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49 welding knowledge points solve the welding problems of common iron-based materials
Release time:
2021-08-30
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Weldability and its test evaluation: 1. Welding: By heating or pressurizing, with or without filler material, the two objects are combined between atoms to form an integral whole process. 2. Weldability: refers to the ability of homogeneous or heterogeneous materials to be welded to form a complete joint and meet the expected use requirements under the manufacturing process conditions. 3. The four factors that affect weldability are: material, design, process and service environment. 4. The principle of evaluating weldability mainly includes: ① evaluating the tendency of welding joint to produce process defects, so as to provide basis for formulating reasonable welding process; ② evaluating whether the welding joint can meet the structural requirements
Weldability and its test evaluation:
1. Welding: By heating or pressurizing, with or without filling materials, the process of combining two objects between atoms to form an indivisible whole.
2. Weldability: refers to the ability of homogeneous or heterogeneous materials to be welded to form a complete joint and meet the expected use requirements under the manufacturing process conditions.
3. The four factors that affect weldability are: material, design, process and service environment.
4. The principles of welding evaluation mainly include: ① the tendency of welding joint to produce process defects, which provides the basis for formulating reasonable welding process; ② whether the welding joint can meet the requirements of structural performance; the design of new welding test methods is in line with the following principles: comparability, pertinence, reproducibility and economy.
5. Carbon equivalent: The content of alloy elements in steel is converted and superimposed according to the equivalent of a number of carbon contents, as a parameter index for rough evaluation of the tendency of cold cracking of steel.
6. Oblique Y groove 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 crack tendency, can also be used to develop welding process.
1) Preparation of test piece, steel plate thickness δ = 9-38mm. The groove of butt joint shall be machined by mechanical method, and the two ends of the test plate shall be welded with restraint weld within 60mm, and double-sided welding shall be adopted. Care shall be taken to prevent corner distortion and incomplete penetration. Ensure that there is a 2mm gap at the weld seam of the intermediate sample to be welded.
2) Test conditions: The welding rod selected for the test weld is matched with the base metal. The welding rod used should be strictly dried. The diameter of the welding rod is 4mm, the welding current (170±10)A, the welding voltage (24±2)V, and the welding speed (150±10)mm/min. The test weld can be welded at various temperatures, and only one test weld is welded without filling the groove. After welding, stand still and cool naturally for 24h, cut out the sample and carry out crack detection.
3) Detection and crack strip rate calculation. Check the surface and section of the weld and heat affected zone for cracks with naked eyes or hand-held 5-10 times magnifying glass. It is generally believed that low alloy steel "small iron research" test surface crack rate is less than 20%, generally do not produce cracks.
7. Pin test: The purpose is to mainly evaluate the hydrogen-induced delayed cracking tendency of steel. Other equipment can also be used to determine reheat cracking sensitivity and layered sensitivity. 1) Preparation of test pieces, processing of welded steel or cylindrical bolt test bars, sampling along the rolling direction and indicating the position of the bolt in the thickness direction. There is an annular or screw-shaped notch near the upper end of the test rod. Insert the pin test bar into the corresponding hole of the base plate to make the notched end flush with the surface of the base plate. For bolt test bars with annular notches, the distance a between the notch and the end face shall be such that the penetration of the weld bead is tangent to or intersects the plane cut by the root of the notch, but the part of the circumference of the root of the notch that is melted through 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 crystal area of the heat affected zone, and the length L of the weld bead is about 100-150mm. When welding, the cooling time value t8/5 value at 800-500 ℃ shall be measured. When welding is not preheated, load when cooling to 100-150 ℃ after welding; When preheating before welding, load when it is 50-70 ℃ higher than the preheating temperature. The load should be applied within 1min and before cooling to 100 ℃ or 50-70 ℃ higher than the preheating temperature. If there is post-heat, it should be loaded before post-heat. When the test bar is loaded, the latch may break during the duration of the load. Record the load time.
weldability of alloy structural steel:
1. high strength steel: yield strength σs ≥ 295MPa strength steel can be called high strength steel.
2. The solid solution strengthening effect of Mn is very significant. When ωMn≤ 1.7%, it can improve toughness and reduce brittle transition temperature. Si will reduce plasticity and toughness. Ni is an element that both solid solution strengthening and at the same time improve toughness and greatly reduce 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, which is generally used in hot rolled or normalized state.
