The Effect of Cooling Rate from Austenitizing Temperature on the Microstructure and Toughness of the 0.4C-5Cr-Mo-V Hot Work Alloy Tool Steel

Abstract

The effect of quenching rate from austenitizing temperature on the microstructure and toughness of the 0.4C-5Cr-Mo-V hot work alloy tool steel was investigated.
(1) As the quenching rate decreases, plane strain fracture toughness and charpy impact value after tempering to H_RC 44 are reduced gradually.
(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 increasing of particle size and tendency of connected distribution of carbide precipitating along bainite grain boundaries compared with carbide precipitating along martensite lath boundaries.
( c ) Increasing of density of extremely fine carbide precipitates.
( d ) Increasing of the effective grain size.
( e ) Transition of bainite configuration from lath to granular type, and increased tendency of carbide precipitation along the grain boundaries of bainite and prior austenite.
The decrease of quenching rate reduces upper bainite toughness through microstructural changes as (b)→(e) in sequence.
(3) Fatigue crack propagation rate are somewhat increased by the formation of upper bainite. This corresponds to the increased distribution density of extremely fine carbide precipitates in tempering of upper bainite.