The Effect of Cooling Rate from Austenitizing Temperature on the Microstructure and Toughness of the 0.35C-3Cr-3Mo-V Hot Work Alloy Tool Steel

Abstract

The effect of quenching rate from austenitizing temperature on the microstructure and toughness of a 0.35C-3Cr-3Mo-V hot work alloy tool steel was investigated.
(1) Plane strain fracture toughness and charpy impact value are reduced abruptly as the quenching rate decreases from the rate by oil quenching to the rate of the half temperature time of 35 min, and gradually reduced as the rate further decreases.
(2) The microstructural changes corresponding to the above behavior of toughness can be summarized as follows:
(a) Formation of upper bainite and increasing of its volume ratio to martensite. (b) Increasing of width of bainite grain, compared with that of martensite lath, and strong tendency to the chain like carbide distribution and agglomeration of carbide particle precipitating along bainite grain boundaries compared with carbide precipitating along martensite lath boundaries. (c) Increasing of the effective grain size. (d) Preffered precipitation of carbide along the grain boundaries of prior austenite. (e) Increasing of distribution density of extremely fine carbide precipitates. (f) Transition of bainite configuration from lath to granular type and further increased volume of carbide precipitated along the grain boundaries of prior austenite, and further increased distribution density of extremely fine cardide precipitates. (Half temperature time 520 min)
(3) Fatigue crack propagation rate are somewhat increased by the formation of upper bainite. This corresponds to the increase of distribution density of extremely fine carbide which precipitates during tempering of upper bainite.