104 Practical Welding Tips You Should Know

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Common defects of welding robot

1. Welding deviation

The welding position may be incorrect or the welding gun may have problems in searching.

At this time, consider whether TCP (welding torch center point position) is accurate and adjust it.

If this happens frequently, check the zero position of each axis of the robot, and calibrate the zero again for correction.

2. Undercut occurs

It may be due to improper selection of welding parameters, incorrect welding gun angle or position, which can be adjusted appropriately.

3. Air hole problem

It may be that the gas protection is poor, the primer of the workpiece is too thick, or the protective gas is not dry enough, so it can be treated after corresponding adjustment.

4. Excessive splash

It may be due to improper selection of welding parameters, gas composition or too long extension length of welding wire, the machine power can be appropriately adjusted to change the welding parameters, the gas proportioner can be adjusted to adjust the proportion of mixed gas, and the relative position of welding gun and workpiece can be adjusted.

5. A crater is formed at the end of the weld after cooling

When programmable, add the function of arc crater in the work step to fill it.

Robot system failure

1. Gun collision

It may be due to the deviation of the workpiece assembly or the inaccurate TCP of the welding gun.

The assembly can be checked or the TCP of the welding gun can be corrected.

2. In case of arc fault, the arc cannot be started

It may be that the welding wire does not touch the workpiece or the process parameters are too small.

The wire can be fed manually, the distance between the welding gun and the weld can be adjusted, or the process parameters can be adjusted appropriately.

3. Protection gas monitoring alarm

If the cooling water or protective gas supply fails, check the cooling water or protective gas pipeline.

Programming Skills of Welding Robot

1. Select a reasonable welding sequence to reduce welding deformation and the length of the welding torch travel path to formulate the welding sequence.

2. The welding torch space transition requires a short, smooth and safe moving track.

3. Optimize welding parameters

In order to obtain the best welding parameters, the working test pieces are made for welding test and process qualification.

4. Reasonable positioner position, welding gun attitude and welding gun position relative to the joint shall be adopted.

After the workpiece is fixed on the positioner, if the welding seam is not the ideal position and angle, it is required to constantly adjust the positioner during programming, so that the welding seam can reach the horizontal position one by one according to the welding sequence.

At the same time, it is necessary to constantly adjust the position of each axis of the robot, reasonably determine the position and angle of the welding gun relative to the joint and the extension length of the welding wire.

After the position of the workpiece is determined, the position of the welding gun relative to the joint must be observed by the programmer with both eyes, which is difficult.

This requires programmers to be good at summarizing and accumulating experience.

5. Timely insert the gun cleaning procedure

After the welding procedure with a certain length is prepared, the gun cleaning procedure shall be inserted in time to prevent welding spatter from blocking the welding nozzle and conductive nozzle, ensure the cleanness of the welding gun, improve the life of the nozzle, ensure reliable arc ignition and reduce welding spatter.

6. Generally, the programming cannot be completed in one step.

A good program can be formed only by constantly checking and modifying the program, adjusting the welding parameters and welding torch posture during the robot welding process.

Design Measures for Reducing Welding Residual Deformation

1. Reasonable selection of weldment size

The length, width and thickness of the weldment have obvious influence on the welding deformation.

For example, the thickness of the plate has a great influence on the angular deformation of the fillet weld.

When the thickness reaches a certain value (about 9mm for steel), the angular deformation is maximum.

When manufacturing T-shaped or I-shaped welded beams, it is a prominent problem that the welding parts are slender, so that the welding area shrinks and deforms, causing bending deformation of the welding parts.

The best way to solve this problem is to carefully design structural size parameters (such as plate thickness, plate width, plate length and rib spacing) and welding parameters (such as unit linear energy, etc.).

2. Reasonably select weld size and groove type

The size of the weld not only relates to the welding workload, but also has a greater impact on the welding deformation.

Large weld size, large amount of welding and large consumption of filler metal cause large welding deformation.

