Austempering versus quenching and tempering: Impact behavior of SAE 4140 and SAE O1 steels

Traditionally it have been reported that bainite microstructure has better impact properties than tempering martensite at the same hardness level. In this work a comparative study was performed using samples of 2 different steels: SAE 4140 and SAE O1 (tool steel) under different heat treatment conditions:  austempering; quenching and tempering followed or not by a cryo bath. At the total 47 samples were Charpy impact tested. Hardness, microscopy (optical and SEM) and fractography analysis were performed. No matter the steel analyzed, better performances were obtained for the tempering martensite followed by the cryo bath.