Knowledge Of Stainless Steel Heat Treatment
Sensitization treatment: Austenitic stainless steel of 18-8 steel series is heated at 450℃~850℃(this range is often called sensitization temperature) for a short time, so that it has the tendency of intergranular corrosion. This is because the solubility of carbon in austenitic stainless steel and temperature have a great impact. Austenitic stainless steel in the temperature range of 400℃~850℃ (sensitization temperature area), there will be high chromium carbide (Cr23C6) precipitation, when the chromium content falls to the corrosion resistance limit, there is poor chromium at the grain boundary, will produce intergranular corrosion, serious when the material can become powder. This method is generally only used in the intergranular corrosion test of stainless steel.
(2) solution heat treatment: the austenitic stainless steel is heated to about 1100℃, so that the carbide phase is completely or basically dissolved, and the carbon is solidly dissolved in austenite, and then it is quickly cooled to room temperature, so that the carbon reaches the susaturation state (the carbon has been stable, and there is no ability and opportunity to form high chromium carbide with chromium). Different stainless steel solid melting temperature burning is different,304, 316 austenitic stainless steel is generally 1050℃, austenitic ferrite duplex stainless steel to a point higher, to 1150℃. Solution heat treatment: austenitic stainless steel is heated to about 1100℃, so that the carbide phase is completely or basically dissolved, and the carbon is solidly dissolved in austenite, and then it is cooled to room temperature quickly, so that the carbon reaches susaturation state (the carbon has been stable, and has no ability and opportunity to form high chromium carbide with chromium). This heat treatment method is solution heat treatment. The rapid cooling in the solution heat treatment may seem like the quenching of ordinary steel, but the "quenching" is different from the quenching of ordinary steel in that the former is the softening treatment and the latter is the hardening (martensite formation). The latter also takes different heating temperatures to obtain different hardness, but not to 1100℃.In the power plant, austenitic stainless steel pipe cold bending processing, easy to produce deformation induced martensite transformation ( in fact, is produced martensite), easy to cause the decline of corrosion resistance. ASME standards stipulate that solution treatment must be carried out when the amount of processing exceeds a certain amount.
(3) Stabilization treatment: in order to avoid the formation of high chromium carbides between carbon and chromium, stabilizing elements (such as Ti and Nb) are added to austenitic steel. When heated to a temperature above 875℃, stable carbides can be formed. This is because Ti(or Nb) can preferentially combine with carbon to form TiC(or NbC), which greatly reduces the concentration (content) of solid solution carbon in austenite, thus sacrificing Ti(or Nb) to protect Cr. Austenitic stainless steels containing Ti (or Nb) (e.g., 1Cr18Ni9Ti, 1Cr18Ni9Nb) have better comprehensive mechanical properties after stabilization than after solution heat treatment.
(4) Therefore, austenitic stainless steel with intergranular corrosion tendency should be treated with solution heat treatment or stabilization.