Therefore, when designing the weld size, the smaller weld size shall be adopted under the condition of ensuring the bearing capacity of the structure.

One sided increase of weld size is extremely unfavorable to reduce welding deformation.

Therefore, for welds that do not bear great working stress, it is not necessary to use large fillet weld, as long as the strength requirements can be met.

In addition, the groove type shall be reasonably designed.

For example, the butt joint should adopt the best X groove size with zero angular deformation.

For T-shaped joints and cross joints with large stress, under the condition of ensuring the same strength, the dynamic load strength of the weld with groove is higher than that of the weld without groove, and the amount of weld metal is less, which is also beneficial to reduce welding deformation, especially for thick plates.

3. Minimize unnecessary welds

In the design of welding structure, the number of welds shall be minimized.

In general, ribbed plates are often used in design to improve the stability and stiffness of the structure.

Especially, sometimes thinner plates are used to reduce the weight of the main structure, which is bound to increase the number of ribbed plates, thus greatly increasing the workload of assembly and welding.

As a result, not only is it not economical, but also the welding deformation is too large.

Therefore, the practice has proved that it is economical to reasonably select the plate thickness, appropriately reduce the rib plate and reduce the weld seam, even though the structure may be slightly heavy.

4. Reasonably arrange the weld position

In order to avoid bending deformation of welded structure, in the structural design, it is necessary to make the weld position symmetrical to or close to the neutral axis of the material connection component.

Because the weld is symmetrical to the neutral axis, it is possible to completely offset or largely offset the bending deformation of the weld axis on both sides of the neutral axis.

Because the weld is close to the neutral axis, the bending moment caused by the weld shrinkage is reduced, so the bending deformation of the member is also reduced.

Therefore, the structure shall be symmetrical when welding the structure.

For some unsymmetrical structural members whose section shape cannot be changed, the deformation can be reduced by adjusting the distance between the center of gravity axis of the weld and the neutral axis while keeping the section shape unchanged.

Technological measures to reduce welding residual deformation

1. Reverse deformation method

During assembly before welding, the size of the deformation shall be estimated according to experience to give members a deformation opposite to the welding deformation direction, so as to offset the welding deformation and make the structure meet the technical requirements after welding.

There are two methods of reverse deformation: ① plastic reverse deformation; ② Elastic anti-deformation.

In actual production, elastic reverse deformation is more reliable than plastic reverse deformation.

Because even if the pre-strain of elastic anti-deformation is not accurate enough, angular deformation can always be reduced.

If plastic reverse deformation is adopted, the selected plastic pre bending amount must be very accurate, otherwise, good results will not be obtained.

2. Welding under external restraint

The weldment is rigidly fixed in the fixture to limit the deformation of the member during welding.

It has a good effect on reducing the angular deformation of the weldment, which can reduce the welding deformation, but the welding stress is high.

3. Reasonable selection of welding methods and welding specifications

In order to reduce welding deformation, welding methods with high energy density shall be adopted as far as possible.

Such as electron beam welding, laser welding, narrow gap welding, etc.

They have low welding line energy and minimal welding deformation.

In general production, CO2 gas shielded welding replaces manual arc welding, which is not only efficient, but also can significantly reduce welding deformation.

When welding thin plates, tungsten pulse argon arc welding or resistance welding and seam welding can be used to prevent buckling deformation.

If the method of low line energy is not available in production and the welding specification is not reduced, direct water cooling or water-cooled copper block can be used to change the thermal field distribution to reduce deformation.

However, for metal materials with high hardenability, this method should be used with caution.

4. Select reasonable assembly welding sequence and welding direction

The design of assembly and welding sequence mainly considers the influence of welding stress and deformation generated by the previous weld on the subsequent weld, as well as how the stress and deformation generated by the subsequent weld interact with the influence of the previous weld.

The practice has proved that the correct selection of assembly welding sequence is a powerful measure to prevent welding deformation.

