Chromium element is a factor that makes such materials rust resistant. Previously, it was found that chromium content must be above 12% to form a dense surface oxide film and achieve corrosion protection. Therefore, any heat treatment of stainless steel must consider whether it will cause any changes in the composition of chromium.
(1) Ma Tian loose iron stainless steel:
This type of stainless steel has a body centered cubic structure (BCC) that can be magnetically attracted. It is made by rapid cooling at Ostian temperature, and its corrosion resistance is. However, the material is hard and brittle, and tempering can increase ductility. However, the corrosion resistance will decrease, especially when tempering between 450 ° C and 650 ° C. This will cause carbon atoms to diffuse and precipitate in the interstitial spaces of the crystal lattice, forming a network of chromium carbide with chromium, resulting in the consumption of chromium elements in the adjacent area, reducing the chromium content and preventing the formation of a protective film, thus losing corrosion resistance. Therefore, special attention should be paid. The following are the heat treatment temperatures for various Ma Tian loose iron stainless steel materials.
(a) The temperature of 403, 410, and 416 is 650-750 ℃.
(b) The temperature of 414 is 650-730 ℃.
(c) The temperature of 431 is 6.
(d)440-A, 440-B, 440-C, The temperature of 420 is 680-750 ℃.
(2) Fatty iron stainless steel:
This type of stainless steel has a body centered cubic structure (BCC), which can be used for magnetic attraction and is commonly used in the automotive or chemical industries. Its strength will not change due to heat treatment, but it can be increased by cold working.
(3) Ostian iron stainless steel:
This type of stainless steel has a face centered cubic structure (FCC) that does not work with magnets. As mentioned earlier, this type of material is easy to process, so different heat treatments can be applied after processing to eliminate residual stress in the material.
(4) Precipitation hardening stainless steel:
This type of stainless steel is quenched at high temperatures and then subjected to low-temperature heat treatment. Due to the precipitation of aluminum or copper elements in the material, inter metallic compounds are formed along the slip surfaces or grain boundaries, which can enhance its strength or hardness. The commonly used precipitation hardening stainless steel is 17-4 PH, while others include 17-7 PH, PH15-7MO, AM-350, AM-355, etc.
(5) Heat treatment after welding of various types of stainless steel:
The chromium element contained in stainless steel often diffuses and precipitates with carbon to form chromium carbide in high-temperature areas (heat affected zones) after welding, resulting in a decrease in local chromium content and the inability to form a protective film. Corrosion such as perforation often occurs in these heat affected zones. To remedy this situation, operators often perform heat treatment on the object after welding, which aims to diffuse chromium elements from other areas to the chromium deficient area for protection.