Stainless steel refers to steel in which the main additive element Cr is higher than 12%, which enables the steel to be passivated and has stainless steel characteristics. Stainless steel is classified into a ferrite type, a martensite type, an austenite type, an austenite + ferrite type, and a precipitation hardening type stainless steel according to its microstructure. Austenitic stainless steels usually have pure austenite at room temperature, and some have austenite + a small amount of ferrite. This small amount of ferrite helps prevent hot cracks.
First, the welding characteristics of austenitic stainless steel
1. It is prone to hot cracks.
Preventive measures:
(1) Try to make the weld metal in a two-phase structure, and the content of ferrite is controlled below 3-5%. Because ferrite can dissolve harmful S and P impurities in large quantities.
(2) Try to use high-quality welding rods of alkaline coatings to limit the content of S, P, C, etc. in the weld metal.
2. Intergranular corrosion: According to the theory of lean chromium, the weld and heat affected zone precipitates chromium carbide on the grain boundary when heated to the sensitized temperature zone of 450-850 °C, resulting in a chromium-depleted grain boundary, insufficient to resist corrosion. .
Preventive measures:
(1) Use low-carbon or ultra-low-carbon welding consumables, such as A002; use electrodes containing stabilizing elements such as titanium and niobium, such as A137 and A132.
(2) A certain amount of ferrite forming elements are melted into the weld by the welding wire or the welding rod, so that the weld metal becomes a two-phase structure of austenite + ferrite (ferrite is generally controlled at 4-12%) .
(3) Reduce the overheating of the weld pool, use a smaller welding current and a faster welding speed to speed up the cooling.
(4) Post-weld stabilization annealing treatment of weldments with high resistance to intergranular corrosion resistance
3. Stress corrosion cracking: Stress corrosion cracking is a delayed cracking phenomenon caused by tensile stress of a welded joint under a specific corrosive environment. The stress corrosion cracking of austenitic stainless steel welded joints is a serious failure form of welded joints, which is characterized by brittle failure without plastic deformation.
Stress corrosion cracking prevention measures:
(1) Reasonably formulate the forming process and assembly process, minimize the cold deformation degree, avoid forced assembly, and prevent various kinds of scratches during the assembly process (all kinds of assembly scars and arc burn marks will become the source of SCC, which is easy to cause Corrosion pit).
(2) Reasonable selection of welding consumables: welds and base metals should have a good match, without any bad structure, such as grain coarsening and hard brittle martensite;
(3) Adopt a suitable welding process: ensure that the weld is well formed, does not produce any stress concentration or pitting defects, such as undercuts; take a reasonable welding sequence to reduce the residual stress level;
(4) Stress relief treatment: post-weld heat treatment, such as complete annealing or annealing after welding; use post-weld hammering or shot peening when it is difficult to perform heat treatment.
(5) Production management measures: control of impurities in the medium, such as O2, N2, H2O in liquid ammonia medium; H2S in liquefied petroleum gas; O2, Fe3+, Cr6+ in chloride solution; anti-corrosion treatment: Coating, lining or cathodic protection, etc.; adding a corrosion inhibitor.
4. Low-temperature embrittlement of weld metal: For austenitic stainless steel welded joints, the plastic toughness of weld metal is a key issue when used at low temperatures. At this time, the presence of ferrite in the weld bead always deteriorates the low temperature toughness.
Preventive measures:
A single austenitic weld is obtained by selecting pure austenitic welding consumables and adjusting the welding process.
5. σ phase embrittlement of welded joints: After a certain period of high temperature heating, the weldment will precipitate a brittle σ phase in the weld, resulting in embrittlement of the joint and a significant decrease in ductility and toughness. The precipitation temperature of the σ phase ranges from 650 to 850 °C. During high temperature heating, the σ phase is mainly transformed from ferrite. The longer the heating time, the more the σ phase precipitates.
Preventive measures:
(1) Limiting the ferrite content in the weld metal (less than 15%); using superalloyed welding materials, ie high nickel welding consumables.
(2) Adopt small specifications to reduce the residence time of the weld metal at high temperatures;
(3) The precipitated σ phase is subjected to solution treatment when conditions permit, and the σ phase is dissolved into austenite.
