Influence Of Heat Treatment To Microstructures And Localized Corrosion In Several Stainless Steels

The ferritic stainless steel is one kind of stainless steel that does not have nickel or low nickel, which performs good on formability, corrosion resistance and oxidation resistance. In recent years the use of the ferritic stainless steel enhanced year by year. Especially in the automobile domain, the research of ferritic stainless steel will become the future trend. In this paper, the influence of different heat treatment on the 409L, 430 and 410 ferritic stainless steel for the microstructure and localized corrosion performance was analyzed, which was contrasted with the 304 austenite stainless steel. This provides the basis to the further study of the influence of heat treatment to the austenite stainless steel and ferritic stainless steel, which can be the theory basis for these four kinds of stainless steel's localized corrosion resistance study and the improvement of process and can also instruct the production.Firstly, the influence of the different heat treatment to 304, 409L ,430 and the 410 experimental steel for the microstructure was studied. Regarding 304: After the sensitization (670℃, 2h, air cooled), the 304 black small particles in banding place increased obviously, which is caused by the carbide compounds along the crystal boundary separation. Melting (1100℃, 1h, water cooling), because the heating temperature is high, the crystal grain grows up obviously with the twin crystals distribution, and the crystal grain is even, also, the crystal boundary place hardly has the carbide compounds separation. Moreover, after several kind of heat treatments the 409L turned out to be the equiaxial grain and the microstructure are the ferrite and very few M23C6. However, because of the different heat treatment the 430 and 410 microstructure transformed from the banding to near equiaxial grain,and microstructure presented few martensite after sensitization processing (1000℃,1h, water cooling).Secondly, in this paper, the electrochemistry (the EPR method) and the immersion testing method were adopted to test the intergranular corrosion performance of several kinds of stainless steel respectively under different heat treatment conditions. Based on the electrochemistry I_r/I_a ratio size, immersion weightlessness percentage and after electrochemistry, the immersion experimental SEM picture, we can draw a conclusion that the intergranular corrosion resistance ability of these four kinds of stainless steel is 304 the strongest, and then 430, 409L the next, 410 the worst. Moreover, the order of the intergranular corrosion performance form strong to weak of these four kinds of stainless steel under different heat treatment can be shown as follows: 304: The melting condition, the sensitization plus goes to the sensitization condition, hot rolling condition, the melting + sensitization condition, the sensitization condition. 409L: The sensitization + goes to the sensitization condition, to go to the sensitization condition, hot rolling condition, stabilizing processing condition, the sensitization condition.430 and 410: goes to the sensitization condition, the sensitization + to go to the sensitization condition, hot rolling condition, sensitization condition.This paper carried out the pitting test to these four kinds of stainless steel under different heat treatment conditions, by analysis of the test result the pitting resistance ability of these four kinds of stainless steel 304, 409L 430 and 410 can be shown in turn from high to low as follows: 430, 410, 409L, 304. Moreover, the SEM corrosion appearance of 304 and 409L, under different heat treatment, also becomes punctual. The SEM corrosion appearance of 430 and 410 is dissimilar after different heat treatment.The hot rolling condition 430(410) corrosion appearance shows stripped, while punctual under other situations. In accordance with the electrochemistry tentative data, the immersion weightlessness percentage and the electrochemistry experimental SEM picture, the pitting resistance ability of these four kinds of stainless steel under different heat treated conditions can be shown from strong to weak in turn as follows: 304: The melting condition, the sensitization + goes to the sensitization condition, hot rolling condition, the melting + sensitization condition, the sensitization condition. 409L: Goes to the sensitization condition, the sensitization + to go to the sensitization condition, stabilizing processing condition, hot rolling condition, the sensitization condition.430 and 410: Hot rolling condition, goes to the sensitization condition, the sensitization + to go to the sensitization condition, the sensitization condition.The essay also calculated the chromium diffusion distance X through the diffusion theory when it was sensitized and gone to the sensitization in austenite and ferrite. Compared with the poor chromium area's width d0 which approximately was (1.5-2.0)×10~-7m, we can find that the sensitization processing had the poor chromium phenomenon, while the poor chromium phenomenon reduced or even eliminated in go to sensitization. Through the experiment we can draw a conclusion that the heat treatment process which causes and eliminates (or reduces) the austenite and the ferritic stainless steel intergranular corrosion (pitting) tendence is opposite. However, what's the same is that the heat treatment of steel can cause the crystals growing up in varying degrees, and cause the steel to bear the intergranular corrosion (pitting) to drop after the sensitization. The difference is that go to the sensitization can reduce or even eliminate the intergranular corrosion tendency of the steel, but cannot eliminate all the steel pitting tendency.