstable;

2, the wire is not stable;

3. Poor quality of welding wire;

Preventive measures

1, try to keep the dry elongation of welding wire unchanged;

2. Check wire feeding during welding to ensure smooth wire feeding;

3. Select welding wire with good quality;       

                          The welding wire adheres to the conductive nozzle

cause

1, the length of the welding wire is too small;

2, the gas flow is too small;

3. The spatter in the nozzle is not removed in time;

4, welding parameters do not match.

Preventive measures

1, choose the appropriate dry elongation;

2. Adjust the appropriate gas flow rate;

3. Clean the spatter in the nozzle in time;

4. Select appropriate and matched welding parameters.

                                                Welding splash

cause

1. Welding current does not match with voltage;

2, the inductance does not match, the larger the current is, the larger the inductance is;

3. The welding wire is extended too long or too short;

4. There are rust and oil stains on the surface of base metal and welding wire.

Preventive measures

1.adjust the matching current and voltage value;

2. adjust the appropriate inductance value according to the current;

3. choose the appropriate dry elongation;

4.Clean the surface of the base metal or replace the welding wire. 

                                               Crack in weld

cause

1. Welding wire or workpiece is not clean;

2. welding wire quality is not good;

3. the first welding seam is too thin;

4. The current and penetration are too large.

Preventive measures

1. Replace the new welding wire and clean the workpiece surface;

2. Select welding wire with good quality;

3. increase the width of the pass;

4. adjust the welding specification, control the penetration.

       A serpentine pass appears during welding                                                     

cause

1, conductive nozzle wear serious;

2. The conductive nozzle is loose;

3. The mouth of the wire feeding tube is too large;

4, dry elongation of welding wire is too large;

5, welding wire alignment adjustment is poor.

Preventive measures

1. Replace the new conductive nozzle;

2. Tighten the conductive nozzle;

3. Regular maintenance and cleaning of wire feeding tube or replacement of new wire feeding tube;

4, choose the appropriate dry elongation;

5, adjust the scale of the pressure arm, choose the right pressure.

                                             Underpenetration

cause

1. Small welding current and shallow melting depth;

2. groove and clearance size is unreasonable, blunt edge is too large;

3. the arc is too long;

4. Poor cleaning between layers and welding roots.

Preventive measures

1. Adjust and select the appropriate welding current;

2. The design of groove and gap size should be reasonable.

3. reduce the arc length;

4. Clean the interlayer and welding root. 

                                              Weld through

cause

1. Welding current is too large;

2, the welding parts heated too much;

3, groove butt clearance is too large;

4, welding speed is slow, arc residence time is long, etc.

Preventive measures

1. Select appropriate current parameters;

2. When heating welding parts, attention should be paid to the temperature should not be too high;

3. Groove and clearance design should be reasonable;

4. Accelerate the welding speed appropriately.

 The main causes of magnetic bias are as follows:

 ① The grounding cable is not connected correctly

For a long workpiece, magnetic bias will occur if one end is grounded, as shown in Figure A.

This is because there is current flowing through the workpiece on the left of the axis of the welding wire. The magnetic field generated by it is superimposed with the magnetic field of the arc, making the magnetic field line density on this side greater than that on the right side of the arc.

In this way, the larger electromagnetic force on the left side pushes the arc to the right side.

The solution is to ground the workpiece on both sides. The welding wire (electrode) tilts to the right at a certain Angle, enlarges the left space, and makes the magnetic field line density on both sides symmetrical, as shown by the dotted line in Figure b.

② Ferromagnetic material

The magnetic conductivity of ferromagnetic materials is much greater than that of air. When there is ferromagnetic material (such as steel plate, iron block, etc.) on one side of the arc, the magnetic force line on this side will be sucked into the ferromagnetic material, so that the density of magnetic force line near the axis of the welding wire (electrode) on this side is less than that on the other side, as shown in the figure.

In this way, the force on the ferromagnetic side is less than on the other side, and the arc is biased toward the ferromagnetic side, just as the ferromagnetic material attracts the arc. The larger the steel plate around the arc or the closer it is to the arc, the more serious the magnetic deflection will be.

