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Abstract
The effect of austenitizing temperature on mechanical properties of the mixed bainite - martensite microstructure in CrMoV steel was studied in the present work. The result showed that at low austenitizing temperature (910°C - 1000°C), the mixed microstructures containing 12-28% volume fraction of lower bainite showed higher yield and tensile strength than fully martensitic microstructure. The partitioning of the prior austenite grain by lower bainite was found to cause a refinement of the martensite packet size. In addition the strength of the lower bainite in the mixed microstructure is enhanced by plastic constraint induced by the surrounding stronger martensite. By increasing the austenitizing temperature from 1000°C to 1200°C (40min), the YS, UTS, %EL, %RA and CVN impact energy decreased for all samples. This is attributed mainly to the increase in austenite grain size and width of bainite sheaves.
Highlights
Ultrahigh strength medium carbon low alloy CrMoV(D6AC) steel has been used as a structural material for several critical applications such as landing gears, shafts and motor cases, etc
Quenching followed by tempering process is the most common heat treatment cycle for the hardening and strengthening of ultrahigh strength medium carbon low alloy steels[2]
Corresponding scanning electron micrographs of the mixed microstructure of the lower bainite - martensite (Fig.1b) reveals that the lower bainite associated with martensite appears in acicular form and partitions the prior austenite grains, refines the martensite packet size
Summary
Ultrahigh strength medium carbon low alloy CrMoV(D6AC) steel has been used as a structural material for several critical applications such as landing gears, shafts and motor cases, etc. Quenching followed by tempering process is the most common heat treatment cycle for the hardening and strengthening of ultrahigh strength medium carbon low alloy steels[2]. Increase in strength is usually associated with decrease in the ductility values limiting the structural applications of these steels[3,4]. An approach to overcome this problem is to develop steels, having mixed or multiphase microstructures[5,6,7,8]. Inferior mechanical properties have been reported for these steel compared with those having conventional microstructures[8]
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