Second, the selection of welding rods for austenitic stainless steel
Stainless steel is mainly used for corrosion resistance, but it is also used as heat resistant steel and low temperature steel. Therefore, when welding stainless steel, the performance of the electrode must match the purpose of the stainless steel. Stainless steel electrodes must be selected according to the base metal and working conditions (including working temperature and contact medium, etc.).
1. Generally speaking, the selection of the electrode can refer to the material of the base material, and the electrode with the same or similar composition as the base material is selected. For example, A102 corresponds to 0Cr19Ni9; A137 corresponds to 1Cr18Ni9Ti.
2. Since the carbon content has a great influence on the corrosion resistance of stainless steel, it is generally preferred to use a stainless steel electrode with a carbon content of the deposited metal not higher than that of the base material. For example, 316L must use A022 electrode.
3. The weld metal of austenitic stainless steel should ensure the mechanical properties. Can be verified by welding process qualification.
4. For heat-resistant stainless steel (austenitic heat-resistant steel) working at high temperature, the selected electrode should mainly meet the thermal cracking resistance of the weld metal and the high temperature performance of the welded joint.
(1) For austenitic heat-resistant steels with Cr/Ni≥1, such as 1Cr18Ni9Ti, austenitic-ferritic stainless steel electrodes are generally used, and 2-5% ferrite is preferably included in the weld metal. When the ferrite content is too low, the weld metal has poor crack resistance; if it is too high, the σ embrittlement phase is easily formed during long-term use or heat treatment at high temperature, causing cracks. Such as A002, A102, A137.
In some special applications, it may be required to use austenitic weld metal, such as A402, A407 electrode.
(2) For austenitic heat-resistant steels with Cr/Ni<1, such as Cr16Ni25Mo6, etc., it is generally necessary to increase the weld metal to be similar to the chemical composition of the base metal, and to increase Mo, W, and The content of elements such as Mn improves the crack resistance of the weld while ensuring the thermal strength of the weld metal. Such as the use of A502, A507.
5. For corrosion-resistant stainless steels working in various corrosive media, the electrode should be selected according to the medium and working temperature, and its corrosion resistance (corrosion performance test of welded joints) should be ensured.
(1) For media with a working temperature above 300 °C and strong corrosiveness, a Ti or Nb stabilizing element or an ultra-low carbon stainless steel electrode shall be used. Such as A137 or A002.
(2) For medium containing dilute sulfuric acid or hydrochloric acid, stainless steel electrodes containing Mo or containing Mo and Cu such as A032, A052, etc. are often used.
(3) Stainless steel welding rods without Ti or Nb may be used for work, equipment with weak corrosiveness or only to avoid rust pollution.
In order to ensure the stress corrosion resistance of the weld metal, the superalloyed welding consumables, that is, the corrosion resistant alloying elements (Cr, Mo, Ni, etc.) in the weld metal are higher than the base metal. For example, 00Cr19Ni10 weldment is welded with 00Cr18Ni12Mo2 type welding material (such as A022).
6. For austenitic stainless steels working under low temperature conditions, the low temperature impact toughness of the welded joints at the use temperature should be ensured, so pure austenitic welding rods are used. Such as A402, A407.
7. Nickel-based alloy electrodes can also be used. For example, Mo6 super austenitic stainless steel is welded with 9% nickel-based welding material.
8, the choice of electrode type:
(1) Since the weld metal of the duplex austenitic steel itself contains a certain amount of ferrite, it has good plasticity and toughness, and is compared with the crack resistance of the weld metal, the alkaline coating and the titanium calcium type coating. The difference in the electrode is not as significant as the carbon steel electrode. Therefore, in practical applications, there are many eyesights in terms of soldering process performance, and most of them use electrode strips with a coating type of 17 or 16 (such as A102A, A102, A132, etc.).
(2) Only when the structural rigidity is large or the weld metal is poor in crack resistance (such as some martensitic chromium stainless steel, pure austenitic chromium-nickel stainless steel, etc.), the selection of the drug code is 15 Alkaline coated stainless steel electrode (such as A107, A407, etc.).
