Microstructure - Toughness relationships in sub-zero treated and tempered Vanadis 6 steel compared to conventional treatment

摘要

Ledeburitic steel; Sub-zero treatment; Microstructure; Carbide particles effect; Fracture toughness; Fracture surface roughness%The influence of sub-zero treatment and tempering on the microstructure, flexural strength, and fracture toughness of Cr-V ledeburitic steel was examined with reference to the same material processed without the subzero period. The experimental results lead to the following findings: (i) Sub-zero treatment induces a considerable reduction of the amount of retained austenite, refinement of newly formed martensite, and increase in population density of small globular carbides, (ii) The hardness is improved by the sub-zero treatment in the prior-to-tempering state as well as in the state after low-temperature tempering; after high-temperature tempering, the situation is changed in favour of conventionally quenched specimens, whereas the secondary hardening peak disappears in sub-zero treated steel. This is due to combined effect of the population density decrease of small globular carbides, accelerated decomposition rate of retained austenite, coarsening of ce-mentitic precipitates, and retardation of precipitation of stable M_7C_3 carbide, (iii) There is almost no effect of sub-zero treatment on the flexural strength in the low-temperature tempering range but this property is improved after tempering into the secondary hardening range. Sub-zero treatment causes a worsening of fracture toughness after low-temperature tempering but a slight improvement after tempering in the secondary hardening temperature range.