1. Weld bias problem occurs

It may be that the weld is not in the correct position or that there is a problem when the torch is looking. In this case, consider whether the TCP (torch center point position) is accurate and adjust it. If this happens frequently check the zero position of each axis of the robot and recalibrate the zero to correct it.

  1. The problem of biting edge

May be improper selection of welding parameters, torch angle or torch position, can be properly adjusted.

  1. The problem of air holes

It is possible that the gas protection is poor, the primer of the workpiece is too thick or the protective gas is not dry enough, and can be dealt with by making corresponding adjustments.

  1. Too much splash problem

The welding parameters may be improperly selected, the gas components or the wire extension length is too long, the machine power can be adjusted to change the welding parameters, adjust the gas proportioning instrument to adjust the gas mixture ratio, adjust the relative position of the welding gun and the workpiece.

  1. The formation of an arc pit at the end of the weld after cooling problem

A buried arc pit feature can be added to the work step when programmed to fill it up.

Robot system failure

  1. Gun bumping occurs

It may be due to the deviation of the workpiece assembly or inaccurate TCP of the torch, which can check the assembly or correct the TCP of the torch.

  1. arc fault, can not lead the arc

It may be due to the welding wire does not touch the workpiece or process parameters are too small, you can manually feed the wire, adjust the distance between the welding gun and the welding seam, or properly adjust the process parameters.

  1. Protective gas monitoring alarm

There is a fault in the cooling water or protective gas supply, check the cooling water or protective gas line.

Design measures to reduce residual welding deformation

  1. Reasonable choice of welding size  

Dimensions such as the length, width and thickness of the weld have a significant effect on the welding deformation. For example, the thickness of the plate has a large effect on the corner deformation of the fillet weld, and the corner deformation is greatest when the thickness reaches a certain value (about 9 mm for steel). In the manufacture of T-shaped or work-shaped welded beams, due to the slender welded parts, so that the welding area shrinkage deformation caused by the weld bending deformation is a prominent problem. The best way to solve this problem is to carefully design the structural dimensional parameters, (such as plate thickness, plate width, plate length and rib spacing, etc.) and welding parameters (such as unit line energy, etc.).

  1. Reasonable choice of weld size and bevel form

The size of the weld seam is not only related to the welding workload, but also has a greater impact on the welding deformation. The weld size is large, the welding volume is also large, and the filler metal consumption is high, resulting in large welding deformation. Therefore, in the design of the weld seam size, under the condition of ensuring the load-bearing capacity of the structure, a smaller weld seam size should be used. Unilaterally increase the size of the weld seam is extremely detrimental to reduce welding deformation. Therefore, the weld does not bear a lot of working stress, do not need to use large size welding angle, as long as it can meet its strength requirements.

In addition, the bevel type should be reasonably designed. For example, the butt joint should be the best X-shaped bevel size with zero angular deformation. For the larger force T-shaped joints and cross joints, under the condition of ensuring the same strength, the use of open bevel welds than open bevel weld dynamic load strength, less weld metal, and to reduce the welding deformation is also beneficial, especially for thick plates, more in the sense.

  1. Minimize unnecessary welding seams  

In the design of the welded structure, should strive to minimize the number of welds. In general, the design is often used to add ribbed plate to improve the stability and stiffness of the structure, especially sometimes to reduce the weight of the main structure and the use of thinner plate, is bound to increase the number of ribs, thus greatly increasing the assembly and welding workload, the result is not only uneconomical, and the weld resulting in welding deformation is too large. So practice has proved that the reasonable choice of plate thickness, appropriate to reduce the rib plate, so that the weld seam is reduced, even though the structure may be slightly heavier, or more economical.

  1. Reasonable arrangement of welding seam position  

In order to avoid bending deformation of the welded structure, in the design of the structure, should strive to make the weld position symmetrical to the neutral axis or close to the neutral axis of the material joint components. Because the weld symmetry in the neutral axis, it is possible to make the neutral axis on both sides of the bending deformation generated by the weld axis completely or largely offset. Because the weld is close to the neutral axis, so that the weld shrinkage caused by the bending moment is reduced, so that the member bending deformation is also reduced. Therefore, when welding the structure should strive to make the structure symmetrical. For some cross-sectional shape can not change the non-symmetrical structural parts, can maintain the same cross-sectional shape, using the method of adjusting the weld center of gravity axis and the distance between the neutral axis to reduce deformation.

