Weldability refers to the ability of homogeneous materials or dissimilar materials to be welded to form complete joints and meet the expected use requirements under the conditions of the manufacturing process. The principles of assessing weldability mainly include: (1) assessing the tendency of welded joints to produce process defects to provide a basis for formulating reasonable welding procedures; (2) assessing whether welded joints can meet the requirements of structural performance.
ONE. Weldability of alloy structural steel
1. High-strength steel: Steel with a 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 will reduce the plasticity and toughness. Ni not only solid solution strengthens, but also improves the toughness and greatly reduces the brittle transition temperature. element, commonly used in low temperature steels.
3. Hot-rolled steel (normalized steel): low-alloy high-strength steel with a yield strength of 295-490 MPa, which is generally supplied and used in a hot-rolled or normalized state.
4. Design principles of high-strength steel welded joints: High-strength steel is selected based on its strength, so the principle of welded joints is that the strength of welded joints is equal to the strength of the base metal (the principle of equal strength), the reasons are:
① The strength of welded joints is greater than that of the base metal Strength, plastic toughness decreased;
② equal to the same life;
③ less than, the joint strength is insufficient.
5. Weldability of hot-rolled and normalized steel: Hot-rolled steel contains a small amount of alloying elements and generally has little tendency to cold cracks. Because normalized steel contains more alloying elements, the hardening tendency increases. With the increase of steel carbon equivalent and plate thickness, the hardenability and cold crack tendency increase. Influencing factors:
(1) carbon equivalent;
(2) hardening tendency;
(3) the highest hardness of the heat-affected zone, the highest hardness of the heat-affected zone is a simple method to evaluate the hardening tendency and cold crack susceptibility of steel.
6. SR cracks (stress crack elimination, reheat cracks): For welded structures such as Mo-containing normalized steel thick-walled pressure vessels, during the post-weld stress-relief heat treatment or the post-weld reheating process, another kind of crack may appear. form of cracks.
7. Toughness is a property that characterizes the ease with which brittle cracks are generated and propagated by metals.
8. Two aspects must be considered when selecting welding materials for low alloy steel:
① There should be no welding defects such as cracks;
②It can meet the performance requirements.
Welding of hot-rolled steel and normalized steel is generally based on the selection of welding materials according to their strength levels. The selection points are as follows:
①Select the corresponding level of welding material that matches the mechanical properties of the base metal;
② Consider the influence of fusion ratio and cooling rate at the same time;
③ Consider the effect of post-weld heat treatment on the mechanical properties of the weld.
9. The principle of determining the 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;
②For tempered materials, avoid the temperature range where temper brittleness occurs.
10. Quenched and tempered steel: quenched + tempered (high temperature).
11. The use of "low-strength matching" for high-strength steel welding can improve the crack resistance of the welded area.
12. Two basic issues should be paid attention to when welding low-carbon quenched and tempered steel:
① The cooling rate during martensite transformation should not be too fast, so that the martensite has a self-tempering effect to prevent the generation of cold cracks;
② It is required to be in The cooling rate between 800°C and 500°C is greater than the critical rate for producing a brittle mixed structure.
The problems to be solved in the welding of low carbon quenched and tempered steel:
① prevent cracks; ② improve the toughness of the weld metal and the heat affected zone while ensuring that the high strength requirements are met.
13. For low-alloy steels with low carbon content, increasing the cooling rate to form low-carbon martensite is beneficial to ensure toughness.
14. The addition of alloying elements in medium carbon quenched and tempered steel mainly plays the role of ensuring hardenability and improving tempering resistance, and 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 The matrix is solid-solution strengthened, and alloying elements are added to strengthen the ferrite matrix. The commonly used Cr, Mo, W, and Nb elements can significantly improve the thermal strength; ② No. Two-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, delay the diffusion of alloy elements along the grain boundary, thereby strengthening Grain boundaries.
16. The main problems existing in the welding of pearlitic heat-resistant steel are cold cracks, hardening and softening of the heat-affected zone, and stress crack elimination in post-weld heat treatment or long-term use at high temperatures.
17. The temperature range from -10 to -196°C is called "low temperature", and when it is lower than -196°C, it is called "ultra low temperature".
TWO. Weldability of cast iron
1. Three major 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, and the matrix structure also has a certain influence.
3. Ductile iron: F matrix + spherical graphite; gray cast iron: F matrix + flake graphite; vermicular graphite iron: matrix + vermicular graphite; malleable iron: F matrix + flocculent graphite.
4. Whether the low carbon steel electrode can weld cast iron: No. During welding, even if the current is small, the proportion of the base metal in the first weld is 25%-30%. If calculated according to C=3% in cast iron, the carbon content in the first weld is 0.75%. %-0.9%, belongs to high carbon steel, high carbon martensite appears immediately after welding cooling, and the welded HAZ will have white mouth structure, which makes machining difficult.
5. Arc thermal welding: The molten castings are preheated to 600-700 ℃, and then welded in a plastic state. The welding temperature is not lower than 400 ℃. In order to prevent cracking during the welding process, stress relief treatment and slow cooling are carried out immediately after welding. This cast iron welding repair process is called arc welding.
