After the austenitic stainless steel pressure vessel is formed by the plate rolling machine, it will be welded. What should we pay attention to?
What is ferritic stainless steel
Ferritic stainless steel is divided into two categories: ordinary ferritic stainless steel and ultra-pure ferritic stainless steel. Among them, ordinary ferritic stainless steel has Cr12 ~ Cr14 type, such as 00Cr12, 0Cr13Al; Cr16 ~ Cr18 type, such as 1Cr17Mo; Cr25 ~ 30 type.
3 welding characteristics of ferritic stainless steel
Due to the high content of carbon and nitrogen in ordinary ferritic stainless steel, it is difficult to process, form and weld, and it is difficult to ensure corrosion resistance, and the use is limited. In ultra-pure ferritic stainless steel, the carbon in the steel is strictly controlled. The total amount of nitrogen and nitrogen is generally controlled at three levels of 0.035% ~ 0.045%, 0.030%, 0.010% ~ 0.015%, and necessary alloying elements are also added to further improve the corrosion resistance and comprehensive properties of the steel. Compared with ordinary ferritic stainless steel, ultra-pure high-chromium ferritic stainless steel has good resistance to uniform corrosion, pitting corrosion and stress corrosion, and is widely used in petrochemical equipment.
Ferritic stainless steel has the following welding characteristics:
Under the action of high welding temperature, the grains in the heat affected zone, especially in the near seam zone, will grow sharply when the heating temperature reaches above 1000 °C. High intergranular corrosion tendency.
Ferritic steel itself contains high chromium content, more harmful elements such as carbon, nitrogen, oxygen, etc., high brittle transition temperature, and strong notch sensitivity. Therefore, the post-weld embrittlement phenomenon is more serious.
When it is heated and cooled slowly for a long time at 400 ℃ ~ 600 ℃, embrittlement at 475 ℃ will occur, which will seriously reduce the normal temperature toughness. After long-term heating at 550 ℃ ~ 820 ℃, the σ phase is easily precipitated from the ferrite, and its plasticity and toughness are also significantly reduced.
How to choose ferritic stainless steel welding consumables
There are basically three types of ferritic stainless steel welding consumables:
- Welding consumables whose composition basically matches the base metal;
- Austenitic welding consumables;
- Nickel-based alloy welding consumables are rarely used due to their high price.
Ferritic stainless steel welding consumables can be made of materials equivalent to the base metal, but when the degree of restraint is large, cracks are easily generated. After welding, heat treatment can be used to restore corrosion resistance and improve joint plasticity.
The use of austenitic welding consumables can avoid preheating and post-weld heat treatment, but for various steels without stabilizing elements, the sensitization of the heat-affected zone still exists, and 309-type and 310-type chromium-nickel austenitic welding consumables are commonly used.
For Cr17 steel, 308 type welding consumables can also be used, and the welding consumables with high alloy content are beneficial to improve the plasticity of welded joints.
Note: Austenitic or austenitic-ferritic weld metal is basically as strong as the ferrite base metal, but in some corrosive media, the corrosion resistance of the weld may be very different from the base metal. This point should be paid attention to when choosing welding consumables.
Welding process focus of ferritic stainless steel
- Ferritic stainless steel has relatively more ferrite-forming elements, relatively few austenite-forming elements, and the material is less prone to hardening and cold cracking.
- Under the action of welding thermal cycle, the grains in the heat-affected zone of ferritic stainless steel grow significantly, and the toughness and plasticity of the joint decrease sharply.
- The degree of grain growth in the heat-affected zone depends on the highest temperature reached during welding and its holding time. For this reason, when welding ferritic stainless steel, small line energy should be used as much as possible, that is, the method of energy concentration should be used, such as Small current TIG, small diameter electrode manual welding, etc., and at the same time, measures such as narrow gap groove, high welding speed and multi-layer welding are adopted as much as possible, and the temperature between layers is strictly controlled.
Due to the effect of welding thermal cycle, general ferritic stainless steel is sensitized in the high temperature zone of the heat-affected zone, and intergranular corrosion occurs in some media. After welding, it is annealed at 700~850℃ to homogenize the chromium and restore its corrosion resistance.
Three measures to prevent cracks after welding
Ordinary high-chromium ferritic stainless steel can be welded by electrode arc welding, gas shielded welding, submerged arc welding and other fusion welding methods. Due to the inherent low plasticity of high-chromium steel, as well as the grain growth in the heat-affected zone and the accumulation of carbides and nitrides at the grain boundaries caused by the welding thermal cycle, the plasticity and toughness of the welded joints are very low. Cracks are easy to occur when welding consumables with similar chemical composition to the base metal are used and the degree of restraint is large. In order to prevent cracks and improve joint plasticity and corrosion resistance, taking electrode arc welding as an example, the following process measures can be taken.
- Preheat about 100 ~ 150 ℃, so that the material can be welded in a tough state. The higher the chromium content, the higher the preheating temperature should be.
- Use small line energy and do not swing welding. When multi-layer welding, the temperature between layers should be controlled not to be higher than 150 °C, and continuous welding should not be performed to reduce the effect of high temperature embrittlement and 475 °C brittleness.
- After welding, annealing at 750 ~ 800 ℃ can restore corrosion resistance and improve joint plasticity due to carbide spheroidization and uniform distribution of chromium. After annealing, it should be cooled quickly to prevent σ phase and brittleness at 475 °C.