The purpose of stainless steel stabilizing treatment
The stabilization process is set up for 18-8 stainless steel which contains titanium or niobium, such as lCr18Ni9T. The main purpose of adding the titanium is to prevent the carbide separating out along the grain boundary. The chemical affinity between titanium and carbon is better than that between chromium and carbon, which makes the titanium easier to form carbide than chromium. As a result, it can eliminate the poor chrome area of grain boundary and reduce the tendency to intergranular corrosion. However, in order to reach the effect, stabilizing treatment should be taken.
All the chrome carbide will dissolve into Austenite after the solid solution treatment of 18-8 steel, at the same time, most of the titanium carbide is dissolved as well. And if the steel is heated at a temperature around 400 - 800℃, chrome carbide will preferentially separate out because that the titanium content is much less than chrome and the diffusion velocity is slower due to the bigger atomic ratio. On this occasion, adding titanium has no effect on prevent chrome carbide from separating out.
Stabilizing treatment is a process that adding a heat treament, which is similar to normalizing, after solid solution treatment of titanium or niobium 18-8 steel. There are two conditons of the noralizing temperature. One is the temperature should be higher than that of chromium carbide dissolution and lower than the melting temperature of titanium carbide. The other is the temperature at the same time is that of intense precipitation of titanium carbide. Usually, the tstabilizing temperature range is 850-950 ℃. For 1Cr18ni9t1 steel, stabilizing temperature is 860-880 ℃. After 4-6h heat preservation, cool the steel in the air when it is released. The purpose of long time heat preseration is to make the chrome carbide dissolve into austenite completely and the titanium carbide separate out as much as possible.
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