For the long workpiece, when the arc walks to the end of the workpiece, the arc will be biased to the inside of the workpiece, which is caused by the asymmetry of ferromagnetic material on both sides of the arc at this time. The magnetic permeability area on the inside of the steel plate is much larger than that on the outside, which is equivalent to placing a piece of ferromagnetic material on the inside, as shown in the figure below.

What are the common methods to reduce arc deflection?

Bias blowing will lead to the deterioration of arc operability, welding process instability, poor weld formation, serious welding defects and arc extinguishing, so it must be avoided in the welding process. The methods to prevent magnetic bias are as follows.

① Try to use short arc, this is because the short arc is not easy to be affected by airflow, the possibility of blowing off. Even when magnetic bias is produced, the deviation is smaller than in the case of long arcs.

② If allowed, try to use AC arc for welding, because AC arc almost will not produce magnetic bias.

(3) Outdoor work, such as strong winds, must take shelter measures to protect the arc. When welding the pipe, try to plug the pipe mouth to prevent the airflow in the pipe caused by blowing.

(4) For the butt weld with large groove clearance, a backing plate is added under the weld to prevent arc deflection caused by heat convection.

⑤ Add a small piece of arc initiation plate and arc extinguishing plate at both ends of the weld, so that the distribution of magnetic force lines on both sides of the arc walking at the end is also as symmetrical as possible.

⑥ Adjust the Angle of the welding wire (electrode) in the operation, so that the magnetic force line on both sides of the wire to maintain symmetry, this method is used in the actual production.

The use of correct wiring methods, for the larger workpiece using both ends of the ground.

⑧ try to use small current for welding.

At present, the most commonly used welding methods are solid cored wire gas welding (GMAW) and flux cored wire gas welding (FCAW). For carbon steel and low alloy steel, the main gas used in GMAW is argon (Ar)+ carbon dioxide (CO2), and the main gas used in FCAW is 100% carbon dioxide (CO2). It is found that FCAW is more prone to cold crack than GMAW. In order to verify the above inference, we carried out relevant experiments. First of all, we use the method of measuring diffused hydrogen of fused metal, and study the influence of different welding methods, welding parameters and gas on diffused hydrogen of fused metal. Secondly, the influence of diffused hydrogen content on the cold crack was studied by the inclined Y-groove welding crack test.

                          Experimental content and results of diffused

hydrogen determination of fused metal

The welding sample includes the front and rear two priming, arc extinguishing test block and the formal sample in the middle. The sample material was composed of carbon steel. The intermediate sample was treated with hydrogen at 650C for 1 hour and then cooled in an inert atmosphere. Then the intermediate sample was weighed with an accuracy of 0.01g. Store in a clean, dry container. The welding methods were GMAW and FCAW. For GMAW, we used gas Arcal 5, Arcal 21 and welding wire ER70S-6. For FCAW, 100% carbon dioxide (CO2) is used as gas, and E71T-1 low hydrogen flux-cored wire with diffused hydrogen content of H4 and H8 is used as welding wire, respectively. Welding equipment adopts automatic welding trolley, in order to ensure the comparability of the test, the welding environment must be controlled constant temperature and humidity. Before welding, we need to purge the welding pipeline for half an hour to ensure that there is no moisture adsorption in the gas pipeline. In addition, the dew-point meter is used to measure the dew-point at the outlet of different gas cylinders to master the moisture content of the above different gases. The welding mode is non-swing welding, and the welding parameters must be 4g+-0.5g of molten metal. Immediately after the welding (within 3-5s), place the welding sample in the ice-water mixture for 20 seconds, and then store the sample in the dry ice alcohol solution or liquid nitrogen below -78C. Before performing the diffused hydrogen content test, clean the intermediate sample with acetone or alcohol solution and quickly dry it with cold air. It is then placed in a collector as shown in the figure and argon gas is added to isolate the sample. The collector was placed in a heating furnace at 45℃ for 72 hours to collect diffused hydrogen from the central sample. Finally, the content of diffused hydrogen was measured by gas image chromatography.