Process measures to reduce residual deformation of welding

1.Anti-deformation method  

The size of the deformation is predicted based on experience when assembling before welding, and the member is given a deformation in the opposite direction from the welding deformation, so that the deformation can be offset with the welding deformation, so that the structure can meet the technical requirements after welding. There are two methods of counter-deformation: ① plastic counter-deformation; ② elastic counter-deformation. In actual production, elastic counter-deformation is more reliable than plastic counter-deformation. Because even if the pre-strain of elastic counter-deformation is not accurate enough, it is always possible to reduce the angular deformation. If plastic counter-deformation is used, the selected plastic pre-bending amount must be very accurate, otherwise it will not get good results.

  1. Welding under external constraint conditions  

Rigidly fixes the welded part in the fixture to limit the deformation of the member during welding. It is very effective in reducing the angular deformation of the welded parts, which can lead to less welding deformation, but higher welding stress.

  1. Reasonable choice of welding methods and welding specifications

To reduce welding distortion, high energy density welding methods should be used as much as possible. Such as electron beam welding, laser welding, narrow gap welding, etc. They have a lower welding line energy, welding deformation is extremely small. In general production, CO2 gas shielded welding to replace manual arc welding, not only high efficiency, but also can significantly reduce the welding deformation. When welding thin plates, tungsten pulse argon arc welding or resistance welding and seam welding can be used, all of which can prevent pressure bending deformation.

If there are no conditions in the production of low line energy methods and do not reduce the welding specification, direct water cooling or the use of water-cooled copper blocks to change the distribution of the heat field to reduce the deformation of the purpose. However, for hardened metal materials, this method is used with caution.

  1. Select a reasonable assembly welding sequence and welding direction  

The design of the assembly welding sequence, mainly considering the impact of the welding stress and deformation generated by the first weld on the subsequent weld, but also consider how the stress and deformation generated by the subsequent weld interacts with the impact of the first weld. Practice has proved that the correct choice of assembly welding sequence is a powerful measure to prevent welding deformation.

In production, the welded structure is usually produced in a small and large way, first welded into a number of parts and components, and then assembled and welded into a whole structure. Since the assembly and welding sequence of the welded parts are different, the incremental structural rigidity and the influence on the welding deformation during the production process are also different, so they should be analyzed and compared to choose the reasonable assembly and welding sequence with the least deformation.

In general, should be welded first shrinkage of the weld, after welding shrinkage of the small weld. When there are both butt welds and fillet welds, generally should first weld butt welds, after welding fillet welds; when there are both transverse welds and longitudinal welds, should first weld transverse welds, after welding longitudinal welds; when there are both thick plate welds and thin plate welds, generally should first weld thick plate welds, after welding thin plate welds; when there are both intermittent and continuous welds in the structure, generally should first weld continuous welds, after welding intermittent Weld seam.

  1. Preheat

Welding uneven heat field is the main cause of welding deformation. Therefore, the use of appropriate preheating; so that the welding temperature distribution tends to be uniform, but also an effective measure to reduce residual welding deformation.

  1. Reducing out-of-plane deformation of welded sheet by stretching and heating method  

The welded wall plate is mechanically or preheated to stretch or elongate, and at the same time the wall plate is welded to the frame of the structure, and after welding, the tensile load is removed. At this time, the contraction of the wall plate is constrained by the welded frame, so that only a small amount of out-of-plane deformation is produced on the wall plate. At this time there is residual tensile stress in the wall plate after welding, while in the frame there is residual compressive stress. This method has a good effect on reducing the compression bending deformation of the welded sheet.

Prevention of cold cracks in welding

  1. Correctly selected materials

Choose alkaline low-hydrogen electrodes and fluxes to reduce the content of hydrogen diffusion in the weld metal; improve the selection of the base material and welding consumables to match; under the premise of technical conditions, the use of good toughness of the material (such as a lower strength level of welding consumables), or the implementation of the “soft” cover to reduce the residual stress on the surface; if necessary, before the manufacture of the base material and welding consumables Weld material for chemical analysis, mechanical properties and weldability, crack sensitivity test.

