The effect of isothermal holding on the microstructures and mechanical properties of a low carbon alloy steel

Abstract

A low carbon high-strength steel was isothermally treated at 900°C for varying times and was immediately quenched into ice water. The microstructures and grain refinement were characterized by the techniques of serial sectioning and computer-aided three-dimensional reconstruction. The microstructures in the specimens that were isothermally maintained for short periods of time consisted of predominantly martensite and much lower bainite. The lower bainite was lath-like, plate-like, rod-like and lenticular in shape. Most were lath-like or plate-like. The lower bainite laths or plates divided the prior austenite grains into many smaller regions, and the martensite that transformed at lower temperatures was thus confined in the smaller zones and thus had a smaller packet size. The specimens that were isothermally held for short periods thus had higher strength, compared with the directly quenched specimens.