1. What is martensitic stainless steel?
Stainless steel with a martensitic microstructure at room temperature and whose mechanical properties can be adjusted by heat treatment. In layman’s terms, it is a type of hardenable stainless steel. The steel grades belonging to martensitic stainless steel include 1Cr13, 2Cr13, 3Cr13, 4Cr13, 3Cr13Mo, 1Cr17Ni2, 2Cr13Ni2, 9Cr18, 9Cr18MoV, etc.
2. Common welding methods
Various arc welding methods can be used to weld martensitic stainless steel. At present, arc welding is still the main method, while the use of carbon dioxide gas shielded welding or argon and carbon dioxide mixed gas shielded welding can greatly reduce the hydrogen content in the weld, thereby reducing the sensitivity of cold cracking of the weld.
3. Commonly used welding materials
(1) Cr13 type martensitic stainless steel welding rods and welding wires. Usually, when the weld has a higher strength requirement, Cr13 type martensitic stainless steel welding rods and welding wires are used, which can make the chemical composition of the weld metal close to that of the parent material, but the weld has a greater tendency to cold cracking.
Notes: a. Preheating is required before welding, and the preheating temperature cannot exceed 450℃ to prevent embrittlement at 475℃. Heat treatment is performed after welding. The heat treatment after welding is to cool to 150-200℃ and keep it warm for 2h to transform the austenite parts into martensite, and then immediately perform high-temperature tempering, heating to 730-790℃, and the holding time is 10min per 1mm plate thickness, but not less than 2h, and finally air cooling.
b. In order to prevent cracks, the content of S and P in the welding rod and welding wire should be less than 0.015%, and the content of Si should be less than 0.3%. The increase in Si content promotes the formation of coarse primary ferrite, resulting in reduced plasticity of the joint. The carbon content should generally be lower than that of the parent material, which can reduce the hardenability.
(2) Cr-Ni austenitic stainless steel welding rods and wires Cr-Ni austenitic steel weld metal has good plasticity, which can alleviate the stress generated during the martensitic transformation of the heat-affected zone. In addition, the Cr-Ni austenitic stainless steel weld has a large solubility in hydrogen, which can reduce the diffusion of hydrogen from the weld metal to the heat-affected zone and effectively prevent cold cracks, so no preheating is required. However, the strength of the weld is low and cannot be improved by post-weld heat treatment.
4. Common welding problems
(1) Welding cold cracks Due to the high chromium content of martensitic stainless steel, its hardenability is greatly improved. Regardless of the original state before welding, welding will always produce martensitic structure in the near-seam area. As the hardening tendency increases, the joint becomes more sensitive to cold cracking, especially in the presence of hydrogen, martensitic stainless steel will also produce more dangerous hydrogen-induced delayed cracking.
Measures: 1) Use high wire energy and high welding current to slow down the cooling rate; 2) The interlayer temperature is different for different steel grades, generally not lower than the preheating temperature; 3) Slowly cool to 150-200℃ after welding, and perform post-weld heat treatment to eliminate welding residual stress and remove diffused hydrogen in the joint, which can also improve the structure and performance of the joint.
(2) Brittle martensitic stainless steel, especially martensitic stainless steel with high ferrite forming elements, has a greater tendency to grow grains. When the cooling rate is low, coarse ferrite and carbide are easily produced in the heat affected zone of welding; when the cooling rate is high, the heat affected zone will harden and form coarse martensite. These coarse structures reduce the plasticity and toughness of the heat affected zone of martensitic stainless steel welding and make it brittle.
Measures: 1) Control the cooling rate reasonably; 2) Select the preheating temperature reasonably. The preheating temperature should not exceed 450℃, otherwise the joint will be at high temperature for a long time, which may cause 475℃ embrittlement; 3) Select the welding material reasonably to adjust the composition of the weld and avoid the generation of coarse ferrite in the weld as much as possible.
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5. Welding process
1) Preheating before welding Preheating before welding is the main process measure to prevent cold cracks. When the mass fraction of C is 0.1% to 0.2%, the preheating temperature is 200 to 260℃, and the high-rigidity weldment can be preheated to 400 to 450℃.
