49 welding technical knowledge about pipeline
49 welding technical knowledge about pipeline
Weldability and test evaluation
1. Welding: a process in which two objects are combined to form an integral whole by heating or pressurizing, adding or not adding 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 factors affecting weldability are: material, design, process and service environment.
4. The principles to evaluate the weldability mainly include: ① to evaluate the tendency of welding joints to produce process defects, so as to provide a basis for making a reasonable welding process; ② to evaluate whether the welding joints can meet the requirements of structural SERVICEABILITY; and to design new welding test methods to meet 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, as a parameter index for rough assessment of 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 weld and HAZ of low-alloy high-strength steel, and it can also be used to draw up the welding process. 1) preparation of test piece, thickness of welded steel plate 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 within 60mm, with double-sided welding. Pay attention to prevent angle deformation and incomplete penetration. Ensure that there is 2mm gap at the weld joint of the sample to be welded in the middle. 2) test conditions: the welding rod selected for the test weld shall match with the base metal. The welding rod shall be dried strictly, with the diameter of 4mm, welding current (170 ± 10) a, welding voltage (24 ± 2) V, and welding speed (150 ± 10) mm / min. The test weld can be welded at different temperatures, and only one test weld can be welded without filling the groove. After 24 hours of standing and natural cooling after welding, cut off the sample and carry out crack detection. 3) detection and crack strip rate calculation. Use naked eye or hand-held 5-10 times magnifying glass to check whether there are cracks on the surface and section of weld and heat affected zone. It is generally believed that when the surface crack rate of "small iron grinding" test of low alloy steel is less than 20%, there is no crack generally.
7. Pin test: the purpose is to evaluate the hydrogen induced delayed cracking tendency of steel. With other equipment, the reheat cracking sensitivity and lamellar sensitivity can also be determined. 1) for the preparation of test piece, take samples along the rolling direction of the pin test bar processed by the welded steel or cylinder, and indicate the position of the pin in the thickness direction. There is a ring or screw gap near the upper end of the test bar. Insert the pin test bar into the corresponding hole of the bottom plate so that the end with notch is flush with the surface of the bottom plate. For the pin test bar with annular notch, the distance a between the notch and the end face shall be such that the weld bead penetration is tangent or intersected with the plane cut by the notch root, but the part of the circumferential penetration of the notch root shall not exceed 20%. For low alloy steel, a value 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, weld a layer of weld bead on the bottom plate, the weld bead center line passes through the center of the sample, and the penetration shall make the notch tip located in the coarse-grained area of the heat affected area, and the weld bead length L is about 100-150 mm. When welding, the cooling time value T8 / 5 of 800-500 ℃ shall be measured. When welding without preheating, it shall be loaded when cooling to 100-150 ℃ after welding; when preheating before welding, it shall be loaded when the preheating temperature is higher than 50-70 ℃. The load shall be applied within 1min and before cooling to 100 ℃ or 50-70 ℃ higher than preheating temperature. If there is afterheat, it should 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 σ s ≥ 295mpa can be called high strength steel.
2. The solution strengthening effect of Mn is very significant. When ω Mn ≤ 1.7%, it can improve the toughness and reduce the brittleness transition temperature. Si can reduce the plasticity and toughness. Ni is the element that both solution strengthening and toughness are improved and brittleness transition temperature is greatly reduced. 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 supplied and used in hot rolling or normalized state.
4. Design principle of welded joint of high strength steel: high strength steel is selected based on 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 welded joint is greater than the strength of base metal, the plastic toughness is reduced, ② the time life is equal to ③ less, the strength of joint is insufficient.
5. Weldability of hot rolled and normalized steel: generally, the tendency of cold crack is not obvious when the hot rolled steel contains a small amount of alloy elements. Because the normalized steel contains more alloy elements, the hardening tendency increases. With the increase of carbon equivalent and plate thickness of normalized steel, the hardenability and the tendency of cold crack increase. Influencing factors: (1) carbon equivalent; (2) hardening tendency: the hardening tendency of hot rolled steel and normalized steel; (3) the highest hardness in the heat affected zone and the highest hardness in the heat affected zone is a simple method to evaluate the hardening tendency and cold crack sensibility of steel.