In production, the welding structure of small parts and large parts is usually used for production. First, several parts and components are welded, and then the whole structure is assembled and welded.

Because the assembly and welding sequence of weldments are different, the increase of structural rigidity in the production process and the impact on welding deformation are also different, so it is necessary to analyze and compare them, and select the reasonable assembly and welding sequence with the minimum deformation.

Generally, the weld with large shrinkage shall be welded first, and then the weld with small shrinkage shall be welded.

When there are butt welds and fillet welds at the same time, the butt welds shall be welded first and then the fillet welds;

When there are transverse welds and longitudinal welds at the same time, the transverse welds shall be welded first and then the longitudinal welds;

When there are thick plate welds and thin plate welds at the same time, the thick plate welds shall be welded first and then the thin plate welds;

When intermittent weld and continuous weld exist in the structure at the same time, the continuous weld shall be welded first and then the intermittent weld.

5. Preheating

Uneven thermal field is the main reason for welding deformation.

Therefore, proper preheating shall be adopted;

It is also an effective measure to reduce welding residual deformation to make the welding temperature distribution more uniform.

6. Reduce out of plane deformation of welded thin plates by stretching and heating

Use mechanical method or preheating method to stretch or extend the wall panel to be welded.

At the same time, weld the wall panel to the frame of the structure.

After welding, remove the tensile load.

At this time, the contraction of the wall panel is restrained by the welded frame, so only a small amount of out of plane deformation occurs on the wall panel.

At this time, there is residual tensile stress in the welded wallboard and residual compressive stress in the frame.

This method has a good effect on reducing the bending deformation of welded thin plates.

Methods to prevent welding cold cracks

1. Select materials correctly

Alkaline low hydrogen electrode and flux are selected to reduce the content of diffusible hydrogen in weld metal;

Select and match the base metal and welding material;

If the technical conditions permit, materials with good toughness (such as welding materials with a lower strength level) can be selected, or “soft” covering can be applied to reduce the surface residual stress;

If necessary, chemical analysis, mechanical property, weldability and crack sensitivity tests shall be conducted on the base metal and welding material before manufacturing.

2. Welding operation shall be carried out in strict accordance with the correct process specification obtained from the test

It mainly includes:

The welding rods shall be dried strictly according to the specifications;

Select appropriate welding specification and line energy, reasonable current, voltage, welding speed, interpass temperature and correct welding sequence;

Check the spot welding;

Do a good job of back gouging of double-sided welding;

Carefully clean the groove and welding wire to remove oil, rust and moisture.

3. Select reasonable welding structure to avoid excessive restraint stress;

Correct groove form and welding sequence;

Reduce the peak value of welding residual stress.

4. Preheating before welding, slow cooling after welding, control of interpass temperature and post welding heat treatment are high strength steel with poor weldability and inevitable high restraint structure forms, which are effective methods to prevent cold cracks.

Preheating and slow cooling can slow down the cooling rate (prolong the residence time of △ t at 800~500 ℃), improve the microstructure of the joint, reduce the hardening tendency and reduce the structural stress;

Post weld heat treatment can eliminate welding residual stress and reduce the content of diffusible hydrogen in the weld.

In most cases, stress relieving heat treatment shall be carried out immediately after welding.

5. Hammer immediately after welding to disperse residual stress and avoid high stress zone, which is one of the effective methods to prevent cold cracks during local repair welding.

6. On the root of the weld and the surface of the weld 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. Inert gas shielded welding can maximally control the hydrogen content of the weld and reduce the cold crack sensitivity.

Therefore, TIG and MIG welding should be vigorously promoted.

Methods to prevent welding hot cracks

1. Limit the content of elements and harmful impurities prone to segregation in steel and welding materials, especially S, P and C, because they not only form low melting point eutectic, but also promote segregation. C ≤ 0.10% hot crack sensitivity can be greatly reduced.

If necessary, chemical analysis and low magnification inspection (such as sulfur print) shall be carried out for materials.

