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31/05/2023Welding quality inspection -- tightness inspection

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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26/05/2023What are the steps of dehydrogenation?

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.

图片1.jpg

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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26/05/202318 argon arc welding procedures!

11/05/2023Huarui recent international exhibitions

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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05/05/2023How to weld fillet welds？

27/04/2023How do you polish tungsten?

14/04/2023Common problems and defects of gas welding and preventive measures

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.

07/04/2023What are the effective ways to reduce arc deflection?

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.

17/03/2023Why is cold cracking more likely to occur in FCAW than in GMAW?

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.

10/03/2023The difference between push pull welding gun and ordinary welding gun

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.

03/03/2023It is important to choose the right electrode

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.

24/02/2023Selection and application of welding protective gas

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.

17/02/2023What are the operations and contraindications of zero-base welders?

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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02/12/2022What is the role of the conductive nozzle?

25/11/2022Why should the electrode be dried before welding

CO2 MIG welding is one of the welding methods, which is based on carbon dioxide gas as a protective gas for welding. In the application of simple operation, suitable for automatic welding and all-round welding. No wind during welding, suitable for indoor work. The full name of carbon dioxide shielded welding is carbon dioxide gas shielded arc welding. The shielded gas is carbon dioxide, and sometimes the mixture of carbon dioxide and argon is used. This welding technology is mainly used for manual welding.

In the application of simple operation, suitable for automatic welding and all-round welding. Poor wind resistance during welding, suitable for indoor work. Due to the special influence of zero thermal and physical properties of carbon dioxide gas, when using conventional welding power supply, it is impossible for the molten metal at the end of the wire to form a balanced axial free transition, usually need to use short circuit and melt drop neck burst, therefore, such as the use of high quality welding machine, the parameter selection is appropriate, you can get a very stable welding process, so that the splash reduced to the minimum degree.

Advantages of CO2 MIG welding:

1. Low welding cost. Its cost is only 40~50% of submerged arc welding and electrode arc welding.

2. High production efficiency. Its productivity is 1~4 times that of electrode arc welding.

3, easy to operate. Open arc, the thickness of the workpiece is not limited, can be welded in all positions and can be welded downward.

4. High cracking resistance of weld. The weld has low hydrogen and less nitrogen content.

5. Small deformation after welding. The angular deformation is 5 parts per thousand, and the unevenness is only 3 parts per thousand.

6, welding splash small. Welding spatter can be reduced when using ultra-low carbon alloy wire or flux-cored wire, or adding argon gas to carbon dioxide.

Harmful substances produced by CO2 MIG welding can be divided into two categories:

One is harmful gases: carbon dioxide (CO2), carbon monoxide (CO), nitrogen dioxide (NO2), ozone (O3)

One is soot: iron trioxide (Fe2O3), silicon dioxide (SiO2), manganese oxide (MnO), other impurities

These harmful substances, in addition to carbon dioxide is in order to protect the arc and weld pool, from the torch ejection, welding is not finished and left around the welding area, the rest of the harmful substances are produced from the welding arc and weld pool.

Due to the low cost of gas shielded welding, carbon dioxide gas is easy to produce, widely used in enterprises of all sizes, the price of the protective gas is low, the short circuit transition weld formation is good, and the use of deoxidizing agent containing welding wire can be obtained without internal defects of high quality welding joint. Therefore, this welding method has become one of the most important welding methods for ferrous metal materials.

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19/11/2022What are the advantages of CO2 MIG welding in use?

1. Electrode arc welding:

Principle - Arc welding method with manual operation of the electrode welding. The stable burning arc established between the electrode and the weldment is used to melt the electrode and the weldment so as to obtain a strong welded joint. It is gas-slag combined protection.

Main features - flexible operation; Low assembly requirements for welded joints; Weldable metal material wide; Low welding productivity; Strong dependence on weld quality (depends on the welder's operating skills and field play).

Application - widely used in shipbuilding, boiler and pressure vessel, machinery manufacturing, building structure, chemical equipment and other manufacturing and maintenance industries. It is suitable for welding of all kinds of metal materials, all kinds of thickness and all kinds of structural shapes.

2. Submerged arc welding (automatic welding) :

Principle - The arc burns under the flux layer. The heat generated by the burning arc between the wire and the weldment is used to melt the wire, flux, and base metal (the weldment) to form a weld. It is slag protection.

Main features - high welding productivity; Good weld quality; Low welding cost; Good working conditions; It is difficult to weld in spatial position; High quality requirements for weldment assembly; Not suitable for welding thin sheet (welding current less than 100A, arc stability is not good) and short weld.

Application - widely used in shipbuilding, boiler, bridge, lifting machinery and metallurgical machinery manufacturing. Submerged arc welding can be used in welds where the weld can be kept horizontal or the inclination Angle is not large. Plate thickness must be greater than 5 mm (burn through). Welding carbon structural steel, low alloy structural steel, stainless steel, heat-resistant steel, composite steel, etc.

3, CO2 Shielded Welding Torch (automatic or semi-automatic welding) :

Principle: Fusion electrode arc welding using carbon dioxide as a protective gas. It is gas protection. Main features - high welding productivity; Low welding cost; Small welding deformation (arc heating concentrated); High welding quality; Simple operation; High splash rate; It is difficult to weld with AC power; Poor wind resistance; Non - ferrous metals which are easily oxidized cannot be welded.

