Martensitic steels must be tempered to increase their toughness and ductility. The tempering process requires heating from room temperature to the desired tempering temperature. In this paper, the effects of heating rates on carbide precipitate size distribution, chemistry, and precipitate density will be discussed. As-quenched martensite in AISI 4140 steel was heated to selected tempering temperatures in air furnaces as well as by induction. The heating rates for tempering vary from 30 seconds to 20 minutes. The experimental results are presented, and carbides were characterized using an extraction technique.

To experimentally investigate the effect of tempering temperature and time on the structure and composition of martensite, AISI 52100 was austenized at 1000°C for 40 minutes and quenched in agitated water at 21°C. The as-quenched steel contained body-centered tetragonal (BCT) martensite with 22% retained austenite. These samples were tempered at 100°C, 200°C, and 300°C with different holding times and then were characterized by x-ray diffraction (XRD) to determine the effect on the structure of the martensite. It was found that the content of retained austenite did not change after tempering at 100°C. Retained austenite decomposed after tempering for 40 minutes at 300°C. The changes in crystal structures and lattice parameters for tempered martensite with different holding times and temperatures were measured. The effect of sample preparation on retained austenite and the structure of martensite and tempered martensite was evaluated. An effective technique for carbide extraction and collection in steel is introduced.