2. Adjust the chemical composition of weld metal, improve the structure, refine the grain, improve the plasticity, change the morphology and distribution of harmful impurities, and reduce segregation, such as using the dual phase structure of austenite plus less than 6% ferrite.

3. Increase the alkalinity of the welding rod and flux to reduce the content of impurities in the weld and improve the degree of segregation.

4. Select a reasonable groove form.

The weld forming coefficient ψ=b/h > 1. Avoid narrow and deep “pear shaped” welds (“pear shaped” welds will also be formed if the welding current is too large), and prevent the columnar crystals from converging in the center of the weld bead, causing central segregation and forming brittle sections;

Multi layer and multi pass welding is adopted to disrupt segregation and aggregation.

5. Use small (appropriate) welding line energy.

For austenitic (nickel base) stainless steel, try to use small welding line energy (no preheating, no swinging or less swinging, fast welding, small current), strictly control the interpass temperature, so as to shorten the residence time of weld metal in the high-temperature zone;

6. Pay attention to the protection during arc extinguishing.

The arc extinguishing shall be slow and the crater shall be filled to prevent the crater segregation from generating hot cracks;

7. Repeated repair shall be avoided as far as possible to prevent the accumulation of lattice defects from generating polygonal hot cracks;

8. Measures shall be taken to reduce the joint stress as much as possible, avoid stress concentration, reduce the stiffness near the weld, properly arrange the welding sequence, and make most of the weld joints welded under a small stiffness as much as possible, so that there is room for shrinkage.

Methods to prevent reheat cracks

1. When selecting materials, pay attention to the carbide-forming elements that can cause precipitation, especially the content of V.

When high V steel must be used, special attention shall be paid to welding and heat treatment.

2. Avoid the reheat-sensitive area during heat treatment to reduce the possibility of reheat cracks. If necessary, conduct heat treatment process test before heat treatment.

3. Minimize residual stress and stress concentration, reduce reinforcement, eliminate undercut, incomplete penetration and other defects, and polish the reinforcement and weld toe smoothly if necessary;

Increase preheating temperature, slow cooling after welding, and reduce residual stress.

4. Appropriate linear energy shall be used to prevent overheating and coarse grains in the heat affected zone.

5. On the premise of meeting the design requirements, select the welding rod with a lower strength level to release some of the stress eliminated by the heat treatment process (let the stress relax in the weld), which is beneficial to reduce reheat cracks.

Methods to prevent incomplete penetration

1. Control the groove size: clearance, blunt edge, angle and staggering;

2. Control current, polarity and welding speed; Fully preheat the joint and establish the first weld pool;

3. Control electrode diameter and welding angle; Overcoming arc deflection;

4. Backgouging of double-sided welding must be thorough;

5. The oil, rust, slag and dirt on the groove and blunt edge must be cleaned.

Weldability and its test evaluation

1. Welding: a process in which two objects are combined to form an indivisible whole by heating or pressurizing, with or without filler materials.

2. Weldability: refers to the ability of homogeneous or heterogeneous materials to weld to form a complete joint and meet the expected use requirements under the manufacturing process conditions.

3. The four major factors affecting weldability are: material, design, process and service environment.

4. The principles for assessing weldability mainly include:

① Evaluate the tendency of welding joint to produce process defects, so as to provide basis for formulating reasonable welding process;

② Evaluate whether the welded joint can meet the requirements of structural performance;

The design of new welding test method conforms to the following principles: comparability, pertinence, reproducibility and economy.

5. Carbon equivalent: the content of alloy elements in steel is converted and superposed according to several carbon contents, which can be used as a parameter index to roughly evaluate the cold crack tendency of steel.

6. Diagonal Y-groove butt crack test: it is mainly used to identify the cold crack tendency of the first layer of low-alloy high-strength steel weld and HAZ, and can also be used to propose welding processes.

1) For the preparation of test piece, the thickness of steel plate to be welded is δ=9-38mm.

