Effect of single and double quenching-tempering heat treatments on microstructures and tensile strength of AISI 4140 in annealing condition

Low alloy of AISI 4140 steel in annealing condition was heat-treated by single quenching-tempering (SQT) and double quenching-tempering (DQT) processes. Quenching process was done by heating steel at 850 °C for soaking time 1 h and followed by immersion into oil bath at 10 °C. Tempering heat treatment was performed at 650 °C for 1 h and followed by cooling sample into oil bath at 10 °C. The tensile strength properties of heat-treated steel were evaluated from quenched and tempered specimens for single and double heat treatment conditions. The microstructures of heat-treated sample were characterized by means of Optical Microscope (OM). The DQT heat treatment results higher tensile strength than those of SQT heat treatment with relatively low ductility. The finer austenite grains resulted by subsequent austenization in DQT heat treatment is believed to produce small many packets consisting of finer lath tempered martensite. The microstructural observation indicates that the finer grain size consisting of solid solutions of tempered-martensite phase in ferrite matrix may be responsible to sharply increase of the steel strength properties in DQT condition.