The end quench as a screening tool for continuous cooling sensitization of type 304 stainless steel

Abstract

Intergranular stress corrosion cracking in type 304 stainless steel is a common failure mode of components in energy conversion systems. Intergranular attack occurs in combination with critical conditions of stress and environment when the steel has been sensitized by a thermal treatment in the 400–850°C range and it occurs, in many practical situations, by continuous cooling through the sensitizing temperature range wherein the variables are the peak temperature reached and the cooling rate. Hence the Jominy end-quench test is recommended as a simple way of obtaining a large range of cooling rates on a single specimen and a correlation with the cooling rates in quenching production rounds and flat. Thus, it can reveal the sensitization behavior of a particular cross section produced from a particular heat and can also simulate cooling rates that occur in pipe welds and heavier section welds. As a screening tool to qualify billet and bar products, results are presented on a number of commercial heats of type 304 after etching according to ASTM Standard A 262A and quantifying the degree of sensitization via counting grid or planimeter measurements. Also, an implant Jominy test fixture is proposed for the evaluation of sheet, strip and plate. As an inspection method for screening heats that would produce varying precipitation rates in welded tubing, results are presented for strip cut from commercial coils and laminated as a strip pack within the fixture. For thin plate or small bars made in a small quantity, results are presented for compositional variations in type 304 stainless steel implanted as rods within the test fixture. One series reveals the effect of sulfur or phosphorus doping and a second series the effect of molybdenum, niobium, vanadium, zirconium and boron in various combinations.