Behaviors of Microstructure and High-temperature Strength of 0.2C-3Cr-W-Mo-V-Co Precipitation Hardening Hot Work Alloy Tool Stee

Abstract

In this study, effects of bainite structure on high-temperature strength, precipitation and aggregation behaviors of carbides during tempering are investigated for 0.2C-3Cr-W-Mo-V-Co steel comparing with martensite structure obtained by particularly rapid quenching.
The results obtained are as follows :
(1) During cooling from austenitized temperature at the rate lower than the rate of half temperature time being 10 min, granular upper bainite structure, without carbide precipitation in boundary is obtained with about 15% stable retained austenite.
(2) During tempering of this structure,
(i) precipitation of M₃C and transformation of M₃C→M₇C₃ in situ are suppressed, while those phenomena take place in martensite tempering,
(ii) boundary precipitation and aggregation of M₆C are suppressed until higher tempering temperature because of the rarer distribution density of bainite grain boundary than martensite lath boundary, and through above (i), (ii),
(iii) increasing of volume of MC, M₂C precipitated, their homogeneous distribution and suppressing of aggregation of them, which are all effective to the strengthening of the steel, are promoted.
(3) As the results, this upper bainite structure shows excellent high-temperature strength, compared with martensite structure.