Abstract
Austenitic stainless steels with excellent corrosion resistance and good weldability have wide applications in industry. These iron-based alloys contain a high level of chromium which form protective oxide film on the surface hence resisting corrosion. The oxide film regenerates when damaged, making the steel 'stainless'. However, carbide precipitation due to a welding process or heat treatment can cause the occurrence of chromium-depleted zones at the boundaries, leading to a phenomenon known as sensitisation, in which the depleted zones become the focus of the intense corrosion. The present work was concerned with the study of the sensitization and desensitisation of 316L steel at the normalizing temperatures of 750- 950 0 C and soaking times of 0.5, 1, 2 and 8 hrs. 316L stainless steel was observed to be sensitized when heated to 750- 850 0 C and held for short soaking times of 0.5 – 2hrs before normalizing. Increasing soaking times at these temperatures to 8 hrs triggered the desensitization process which was fully accomplished at 750 0 C but ongoing at 800 and 850 0 C. At 900 0 C, sensitization did not occur at 30 mins soaking time but observed at soaking times of 1 and 2hrs. At a longer soaking time of 8 hrs, there was full desensitization. At 950 0 C, sensitization was already observed at 30 mins. Soaking time and desensitization was observed to be in progress at 1 and 2 hrs soaking time. By 8 hrs there was full desensitization. Thus it was observed that at 950 0 C, diffusion of Cr was thermally aided making desensitization fast. The hardness of normalized 316L stainless steel was also observed to decrease with soaking time and normalization temperature.
Highlights
The basic 18Cr8Ni (18/8) austenitic stainless steel is so widely used that it accounts for about 50% of all stainless steel production
The carbon content is kept to 0.03% or less to avoid grain boundary precipitation of chromium carbide in the critical range (430 to 900oC)
316L stainless steel was observed to go into sensitization when heated to 750- 8500 C and held for short soaking times of 0.5 – 2 hrs before normalizing
Summary
The basic 18Cr8Ni (18/8) austenitic stainless steel is so widely used that it accounts for about 50% of all stainless steel production. Grain boundary precipitation deprives the steel of the chromium in solution and promotes corrosion adjacent to grain boundaries. Other elements can influence the effectiveness of chromium in forming or maintaining the film, but no other element can, by itself, create the stainless steel. When nickel is increased to about 8 to 10% this is a level required to ensure austenitic structures in a stainless steel that has about 18% chromium. Molybdenum in moderate amounts in combination with chromium is very effective in terms of stabilizing the passive film in the presence of chlorides. Molybdenum is especially effective in enhancing the resistance to pitting and crevice corrosion[4]. Nitrogen is beneficial to austenitic stainless in that it enhances pitting resistance, retards formation of sigma phase. The change in the microstructure of an alloy can be achieved by heat treatment
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