4. Design principle of high-strength steel welded joint: high-strength steel takes its strength as the selection basis, 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 welded joint is greater than the strength of base metal, and the plastic toughness is reduced; ② When the service life is equal to that, the strength of joint is insufficient.
5. Hot rolled and normalized steel weldability: hot rolled steel contains a small amount of alloying elements in general, the tendency of cold cracking is not large, normalized steel due to more alloying elements, hardening tendency increases, with the increase of normalized steel carbon equivalent and plate thickness, hardening and cold cracking tendency increases. Influencing factors: (1) carbon equivalent (2) hardening tendency: hardening tendency of hot rolled steel and hardening tendency of normal fire steel (3) the highest hardness of heat affected zone, the highest hardness of heat affected zone is a simple method to evaluate the hardening tendency and cold crack sensibility of steel.
6.SR crack (stress elimination crack, reheat crack): another form of crack may occur in the process of stress elimination heat treatment after welding or high temperature heating after welding for welded structures such as thick-walled pressure vessels containing Mo normalized steel.
7. Toughness is a property that characterizes the ease with which a metal is susceptible to brittle crack initiation and propagation.
8. When selecting welding materials for low alloy steel, two aspects must be considered: ① there can be no welding defects such as cracks; ② can meet the performance requirements. Hot rolled steel and normalized steel welding is generally based on its strength level to select welding materials, the selection of the main points are as follows: ① select the corresponding level of welding materials to match the mechanical properties of the base metal ② consider the 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. Determine the principle of 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: quenching and tempering (high temperature).
11. High strength steel welding using "low strength matching" can 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 The cooling speed during martensite transformation should not be too fast, so that martensite has self-tempering effect to prevent cold cracks; ② The cooling speed between 800 ℃ and 500 ℃ is greater than the critical speed for brittle mixed structure. Low carbon quenched and tempered steel welding problems to be solved: ① to prevent cracks ② to ensure that high strength requirements are met at the same time, improve the toughness of weld metal and heat affected zone.
13. For low alloy steel 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 a role in ensuring hardenability and improving 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 solid solution strengthening, adding alloying elements to strengthen ferrite matrix, commonly used Cr,Mo,W,Nb elements can significantly improve the thermal strength; ② second phase precipitation strengthening: in heat-resistant steel with ferrite matrix, the strengthening phase is mainly alloy carbide; ③ grain boundary strengthening: adding trace elements can adsorb on grain boundaries, the diffusion of alloying elements along the grain boundaries is retarded, thereby strengthening the grain boundaries.
16. The main problems in the welding of pearlitic heat-resistant steel are cold cracks, hardening and softening of the heat-affected zone, and stress-relieving cracks in post-weld heat treatment or long-term use at high temperatures.
17. The temperature range of -10 to -196 ℃ is called "low temperature", and when it is lower than -196 ℃, it is called "ultra-low temperature".
Stainless steel welding:
1. Stainless steel: stainless steel refers to the general term of alloy steel with high chemical stability that can resist corrosion of air, water, acid, alkali, salt and its solution 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 corrosive medium are corroded. Pitting corrosion refers to the scattered local corrosion that does not corrode or corrodes slightly on the surface of most metal materials. Crevice corrosion, when there is a gap between stainless steel or surfaces in contact with foreign matter in an oxygen ion environment, the solution flow in the gap will be delayed, resulting in local Cl-of the solution-, the formation of concentration cell, which leads to the phenomenon that the stainless steel passivation film in the gap adsorbs Cl-and is locally destroyed. Intergranular corrosion, a selective corrosion phenomenon that occurs near the grain boundary; Stress corrosion refers to the phenomenon of brittle cracking of stainless steel that is lower than the strong strength under the action of specific corrosive medium and tensile stress.
3. Measures to prevent pitting corrosion:
1) Reduce the content of chloride ion and oxygen ion;
2) Adding chromium, nickel, molybdenum, silicon, copper and other alloying elements to stainless steel;
3) Try not to cold work to reduce the possibility of pitting corrosion at the dislocation outcrop;
4) Reduce the carbon content in steel.
4. High temperature properties of stainless steel and heat resistant steel: brittleness at 475 ℃, mainly occurring in ferrite with Cr>13%, long-term heating and slow cooling between 430-480 ℃, resulting in strength increase and toughness decrease at normal temperature or negative temperature; σ phase embrittlement is typical of 45% of 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 sensitized zone of heat affected zone;
3) knife-like corrosion.
