Weldability, refers to homogeneous or heterogeneous materials in the manufacturing process conditions, can be welded to form a complete joint, and to meet the expected use of the ability of the material, design, process and service environment for the four major factors affecting weldability.

　　Principles of assessing weldability mainly include:

　　① assess the tendency of welded joints to produce process defects, to provide a basis for the development of a reasonable welding process;

　　② assessment of welded joints to meet the structural performance requirements.

　　First, the weldability of alloy structural steel

　　1. high-strength steel: yield strength σs ≥ 295MPa strength steel can be called high-strength steel. 2.

　　2. Mn solid solution strengthening is very significant, ωMn ≤ 1.7%, can improve toughness, reduce the brittle transition temperature, Si will reduce the plasticity, toughness, Ni both solid solution strengthening and at the same time to improve the toughness and significantly reduce the brittle transition temperature of the elements, commonly used in low-temperature steels.

　　3. hot rolled steel (normalized steel): yield strength of 295-490MPa low alloy high-strength steel, generally in the hot rolled or normalized state supply and use.

　　4. High-strength steel welded joints design principles: high-strength steel to its strength as the basis for selection, and therefore the principle of welded joints for the welded joints strength is equal to the strength of the base material (the principle of equal strength), the reasons are:

　　① welded joint strength is greater than the base material strength, plastic toughness is reduced;

　　② equal to the life of the same;

　　③ less than when the joint strength is insufficient.

　　5. hot-rolled and normalized steel weldability: hot-rolled steel contains a small number of alloying elements in general, cold cracking tendency is not large, normalized steel due to the alloying elements contained in more hardening tendency has increased, with the increase in the carbon equivalent of normalized steel and plate thickness, hardening and cold cracking tendency increases.

　　Factors affecting:

　　(1) carbon equivalent;

　　(2) Hardening tendency;

　　(3) the maximum hardness of the heat-affected zone, the maximum hardness of the heat-affected zone is an easy way to assess the steel hardening tendency and cold crack susceptibility.

　　6. SR cracks (stress relief cracks, reheat cracks): Mo-containing annealed steel thick-walled pressure vessels and other welded structures, post-weld stress relief heat treatment or post-weld high-temperature heating again in the process of another form of cracking may occur.

　　7. Toughness is characterized by the metal of brittle cracks and expansion of the ease of performance.

　　8. Low-alloy steel welding material selection must be considered in two ways: ① can not have cracks and other welding materials.

　　① can not have cracks and other welding defects; ② to meet the performance requirements.

　　② can meet the performance requirements.

　　Hot-rolled steel and normalized steel welding, generally based on its strength level selection of welding materials, the selection of points are as follows: ① Select the mechanical properties of the base material with the mechanical properties of the base material.

　　① Select the corresponding level of welding consumables to match the mechanical properties of the base material; ② Consider both the fusion ratio and the welding consumables to match the mechanical properties of the base material.

　　② at the same time consider the fusion ratio and the effect of cooling rate; ③ consider the impact of post-weld heat treatment on the welding material.

　　③ consider the impact of post-weld heat treatment on the mechanical properties of the weld.

　　9. Determine the principle of post-weld tempering temperature:

　　① Do not exceed the original tempering temperature of the base material so as not to affect the properties of the base material itself.

　　② for tempered materials, to avoid the tempering brittleness of the temperature range.

　　10. Tempered steel: quenching + tempering (high temperature). 11.

　　11, high-strength steel welding using "low-strength matching" can improve the crack resistance of the weld zone. 12, low carbon tempered steel welding.

　　12, low-carbon tempered steel welding should pay attention to two basic issues:

　　① requirements of the martensitic transformation of the cooling rate can not be too fast, so that the martensite has a self-tempering effect, in order to prevent the emergence of cold cracks;

　　② requires the cooling rate between 800 ℃ -500 ℃, greater than the critical speed of producing brittle mixed tissue.

　　Low carbon tempered steel welding to solve the problem:

　　① to prevent cracking;

　　② in order to ensure that meet the high strength requirements at the same time, improve the toughness of the weld metal and heat affected zone.

　　13. For low carbon content of low alloy steel, increase the cooling rate to form low carbon martensite, to ensure that the toughness is favorable.

　　14. Medium carbon tempered steel alloying elements added to ensure that the hardenability and improve the role of tempering resistance, and the true strength properties are still mainly dependent on the carbon content. Main features: high specific strength and high hardness. 15.