  1. Strictly follow the correct process specification derived from the test to carry out the welding operation

Mainly including: strictly according to the specifications for welding rod drying; choose the appropriate welding specification and line energy, reasonable current, voltage, welding speed, interlayer temperature and the correct welding sequence; spot welding inspection processing; good double-sided welding root cleaning, etc.; carefully clean the bevel and wire, remove oil, rust and moisture.

  1. Select a reasonable welding structure to avoid excessive constraint stress; correct bevel form and welding sequence; reduce the peak of residual stress in welding.
  2. Preheating before welding, slow cooling after welding, control of interlayer temperature and post-weld heat treatment, is poor weldability of high-strength steel and unavoidable high constraint structural form, to prevent cold cracking effective method. Preheating and slow cooling can slow down the cooling rate (extended △ t 800 ~ 500 ℃ residence time), improve the organizational state of the joint, reduce the tendency to harden, reduce tissue stress; post-weld heat treatment can eliminate residual stresses in welding, reduce the content of diffusion hydrogen in the weld. In most cases, the stress relief heat treatment should be carried out immediately after welding.
  3. Hammering immediately after welding, so that the residual stress dispersion, to avoid causing high stress areas, is one of the effective methods to prevent cold cracking when local welding.
  1. In the root of the weld and the stress is more concentrated on the surface of the weld, (the heat-affected zone is subjected to lower constraint stress), the use of a lower strength level of welding rod, often at high constraint to achieve good results.
  2. The use of inert gas shielded welding, the maximum control of the weld hydrogen content, reduce the sensitivity of cold cracking, so TIG, MIG welding should be vigorously promoted.

Prevention of welding heat cracks

  1. limit the steel and welding materials, easy to produce bias elements and the content of harmful impurities, especially the content of S, P, C, because they not only form low melting point eutectic, but also promote bias. c ≤ 0.10% hot crack sensitivity can be greatly reduced. Chemical analysis of the material, if necessary, low times the test (such as sulfur seal, etc.).
  2. Adjust the chemical composition of the weld metal, improve the organization, refine the grain, improve plasticity, change the morphology and distribution of harmful impurities, reduce segregation, such as the use of austenite plus less than 6% of the ferrite two-phase organization.
  3. Improve 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 bevel form, weld forming factor ψ = b / h> 1, avoid narrow and deep “pear-shaped” weld, (excessive welding current will also form a “pear-shaped” weld), to prevent columnar crystals in the center of the weld channel convergence, resulting in the formation of brittle fracture center bias The weld is a multi-layer multi-pass weld that disrupts the gathering of segregation.
  5. the use of smaller (appropriate) welding line energy, for austenitic (nickel-based) stainless steel should try to use a small welding line energy (no preheat, no or less swing, fast welding, small current), strict control of the interlayer temperature to shorten the residence time of the weld metal in the high temperature zone;
  6. pay attention to the protection of the closing arc, the closing arc should be slow and fill the arc pit, to prevent the arc pit deviations produce hot cracks;
  7. avoid multiple reworking as much as possible to prevent lattice defects from gathering to produce polygonal thermal cracks;
  8. Take measures to minimize joint stress, avoid stress concentration, and reduce the stiffness near the weld, properly arrange the welding sequence, and try to make most of the welds welded at a smaller stiffness, so that there is room for shrinkage.

Prevention of reheat cracks

  1. When selecting the material should pay attention to the carbide forming elements that can cause precipitation precipitation, especially the content of V. When high V steel must be used, special attention should be paid to welding and heat treatment.
  2. Heat treatment to avoid reheat sensitive areas, can reduce the possibility of reheat cracks, if necessary, before the heat treatment process test.
  3. Minimize residual stress and stress concentration, reduce the residual height, eliminate the biting edge, not weld through and other defects, if necessary, the residual height and weld toe grinding round and smooth; improve the preheating temperature, slow cooling after welding, reduce residual stress.
  4. Proper line energy to prevent overheating of the heat affected zone and coarse grain size.
  5. In the premise of meeting the design requirements, the choice of a lower strength level of welding rod, let it release part of the stress eliminated by the heat treatment process, (let the stress relax in the weld), to reduce reheat cracking is beneficial.

