Delayed Fracture Characteristics of Isothermally Transformed Steel

Abstract

As a part of study of the influence of metal-structure on the delayed fracture strength of low alloy high strength steel in moist atmosphere, the delayed fracture characteristics in 0-1N-HC1 solution was examined for Cr-Mo-V steel of bainitic structure obtained by isothermal heat treatment, and the structure tempered after the treatment.
At the hardness levels over Hv 350, the delayed fracture limit and the delayed fracture ratio of the materials isothermally transformed were higher than those of the materials conventionally quenched and tempered. Only in the case of tempered at 550°C after isothermal transformation, the daleyed fracture strength decreased by the amount equal to that of the quench-tempered material with the same hardness.
Electron microscopic observation revealed that carbides precipitated densely along the prior austenitic grain boundaries in the steel tempered at 550°C after isothermal transformation. A very small amounts of carbides was observed along the grain boundaries in the steel transformed isothermally, and grain boundary carbides of the steel tempered at the temperature over 600°C were globulized.
The comparison of these results with those of quench-tempered material suggested that the difference in the delayed fracture strength between both materials at the high hardness levels attributed to the difference of the precipitation behavior of carbides along the austenitic grain boundaries.