The test results show that:

1) The diffused hydrogen content of fused metal using FCAW is significantly higher than that of solid cored gas shielded welding (GMAW). The main reason is that we believe that the water contained in some raw materials (such as rutile and fluorite) of powder inside the flux-cored wire exists in the form of crystal water. These crystalline waters have strong ion-dipole interaction and can remain stable below 1000C. When under the action of an electric arc, it breaks down into atomic hydrogen and enters the molten metal.

2) For gas shielded welding of solid cored wire, the diffused hydrogen content of fused metal with short circuit transition is lower than that of fused metal with jet transition. The main reason is that compared with the injection transition, the short circuit transition has a shorter arc length, and the contact range between the atmospheric water and the arc around the arc is smaller, which will produce less hydrogen atoms to dissolve into the molten metal.

3) For gas shielded welding with solid cored wire, the content of diffused hydrogen of cladding metal decreases with the increase of CO2 in gas. The main reason is that CO2 is decomposed into oxygen atoms under the action of electric arc, and oxygen atoms in the atmosphere of electric arc combine with hydrogen atoms, thus reducing the number of hydrogen atoms entering the interior of the fused metal.

Experimental content and result of welding crack of inclined Y groove

It is known from the above experiments that FCAW has a higher diffused hydrogen content in fused metal than GMAW. Whether the more diffused hydrogen content, the more likely to cause cold crack? Now let's look at the following experiment.

The sample is a sample of inclined Y-groove welding crack. The material is low carbon steel and the plate thickness is 25mm. The middle groove is inclined Y groove, and the two slope mouths are X groove. Before the formal welding test, it is necessary to complete the welding of X groove on both sides. The weld shall be 100% fully permeable. The weld can also be called constrained weld, whose purpose is to make the intermediate formal weld receive greater restraint during welding. Formal welds only require one weld.

We are going to use three samples.

Sample ① : Welding method is GMAW, gas is Arcal 5;

Sample ② : The welding method is GMAW, and the gas is Arcal 21;

Sample ③ : The welding method is FCAW, and the gas is 100%CO2.

The welding parameters shall be consistent with those used in the diffused hydrogen content test.

When PT test (penetration test) was carried out on the surface weld, it was found that cracks had appeared on the surface of the weld using FCAW (there was a purple line in the middle of sample ③, where was the crack). Therefore, it can be shown from the test that the sensitivity of cold crack is proportional to the content of diffused hydrogen in the weld.

                                                                Technical background

Push and draw fuse gas shielded welding gun is widely used in industrial manufacturing. Compared with the ordinary push-wire welding gun, the push-wire welding gun comes with a motor. When working, the motor on the front of the welding gun pulls the welding wire, and the motor on the welding machine pushes the welding wire, the wire feeding distance can reach 12 meters or even farther, and the longest of the ordinary push-wire welding gun can only do about 5 meters. Because the welding wire tray is on the welding machine, you can use a large welding wire tray, and the melting rate is the same as that of the ordinary push wire welding gun.

In aluminum or copper welding, because the welding wire is soft, it is easy to plug the wire in the process of pushing the wire, so the length of the welding gun for soft welding wire can only be about 3 meters, then the advantages of pushing and drawing the welding gun in length are more obvious. In the actual industrial production, the push and draw welding gun is mainly used for welding with soft welding wire. In recent years, with the national efforts to promote energy saving and efficiency improvement and the development of molten gas welding technology, molten gas welding is more and more applied in the welding field of aluminum alloy, and more and more application places of push-draw welding gun.

Push-draw welding gun, as a kind of welding auxiliary equipment, must be used with welding machine and related protective gas, but most of the existing fuse gas shielded welding machines do not support push-draw welding gun. If the user needs to use the push-draw type welding gun, it is necessary to buy a welding machine supporting the push-draw type welding gun or install synchronous drive on the existing machine. The former requires a large amount of capital, which is not allowed economically when the usage rate is not high, the latter requires complex wiring and installation, the need for independent power supply, and the motor drive tension is unstable, the effect is not ideal. Attached Figure 1 is the wiring diagram of the independent control circuit board of the prior art wire-drawing welding gun. The control board of the wire-drawing motor 2 is fixed in the wire feeder 1 at the side of the welding machine. The independent power supply is connected through relay 6, and the welding gun is connected through the cable. Wire pusher motor controller 5 connects to wire pusher motor 4 through wire pusher motor connection, and welding gun 3 through welding gun control line. In this connection mode, an independent power supply is needed, and a relay needs to be connected to control the on and off of the power supply. The relay is generally controlled by the power supply of the air supply valve. The wiring mode is complex, and the power of pulling the welding wire is not constant, and the current limiting resistance above is easy to heat.

