Effects of Co and Ni on secondary hardening and fracture behavior of martensitic steels bearing W and Cr

Abstract

The purpose of this study was to analyze the effect of Co and Ni additions on secondary hardening and fracture behavior of the martensitic steels bearing W and Cr, where W exhibits a much weaker effect on the secondary hardening compared with Mo and Cr can contribute to an increase in toughness even though the Cr additions weaken the secondary hardening in Mo and W steels. The chemical compositions of the alloys used in this study are presented in Table 1. Impact specimens were austenitized in a flowing argon atmosphere at 1,200 C for 1 hour and then water- or oil-quenched. Austenitized specimens were aged in a neutral salt bath at 400 C and 650 C for 1 hour and then water-quenched. Fractography was conducted on fracture surfaces cut off the impact-tested samples, while samples for hardness measurements were obtained from a second cut 5 to 10 mm below the fracture surface. Hardness was measured using the Rockwell C scale (HRc), and average values of five readings are reported. In order to observe martensitic substructures (M{sub 2}C and M{sub 3}C carbides), thin foils were examined in a JEOL transmission electron microscope (TEM), operated at 120 kV. The variations in hardnessmore » for 6W, 3W-3Cr, 3W-3Cr-14Co, and 3W-3Cr-14Co-10Ni steels are shown.« less