6. Measures to prevent intergranular corrosion of welds:
1) Through the welding material, the weld metal is either ultra-low carbon or contains sufficient stabilizing element Nb.
2) Adjust the weld composition to obtain a certain δ phase. The intergranular corrosion theory is essentially the chromium-poor theory.
7. Heat-affected zone Sensitized zone intergranular corrosion: refers to the welding heat-affected zone in the heating peak temperature in the sensitized heating zone of the parts of the intergranular corrosion.
8. Knife-like corrosion: Intergranular corrosion in the fusion zone is like a knife cutting incision, so it is called "knife-like corrosion".
9. Measures to prevent knife-like corrosion:① Select low-carbon base metal and welding material ② Use stainless steel with phase structure ③ Use low-current welding to reduce the overheating degree and width of the welded coarse-grained zone ④ Finally weld ⑤ Cross welding ⑥ Increase the content of Ti and Tb in the steel so that the grain boundary of the welded coarse-grained zone has sufficient Ti,Tb and carbide bonding.
In order to reduce the temperature of the welding heat affected zone, prevent the generation of intergranular corrosion of the weld, prevent the welding rod, welding wire overheating, welding deformation, welding stress, can reduce the heat input.
11. Three conditions that cause stress corrosion cracking: environment, selective corrosion medium, and 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, the matching problem of composition and medium;
2) Remove welding residual stress;
3) electrochemical corrosion, regular inspection and timely repair.
13. To improve pitting corrosion 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. Austenitic stainless steel welding will produce thermal cracks, stress corrosion cracks, welding deformation, intergranular corrosion.
15. Austenitic steel welding hot crack causes:
1) The thermal conductivity of austenitic steel is small, the linear expansion coefficient is large, and the tensile stress is large;
2) Austenitic steel is easy to form a directional columnar crystal weld tissue, which is conducive to the segregation of harmful impurities;
3) Austenitic steel alloy composition is more complex, soluble eutectic.
16. Measures to prevent hot cracks:① Strictly limit P in base metal and welding material,S content ② Try to make the weld form a dual-phase structure ③ Control the chemical composition of the weld ④ Small current welding.
What is the difference between the weld microstructure of type 17.18-8 and type 25-20 in preventing hot cracking? 18-8 steel weld forms A + δ structure, δ phase can dissolve a large amount of P,S,δ phase is generally 3%-7%,25-20 steel weld forms A + primary carbide structure.
18. Attention should be paid when selecting austenitic stainless steel: ① Adhere to the "applicability principle"; ② Determine whether it is applicable according to the specific composition of each selected welding material; ③ Consider the fusion ratio that may be caused by the welding method and process parameters of the specific application; ④ Determine the alloying degree according to the overall weldability requirements stipulated in the technical conditions; ⑤ Pay attention to the weld metal alloy system and the role of specific alloy composition in the alloy system, consider the performance requirements and process weldability requirements.
19. Ferritic stainless steel weldability analysis:
1) Intergranular corrosion of welded joints;
2) Embrittlement of welded joints, high temperature embrittlement, σ phase embrittlement, 475 ℃ embrittlement.
Cast iron welding:
1. The three characteristics of cast iron: vibration damping, oil absorption, wear resistance.
2. The performance of cast iron mainly depends on the shape, size, quantity and distribution of graphite, and the matrix organization also has a certain influence.
3. Ductile iron: F matrix spherical graphite; Gray cast iron: F matrix flake graphite; Vermicular graphite cast iron: matrix worm-like graphite; Malleable cast iron: F matrix floc 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 calculated according to C = 3% in cast iron, the carbon content in the first weld is 0.75-0.9, which belongs to high carbon steel. high carbon martensite will appear immediately after welding cooling, and white mouth structure will appear in welding HAZ, making it difficult to machine.
5. Arc hot welding: the molten castings are preheated to 600-700 ℃, and then welded in a plastic state at a welding temperature of not less 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 arc hot welding.
6. Semi-hot welding: when the preheating temperature is 300-400 ℃, it is called semi-hot welding.
Key words:
Welding, corrosion, crack, weld, influence, stainless steel, stress, strength, alloy
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