Although some welding machines support push-draw welding gun, but because of the different welding machine manufacturers design wire-drawing welding gun drive circuit, users purchase push-draw welding gun need to inform the manufacturer of motor parameters, potentiometer parameters and wiring mode and other information, the manufacturer begins production after receiving the order. This causes long order cycle, small batch, inventory pressure and high production costs.

Moreover, different motor parameters are not the same, the control mode of the push and draw welding gun has very strict requirements on the parameters of the welding gun motor, sometimes only the same parameters can be used, otherwise it can not work normally.

                       Basic method of welding with push-draw welding gun

1. welding temperature

TBi air cooled push wire drawing welding gun material polyethylene polypropylene ABS hard PVC polyformaldehyde polycarbonate

TBi air cooled push wire drawing welding gun Temperature: 250-280 220-230 230-250 220-240 270-300 260-280

2. welding method:

The welding gun nozzle is aligned with the electrode, in order to make the electrode and the welding place heated at the same time, push the wire drawing welding gun, the welding gun should be fan-shaped swing between the weld and the electrode, when the electrode is softened, the constant pressure is applied to the welding strip, and the nozzle of the welding gun is slowly welded from left to right. The Angle of the nozzle of the plastic welding gun and the surface of the base material depends on the thickness of the base material and the temperature of the hot air. Generally, the tilt Angle is 25°-45°, and the distance between the nozzle and the weld surface should be about 10mm. The swing speed of the plastic welding gun is generally 2 times per second, and the swing is about 10mm.

                                         Attention

Spot welding Electrode spot welding is usually divided into two sided spot welding and single side spot welding. Push the wire drawing welding gun to push the wire drawing welding gun to push the wire drawing welding gun, the electrode from both sides of the workpiece to the welding. Typical double-sided spot welding is commonly used, when both sides of the workpiece have electrode indentation. Spot welding of large welding area of conductive plate as the lower electrode, so as to eliminate or reduce the indentation of the workpiece below. Generally used for spot welding of decorative panels. At the same time welding two or more double-sided spot welding, with a transformer to connect the electrodes in parallel, at this time, the impedance of all current paths must be basically equal, and the surface state of each welding position, material thickness, electrode pressure must be the same, push and draw wire welding gun to ensure that the current through each spot is basically the same, using multiple transformers double-sided multi-point spot welding, This will avoid shortfalls

The fault is divided into mechanical fault and electrical fault

(1) Mechanical failure

 Fault symptoms.

1, the wind pressure is reduced, the air volume is reduced;

2, the air pump shell is crackling;

3, vibration is too large;

These failures are caused by the wear of the motor bearing, the use of the carbon brush of the push and draw welding gun, the deformation of the impeller or the foreign body in the impeller and other reasons; This fault can be checked around the motor shaft

Whether the swing is too large, wind leaf and other parts can be solved; If the blade or rectifying ring should be replaced, special attention should be paid to their installation direction push wire drawing welding gun

(2) Electrical faults

1, the temperature is too high, may be silicon controlled short circuit (use a multimeter to measure T1, T2 resistance is 0Ω) or temperature controller failure; Push and draw welding gun

2, Adjust the temperature regulator knob, can adjust the temperature, but can not be adjusted to the cold air, TBi360 push wire drawing welding gun, with ammeter measurement, ammeter pointer between 2A-4.5A, can not go back At about 0.5A, the temperature controller fails; Push and draw welding gun

3, the use of sudden cooling, temperature controller failure, heater circuit (use a multimeter to measure the resistance between the heater pins, the normal value is about 50Ω) or Thyristor damage;

4, in the use of the process, the temperature is high and low, it may be damaged potentiometer or a wire joint loose resulting in poor contact.