In summary, the welding of austenitic stainless steel has its own unique characteristics. The welding rod selection of austenitic stainless steel is particularly noteworthy. Only in this way can different welding methods and different materials be applied to different materials, stainless steel. The electrode must be selected according to the base metal and working conditions (including working temperature and contact medium). This will make it possible to achieve the desired weld quality.
First, the welding characteristics of austenitic stainless steel
1. It is prone to hot cracks.
Preventive measures:
(1) Try to make the weld metal in a two-phase structure, and the content of ferrite is controlled below 3-5%. Because ferrite can dissolve harmful S and P impurities in large quantities.
(2) Try to use high-quality welding rods of alkaline coatings to limit the content of S, P, C, etc. in the weld metal.
2. Intergranular corrosion: According to the theory of lean chromium, the weld and heat affected zone precipitates chromium carbide on the grain boundary when heated to the sensitized temperature zone of 450-850 °C, resulting in a chromium-depleted grain boundary, insufficient to resist corrosion. .
Preventive measures:
(1) Use low-carbon or ultra-low-carbon welding consumables, such as A002; use electrodes containing stabilizing elements such as titanium and niobium, such as A137 and A132.
(2) A certain amount of ferrite forming elements are melted into the weld by the welding wire or the welding rod, so that the weld metal becomes a two-phase structure of austenite + ferrite (ferrite is generally controlled at 4-12%) .
(3) Reduce the overheating of the weld pool, use a smaller welding current and a faster welding speed to speed up the cooling.
(4) Post-weld stabilization annealing treatment of weldments with high resistance to intergranular corrosion resistance
3. Stress corrosion cracking: Stress corrosion cracking is a delayed cracking phenomenon caused by tensile stress of a welded joint under a specific corrosive environment. The stress corrosion cracking of austenitic stainless steel welded joints is a serious failure form of welded joints, which is characterized by brittle failure without plastic deformation.
Stress corrosion cracking prevention measures:
(1) Reasonably formulate the forming process and assembly process, minimize the cold deformation degree, avoid forced assembly, and prevent various kinds of scratches during the assembly process (all kinds of assembly scars and arc burn marks will become the source of SCC, which is easy to cause Corrosion pit).
(2) Reasonable selection of welding consumables: welds and base metals should have a good match, without any bad structure, such as grain coarsening and hard brittle martensite;
(3) Adopt a suitable welding process: ensure that the weld is well formed, does not produce any stress concentration or pitting defects, such as undercuts; take a reasonable welding sequence to reduce the residual stress level;
(4) Stress relief treatment: post-weld heat treatment, such as complete annealing or annealing after welding; use post-weld hammering or shot peening when it is difficult to perform heat treatment.
(5) Production management measures: control of impurities in the medium, such as O2, N2, H2O in liquid ammonia medium; H2S in liquefied petroleum gas; O2, Fe3+, Cr6+ in chloride solution; anti-corrosion treatment: Coating, lining or cathodic protection, etc.; adding a corrosion inhibitor.
4. Low-temperature embrittlement of weld metal: For austenitic stainless steel welded joints, the plastic toughness of weld metal is a key issue when used at low temperatures. At this time, the presence of ferrite in the weld bead always deteriorates the low temperature toughness.
Preventive measures:
A single austenitic weld is obtained by selecting pure austenitic welding consumables and adjusting the welding process.
5. σ phase embrittlement of welded joints: After a certain period of high temperature heating, the weldment will precipitate a brittle σ phase in the weld, resulting in embrittlement of the joint and a significant decrease in ductility and toughness. The precipitation temperature of the σ phase ranges from 650 to 850 °C. During high temperature heating, the σ phase is mainly transformed from ferrite. The longer the heating time, the more the σ phase precipitates.
Preventive measures:
(1) Limiting the ferrite content in the weld metal (less than 15%); using superalloyed welding materials, ie high nickel welding consumables.
(2) Adopt small specifications to reduce the residence time of the weld metal at high temperatures;
(3) The precipitated σ phase is subjected to solution treatment when conditions permit, and the σ phase is dissolved into austenite.
Second, the selection of welding rods for austenitic stainless steel
Stainless steel is mainly used for corrosion resistance, but it is also used as heat resistant steel and low temperature steel. Therefore, when welding stainless steel, the performance of the electrode must match the purpose of the stainless steel. Stainless steel electrodes must be selected according to the base metal and working conditions (including working temperature and contact medium, etc.).