4, MIG/MAG welding (melting extremely inert gas/active gas welding) :

MIG welding principle -- an arc welding method using inert gas as protection gas and welding wire as melting electrode. The shielding gas is usually argon or helium or a combination thereof. MIG uses an inert gas, while MAG adds a small amount of active gas, such as oxygen and carbon dioxide gas, to the inert gas.

5. TIG welding (tungsten inert gas shielded welding)

Principle - Under the protection of inert gas, using the arc generated between the tungsten pole and the weldment to melt the base metal and the filler wire (or no filler wire) to form a welding seam. Electromelting is extremely low during welding.

6. Plasma welding

Principle -- A method of welding plasma arc with high energy density by means of the constraint effect of water cooled nozzle on the arc.

Welding precautions:

1、the length of the arc

The arc length is related to the type of electrode coating and the thickness of electrode coating. But all should be taken as short arc as possible, especially low hydrogen electrode. Long arcing may cause air holes. The short arc can avoid the harmful gases such as O2 and N2 from the atmosphere from invading the weld metal, forming bad impurities such as oxides and affecting the quality of the weld.

2. Speed of welding

The appropriate welding speed is based on the diameter of the electrode, the type of coating, welding current, the heat capacity of the welded object, the beginning of the structure and other conditions have their corresponding changes, can not be made standard provisions. Keep proper welding speed, molten slag can cover the molten pool well. All kinds of impurities and gases in the molten pool have sufficient time to emerge, to avoid the formation of slag inclusion and porosity of the weld. If the rod speed is too fast in welding, the shrinkage stress will increase when the welding part is cooled, so that the weld will have cracks.

The main points of welding wire selection

The selection of welding wire should be based on the type of steel to be welded, quality requirements of welding parts, welding construction conditions (plate thickness, groove shape, welding position, welding conditions, post-welding heat treatment and welding operation, etc.), cost and other comprehensive considerations.

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11/11/2022There are several common welding methods

1）This process is suitable for the welding of various large steel structures of low carbon steel and low alloy high strength steel. It has high welding productivity, good crack resistance, small welding deformation, large deformation range, and can be used for welding of thin and medium thickness plate parts.

1. Joint cleaning before welding requires that burrs, oil, rust and dirt affecting weld quality within 30mm on both sides of the groove must be cleaned.

2. When the construction environment temperature is below zero or the carbon equivalent of steel is greater than 0.41%, and the structural rigidity is too large, the object is thick, preheating measures should be adopted before welding, preheating temperature is 80℃~100℃, preheating range is 5 times of plate thickness, but not less than 100mm.

3. When the thickness of the workpiece is greater than 6mm, in order to ensure the penetration strength, the butt edge of the plate should be cut V-shaped or X-shaped groove, groove Angle is 60°, blunt edge p is 0~1mm, assembly gap b is 0~1mm; When plate thickness difference ≥4. Depending on your welding position if it is full position welding, the plate is not thick, the current is between 100 and 200. Voltage BETWEEN 18 and 25. It depends on the performance of your welder and the wire used.

2）How should the welding current of the second welding technology be adjusted?

Two welding arc position: generally at the beginning of the welding end 15-20mm arc, from the end of the normal welding, this is based on the thermal ionization of gas away from the design, because the higher the temperature, the easier the thermal separation, arc easy to produce. Call it: preheating method arc. Render. Generally, the current coarse adjustment and fine adjustment 1 grade, 2 grade, fine adjustment has 10 grade and wire feeding speed is proportional to, the more crisp the welding sound, the better the welding effect of the specific current adjustment has a formula, familiar with the current can be large or small, see their own technique. To see the thickness of the welding base metal and adjust, the voltage is proportional to the current, the voltage is too high splash will be more, and too low, the welding wire and the base metal does not melt.

3）How to operate the technology of secondary protection welding

Left welding, current voltage with good, stable arc combustion, welding wire end with a slightly larger than the wire diameter of the small molten ball is appropriate. As for how thick the base material with how much current can only be mastered in practice. Two welding welding skills, welding torch low, the protection of the gas, generally used left and right swing or swing back and forth, forward welding. Keep your hands steady.

Matters needing attention:

1. The joints with vertical or inclined openings must be welded from bottom to top, and the thin plate butt and vertical fillet welding without opening openings can be welded downward; Flat, horizontal and vertical butt joints can be welded to the left.

2. When the wind speed is greater than 1m/s, take windproof measures.

3. A reasonable welding sequence must be selected according to the structure of the welder.

4. Arc-starting and arc extinguishing plates with appropriate size should be set at both ends of the butt joint.

5. Clean up the dirt and spatter in the hose regularly.

6. Groove groove seam, especially thick plate multi weld, welding wire swing on both sides of the groove should be slightly stopped, the thickness of each layer of serrated strip is not more than 4mm, in order to make the weld fusion good.

7. According to the diameter of the welding wire, choose the electrode of the welding wire correctly, and the length of the welding wire should be controlled within 10 times the diameter of the welding wire.

8. When welding the wire feeding hose, it must be pulled smoothly, not coiled, and the radius of the wire feeding hose should not be less than 150mm. The remaining impure gas in the air supply hose should be discharged before welding.

9. The aperture of the conductive nozzle increases after wear, resulting in unstable welding. It is necessary to replace the conductive nozzle.

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