Effects of Purification on Fracture Toughness of Steels

The effects of purification on the fracture toughness of steels, as researched by Kawasaki Steel and other companies in joint forums in Japan, are presented. The steels studied are 3.5% NiCrMoV steel for steam turbine rotors, 9% Ni steel, aluminium-killed steel for low-temperature use, Si--Mn steel for welded structures, JIS SCr 420 steel (simulation case-hardened), the JIS steels SCM 420, SNCM 439, SCr 430 and 435, 3% Cr direct-quenched steel, Nb-added 3% Cr direct-quenched steel, HT-60 direct-quenched grade steel, X 60 and medium-C (JIS SCr 430 type) steel. The effect of sulphur content was studied by forcing, homogenising, tempering and quenching from 900 deg C, resulting in a tensile strength of 104 kgf/mm exp 2 at 610 deg C and 86 kgf/mm exp 2 at 660 deg C, with study also of the effect of S content on Charpy absorption energy. All the above steels were tested for effect of S content in similar way and fracture modes illustrated. Phosphorus and nirtogen content effects were also studied, with electron-micrograph photograph illustration of fracture surface. As regards the effects of such "impurities" on the manufacturing process, these steels were heat-simulation treated and the effects studied by monitoring the cooling rate. It was found that purification techniques as used in Japan have already been used to near optimum effect, as reduction in the incidence of the S, P and N "impurities" tends to produce corresponding decrease in teh fracture toughness and heat-treated hardness of all the steels, especially the medium-C and HT-60 grade steels. 20 ref.--J.G.