5. When running, the speed drops sharply and the wind pressure drops significantly, it may be the motor is damaged. The temperature controller knob should be immediately adjusted to "1" to push and draw the wire welding gun, and the TBi air cold push and draw the wire welding gun should be repaired after the barrel is cooled.

The selection of electrode must be in ensure the safety and reliable use of welding structure, according to the chemical composition of the welding material, mechanical properties, plate thickness and joint form, welding structure characteristics, stress state, structural conditions, the requirements of weld performance, welding construction conditions and technical and economic benefits, targeted selection, if necessary, also need to carry out welding performance test.

                                              Same steel welding electrode selection points

1. Consider the mechanical properties and chemical composition of weld metal

For common structural steel, the strength of weld metal and base metal is usually required, and the tensile strength of fused metal equal to or slightly higher than that of base metal should be selected. For alloy structural steels, the alloy composition is sometimes required to be the same or close to that of the base metal. The welding rod with lower strength than the base metal should be considered when the welding structure is rigid, the joint stress is high and the weld is easy to crack. When the content of carbon, sulfur, phosphorus and other elements in the base metal is high, cracks are easy to occur in the weld, and alkaline low hydrogen electrode with good cracking resistance should be selected.

1. Consider the performance and working conditions of welded components

For the welding parts under dynamic load and impact load, in addition to meeting the strength requirements, the main thing should be to ensure that the weld metal has high impact toughness and plasticity, and the low hydrogen electrode with high plasticity and toughness index can be selected. For welds in contact with corrosive media, stainless steel electrode or other corrosion resistant electrode should be selected according to the nature and corrosion characteristics of the medium. For welding parts working under high temperature, low temperature or other special conditions, corresponding heat resistant steel, low temperature steel, surfacing or other special diagnostic electrodes shall be selected.

1. Consider the characteristics of welding structure and stress conditions

For the thick and rigid welding parts with complex structure and shape, due to the great internal stress produced in the welding process, it is easy to crack the weld, so the alkaline electrode with good cracking resistance should be selected. On the force is not large, welding parts difficult to clean up, should choose to rust, oxide skin, oil sensitive acid electrode. For welding parts that cannot be turned over due to conditions, the electrode suitable for all-position welding should be selected.

4.  Consider construction conditions and economic benefits

In the case of meeting the requirements of product performance, the acid electrode with good technology should be selected. In narrow or poor ventilation conditions, should choose acid electrode or low dust electrode. For the structure of large welding workload, conditions should be as far as possible to use high efficiency electrode, such as iron powder electrode, high efficiency gravity electrode, or the selection of the bottom electrode, vertical downward electrode and other special electrode, in order to improve welding productivity.

          Key points of welding rod selection in dissimilar steel welding

1. Carbon steel ten low alloy steel with different strength levels

  （Or low alloy steel ten low alloy high strength steel）

1.  Low alloy gold steel + austenitic stainless steel

Low alloy steel + austenitic stainless steel should be selected in accordance with the limited value of the chemical composition of the molten metal, the general selection of chromium, nickel content is higher, plastic, crack resistance is better Cr25-N; Type 13 austenitic steel electrode to avoid cracks due to the formation of brittle hardened microstructure. But the welding process and specification should be determined according to the poor welding of stainless steel.

3.  Stainless steel composite steel plate

Stainless steel composite steel plate should be considered for the base, cladding, transition layer welding requirements of three different performance of the electrode. For the welding of the base (carbon steel or low alloy steel), the corresponding strength grade of structural steel electrode is selected; The coating is directly in contact with the corrosive medium, and the austenitic stainless steel electrode of the corresponding composition should be selected. The key is the welding of the transition layer (that is, the interface between the coating and the base layer). The dilution effect of the substrate material must be considered, and the Cr25-Ni13 austenitic steel electrode with high chromium and nickel content, good plasticity and crack resistance should be selected.

  Abstract: The properties and components of welding shielding gases have a great impact on welding quality. Different properties of common welding shielding gases have different effects on welding speed, weld penetration and arc stability. Therefore, in the actual production process, a variety of influencing factors should be considered comprehensively to select appropriate shielding gases to provide reliable guarantee for welding quality.

