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

It is well known that austempering, producing a microstructure bainitic, is the preferred heat treatment when toughness is the main requirement of the component. Literature shows a several works where mechanical properties for tempered martensite are compared with bainite presenting both same hardness. Most part of the results demonstrate better properties for the bainite microstructure. There are 2 types of bainite, upper bainite formed from austenite transformation in a isothermal bath using higher range of temperature, usually 5000C. Lower bainite is formed at lower temperatures, about 3500C, depending on the alloy steel. Lower bainite has hardness very close to the tempered martensite and it is used when elastic characterisitics are wished.