2) Cooling after welding The weldment should not be directly heated from the welding temperature for tempering after welding, because the austenite may not be completely transformed during the welding process. If the temperature is immediately raised and tempered after welding, carbides will precipitate along the austenite grain boundary and austenite will transform into pearlite, resulting in a coarse grain structure, which seriously reduces the toughness. Therefore, the weldment should be cooled before tempering to allow the austenite in the weld and heat-affected zone to be basically decomposed. For welds with low rigidity, they can be cooled to room temperature and then tempered; for welds with large thickness, more complicated processes are required; after welding, they are cooled to 100-150℃, kept warm for 0.5-1h, and then heated to the tempering temperature.
3) The purpose of post-weld heat treatment is to reduce the hardness of the weld and heat-affected zone, improve plasticity and toughness, and reduce welding residual stress. Post-weld heat treatment is divided into tempering and full annealing. The tempering temperature is 650-750℃, kept warm for 1h, and air-cooled; if the weld needs to be machined after welding, in order to obtain the lowest hardness, full annealing can be used, the annealing temperature is 830-880℃, kept warm for 2h, furnace cooled to 595℃, and then air-cooled.
4) Selection of welding rods Welding rods for martensitic stainless steel are divided into two categories: chromium stainless steel welding rods and chromium-nickel austenitic stainless steel welding rods. Commonly used chromium stainless steel electrodes include E1-13-16 (G202) and E1-13-15 (G207); commonly used chromium-nickel austenitic stainless steel electrodes include E0-19-10-16 (A102), E0-19-10-15 (A107), E0-18-12Mo2-16 (A202), E0-18-12Mo2-15 (A207), etc.
Welding of duplex stainless steel
1. Weldability of duplex stainless steel
The weldability of duplex stainless steel combines the advantages of austenitic steel and ferritic steel, and reduces their respective shortcomings. (1) The sensitivity to hot cracks is much smaller than that of austenitic steel; (2) The sensitivity to cold cracks is also much smaller than that of general low-alloy high-strength steel; (3) After the heat-affected zone cools, more ferrite is always retained, thereby increasing the corrosion tendency and sensitivity to hydrogen-induced cracking (embrittlement); (4) Duplex stainless steel welded joints may precipitate δ phase embrittlement. δ phase is an intermetallic compound of Cr and Fe. Its formation temperature ranges from 600 to 1000℃. Different steel grades have different formation temperatures for δ phase;(5) Duplex stainless steel contains 50% ferrite and also has brittleness at 475℃, but it is not as sensitive as ferritic stainless steel;
2. Selection of welding method
The preferred welding method for duplex steel is TIG welding, followed by arc welding. When using submerged arc welding, the heat input and interlayer temperature should be strictly controlled, and a large dilution rate should be avoided.
Note: When using TIG welding, it is advisable to add 1-2% nitrogen to the shielding gas (if N exceeds 2%, the porosity tendency will increase and the arc will be unstable) to allow the weld metal to absorb nitrogen (to prevent the surface area of the weld from losing nitrogen due to diffusion), which is beneficial to stabilize the austenite phase in the weld joint.
3. Selection of welding materials
Select welding materials with higher austenite-forming elements (Ni, N, etc.) to promote the transformation of ferrite to austenite in the weld. 2205 steel mostly uses 22.8.3L welding rods or wires, and 2507 steel mostly uses 25.10.4L welding wires or 25.10.4R welding rods.
4. Key points of welding
(1) Control of welding heat process Welding line energy, interlayer temperature, preheating and material thickness will affect the cooling rate during welding, thereby affecting the structure and performance of the weld and heat-affected zone. In order to obtain the best weld metal performance, it is recommended that the maximum interlayer temperature be controlled at 100°C. When heat treatment is required after welding, the interlayer temperature can be unlimited.
(2) Post-weld heat treatment Duplex stainless steel is preferably not heat treated after welding. When heat treatment is required after welding, the heat treatment method used is water quenching. Heating should be as fast as possible during heat treatment, and the holding time at the heat treatment temperature is 5 to 30 minutes, which should be sufficient to restore the phase balance. Metal oxidation is very serious during heat treatment, and inert gas protection should be considered.
Post time: Nov-11-2024
