The Effect of Microstructure on the Toughness and High-temperature Strength of 0.2C-3Ni-3Mo Precipitation Hardening Hot Work Alloy Tool Steel

Abstract

The effects of microstructure on toughness and high temperature strength of the 0.2C-3Ni-3Mo steel were investigated.
(1) The granular upper bainite structure, which was obtained by quenching at the rate of the half temperature time of 60 min, provides the following differences in properties compared with the martensite structure.
(a) After tempering at 400°C;
(i) The plane strain fracture toughness was lower but the difference was small. (ii) The fatigue crack propagation rate was smaller. (iii) The uniform elongation in tensile testing was higher.
(b) After tempering at precipitation hardening temperature of 550600°C, the plane strain fracture toughness and Charpy impact value were lower but the differences were small.
(c) The high-temperature strengths at the temperature higher than 600650°C were higher.
(2) Followings are microstructural factors relating to the above three behaviors ((a)(c)), respectively.
(a); (i) Larger effective grain size. (ii) Formation of excessively stable retained austenite located both along grain boundaries and in grains. (iii) Suppressed precipitation of M₃C.
(b); (i) Not a large increase of tendency of carbide precipitation along the grain boundaries of prior austenite. (ii) Existing of retained austenite to some extent.
(c); Suppressed agglomeration of M₂C carbides to higher temperature.
(3) This steel is superior in room temperature toughness and inferior in high temperature strength to the 0.2C-3Cr-W-Mo-V-Co steel.