The Bainite in Low Carbon Low Alloy High Strength Steels

Abstract

The morphology of the bainite in some low carbon, low alloy high strength steels has been investigated by means of dilatometry and both optical and electron microscopies. The results indicate that the bainite can be divided into three distinct types by the morphology of the cementite precipitation.
The bainite I is formed above 500°C and is the carbide-free bainitic ferrite. The bainite II is formed in the intermediate temperature range or by cooling at intermediate cooling rate. Although some amount of cementite particles are formed within the grains, the bainite II consists mainly of ferrite laths with cementite layers between them, being a typical of upper bainite. The bainite III is formed at temperatures close to the Ms or during cooling as fast as the upper critical cooling rate, the morphology of cementite formation is similar in appearance to the high carbon lower bainite. However, these bainites have the lath-like ferrite morphology with the same ‹111›α{110}α habit. This is different from the high carbon lower bainite with plate-like ferrite. Therefore, it seems reasonable to classify the bainites observed in low carbon low alloy high strength steels as the upper bainite.