The groove of butt joint shall be machined by mechanical method, and both ends of the test plate shall be welded within 60mm to restrain the weld, and double-sided welding shall be adopted.

Pay attention to prevent angular deformation and incomplete penetration.

Ensure that there is a 2mm gap at the weld of the sample to be welded in the middle.

2) Test conditions: the welding rod selected for the test weld shall match the base metal. The welding rod used shall be dried strictly.

The diameter of the welding rod shall be 4mm, the welding current shall be (170 ± 10) A, the welding voltage shall be (24 ± 2) V, and the welding speed shall be (150 ± 10) mm/min.

The test weld can be welded at different temperatures. Only one test weld is welded without filling the groove.

After standing and natural cooling for 24h after welding, take samples and conduct crack detection.

3) Detection and crack rate calculation.

Use the naked eye or hand-held 5-10 times magnifier to detect whether there are cracks on the surface and section of the weld and heat affected zone.

It is generally believed that when the surface crack rate of low alloy steel in the “small iron lapping” test is less than 20%, there is generally no crack.

7. Plug test: the purpose is to evaluate the tendency of hydrogen induced delayed cracking of steel, and to add other equipment to measure the reheat crack sensitivity and layered sensitivity.

1) For the preparation of test piece, the steel to be welded shall be processed or the cylindrical pin test bar shall be sampled along the rolling direction and the position of the pin in the thickness direction shall be indicated.

There is an annular or spiral notch near the upper end of the test bar. Insert the pin test bar into the corresponding hole of the base plate, so that the notched end is flush with the surface of the base plate.

For the pin test bar with an annular notch, the distance a between the notch and the end face shall make the weld bead penetration tangent or intersect with the plane cut by the notch root, but the circumferential penetration of the notch root shall not exceed 20%.

For low alloy steel, the value of a is 2mm when the welding heat input is E=15KJ/cm.

2) During the test, according to the selected welding method and strictly controlled process parameters, a layer of surfacing weld bead is melted on the bottom plate.

The center line of the weld bead passes through the center of the sample.

The penetration depth should make the notch tip located in the coarse grain zone of the heat affected zone. The weld bead length L is about 100-150 mm.

During welding, the t8/5 value of 800-500 ℃ cooling time shall be measured.

When welding is not preheated, the load shall be loaded when the temperature is cooled to 100-150 ℃ after welding;

When preheating before welding, it shall be loaded at 50-70 ℃ higher than the preheating temperature.

The load shall be applied within 1min and before cooling to 100 ℃ or 50-70 ℃ higher than the preheating temperature.

If there is afterheat, it shall be loaded before afterheat.

When the test bar is loaded, the pin may break within the load duration, and the bearing time shall be recorded.

Weldability of alloy structural steel

1. High strength steel: steel with yield strength σs ≥ 295MPa can be called high strength steel.

2. The solid solution strengthening effect of Mn is very significant.

When ωMn ≤ 1.7%, it can improve the toughness and reduce the brittle transition temperature. Si can reduce the plasticity and toughness.

Ni is an element that can both strengthen the solid solution and improve the toughness and greatly reduce the brittle transition temperature. It is commonly used in low temperature steel.

3. Hot rolled steel (normalized steel): low alloy high-strength steel with yield strength of 295-490MPa, generally supplied in hot rolled or normalized state.

4. Design principles of high-strength steel welded joints:

High strength steel is selected on the basis of its strength, so the principle of welded joint is: the strength of welded joint is equal to the strength of base metal (equal strength principle).

analysis:

① The strength of the welded joint is higher than that of the base metal, and the plastic toughness decreases;

② Equal to the life of the same time;

③ When less than, the joint strength is insufficient.

5. Weldability of hot rolled and normalized steels: hot rolled steels contain a small amount of alloy elements.

Generally, the cold crack tendency is small. Normalized steels contain more alloy elements, so the hardening tendency is increased.

With the increase of carbon equivalent and plate thickness of normalized steels, the hardenability and cold crack tendency are increased.