一、Introduction

In the process of arc welding, if no protector is added, oxygen (O2) or other gases in the atmosphere will invade the arc and molten pool and react with the metal melted at high temperature, resulting in welding defects and affecting the adaptability of the product. The main function of welding gas is to protect the metal to be welded from pollution of other gases and impurities, to ensure the quality of welding products; On the other hand, the properties of welding protective gas have corresponding effects on welding speed, weld penetration, forming, welding dust, arc stability and so on. Welding protective gas can be divided into inert gas (e.g., He) and non-inert gas (e.g., CO2) according to the degree of gas activity. According to the component composition, it can be divided into a single component gas (such as: GTAW- using pure Ar as a protective gas) and multiple gas mixture (such as: GMAW- using pure 75%Ar+25%CO2 as a protective gas); According to the oxidation strength of gas can be divided into strong oxidizing gas, weak oxidizing gas, reducing gas, neutral gas; In addition, in addition to gas activity degree, component composition, oxidation tendency, ionization energy and thermal conductivity of protective gas are also important basis for selection of protective gas.

一、Selection and application of welding protective gas

（一）Selection and application of welding gas in GTAW

Argon (Ar) is an inert gas, does not react with other elements at normal and high temperature, and has good arc stability, low ionization voltage, beautiful weld formation and low cost, so it is suitable for various metal welding protection gas in GTAW. At present, the metals that can use argon as welding protection gas in GTAW include all carbon steel, stainless steel and almost all non-ferrous metals such as aluminum, copper, nickel, titanium, zirconium and their alloys, and the welding effect is excellent. Although using pure argon as GTAW protective gas can obtain excellent welded joints, due to its low arc energy, the welding penetration force and welding speed cannot meet the actual needs. In this case, argon can be mixed with other gases to improve the welding penetration force and welding speed, such as Ar+He or Ar+H2.

（二）Selection and application of GMAW welding gas

   1, ordinary carbon steel (structural steel) GMAW protective gas selection

For ordinary carbon steel, or structural steel used for the production of steel structure, if there are not strict requirements on welding quality and welding appearance, CO2 is usually used as a protective gas, also known as CO2 gas shielded welding, this method has the advantages of high production efficiency, good welding quality, low cost, strong practicability and so on. It should be noted that the soluble mass fraction of water in liquid CO2 is 0.05%, and the excess water is free to sink at the bottom of the bottle. This water evaporates with CO2 in the welding process and mixes with CO2, directly into the welding zone. Therefore, water is the most important harmful impurity in CO2 gas. The purity and quality of the protective gas CO2 shall meet the requirements.

Ar+CO2 can be used as a protective gas when non-destructive testing or pressure test is needed in situations with high welding quality. The change of the component of the protective gas can affect the spatial form of welding arc, arc energy density, droplet transition mode, welding wire melting characteristics and splashing during welding. It can also improve the stability of arc and transition in the welding process and the wetting of liquid metal and molten pool, improve weld forming, reduce spatter, eliminate and prevent defects, and improve the performance of weld joint.

 2, stainless steel GMAW protective gas selection

Stainless steel GMAW welding gas selection, not only according to the type of stainless steel and welding position and other factors, but also must consider back forming, welding combination, droplet transition form and other factors, in order to obtain the best welding effect.

With pure argon only suitable for TIG welding stainless steel, but not suitable for MIG welding stainless steel. In the process of gas shield welding under pure argon, the surface tension of stainless steel droplet and molten pool is large, and the fluidity of liquid metal in molten pool is very poor. The surface of weld can not be spread and moistened, and the forming of weld pass is poor.

When 1-2% oxygen is added into argon, the surface tension of stainless steel droplet and molten pool is reduced, the fluidity of liquid metal in molten pool is enhanced, and the spreading wettability of weld surface is improved. The weld depth and width are moderate, and the weld path is beautiful. 0-1% suitable for austenitic stainless steel, 0-2% suitable for ferritic stainless steel; The 0-2% molten pool has better fluidity than the 0-1% molten pool, suitable for the spray transition and pulse transition of stainless steel welding wire, suitable for the flat welding and flat fillet welding of stainless steel welding parts.       

