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 mod ify inclusions was performed, Cr sulfides were formed. While the secondary hardening peak was investigated at 510 degrees C in the 1200 and 1000 degre es C austenitizing conditions, it was shifted down to 475 degrees C in the 899-816 degrees C double-austenitizing condition. The aging acceleration in the tatter 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 su rfaces of impact specimens exhibited the transgranular dimple mode. While t he combined action of impurities and coarse cementite at the grain boundari es 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-a ging condition in spite of an increase in hardness. In the 1000 degrees C a ustenitizing and peak-aging condition, the impact toughness to hardness rat io was relatively high in the presence of large, spheroidal La sulfides wit h 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 t he small interparticle spacing and the easy stress concentrations at the in terfaces, respectively. (C) 1999 Elsevier Science S.A. All rights reserved.