Influencing factors:

⑴ Carbon equivalent;

(2) Hardening tendency: hardening tendency of hot rolled steel and normalizing steel;

⑶ The highest hardness in the heat affected zone is a simple method to evaluate the hardening tendency and cold crack susceptibility of steel.

6. SR crack (stress relief crack, reheat crack): another type of crack may appear in the process of post weld stress relief heat treatment or post weld high temperature heating for welded structures such as thick wall pressure vessels made of Mo normalized steel.

7. Toughness is a property that characterizes the difficulty of metal to generate and expand brittle cracks.

8. Two aspects must be considered when selecting welding materials for low alloy steel:

① No welding defects such as cracks;

② It can meet the performance requirements.

Welding materials for hot rolled steel and normalized steel are generally selected according to their strength levels. The key points for selection are as follows:

① Select the welding materials of the corresponding level matching the mechanical properties of the base metal;

② The effects of fusion ratio and cooling rate are also considered;

③ Consider the effect of post weld heat treatment on the mechanical properties of the weld.

9. Principles for determining tempering temperature after welding:

① Do not exceed the original tempering temperature of the base metal so as not to affect the performance of the base metal itself;

② For tempered materials, avoid the temperature range where temper brittleness occurs.

10. Quenched and tempered steel: quenched+tempered (high temperature).

11. High strength steel welding adopts “low strength matching” to improve the crack resistance of the welding zone.

12. Two basic problems should be paid attention to when welding low carbon quenched and tempered steel:

① It is required that the cooling rate during martensite transformation shall not be too fast, so that martensite can have self tempering effect to prevent cold cracks;

② The cooling rate between 800 ℃ and 500 ℃ is required to be greater than the critical rate for producing brittle mixed structure.

Problems to be solved in welding of low-carbon quenched and tempered steel:

① Prevent cracks;

② The toughness of weld metal and heat affected zone shall be improved while meeting the requirements of high strength.

13. For low alloy steel with low carbon content, increasing the cooling rate to form low carbon martensite is beneficial to ensuring toughness.

14. The addition of alloy elements in medium carbon quenched and tempered steel mainly ensures the hardenability and improves the tempering resistance, while the true strength performance mainly depends on the carbon content.

Main features: high specific strength and high hardness.

15. There are three ways to improve the thermal strength of pearlitic heat-resistant steel:

① Matrix solution strengthening, adding alloying elements to strengthen ferrite matrix, common Cr, Mo, W, Nb elements can significantly improve the thermal strength;

② The second phase precipitation strengthening: in the heat-resistant steel with ferrite as the matrix, the strengthening phase is mainly alloy carbide;

③ Grain boundary strengthening: adding trace elements can adsorb on the grain boundary, delaying the diffusion of alloy elements along the grain boundary, thus strengthening the grain boundary.

16. The main problems existing in the welding of pearlitic heat-resistant steel are cold cracks, hardening and softening of heat affected zone, and stress relieving cracks in post weld heat treatment or long-term use at high temperature.

17. The temperature range from – 10 to – 196 ℃ is called “low temperature”, and the temperature below – 196 ℃ is called “ultra-low temperature”.

Austenite welding

1. Stainless steel:

Stainless steel refers to the general name of alloy steel with high chemical stability that can withstand the corrosion of air, water, acid, alkali, salt and other corrosive media.

2. The main corrosion forms of stainless steel include uniform corrosion, spot corrosion, crevice corrosion and stress corrosion.

Uniform corrosion refers to the phenomenon that all metal surfaces in contact with corrosive media are corroded;

Spot corrosion refers to the local corrosion that occurs dispersedly on the surface of metal materials, but most of them do not corrode or are slightly corroded;

Crevice corrosion: when there is a gap between stainless steels or surfaces contacting foreign matters in the electrolyte, such as in the oxygen ion environment, the solution flow in the gap will be sluggish, so that local Cl – of the solution will form a concentration cell, which will lead to the phenomenon that the stainless steel passive film in the gap will absorb Cl – and be damaged locally;

Intergranular corrosion, a selective corrosion phenomenon near the grain boundary;

Stress corrosion refers to brittle cracking of stainless steel under the action of specific corrosion medium and tensile stress, which is lower than the extremely strong strength.