6. SR crack (stress relief crack, reheat crack): another form of crack may appear in the process of post weld stress relief heat treatment or post weld high temperature reheating for welded structures such as thick wall pressure vessels of Mo normalized steel.
7. Toughness is the property that characterizes the difficulty of metal to produce and expand brittle crack.
8. When selecting welding materials for low alloy steel, two problems must be considered: ① no welding defects such as cracks; ② meeting the requirements of service performance. The welding of hot rolled steel and normalized steel is generally based on its strength level. The key points of selection are as follows: ① select the corresponding level of welding materials matching the mechanical properties of the base metal; ② consider the influence of fusion ratio and cooling speed at the same time; ③ consider the influence 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 to avoid affecting the performance of the base metal itself; ② for materials with tempering, avoid the temperature range where tempering brittleness occurs.
10. Quenched and tempered steel: quenching + tempering (high temperature).
11. "Low strength matching" can improve the crack resistance of the welding area.
12. Two basic problems should be paid attention to when welding low carbon quenched and tempered steel: ① the cooling speed of martensite transformation should not be too fast, so that martensite has self tempering effect to prevent cold crack; ② the cooling speed between 800 ℃ - 500 ℃ is required to be greater than the critical speed of brittle mixed structure. The problems to be solved in the welding of low carbon quenched and tempered steel are as follows: ① preventing cracks; ② improving the toughness of weld metal and heat affected zone while meeting the requirements of high strength.
13. For low alloy steel with low carbon content, increasing cooling rate to form low carbon martensite is beneficial to ensure toughness.
14. The addition of medium carbon quenched and tempered steel alloy elements is mainly to ensure the hardenability and improve 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 alloy elements to strengthen the ferrite matrix, common Cr, Mo, W, Nb elements can significantly improve the thermal strength ② second phase precipitation strengthening: in the ferrite based heat-resistant steel, the strengthening phase is mainly alloy carbide ③ grain boundary strengthening: adding trace elements can adsorb on the grain boundary and delay the alloy elements along the grain boundary The diffusion of the grain boundary strengthens the grain boundary.
16. The main problems in welding of pearlitic heat-resistant steel are cold crack, hardening and softening of heat affected zone, and stress relief crack in post weld heat treatment or high temperature long-term use.
17. The temperature range from - 10 to - 196 ℃ is called "low temperature", and below - 196 ℃ 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 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 the metal surfaces in contact with the corrosive medium produce corrosion; spot corrosion refers to the local corrosion that most of the metal surfaces do not corrode or slightly corrode, but occur dispersedly; crevice corrosion, in the electrolyte, such as in the oxygen ion environment, when there is a gap between stainless steel and the surface in contact with foreign matters, the solution flow in the gap will be sluggish So that local Cl - in the solution forms a concentration cell, leading to the phenomenon that the passivation film of stainless steel in the gap adsorbs Cl - and is locally destroyed; intergranular corrosion, selective corrosion near the grain boundary; stress corrosion, refers to the brittle cracking phenomenon 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 spot corrosion: 1) reduce chloride ion content and oxygen ion content; 2) add alloy elements such as chromium, nickel, molybdenum, silicon and copper into stainless steel; 3) try not to carry out cold working to reduce the possibility of spot corrosion at dislocation outcrop; 4) reduce carbon content in steel.
4. High temperature properties of stainless steel and heat-resistant steel: 475 ℃ brittleness, mainly occurs in the ferrite with Cr > 13%, which is heated and cooled slowly for a long time between 430-480 ℃, resulting in the increase of strength and decrease of toughness at normal or negative temperature; σ phase embrittlement is a typical example of 45% of Cr mass fraction, FeCr intermetallic compound, nonmagnetic, hard and brittle.
5. Corrosion resistance of austenitic stainless steel welded joints: 1) intergranular corrosion, 2) intergranular corrosion in the sensitized zone of the heat affected zone, 3) knife corrosion.