Prevention of underweld

  1. control the bevel size: gap, blunt edge, angle and misalignment, etc.;
  2. control the current, polarity and welding speed; make the joint fully preheated and establish a good first melt pool;
  3. control the welding rod diameter and welding angle; overcome arc bias blowing;
  4. Double-sided welding root clearance must be thorough;
  5. The oil, rust, slag and scale on the bevel and blunt edge must be cleaned up.

Weldability and its test evaluation

1.Welding: The process of forming an inseparable whole by the interatomic union of two objects through heating or pressure, with or without the addition of filler material.

  1. Weldability: refers to homogeneous or heterogeneous materials in the manufacturing process conditions, the ability to weld to form a complete joint and meet the expected use requirements.
  2. The four major factors affecting weldability are: material, design, process and service environment.
  3. Assessment of weldability principles include: ① assess the tendency of the welded joint to produce process defects, to provide a basis for the development of a reasonable welding process; ② assess whether the welded joint can meet the requirements of structural performance; design new welding test methods to meet the following principles: comparability, relevance, reproducibility and economy.
  4. Carbon equivalent: the content of the alloying elements in the steel is converted and superimposed on the equivalent of a number of carbon content, as a rough assessment of the steel cold cracking tendency of the parameter index.
  5. Oblique Y-bevel butt crack test: The purpose is mainly used to identify the first layer of low-alloy high-strength steel weld and HAZ formation of cold cracking tendency, can also be used to develop the welding process.

(1) test preparation, welded steel plate thickness δ = 9-38mm. butt joint bevel processing by mechanical means, test plate ends in the range of 60mm at each end of the application of constrained welding seam, using double-sided welding. Attention to prevent corner deformation and not welded through. Ensure that there is a 2mm gap in the middle of the sample weld to be welded.

(2) test conditions: the test weld selected welding rod on the match with the parent material, the welding rod should be strictly dried, the diameter of the welding rod 4mm, welding current (170 ± 10) A, welding voltage (24 ± 2) V, welding speed (150 ± 10) mm / min. test weld can be applied at various temperatures, the test weld only one, not filled bevel. After welding and natural cooling 24h after interception of specimens and crack detection.

3) Detection and crack bar rate calculation. With the naked eye or handheld 5-10 times magnification to detect the surface and section of the weld and heat-affected zone whether there are cracks. It is generally believed that low-alloy steel “small iron research” test surface cracking rate of less than 20%, generally do not produce cracks.

  1. Pin test: purpose, mainly to assess the hydrogen-induced delayed cracking tendency of steel, additional other equipment, but also to determine the reheat cracking sensitivity and laminar sensitivity.

(1) test preparation, will be welded steel processing or cylindrical pin test rod, sampling along the rolling direction and indicate the location of the pin in the thickness direction. The test bar has a ring or screw-shaped notch near the upper end. Insert the pin test bar into the corresponding hole in the base plate so that the notched end is flush with the surface of the base plate. For the ring-shaped notch pin test rod, the distance between the notch and the end surface a should make the weld depth of fusion and the notch root cut plane tangent or intersection, but the notch root circumference is melted through the part of not more than 20%. For low-alloy steel, a value of 2mm when the welding heat input is E = 15KJ/cm.

(2) test process, according to the selected welding method and strictly controlled process parameters, the base plate melt a layer of overlay weld channel, the centerline of the weld channel through the center of the specimen, the melt depth should be so that the tip of the notch is located in the heat-affected zone of the coarse crystal zone, the length of the weld channel L about 100-150mm. application of welding should be measured 800-500 ℃ cooling time value t8/5 value, not preheat welding, after welding cooling to 100- 150 ℃ when loading; preheating before welding, should be loaded at 50-70 ℃ above the preheating temperature. The load should be applied within 1min and before cooling to 100℃ or 50-70℃ above the preheating temperature. If there is post-heat, it should be loaded before post-heat. When the test bar is loaded, the pin may break during the load duration, note down the bearing time.