　　15. Improve the thermal strength of pearlite heat-resistant steel has 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 heat-resistant steel, the strengthening phase is mainly alloy carbides;

　　③ grain boundary strengthening: adding trace elements can be adsorbed at the grain boundary, slowing down the diffusion of alloying elements along the grain boundary, thereby strengthening the grain boundary.

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

　　17. -10 to -196 ℃ temperature range is called "low temperature", below -196 ℃ is called "ultra-low temperature".

　　Second, the weldability of cast iron

　　1. The three main characteristics of cast iron: vibration damping, oil absorption, wear resistance. 2.

　　2. the performance of cast iron depends on the shape, size, quantity and distribution of graphite, etc., while the matrix organization also has a certain impact. 3.

　　3. Ductile cast iron: F matrix + spheroidal graphite;

　　Gray cast iron: F matrix + flake graphite.

　　Peristaltic cast iron: F matrix + worm-like graphite; and

　　Malleable cast iron: F matrix + flocculated graphite.

　　4. Whether mild steel welding electrodes can be welded cast iron: no.

　　In welding, even if the small current, the base material in the first weld in the proportion of 25% -30%, according to the cast iron C = 3%, the first weld in the carbon content of 0.75% -0.9%, belonging to the high-carbon steel, welding cooling immediately after the emergence of high-carbon martensite, and welding HAZ will appear white mouth organization, machining difficulties.

　　5. arc heat welding: fused 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 in the welding process, welded immediately after the stress relieving treatment and slow cooling, this cast iron welding process known as arc heat welding.

　　6. Semi-hot welding: preheating temperature of 300-400 ℃ when called semi-hot welding.

　　Third, the weldability of stainless steel

　　1. stainless steel: stainless steel refers to air, water, acid, alkali, salt and its solutions and other corrosive media corrosion, with a high degree of chemical stability of the alloy steel.

　　2. The main forms of corrosion of stainless steel are uniform corrosion, pitting corrosion, crevice corrosion and stress corrosion. Uniform corrosion, refers to the metal surface in contact with corrosive media all produce corrosion phenomenon; point corrosion, refers to most of the surface of the metal material does not corrode or corrosion is slight, and dispersed occurrence of local corrosion; crevice corrosion, in the electrolyte, such as in the oxygen ions in the environment, stainless steel or contact with the surface of the surface of the existence of gaps between the gap in the gap in the flow of the solution will be delayed phenomenon, to the extent that the solution of localized Cl-, forming a concentration cell, which leads to the formation of a concentration cell. Concentration battery, which leads to the gap in the stainless steel passivation film adsorption Cl- and the phenomenon of localized destruction; intergranular corrosion, selective corrosion phenomena occurring near the grain boundaries; stress corrosion, refers to the stainless steel in the specific corrosive medium and tensile stress under the action of the phenomenon of brittle cracking below the strength of the very strong.

　　3. measures to prevent pitting corrosion: 1) reduce the content of chloride ions and oxygen ions; 2) add chromium, nickel, molybdenum, silicon, copper and other alloying elements in stainless steel; 3) try not to carry out cold working, in order to reduce the dislocation outcrops at the possibility of pitting corrosion; 4) reduce the carbon content in steel.

　　4. stainless steel and heat-resistant steel high temperature performance: 475 ℃ embrittlement, mainly in the Cr> 13% ferrite, 430-480 ℃ between long-term heating and slow cooling, resulting in room temperature or negative temperature when the strength increases and toughness decreases; σ-phase embrittlement, is the Cr mass fraction of 45% of the typical, FeCr intermetallic compounds, non-magnetic, hard and brittle.

　　5. corrosion resistance of austenitic stainless steel welded joints: 1) intergranular corrosion; 2) heat-affected zone sensitized zone intergranular corrosion; 3) knife corrosion.

　　6. Measures to prevent intergranular corrosion of the weld: 1) through the weld material, so that the weld metal or become ultra-low carbon, or contain sufficient stabilization element Nb; 2) adjust the composition of the weld to obtain a certain δ-phase. 7.

　　7. heat-affected zone sensitized zone intergranular corrosion: refers to the welding heat-affected zone in the peak heating temperature in the sensitized heating zone parts of the intergranular corrosion occurs.

　　8. Knife corrosion: intergranular corrosion in the fusion zone, such as knife incision form, so called "knife corrosion". 9.

　　9. prevent knife corrosion measures.

　　(1) the use of low-carbon base material and welding materials; (2) the use of phase-organized stainless steel.