When adding 2-5%CO2 to argon, it is concerned that there is a tendency to carburize. The test proved that CO2≤5%, carbon content of weld ≤0.03%, still below the level of ultra-low carbon. The arc stability is good, the oxidation ability is weakened, the alloying elements are burned less, and there is no carburizing tendency. Suitable for short circuit transition and pulse transition of stainless steel welding wire. When 2-5%CO2 is added in argon, it is suitable for the combination process of TIG bottom welding and MAG filling cover welding of stainless steel pipes. The welding process is all-position, short circuit transition, and the weld is smooth and beautiful.

The advantages of three-way gas mixture are more prominent, such as the three-way gas mixture divided into Ar+5%CO2+2%O2, strong arc concentration, good single-side welding and double-sided welding, suitable for stainless steel welding with higher technical requirements; It is divided into Ar+He+CO2 gas mixture, in which helium gas can increase weld penetration, improve welding speed and reduce the deformation of welds. The gas mixture composed of Ar+CO2+N2 is a new technology developed in Europe and America, in which nitrogen can increase the penetration depth and width of the weld.     

3, aluminum alloy GMAW protective gas selection    

For aluminum alloy suitable for welding, GMAW usually uses Ar as a protective gas. It is worth noting that aluminum alloy has higher requirements for the purity of welding gas. If the protective gas cannot reach, black oxides will appear on both sides of the weld, affecting the appearance quality of welding. A certain proportion of He can be added to Ar if the welding depth and welding speed are to be obtained. Because of the large heat transfer coefficient of He, the arc voltage is higher than that of Ar at the same arc length. High arc temperature, large base metal heat input, high melting speed. Suitable for welding thick aluminum plate, can increase penetration, reduce porosity, improve production efficiency. However, if the proportion of He is too large, there will be more spatter.   

4, other metal and alloy GMAW protective gas selection

   For copper and copper alloy GMAW, in addition to using pure Ar as welding protection gas, a certain proportion of nitrogen can be added to Ar gas, which can reduce the production cost and also play a protective role, but there is a certain splash and smoke, forming is poor. For nickel and nickel alloy GMAW, in addition to using pure Ar and Ar+He as welding protection gas, a small amount of hydrogen can also be added to Ar gas, which can also improve the welding efficiency. For titanium and titanium alloy GMAW, pure Ar and Ar+He can only be used as welding protective gas because Ti, N, H and O have strong tuberculosis. 

（一）conclusion                        

In summary, welding shielding gas plays a particularly important role in gas shielding welding. The selection of welding shielding gas directly affects the quality, efficiency and cost of welding production. Because of the diversity of welding materials, the selection of welding gas is more complex. In actual production, it is necessary to comprehensively consider welding material, welding method, droplet transition form, welding position and required welding effect, so as to select the most suitable welding gas and achieve the best welding result. 

In the structure of the boiler, pressure vessel, pressure pipe welding, the joint must be completely melted through, but some joints can not be in the back root or rewelding can not meet the requirements, then we must use single-side welding double-sided forming technology.

Single-side welding and double-sided forming technology refers to the operation process of welding from the front of the groove of the welded part to achieve the formation of a dense and uniform weld at the same time.

It is the selection of ordinary electrode or special bottom electrode welding, using different operation techniques to make the base metal groove blunt edge metal regularly melted into a certain size of the weld hole, in the arc action on the front weld pool at the same time, so that 1/3~2/3 arc through the weld hole and the formation of both sides are uniform and neat, forming good, in line with the quality requirements.

                                                    Two bottom operation

Continuous arc welding method and arc extinguishing welding (also known as broken arc welding) method of two categories:

(1) Even arc welding at the bottom, after the arc is ignited, artificial arc extinguishing is not allowed in the middle, and it must be a short arc continuous transport until the replacement of another electrode is extinguished.

(2) When the arc extinguishing welding method is laid, the arc is rhythmically started and extinguished, so as to control the temperature of the molten pool and form the single-side welding and double-sided forming.