Weldability of alloy structural steel

  1. High-strength steel: yield strength σs ≥ 295MPa strength of steel can be called high-strength steel.
  2. Mn solution strengthening effect is very significant, ωMn ≤ 1.7%, can improve toughness, reduce the brittle transition temperature, Si will reduce plasticity, toughness, Ni both solution strengthening and at the same time improve toughness and significantly reduce the brittle transition temperature of the elements, commonly used in low temperature steel.
  3. Hot rolled steel (normalized steel): low alloy high strength steel with yield strength of 295-490 MPa, generally supplied in the hot rolled or normalized condition.
  4. Design principles of high-strength steel welded joints: high-strength steel is selected on the basis of its strength, and thus the principle of welded joints: the strength of the welded joint is equal to the strength of the parent material (equal strength principle), analysis: ① welded joint strength is greater than the strength of the parent material, plastic toughness is reduced, ② equal to when the life is equivalent ③ less than when the strength of the joint is insufficient.
  5. Hot-rolled and normalized steel weldability: hot-rolled steel contains a small amount of alloying elements in general, the cold cracking tendency is not large, normalized steel due to the inclusion of more alloying elements, hardening tendency increased, with the increase in carbon equivalent and plate thickness of normalized steel, hardening and cold cracking tendency increased. Influencing factors: (1) carbon equivalent (2) hardening tendency: the hardening tendency of hot-rolled steel and normalized steel (3) the maximum hardness of the heat-affected zone, the maximum hardness of the heat-affected zone is an easy way to assess the hardening tendency and cold cracking susceptibility of steel.

6.SR cracks (stress relief cracks, reheat cracks): Mo-containing normalized steel thick-walled pressure vessels and other welded structures, post-weld stress relief heat treatment or post-weld reheat at high temperatures, another form of cracking may occur.

  1. Toughness is a property that characterizes the ease of generation and expansion of brittle cracking of metals.
  1. Low-alloy steel must consider two aspects when selecting welding materials: ① can not have cracks and other welding defects ② can meet the performance requirements. Hot-rolled steel and normalized steel welding is generally based on its strength level selection of welding materials, the selection points are as follows: ① choose the corresponding level of mechanical properties of the parent material to match the welding material ② consider the fusion ratio and cooling rate ③ consider the impact of post-weld heat treatment on the mechanical properties of the weld.
  2. Determine the principle of tempering temperature after welding: ① Do not exceed the original tempering temperature of the base material to avoid affecting the performance of the base material itself ② For materials with tempering, to avoid the temperature range of tempering brittleness.
  3. Tempered steel: quenched + tempered (high temperature)
  4. High-strength steel welding using “low strength matching” can improve the crack resistance of the welded area.
  5. Low carbon tempered steel welding to pay attention to two basic issues: ① require the cooling rate of martensite transformation can not be too fast, so that the martensite has a self-tempering effect to prevent the generation of cold cracking ② require the cooling rate between 800 ℃ -500 ℃ is greater than the critical speed of brittle mixed tissue. Low-carbon tempered steel welding to solve the problem: ① to prevent cracking ② to ensure that the requirements of high strength at the same time, improve the weld metal and heat-affected zone toughness.
  6. For low alloy steel with low carbon content, increasing the cooling rate to form low carbon martensite is beneficial to ensure toughness.
  7. Medium carbon tempered steel alloying elements are mainly added to ensure hardenability and improve the role of tempering resistance, while the true strength performance is mainly dependent on the carbon content. Main features: high specific strength and high hardness.
  1. improve the thermal strength of pearlite heat-resistant steel in three ways: ① matrix solid solution strengthening, adding alloying elements to strengthen the ferrite matrix, commonly used Cr, Mo, W, Nb elements can significantly improve the thermal strength ② second-phase precipitation strengthening: in the ferrite matrix of heat-resistant steel, the strengthening phase is mainly alloy carbide ③ grain boundary strengthening: adding trace elements can be adsorbed in the grain boundary, slowing the diffusion of alloying elements along the grain boundary The grain boundary is strengthened by the addition of trace elements.
  2. The main problems in welding pearlite heat-resistant steel are cold cracking, hardening of heat-affected zone, softening, and stress relief cracking in post-weld heat treatment or high temperature long-term use.
  3. -10 to -196 ℃ temperature range is called “low temperature”, below -196 ℃ is called “ultra-low temperature”.