3. Measures to prevent pitting corrosion:

1) Reduce the content of chloride ion and oxygen ion;

2) Add chromium, nickel, molybdenum, silicon, copper and other alloy elements into stainless steel;

3) Cold working shall not be carried out as far as possible to reduce the possibility of spot corrosion at the dislocation outcrop;

4) Reduce the carbon content in steel.

4. High temperature performance of stainless steel and heat-resistant steel:

475 ℃ brittleness, mainly occurs in ferrite with Cr > 13%.

Long term heating and slow cooling between 430-480 ℃ lead to strength increase and toughness decrease at room temperature or negative temperature;

σ phase embrittlement is typical of 45% Cr mass fraction. FeCr intermetallic compound is non-magnetic, hard and brittle.

5. Corrosion resistance of austenitic stainless steel welded joints:

1) Intergranular corrosion;

2) Intergranular corrosion in heat affected zone;

3) Knife corrosion.

6. Measures to prevent intergranular corrosion of welds:

1) Through welding materials, the weld metal can either become ultra-low carbon or contain enough stabilizing element Nb.

2) Adjust the weld composition to obtain a certain phase δ.

The theory of intergranular corrosion is essentially the theory of chromium depletion.

7. Intergranular corrosion in the sensitized zone of the heat affected zone:

It refers to the intergranular corrosion occurring in the part where the peak heating temperature in the welding heat affected zone is in the sensitized heating zone.

8. Knife corrosion:

The intergranular corrosion in the fusion zone is like a knife cut, so it is called “knife corrosion”.

9. Measures to prevent knife corrosion:

① Low carbon base metal and welding materials shall be selected;

② Adopt stainless steel with duplex structure;

③ Low current welding is adopted to reduce the overheating degree and width of the coarse grain zone;

④ The weld in contact with corrosive medium shall be welded finally;

⑤ Cross welding;

⑥ Increase the content of Ti and Tb in the steel, so that there is enough Ti, Tb and carbonization at the grain boundary of the welding coarse grain zone.

10. Why does stainless steel adopt low current welding?

To reduce the temperature of the welding heat affected zone, prevent the occurrence of intergranular corrosion of the weld, prevent the overheating of the welding rod and wire, welding deformation, welding stress, and reduce the heat input.

11. Three conditions causing stress corrosion cracking:

Environment, selective corrosion medium and tensile stress.

12. Measures to prevent stress corrosion cracking:

1) Adjusting the chemical composition, ultra-low carbon is conducive to improving the ability to resist stress corrosion, and matching the composition with the medium;

2) Remove welding residual stress;

3) Electrochemical corrosion, regular inspection and timely repair.

13. To improve pitting resistance:

1) On the one hand, segregation of Cr and Mo must be reduced;

2) On the one hand, the so-called “super alloyed” welding materials with higher Cr and Mo contents than the base metal are used.

14. Austenitic stainless steel

Austenitic stainless steel will produce hot cracks, stress corrosion cracks, welding deformation and intergranular corrosion during welding.

15. Reasons for welding hot cracks of austenitic steel:

1) Austenitic steel has small thermal conductivity, large linear expansion coefficient and large tensile stress;

2) Austenitic steel is easy to form columnar crystal with strong directivity by intergrowth crystallization, which is beneficial to segregation of harmful impurities;

3) Austenitic steel alloy has complex composition and is easy to be dissolved and eutectic.

16. Measures to prevent hot cracks:

① Strictly limit the content of P and S in the base metal and welding materials;

② Dual phase structure shall be formed in the weld as far as possible;

③ Control the chemical composition of the weld;

④ Low current welding.