1. Generally speaking, the selection of the electrode can refer to the material of the base material, and the electrode with the same or similar composition as the base material is selected. For example, A102 corresponds to 0Cr19Ni9; A137 corresponds to 1Cr18Ni9Ti.
2. Since the carbon content has a great influence on the corrosion resistance of stainless steel, it is generally preferred to use a stainless steel electrode with a carbon content of the deposited metal not higher than that of the base material. For example, 316L must use A022 electrode.
3. The weld metal of austenitic stainless steel should ensure the mechanical properties. Can be verified by welding process qualification.
4. For heat-resistant stainless steel (austenitic heat-resistant steel) working at high temperature, the selected electrode should mainly meet the thermal cracking resistance of the weld metal and the high temperature performance of the welded joint.
(1) For austenitic heat-resistant steels with Cr/Ni≥1, such as 1Cr18Ni9Ti, austenitic-ferritic stainless steel electrodes are generally used, and 2-5% ferrite is preferably included in the weld metal. When the ferrite content is too low, the weld metal has poor crack resistance; if it is too high, the σ embrittlement phase is easily formed during long-term use or heat treatment at high temperature, causing cracks. Such as A002, A102, A137.
In some special applications, it may be required to use austenitic weld metal, such as A402, A407 electrode.
(2) For austenitic heat-resistant steels with Cr/Ni<1, such as Cr16Ni25Mo6, etc., it is generally necessary to increase the weld metal to be similar to the chemical composition of the base metal, and to increase Mo, W, and The content of elements such as Mn improves the crack resistance of the weld while ensuring the thermal strength of the weld metal. Such as the use of A502, A507.
5. For corrosion-resistant stainless steels working in various corrosive media, the electrode should be selected according to the medium and working temperature, and its corrosion resistance (corrosion performance test of welded joints) should be ensured.
(1) For media with a working temperature above 300 °C and strong corrosiveness, a Ti or Nb stabilizing element or an ultra-low carbon stainless steel electrode shall be used. Such as A137 or A002.
(2) For medium containing dilute sulfuric acid or hydrochloric acid, stainless steel electrodes containing Mo or containing Mo and Cu such as A032, A052, etc. are often used.
(3) Stainless steel welding rods without Ti or Nb may be used for work, equipment with weak corrosiveness or only to avoid rust pollution.
In order to ensure the stress corrosion resistance of the weld metal, the superalloyed welding consumables, that is, the corrosion resistant alloying elements (Cr, Mo, Ni, etc.) in the weld metal are higher than the base metal. For example, 00Cr19Ni10 weldment is welded with 00Cr18Ni12Mo2 type welding material (such as A022).
6. For austenitic stainless steels working under low temperature conditions, the low temperature impact toughness of the welded joints at the use temperature should be ensured, so pure austenitic welding rods are used. Such as A402, A407.
7. Nickel-based alloy electrodes can also be used. For example, Mo6 super austenitic stainless steel is welded with 9% nickel-based welding material.
8, the choice of electrode type:
(1) Since the weld metal of the duplex austenitic steel itself contains a certain amount of ferrite, it has good plasticity and toughness, and is compared with the crack resistance of the weld metal, the alkaline coating and the titanium calcium type coating. The difference in the electrode is not as significant as the carbon steel electrode. Therefore, in practical applications, there are many eyesights in terms of soldering process performance, and most of them use electrode strips with a coating type of 17 or 16 (such as A102A, A102, A132, etc.).
(2) Only when the structural rigidity is large or the weld metal is poor in crack resistance (such as some martensitic chromium stainless steel, pure austenitic chromium-nickel stainless steel, etc.), the selection of the drug code is 15 Alkaline coated stainless steel electrode (such as A107, A407, etc.).
In summary, the welding of austenitic stainless steel has its own unique characteristics. The welding rod selection of austenitic stainless steel is particularly noteworthy. Only in this way can different welding methods and different materials be applied to different materials, stainless steel. The electrode must be selected according to the base metal and working conditions (including working temperature and contact medium). This will make it possible to achieve the desired weld quality.
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