6. Measures to prevent intergranular corrosion of welds: 1) make the weld metal become ultra-low carbon or contain enough stable element NB through welding materials. 2) adjust the weld composition to obtain a certain δ phase. The theory of intergranular corrosion is essentially the theory of chromium deficiency.
7. Intercrystalline corrosion in the sensitized zone of HAZ: it refers to the intercrystalline corrosion in the position where the heating peak temperature in the HAZ is in the sensitized heating zone.
8. Knife like corrosion: intergranular corrosion produced in the fusion area is like knife cutting, so it is called "knife like corrosion".
9. Measures to prevent knife like corrosion: ① select low-carbon base metal and welding material ② adopt stainless steel with phase structure ③ adopt small current welding to reduce the superheat degree and width of welding coarse-grained area ④ weld the last weld contacting with corrosive medium ⑤ cross welding ⑥ increase the content of Ti and TB in the steel, so that the grain boundary of welding coarse-grained area has enough Ti, TB and carburization.
10. Why does stainless steel adopt low current welding? In order to reduce the temperature of the heat affected zone, prevent the intergranular corrosion of the weld, prevent the electrode, welding wire from overheating, 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) adjust chemical composition, ultra-low carbon is conducive to improving the ability of stress corrosion resistance, matching of composition and medium, 2) remove welding residual stress, 3) electrochemical corrosion, regular inspection and timely repair, etc.
13. In order to improve pitting resistance: 1) on the one hand, Cr and Mo segregation must be reduced; 2) on the other hand, the so-called "super alloying" welding material with higher CR and Mo content than the base metal shall be used.
14. Austenitic stainless steel will produce hot crack, stress corrosion crack, welding deformation and intergranular corrosion during welding.
15. The causes of welding hot crack of austenitic steel: 1) the thermal conductivity of austenitic steel is small, the coefficient of linear expansion is large, the tensile stress is large, 2) the austenitic steel is easy to form the weld structure of columnar crystal with strong directionality, which is conducive to the segregation of harmful impurities 3) the alloy composition of austenitic steel is complex and easy to dissolve 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 form a two-phase structure of the weld; ③ control the chemical composition of the weld; ④ low current welding.
17.What is the difference of weld structure between 17.18-8 and 25-20 in preventing hot crack? The weld of 18-8 type steel forms a + δ structure, and the δ phase can dissolve a large number of P, s, δ phase is generally 3% - 7%. The weld of 25-20 type steel forms a + primary carbide structure.
18. Attention shall be paid to the selection of austenitic stainless steel materials: ① adhere to the "applicability principle"; ② determine the applicability according to the specific composition of the selected welding materials; ③ consider the possible fusion ratio caused by the specific welding methods and process parameters; ④ determine the degree of alloying according to the overall weldability requirements specified in the technical conditions; ⑤ pay attention to the weld metal alloy system, where the specific alloy composition is For the role of alloy system, the requirements of service performance and process weldability shall be considered.
19. Weldability analysis of ferritic stainless steel: 1) intergranular corrosion of welded joint 2) embrittlement of welded joint, high temperature embrittlement, σ phase embrittlement, 475 ℃ embrittlement.
Cast iron welding
1. Three main 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. Nodular cast iron: F matrix + spheroidal graphite; grey cast iron: F matrix + flaky graphite; vermicular cast iron: matrix + vermicular graphite; malleable cast iron: F matrix + flocculent graphite.
4. Can low carbon steel electrode weld cast iron? No, in welding, even if the current is small, the proportion of base metal in the first weld is 25% - 30%. If calculated by 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 the HAZ will show white structure, which makes machining difficult.
5. Arc heat welding: the molten iron is preheated to 600-700 ℃, and then welded in the plastic state. The welding temperature is not lower than 400 ℃. In order to prevent cracking in the welding process, stress relief treatment and slow cooling shall be carried out immediately after welding. This cast iron welding repair process is called arc heat welding.