17. What is the difference between the weld microstructure of 17.18-8 type and 25-20 type in preventing hot cracks?

A+δ structure is formed in the weld of 18-8 type steel, and a large amount of P, S and δ phases can be dissolved in the δ phase, which is generally 3% – 7%.

A+primary carbide structure is formed in the weld of 25-20 type steel.

18. When selecting austenitic stainless steel materials, attention shall be paid to:

① Adhere to the “applicability principle”;

② Determine whether it is applicable according to the specific composition of each welding material selected;

③ Consider the fusion ratio that may be caused by the specific welding method and process parameters;

④ Determine the alloying degree according to the overall weldability requirements specified in the technical conditions;

⑤ Attention shall be paid to the weld metal alloy system, the role of specific alloy components in the alloy system, and the requirements for service performance and process weldability shall be considered.

19. Weldability analysis of ferritic stainless steel:

1) Intergranular corrosion of welded joints;

2) Embrittlement of welded joint, high temperature embrittlement, phase σ embrittlement, 475 ℃ embrittlement.

Cast iron welding

1. Three characteristics of cast iron: shock absorption, oil absorption and wear resistance.

2. The properties of cast iron mainly depend on the shape, size, quantity and distribution of graphite, and the matrix structure also has some influence.

3. Ductile iron: F matrix+spherical graphite;

Grey cast iron: F matrix+flake graphite;

Vermicular graphite cast iron: matrix+vermicular graphite;

Malleable cast iron: F matrix+flocculent graphite.

4. Can low carbon steel electrode weld cast iron?

No, during welding, even if the current is small, the proportion of base metal in the first weld is 25% – 30%.

If C=3% in cast iron is calculated, the carbon content in the first weld is 0.75% – 0.9%, belonging to high carbon steel.

High carbon martensite appears immediately after welding cooling, and the welding HAZ will show white microstructure, which makes machining difficult.

5. Electric arc welding: the molten castings are preheated to 600-700 ℃, and then welded in the plastic state.

The welding temperature is not lower than 400 ℃.

In order to prevent cracking during welding, stress relief treatment and slow cooling are carried out immediately after welding.

This cast iron welding repair process is called electric arc welding.

6. Semi hot welding:

it is called semi hot welding when the preheating temperature is 300-400 ℃.

Weldability of magnesium and magnesium alloys

1. Oxidation and evaporation

Due to the strong oxidizability of magnesium, it is easy to form oxide film (MgO) during welding. MgO has a high melting point (2500 ℃) and a large density (3.2g/cm3), which is easy to form inclusions in the weld, reducing the performance of the weld.

At high temperature, magnesium is easy to react with nitrogen in the air to form magnesium nitride, which weakens the performance of the joint.

The boiling point of magnesium is not high, which will result in easy evaporation at high arc temperature.

2. Coarse grains

Due to the high thermal conductivity, high power heat source and high speed welding are required for magnesium alloy welding, which is easy to cause overheating and grain growth of the weld metal and near weld metal.

3. Thermal stress

The thermal expansion coefficient of magnesium alloy is relatively large, which is about 1~2 times of that of aluminum.

It is easy to produce large welding deformation and cause large residual stress during welding.

4. Collapse of weld metal

Because the surface tension of magnesium is smaller than that of aluminum, it is easy to produce weld metal collapse during welding, which affects the weld forming quality.

5. Air hole

Similar to aluminum alloy welding, magnesium alloy welding is prone to generate hydrogen porosity.

The solubility of hydrogen in magnesium decreases with the decrease of temperature, and the density of magnesium is smaller than that of aluminum, so the gas is not easy to escape, and pores will be formed during the solidification of weld.

6. Hot cracks

Magnesium alloys are easy to form low melting point eutectic structure with other metals, and crystal cracks are easy to form in welded joints.

When the temperature at the joint is too high, the low melting point compounds in the joint structure will melt and appear holes at the grain boundary, or produce grain boundary oxidation, which is the so-called “overburning” phenomenon.

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