Stainless steel welding

Stainless steel: Stainless steel is a general term for alloy steel that can resist corrosion by air, water, acids, alkalis, salts and their solutions and other corrosive media, and has a high degree of chemical stability.

  1. The main forms of corrosion of stainless steel are uniform corrosion, pitting corrosion, crevice corrosion and stress corrosion. Uniform corrosion, refers to contact with the corrosive medium of all the metal surface corrosion phenomenon; point corrosion, refers to the majority of the surface of the metal material does not corrode or corrosion is slight, but scattered local corrosion occurs; gap corrosion, in the electrolyte, such as in the oxygen ion environment, between the stainless steel or contact with foreign objects between the surface when there is a gap, the gap in the solution flow will occur hysteresis phenomenon, so that the solution local Cl-, the formation of The concentration difference battery, which leads to the gap in the stainless steel passivation film adsorption Cl- and the phenomenon of local destruction; intergranular corrosion, selective corrosion phenomena occurring near the grain boundaries; stress corrosion, refers to the stainless steel in a specific corrosive medium and tensile stress under the action of the phenomenon of brittle cracking below the strength of very strong.
  1. Measures to prevent pitting corrosion: 1) reduce the chloride ion content and oxygen ion content 2) add chromium, nickel, molybdenum, silicon, copper and other alloying elements in stainless steel 3) try not to cold work to reduce the possibility of pitting corrosion at the dislocation outcrop 4) reduce the carbon content of the steel.
  2. Stainless steel and heat-resistant steel high-temperature properties: 475 ℃ embrittlement, mainly in Cr> 13% of the ferrite, 430-480 ℃ between long-term heating and slow cooling, resulting in increased strength and reduced toughness at room temperature or negative temperature; σ-phase embrittlement, is typical of 45% of the mass fraction of Cr, FeCr intermetallic compounds, non-magnetic, hard and brittle.
  3. Austenitic stainless steel welded joints corrosion resistance: 1) intergranular corrosion, 2) heat-affected zone sensitization zone intergranular corrosion, 3) knife-like corrosion.
  4. measures to prevent intergranular corrosion of the weld: 1) by welding materials, so that the weld metal either becomes ultra-low carbon situation, or contains sufficient stabilizing elements Nb. 2) adjust the weld composition to obtain a certain δ phase. Intergranular corrosion theory is essentially chromium-poor theory.
  5. Heat-affected zone sensitization zone intergranular corrosion: refers to the welding heat-affected zone in the peak heating temperature in the sensitized heating zone between the parts of the intergranular corrosion occurs.
  6. Knife-like corrosion: in the fusion zone produced by intergranular corrosion, such as knife cutting form, so called “knife-like corrosion”.
  7. Measures to prevent knife-like corrosion: ① choose low-carbon base material and welding materials ② use and phase organization of stainless steel ③ using low current welding, reduce the degree of overheating and width of the welded coarse crystal zone ④ contact with the corrosive medium weld last ⑤ cross-welding ⑥ increase the Ti, Tb content in the steel, so that the welded coarse crystal zone grain boundaries have enough Ti, Tb and carbonization together.
  8. Why is stainless steel welding with low current? To reduce the temperature of the weld heat-affected zone, to prevent intergranular corrosion of the weld, to prevent overheating of the welding rod, wire, welding deformation, welding stress, can reduce heat input, etc.
  1. Three conditions that cause stress corrosion cracking: environment, selective corrosive media, tensile stress.
  2. Measures to prevent stress corrosion cracking: 1) adjust the chemical composition, ultra-low carbon is conducive to improving the ability to resist stress corrosion, the composition and medium matching issues, 2) remove residual stress from welding 3) electrochemical corrosion, regular inspection and timely repair, etc.
  3. To improve the pitting resistance: 1) on the one hand, we must reduce the Cr, Mo segregation 2) on the one hand, the use of higher Cr, Mo content than the parent material of the so-called “super-alloyed” welding materials.
  4. Austenitic stainless steel welding will produce thermal cracking, stress corrosion cracking, welding distortion, intergranular corrosion.
  5. Austenitic steel welding heat cracking causes: 1) austenitic steel thermal conductivity is small, large coefficient of linear expansion, tensile stress to large, 2) austenitic steel is easy to crystallize the formation of strong directional columnar crystal weld organization, which is conducive to harmful impurity segregation 3) austenitic steel alloy composition is more complex, easy to solve eutectic.
  6. Measures to prevent thermal cracking: ① Strictly limit the P, S content in the base material and weld material ② Try to make the weld form a two-phase organization ③ control the chemical composition of the weld ④ small current welding.