6. Semi hot welding: when the preheating temperature is 300-400 ℃, it is called semi hot welding.
Weldability and test evaluation
1. Welding: a process in which two objects are combined to form an integral whole by heating or pressurizing, adding or not adding 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 factors affecting weldability are: material, design, process and service environment.
4. The principles to evaluate the weldability mainly include: ① to evaluate the tendency of welding joints to produce process defects, so as to provide a basis for making a reasonable welding process; ② to evaluate whether the welding joints can meet the requirements of structural SERVICEABILITY; and to design new welding test methods to meet 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, as a parameter index for rough assessment of 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 weld and HAZ of low-alloy high-strength steel, and it can also be used to draw up the welding process. 1) preparation of test piece, thickness of welded steel plate 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 within 60mm, with double-sided welding. Pay attention to prevent angle deformation and incomplete penetration. Ensure that there is 2mm gap at the weld joint of the sample to be welded in the middle. 2) test conditions: the welding rod selected for the test weld shall match with the base metal. The welding rod shall be dried strictly, with the diameter of 4mm, welding current (170 ± 10) a, welding voltage (24 ± 2) V, and welding speed (150 ± 10) mm / min. The test weld can be welded at different temperatures, and only one test weld can be welded without filling the groove. After 24 hours of standing and natural cooling after welding, cut off the sample and carry out crack detection. 3) detection and crack strip rate calculation. Use naked eye or hand-held 5-10 times magnifying glass to check whether there are cracks on the surface and section of weld and heat affected zone. It is generally believed that when the surface crack rate of "small iron grinding" test of low alloy steel is less than 20%, there is no crack generally.
7. Pin test: the purpose is to evaluate the hydrogen induced delayed cracking tendency of steel. With other equipment, the reheat cracking sensitivity and lamellar sensitivity can also be determined. 1) for the preparation of test piece, take samples along the rolling direction of the pin test bar processed by the welded steel or cylinder, and indicate the position of the pin in the thickness direction. There is a ring or screw gap near the upper end of the test bar. Insert the pin test bar into the corresponding hole of the bottom plate so that the end with notch is flush with the surface of the bottom plate. For the pin test bar with annular notch, the distance a between the notch and the end face shall be such that the weld bead penetration is tangent or intersected with the plane cut by the notch root, but the part of the circumferential penetration of the notch root shall not exceed 20%. For low alloy steel, a value 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, weld a layer of weld bead on the bottom plate, the weld bead center line passes through the center of the sample, and the penetration shall make the notch tip located in the coarse-grained area of the heat affected area, and the weld bead length L is about 100-150 mm. When welding, the cooling time value T8 / 5 of 800-500 ℃ shall be measured. When welding without preheating, it shall be loaded when cooling to 100-150 ℃ after welding; when preheating before welding, it shall be loaded when the preheating temperature is higher than 50-70 ℃. The load shall be applied within 1min and before cooling to 100 ℃ or 50-70 ℃ higher than preheating temperature. If there is afterheat, it should 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 σ s ≥ 295mpa can be called high strength steel.
2. The solution strengthening effect of Mn is very significant. When ω Mn ≤ 1.7%, it can improve the toughness and reduce the brittleness transition temperature. Si can reduce the plasticity and toughness. Ni is the element that both solution strengthening and toughness are improved and brittleness transition temperature is greatly reduced. 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 supplied and used in hot rolling or normalized state.
4. Design principle of welded joint of high strength steel: high strength steel is selected based on 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 welded joint is greater than the strength of base metal, the plastic toughness is reduced, ② the time life is equal to ③ less, the strength of joint is insufficient.
5. Weldability of hot rolled and normalized steel: generally, the tendency of cold crack is not obvious when the hot rolled steel contains a small amount of alloy elements. Because the normalized steel contains more alloy elements, the hardening tendency increases. With the increase of carbon equivalent and plate thickness of normalized steel, the hardenability and the tendency of cold crack increase. Influencing factors: (1) carbon equivalent; (2) hardening tendency: the hardening tendency of hot rolled steel and normalized steel; (3) the highest hardness in the heat affected zone and the highest hardness in the heat affected zone is a simple method to evaluate the hardening tendency and cold crack sensibility of steel.
