Sensitization of continuously cooled austenitic stainless steels

Abstract

Stainless steel bar is generally used in the ‘quench-and-annealed’ condition, obtained by cooling rapidly from the solution-treatment temperature, the purpose of which is to obviate sensitization to intergranular corrosion, yet very little information is available on the effects of rate of cooling on such sensitization. The authors have therefore looked at a number of type 304L and 304 steels with various concentrations of carbon and nitrogen. After solution heat treatment at 1050°C, the steels were cooled at different rates. Various methods were then used to determine their ability to withstand intergranular corrosion. For each steel a critical rate of cooling was determined, and was found to depend on carbon and nitrogen concentrations. Observed critical cooling rates were slower than expected: e.g. for concentrations of 0·080%C and 0·040%N the rate was 400 Kh⁻¹, corresponding to the rate of cooling in air of 80 mm dia. bar. Nitrogen concentrations of up to 0·030% were found to be beneficial in reducing sensitivity to intergranular corrosion; the additional effect of concentrations between 0·030 and 0·080%N was, however, negligible. These findings can be applied in industrial practice by cooling bars individually in air as they leave the rolling mill, instead of quench-annealing them, provided their diameter is less than a critical value which is determined by the carbon and nitrogen concentrations. This does away with the need for solution treatment, thereby improving the energy balance. The use of nitrogen in modern steelmaking processes of the AOD type is a further positive factor.