Effects of austenitizing treatments and inclusions on secondary hardening and fracture behavior for high Co–Ni steels containing W

When the inclusions were modified by additions of La and Mn in the WCrCoNi steel, La and Mn sulfides were formed, respectively. When no attempt to modify inclusions was performed, Cr sulfides were formed. While the secondary hardening peak was investigated at 510°C in the 1200 and 1000°C austenitizing conditions, it was shifted down to 475°C in the 899–816°C double-austenitizing condition. The aging acceleration in the latter condition of low-temperature austenitizing correlated with the presence of undissolved carbides containing great amounts of W, increasing the Cr/W ratio in the matrix. In the as-quenched condition, the fracture surfaces of impact specimens exhibited the transgranular dimple mode. While the combined action of impurities and coarse cementite at the grain boundaries leading to the intergranular fracture decreased the impact toughness in the underaging condition, however, the dissolution of cementite eliminated the intergranular fracture and recovered the impact toughness in the peak-aging condition in spite of an increase in hardness. In the 1000°C austenitizing and peak-aging condition, the impact toughness to hardness ratio was relatively high in the presence of large, spheroidal La sulfides with the large interparticle spacing. Its ratio was lowered in the presence of small, spheroidal Cr sulfides and large Mn sulfides of rod shape, due to the small interparticle spacing and the easy stress concentrations at the interfaces, respectively.