                   Arc extinguishing welding base operation method

One-click threading method: when the arc is burned on both sides of the groove, the metal on both sides of the blunt edge is melted to form a melting hole at the same time, and then the arc is quickly extinguished. When the molten pool is about to solidify (dark red), the arc is ignited at the arc extinguishing place, and the operation is repeated in turn.

Suitable for: One click threading method is suitable for the welding of thin plate, small diameter pipe (≤φ60mm) and small gap (1.5~2.5mm).

Advantages: One click threading is not easy to appear slag inclusion, porosity and other welding defects.

Disadvantages: the temperature of molten pool is not easy to control, low temperature, easy to appear not welded through; High temperature, the back of the residual high is too large, even appear welding tumor.

Two-click threading method: The arc is ignited alternately on both sides of the groove, that is, a drop of metal is melted at the right blunt edge, and then a drop of metal is melted at the left blunt edge, and so on.

Suitable for：welding of thick plate, large diameter tube and large gap.

Advantages: Two-click threading is easy to master, the temperature of the molten pool is easy to control, and the blunt edge fusion is good.

Disadvantages: easy to appear slag inclusion, porosity and other welding defects.

Three-click threading method: After the arc is ignited, a drop of metal is melted at the left blunt edge, a drop of metal is melted at the right blunt edge, and then a drop of metal is melted at the middle gap.

Advantages: Three click threading method is suitable for the base of the larger gap.

Disadvantages: the back is prone to cold shrinkage hole defects.

              Essentials of arc extinguishing welding operation

First: look at the shape of the molten pool and the size of the molten hole, and distinguish between molten slag and molten metal.

Second: listen to the sound of arc breakdown groove root, when the arc breakdown groove root, will emit a "poof poof" sound, which indicates that the weld penetration is good. If there is no such sound, it indicates that the groove root is not broken down by arc. If welding continues, it will cause incomplete welding.

Three :accurately grasp the size of the formation of the weld hole, generally on both sides of the blunt edge need to melt 0.1~1 mm.

Four : ensure that the arc break and re-ignition time interval is short, generally not more than 1s.

                                         Arc extinguishing welding and arc closing technique

In the replacement of the electrode arc, should be the electrode to the root of the top pressure, so that the weld hole in front of the molten pool slightly expanded, at the same time to improve the arc extinguishing, arc burning frequency, filled with arc pit, so that the molten pool slow cooling and full, prevent shrinkage hole and arc pit cracks.

What is a conductive mouth

Conductive nozzle is: welding equipment wearing parts, belong to welding consumables, the tail part of the welding gun, the metal nozzle that sends the welding wire, called conductive nozzle.

Generally brass, copper, chrome zirconium copper, beryllium copper. Copper conductive nozzle is commonly used, chromium-zirconium copper material is better than copper, more durable. Ceramic makes conductive mouth, later due to performance and other problems were eliminated. The conductive nozzle is classified according to the specification diagram. Commonly used are co2 gas welding machine with conductive nozzle, submerged arc welding conductive nozzle, electrode.

In the European and American welding gun, mainly with copper conductive nozzle, in the Japanese and Korean welding gun is mainly equipped with chromium zirconium copper conductive nozzle. It is mainly used in Europe and the United States as a protective gas with argon as the main mixed gas, with long arc and poor gas cooling performance. The main way of the failure of the conductive nozzle is caused by the micro arc between the inner hole wall of the conductive nozzle and the welding wire. Therefore, red copper with better conductivity and thermal conductivity is used as the conductive nozzle, while CO2 gas is used as the protective gas in Asia, with short arc and strong gas cooling ability. Japan believes that the failure of the conductive nozzle is mainly caused by the friction loss between the inner hole wall of the conductive nozzle and the welding wire, so the rigid chrome-zirconium copper material is mainly used. As China mainly uses CO2 gas as a protective gas, chrome-zirconium copper conductive nozzle should be selected.

What are the functions and structural forms of gas welding conductive nozzle

In order to more steadily direct the current to the arc area, a copper nozzle is installed at the outlet of the torch. It can increase the service life of the conductive nozzle, enhance the stability of the welding process and improve the ability of conducting current. According to different requirements, the conductive nozzle is designed into seven forms: cone, oval, insert, cone table, cylinder, semicircle and roller.

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