Thermal treatment of PM-tool steel alloyed with niobium

The work is aimed at describing the influence of thermal treatment on the properties of a tool steel containing (in wt.%) 2.5% C, 3.3% Si, 6.2% Cr, 2.2% Mo, 2.6% V, 2.6% Nb, 1.0% W prepared by the powder metallurgy process. Alloying with niobium positively affects the microstructure, the mechanical properties and the performance of the steel. The microstructure of the hardened steel is composed of a martensitic matrix, retained austenite and various types of carbide. During tempering of the hardened steel at the temperatures above 480 °C precipitation of very fine carbides occurs, which results in secondary hardening. The optimum thermal treatment of the steel is austenitisation at 1050–1100 °C, nitrogen cooling and tempering at 540 °C (3×1 h).