17.18-8 and 25-20 in the prevention of thermal cracking, what is the difference between the weld organization? 18-8 steel weld formation A + δ organization, δ phase can dissolve a large number of P, S, δ phase is generally 3%-7%, 25-20 steel weld formation A + primary carbide organization.

  1. Austenitic stainless steel selection should pay attention to:① adhere to the “applicability principle” ② according to the specific composition of the selected welding consumables to determine whether the applicable ③ consider the specific application of welding methods and process parameters may cause the fusion ratio size ④ according to the technical conditions of the comprehensive weldability requirements to determine the degree of alloying ⑤ to pay attention to the weld metal Alloy system, the role of specific alloy components in that alloy system, consider the use of performance requirements and process weldability requirements.
  1. Ferrite stainless steel weldability analysis: 1) intergranular corrosion of welded joints 2) embrittlement of welded joints, high temperature embrittlement, σ-phase embrittlement, 475°C embrittlement.

Cast iron welding

  1. Three main characteristics of cast iron: vibration damping, oil absorption, and wear resistance.
  2. The performance of cast iron mainly depends on the shape, size, quantity and distribution of graphite, etc., while the matrix organization also has some influence.
  3. ductile iron: F matrix + round spherical graphite; gray cast iron: F matrix + flake graphite; worm cast iron: matrix + worm-like graphite; malleable cast iron: F matrix + mass flocculent graphite.
  4. Is it possible to weld cast iron with low carbon steel welding rod? No, in welding, even if the small current, the proportion of parent material in the first weld is 25%-30%, if calculated according to the cast iron C = 3%, the first weld in the carbon content of 0.75%-0.9%, is a high carbon steel, welding immediately after cooling high carbon martensite, and welding HAZ will appear white mouth organization, mechanical processing difficulties.
  5. Arc heat welding: molten castings preheated to 600-700 ℃, and then welded in the plastic state, the welding temperature is not less than 400 ℃, in order to prevent cracking during the welding process, immediately after welding stress relief treatment and slow cooling, this cast iron welding and repair process is called arc heat welding.
  6. Semi-heat welding: preheating temperature at 300-400 ℃ is called semi-heat welding.

Weldability of magnesium and magnesium alloys

  1. Oxidation and evaporation

Due to the extremely strong oxidation of magnesium, easy to form oxide film (MgO) in the welding process, MgO high melting point (2500 ℃), high density (3.2g/cm3), easy to form inclusions in the weld, reducing the performance of the weld. At high temperatures, magnesium is also prone to chemical reactions with nitrogen in the air to generate magnesium nitride, weakening the performance of the joint. The boiling point of magnesium is not high, which will lead to high temperatures in the arc is easy to evaporate.
 2. Coarse grain
Due to the large thermal conductivity, so when welding magnesium alloy to use a high-powered heat source, high-speed welding, easy to cause the weld and near the weld area metal overheating and grain growth.
 3. Thermal stress
Magnesium alloy coefficient of thermal expansion is large, about 1 to 2 times that of aluminum, in the welding process is prone to large welding deformation, causing a large residual stress.
 4. Weld metal collapse
Because the surface tension of magnesium is smaller than aluminum, welding is easy to produce weld metal collapse, affecting the quality of weld forming.
 5. Porosity
Similar to welded aluminum alloy, magnesium alloy welding is easy to produce hydrogen porosity. Hydrogen solubility in magnesium decreases with the temperature, and the density of magnesium is smaller than aluminum, gas is not easy to escape, in the process of solidification of the weld will form pores.
 6. Thermal cracking
Magnesium alloy is easy to form a low melting point eutectic organization with other metals, easy to form crystalline cracks in the welded joints. When the joint temperature is too high, the joint organization of the low melting point compounds at the grain boundaries will melt cavities, or produce grain boundary oxidation, etc., that is, the so-called “overburning” phenomenon.