　　② the use of stainless steel and phase organization; ③ the use of low-current welding, and the welding material.

　　(iii) the use of low-current welding, reduce the degree of overheating and width of the welded coarse crystalline zone; (iv) the corrosion medium in contact with the corrosion of stainless steel.

　　④ The weld in contact with the corrosive medium is welded last; ⑤ Cross-welding; and

　　⑤ Cross-welding

　　⑥Increase the Ti, Tb content in the steel, so that the grain boundary of the welded coarse grain area has enough Ti, Tb and carbonization.

　　10. stainless steel why use small current welding? To reduce the temperature of the weld heat-affected zone, to prevent the generation of intergranular corrosion of the weld, to prevent overheating of the electrode, wire, welding deformation, welding stress, reduce heat input and so on.

　　11. three conditions that cause stress corrosion cracking: environment, selective corrosive media, tensile stress.

　　12. Measures to prevent stress corrosion cracking:

　　1) adjust the chemical composition, ultra-low carbon is conducive to improving the ability to resist stress corrosion, composition and media matching problems;

　　2)Removal of welding residual stress;

　　3) electrochemical corrosion, regular inspection and timely repair, etc..

　　13. In order to improve pitting resistance.

　　1) on the one hand, must reduce Cr, Mo segregation; 2) on the one hand, the use of more than the parent material.

　　2) on the one hand, the use of higher Cr, Mo content than the base material of the so-called "superalloyed" welding consumables. 14.

　　14. Austenitic stainless steel welding will produce thermal cracking, stress corrosion cracking, welding deformation, intergranular corrosion. 15.

　　15. Austenitic steel welding hot cracking causes.

　　1) austenitic steel thermal conductivity is small, the coefficient of linear expansion is large, tensile stress to large.

　　2) austenitic steel is easy to associate crystallization to form a strong direction of the columnar crystals of the weld organization, conducive to harmful impurities segregation; 3) austenitic steel is easy to associate crystallization to form a strong direction of the columnar crystals.

　　3) austenitic steel alloy composition is more complex, easy to dissolve eutectic.

　　16. Measures to prevent thermal cracking: ① strictly limit the base material and welding materials in the P, S content; ② try to make the weld to form a two-phase organization; ③ control the chemical composition of the weld; ④ small current welding.

　　17. Austenitic stainless steel selection should pay attention to: ① adhere to the "applicability".

　　① adhere to the "applicability principle"; ② according to the selected welding consumables.

　　② According to the specific composition of the selected welding consumables to determine the applicability; ③ Consider the specific application of the welding method.

　　③ Consider the specific application of welding methods and process parameters may cause the size of the fusion ratio; ④ According to the technical conditions of the comprehensive

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

　　⑤ to pay attention to the weld metal alloy system, the role of specific alloying components in the alloy system, consider the use of performance requirements and process weldability requirements.

　　18. ferritic stainless steel weldability analysis.

　　1) intergranular corrosion of welded joints.

　　2) embrittlement of welded joints, high temperature embrittlement, σ-phase embrittlement, 475 ℃ embrittlement.

　　Fourth, magnesium and magnesium alloy weldability

　　1. Oxidation and evaporation

　　Due to the extreme oxidizing property of magnesium, it is easy to form oxide film (MgO) in the process of welding, MgO melting point is high (2500 ℃), density is large (3.2g/cm3), it is easy to form inclusions in the weld, which reduces 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

　　Because of the thermal conductivity, so the welding of magnesium alloy to use high-power heat source, high-speed welding, easy to cause the weld and near weld area metal overheating and grain growth.

　　3. Thermal stress

　　Magnesium alloy thermal expansion coefficient is large, about 1 to 2 times of aluminum, in the welding process is easy to produce large welding deformation, causing large residual stress.

　　4. Weld metal collapse

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

　　5. Porosity

　　Similar to welding aluminum alloys, magnesium alloys are prone to hydrogen porosity when welding. The solubility of hydrogen in magnesium decreases with the decrease of temperature, and the density of magnesium is smaller than aluminum, the gas is not easy to escape, and gas holes will be formed during the solidification of the weld.

　　6. Thermal cracking

　　Magnesium alloy is easy to form low melting point eutectic organization with other metals, in the welded joints are easy to form crystalline cracks.

　　When the joint temperature is too high, the joint organization of low melting point compounds at the grain boundary will melt cavities, or produce grain boundary oxidation, etc., that is, the so-called "overcooking" phenomenon.

　　Translated with DeepL.com (free version)