Preparation before welding
- Cleaning before welding: remove impurities such as oil stains and adsorbed moisture on the groove and both sides, and reduce the source of hydrogen.
- Drying electrode: The electrode should be dried at a high temperature of 350-400 ℃ before welding, so as to altogether remove moisture, reduce the content of diffusible hydrogen and reduce the sensitivity of cold cracks.
- Correct selection of preheating temperature: The preheating temperature should be lower than the starting temperature of martensite, generally 150-400℃. The carbon content is the most important factor in determining the preheating temperature. The higher the carbon content, the higher the preheating temperature. Other factors that affect the choice of preheating temperature are base metal thickness, filler metal type, etc. When the carbon content is 0.1%~0.2%, the preheating temperature is 200! 260℃. When the carbon content exceeds 0.2%, the interlayer temperature needs to be maintained. For a thick plate structure with high rigidity, a set of steel with a high tendency to harden, the preheating temperature is correspondingly higher, usually above the martensite transformation temperature Ms point.
Three major martensitic stainless steel welding process measures
- Correct selection of welding materials: The chemical composition mainly depends on the welding material, so the composition of the welding material should be the same as that of the base metal.
- Correct selection of process parameters: due to the large linear expansion coefficient of martensitic stainless steel, large welding deformation is easy to occur during welding in a free state. Therefore, try to concentrate the welding heat during welding, and do not make lateral swing when welding with a high current, which can reduce the cooling rate. A welding seam forming does not exceed 3 times the diameter of the electrode.
- Low-voltage short-arc operation: In order to prevent the burning of alloying elements during welding and improve the overall performance of the weld, low-voltage short-arc operation should be used, and the arc length should be kept constant during the entire welding process, and the arc crater should be filled when ending the arc. , to prevent the occurrence of crater cracks. Strictly control the interlayer temperature to prevent strict slag cleaning after deposition and ensure full penetration. At the same time, the number of welding layers should be reduced as much as possible to avoid repeated heating of the welding pool and enlarge the heat-affected zone, thereby reducing the comprehensive mechanical properties and corrosion resistance of the weld.
Post-heat and post-weld heat treatment
Most martensitic stainless steels are not allowed to be directly cooled to room temperature after welding to prevent cold cracks. After welding is interrupted or completed, post-heating should be carried out immediately so that the austenite is completely transformed into martensite at a temperature that is not too low. The timing of post-heating is very important. It can neither wait until it cools to room temperature nor can it go beyond the Ms point. If heat treatment can be performed immediately after welding, post-heating can be eliminated.
The purposes of post-weld heat treatment are:
- Eliminate welding residual force and remove diffusible hydrogen in joints to prevent delayed cracking;
- Temper the weld to reduce hardness and improve the microstructure and mechanical properties.
The selection of the tempering temperature should meet the requirements of the components on the mechanical properties of the joint, and the tempering temperature is generally selected at 650~750℃. The tempering temperature should not be higher than the Ac1 point to prevent re-austenite transformation. For welded structures used at high temperatures, a higher tempering temperature is often used.
During high-temperature tempering, more carbides are precipitated, which is detrimental to corrosion resistance. For welds mainly used for corrosion resistance, lower temperature stress relief annealing should be performed. For the post-weld tempering treatment that is no longer adjusted to the treatment after welding, the welding seam should not be carried out at a high temperature, and the tempering temperature should be carried out immediately when the joint is cooled to the temperature M at which the martensitic transformation is basically completed. Generally do not choose at 45-50 ℃. The toughness of steel tempered at this temperature is very low, and the reason for the low toughness is related to the change of microstructure and temper embrittlement. , V, N and reducing the content can improve the tillage, and the tempering treatment needs enough holding time to make the brittle and hard bainite that may exist in the joints to be converted into Somin body type, and then cooled to 300 ℃ according to shape calculation and then air-cooled.