6. Semi-thermal welding: When the preheating temperature is 300-400 ℃, it is called semi-thermal welding.
THREE. Weldability of stainless steel
1. Stainless steel: Stainless steel refers to the general term for alloy steels with high chemical stability that are resistant to corrosion by air, water, acids, alkalis, salts and their solutions and other corrosive media.
2. The main corrosion forms of stainless steel are uniform corrosion, pitting corrosion, crevice corrosion and stress corrosion. Uniform corrosion refers to the phenomenon that all metal surfaces in contact with the corrosive medium are corroded; pitting corrosion refers to the local corrosion that occurs in most parts of the metal material without corrosion or slight corrosion, but scattered; crevice corrosion, in the electrolyte, such as in oxygen In the ionic environment, when there is a gap between the stainless steel or between the surfaces in contact with foreign objects, the flow of the solution in the gap will be sluggish, so that the local Cl- of the solution will form a concentration battery, which will cause the stainless steel passivation film in the gap to adsorb Cl- and be absorbed by the passivation film. The phenomenon of local failure; intergranular corrosion, a selective corrosion phenomenon that occurs near grain boundaries; stress corrosion, refers to the phenomenon of brittle cracking of stainless steel under the action of specific corrosive medium and tensile stress, which is lower than the strength .
3. Measures to prevent pitting corrosion:
1) Reduce the content of chloride ions and oxygen ions;
2) Add alloying elements such as chromium, nickel, molybdenum, silicon, and copper to stainless steel;
3) Try not to cold work to reduce dislocation outcrops The possibility of pitting corrosion at the place;
4) reduce the carbon content in the steel.
4. High-temperature properties of stainless steel and heat-resistant steel: brittleness at 475°C, mainly in ferrite with Cr>13%, long-term heating and slow cooling between 430-480°C, resulting in an increase in strength at room temperature or negative temperature. High and toughness decreased; σ phase embrittlement, which is typical of 45% of the mass fraction of Cr, FeCr intermetallic compound, non-magnetic, hard and brittle.
5. Corrosion resistance of austenitic stainless steel welded joints:
1) intergranular corrosion;
2) intergranular corrosion in heat-affected zone sensitization zone;
3) knife-like corrosion.
6. Measures to prevent intergranular corrosion in welds:
1) Through the 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 delta phase.
7. Intergranular corrosion in the sensitized zone of the heat-affected zone: refers to the intergranular corrosion that occurs at the position where the heating peak temperature is in the sensitized heating range in the welding heat-affected zone.
8. Knife-shaped corrosion: The intergranular corrosion generated in the fusion zone is like a knife-cut incision, so it is called "knife-shaped corrosion".
9. Measures to prevent knife-like corrosion:
①Choose low-carbon base metal and welding materials;
② Adopt stainless steel with phase structure;
③Using small current welding to reduce the degree of overheating and width of the welding coarse-grained area;
④ The welds in contact with the corrosive medium are finally welded;
⑤Cross welding; ⑥Increase the content of Ti and Tb in the steel, so that there is enough Ti, Tb and carbon in the grain boundary of the welded coarse-grained region.
10. Why is low current welding used for stainless steel? In order to reduce the temperature of the welding heat affected zone, prevent the occurrence of intergranular corrosion of the weld, prevent the electrode and wire from overheating, welding deformation, welding stress, reduce heat input, etc.
11. Three conditions that cause stress corrosion cracking: environment, selective corrosive medium, and tensile stress.
12. Measures to prevent stress corrosion cracking:
1) Adjusting the chemical composition, ultra-low carbon is beneficial to improve the ability to resist stress corrosion, and the matching problem of composition and medium;
2) Removing welding residual stress;
3) Electrochemical corrosion, regular inspection and timely Patching etc.
13. To improve pitting resistance:
1) On the one hand, the segregation of Cr and Mo must be reduced;
2) On the one hand, the so-called "superalloyed" welding material with higher Cr and Mo content than the base metal is used.
14. Hot cracks, stress corrosion cracks, welding deformation and intergranular corrosion will occur when austenitic stainless steel is welded.
15. Causes of hot cracks in austenitic steel welding:
1) The thermal conductivity of austenitic steel is small, the coefficient of linear expansion is large, and the tensile stress is large;
2) Austenitic steel is easy to co-crystallize to form a weld structure with strong directional columnar crystals, which is conducive to the segregation of harmful impurities;
3) The alloy composition of austenitic steel is more complex and soluble eutectic.
16. Measures to prevent hot cracks: ① Strictly limit the content of P and S in the base metal and welding materials; ② Try to make the weld form a dual-phase structure; ③ Control the chemical composition of the weld; ④ Small current welding.
17. Attention should be paid to the selection of austenitic stainless steel materials: ①Adhere to the "applicability principle"; ②Determine whether it is suitable or not according to the specific composition of each selected welding material; ③Consider the welding method and process parameters of the specific application The size of the fusion ratio that may be caused; ④ Determine the degree of alloying according to the overall weldability requirements specified in the technical conditions;
18. Weldability analysis of ferritic stainless steel:
1) Intergranular corrosion of welded joints;
2) Embrittlement of welded joints, high temperature embrittlement, σ phase embrittlement, and 475 ℃ embrittlement.
