Hirooki Nakajima

Effect of nickel addition on the toughness of 18Cr-1 Mo stainless steels

The toughness of ferritic stainless steels has been thought to deteriorate due to carbide and nitride precipitation and also due to grain coarsening. The effect of martensite dispersion on the toughness of 18Cr−1 Mo stainless steel was studied by adding nickel up to 4%, by which theγ-loop was extended and anα-γ transformation was introduced. To precipitate austenite homogeneously, proper thermomechanical processing was conducted: rapid hot-rolling without recrystallization in the temperature range just above the two-phase region, followed by precipitation treatment of austenite at the temperature of the two phase region. The ferrite single phase of grain size 100/gmm was developed by nickel addition up to 2%, and a microduplex structure composed of ferrite and martensite of 10μm was developed by above 3%. The impact transition temperature in the ferritic single phase decreased remarkably only with 1 to 2% nickel addition and water quenching from solution treatment temperature. In the microduplex structure, the transition temperature decreased remarkably with 4% nickel addition and tempering treatment. Martensite has the effect of stopping the crack propagation, and the transition temperature depends mainly on the fracture facet size, which decreases with the amount of martensite.