6. SR crack (stress relief crack, reheat crack): another form of crack may appear in the process of post weld stress relief heat treatment or post weld high temperature reheating for welded structures such as thick wall pressure vessels of Mo normalized steel.
7. Toughness is the property that characterizes the difficulty of metal to produce and expand brittle crack.
8. When selecting welding materials for low alloy steel, two problems must be considered: ① no welding defects such as cracks; ② meeting the requirements of service performance. The welding of hot rolled steel and normalized steel is generally based on its strength level. The key points of selection are as follows: ① select the corresponding level of welding materials matching the mechanical properties of the base metal; ② consider the influence of fusion ratio and cooling speed at the same time; ③ consider the influence 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 to avoid affecting the performance of the base metal itself; ② for materials with tempering, avoid the temperature range where tempering brittleness occurs.
10. Quenched and tempered steel: quenching + tempering (high temperature).
11. "Low strength matching" can improve the crack resistance of the welding area.
12. Two basic problems should be paid attention to when welding low carbon quenched and tempered steel: ① the cooling speed of martensite transformation should not be too fast, so that martensite has self tempering effect to prevent cold crack; ② the cooling speed between 800 ℃ - 500 ℃ is required to be greater than the critical speed of brittle mixed structure. The problems to be solved in the welding of low carbon quenched and tempered steel are as follows: ① preventing cracks; ② improving the toughness of weld metal and heat affected zone while meeting the requirements of high strength.
13. For low alloy steel with low carbon content, increasing cooling rate to form low carbon martensite is beneficial to ensure toughness.
14. The addition of medium carbon quenched and tempered steel alloy elements is mainly to ensure the hardenability and improve 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 alloy elements to strengthen the ferrite matrix, common Cr, Mo, W, Nb elements can significantly improve the thermal strength ② second phase precipitation strengthening: in the ferrite based heat-resistant steel, the strengthening phase is mainly alloy carbide ③ grain boundary strengthening: adding trace elements can adsorb on the grain boundary and delay the alloy elements along the grain boundary The diffusion of the grain boundary strengthens the grain boundary.
16. The main problems in welding of pearlitic heat-resistant steel are cold crack, hardening and softening of heat affected zone, and stress relief crack in post weld heat treatment or high temperature long-term use.
17. The temperature range from - 10 to - 196 ℃ is called "low temperature", and below - 196 ℃ 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 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 the metal surfaces in contact with the corrosive medium produce corrosion; spot corrosion refers to the local corrosion that most of the metal surfaces do not corrode or slightly corrode, but occur dispersedly; crevice corrosion, in the electrolyte, such as in the oxygen ion environment, when there is a gap between stainless steel and the surface in contact with foreign matters, the solution flow in the gap will be sluggish So that local Cl - in the solution forms a concentration cell, leading to the phenomenon that the passivation film of stainless steel in the gap adsorbs Cl - and is locally destroyed; intergranular corrosion, selective corrosion near the grain boundary; stress corrosion, refers to the brittle cracking phenomenon 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 spot corrosion: 1) reduce chloride ion content and oxygen ion content; 2) add alloy elements such as chromium, nickel, molybdenum, silicon and copper into stainless steel; 3) try not to carry out cold working to reduce the possibility of spot corrosion at dislocation outcrop; 4) reduce carbon content in steel.
4. High temperature properties of stainless steel and heat-resistant steel: 475 ℃ brittleness, mainly occurs in the ferrite with Cr > 13%, which is heated and cooled slowly for a long time between 430-480 ℃, resulting in the increase of strength and decrease of toughness at normal or negative temperature; σ phase embrittlement is a typical example of 45% of Cr mass fraction, FeCr intermetallic compound, nonmagnetic, hard and brittle.
5. Corrosion resistance of austenitic stainless steel welded joints: 1) intergranular corrosion, 2) intergranular corrosion in the sensitized zone of the heat affected zone, 3) knife corrosion.
