Process and advantages of 304 stainless steel softening

Stainless steel applications are now very common, among which the most commonly used stainless steel models are 304 and 316. However, it is estimated that few people understand the softening process of stainless steel. This article will introduce this knowledge.
If it is stainless steel with a carbon content of less than 0.15%, there are two softening processes.
One is that when the hardness of the steel does not need to be reduced to a minimum, the high-temperature tempering process can be used to soften it, heat it to 750-800°C, keep it warm for 1 to 3 hours, and then naturally cool it in the air. After this treatment, the stainless steel The hardness of the material will be reduced to HB170-200.
The other is that when the hardness needs to be lower, the material can be heated to 850-900°C, kept warm for 1-3 hours, slowly cooled to 600°C, and then naturally cooled in the air. This can reduce the hardness of the stainless steel material. to HB160. Alternatively, the isothermal annealing process can be used to reduce the hardness. For example, after heating to 900°C and holding heat, it is isothermal at 725°C for 1 hour and then cooled in the air. However, this method is not used much in actual production.
The weak corrosion resistance of annealed stainless steel materials is closely related to the structure of the annealed state. This can be understood from two perspectives. One is that in the annealed state, the carbon in the stainless steel can fully combine with chromium to form Chromium carbide reduces the chromium content in the ferrite matrix. The corrosion resistance of stainless steel in the annealed state will decrease even more as the carbon content in the stainless steel continues to increase.
In addition, because there are many carbides in the annealed structure, the matrix near the carbides will be in a chromium-poor state. In the corrosive medium, the electrode potential of the carbides and the matrix around it is different. The chromium-rich carbide is the cathode, and the chromium-poor area is The presence of a large number of micro-batteries on the anode also accelerates the corrosion of stainless steel materials.
The following is an example of annealing and softening of 304 stainless steel strips. After work hardening, 304 stainless steel can use high-temperature and low-temperature annealing processes to restore plasticity, reduce the hardening degree of 304 stainless steel, and remove or reduce the residual stress of the material. In order to prevent the material from generating a sensitization reaction, do not Sensitization temperature range of 500℃-850℃.
Low-temperature annealing has a low impact on the yield strength of 304 stainless steel. It will be annealed at 500°C. The yield strength value does not change much after annealing. High-temperature annealing will have a greater impact on the yield strength of the sample. The pre-deformation variable is At 15%, Re dropped to 260MPa after annealing at 1050°C. Rm also dropped almost linearly with the annealing temperature, but the amplitude of the change was much smaller than Re. And the Vickers hardness value of the sample decreases with the increase of annealing temperature.