6. Measures to prevent intergranular corrosion of welds: 1) make the weld metal become ultra-low carbon or contain enough stable element NB through welding materials. 2) adjust the weld composition to obtain a certain δ phase. The theory of intergranular corrosion is essentially the theory of chromium deficiency.
7. Intercrystalline corrosion in the sensitized zone of HAZ: it refers to the intercrystalline corrosion in the position where the heating peak temperature in the HAZ is in the sensitized heating zone.
8. Knife like corrosion: intergranular corrosion produced in the fusion area is like knife cutting, so it is called "knife like corrosion".
9. Measures to prevent knife like corrosion: ① select low-carbon base metal and welding material ② adopt stainless steel with phase structure ③ adopt small current welding to reduce the superheat degree and width of welding coarse-grained area ④ weld the last weld contacting with corrosive medium ⑤ cross welding ⑥ increase the content of Ti and TB in the steel, so that the grain boundary of welding coarse-grained area has enough Ti, TB and carburization.
10. Why does stainless steel adopt low current welding? In order to reduce the temperature of the heat affected zone, prevent the intergranular corrosion of the weld, prevent the electrode, welding wire from overheating, 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) adjust chemical composition, ultra-low carbon is conducive to improving the ability of stress corrosion resistance, matching of composition and medium, 2) remove welding residual stress, 3) electrochemical corrosion, regular inspection and timely repair, etc.
13. In order to improve pitting resistance: 1) on the one hand, Cr and Mo segregation must be reduced; 2) on the other hand, the so-called "super alloying" welding material with higher CR and Mo content than the base metal shall be used.
14. Austenitic stainless steel will produce hot crack, stress corrosion crack, welding deformation and intergranular corrosion during welding.
15. The causes of welding hot crack of austenitic steel: 1) the thermal conductivity of austenitic steel is small, the coefficient of linear expansion is large, the tensile stress is large, 2) the austenitic steel is easy to form the weld structure of columnar crystal with strong directionality, which is conducive to the segregation of harmful impurities 3) the alloy composition of austenitic steel is complex and easy to dissolve 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 form a two-phase structure of the weld; ③ control the chemical composition of the weld; ④ low current welding.
17.What is the difference of weld structure between 17.18-8 and 25-20 in preventing hot crack? The weld of 18-8 type steel forms a + δ structure, and the δ phase can dissolve a large number of P, s, δ phase is generally 3% - 7%. The weld of 25-20 type steel forms a + primary carbide structure.
18. Attention shall be paid to the selection of austenitic stainless steel materials: ① adhere to the "applicability principle"; ② determine the applicability according to the specific composition of the selected welding materials; ③ consider the possible fusion ratio caused by the specific welding methods and process parameters; ④ determine the degree of alloying according to the overall weldability requirements specified in the technical conditions; ⑤ pay attention to the weld metal alloy system, where the specific alloy composition is For the role of alloy system, the requirements of service performance and process weldability shall be considered.
19. Weldability analysis of ferritic stainless steel: 1) intergranular corrosion of welded joint 2) embrittlement of welded joint, high temperature embrittlement, σ phase embrittlement, 475 ℃ embrittlement.
Cast iron welding
1. Three main 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. Nodular cast iron: F matrix + spheroidal graphite; grey cast iron: F matrix + flaky graphite; vermicular cast iron: matrix + vermicular graphite; malleable cast iron: F matrix + flocculent graphite.
4. Can low carbon steel electrode weld cast iron? No, in welding, even if the current is small, the proportion of base metal in the first weld is 25% - 30%. If calculated by 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 the HAZ will show white structure, which makes machining difficult.
5. Arc heat welding: the molten iron is preheated to 600-700 ℃, and then welded in the plastic state. The welding temperature is not lower than 400 ℃. In order to prevent cracking in the welding process, stress relief treatment and slow cooling shall be carried out immediately after welding. This cast iron welding repair process is called arc heat welding.
6. Semi hot welding: when the preheating temperature is 300-400 ℃, it is called semi hot welding.
