High-speed quenching of high carbon steel

Medium and high carbon steels are usually quenched in polymer and oil in order to avoid cracking and distortion; however, recent studies have proved that it is possible to minimize cracking using water as a coolant of these steels by promoting extremely high cooling rates. By great agitation and velocity of quenchant, the vapor blanket is reduced or prevented during water quench, allowing uniform hardening of the surface. In this study, the cooling severity of a spray quenching system and a high-speed quenching chamber are studied. Cylindrical samples of AISI 304 stainless steel (20 mm in diameter and 100 mm length) were employed to characterize the cooling severity. Thermal data was acquired through K-type thermocouples placed in the sample at three positions, 1mm below surface, mid-radius and at center of the specimen, connected to a data acquisition system. High thermal gradients were observed in both systems, being the high-speed chamber the severest cooling. The maximum cooling rate obtained at the surface was 470 and 300°C/s for the high-speed chamber and the spray system, respectively. In addition, 5160 spring steel samples were quenched for short times in both systems; the cooling was interrupted to avoid through transformation and to produce a case-core type microstructure. Different cooling times were used for the interrupted quenching to modify the martensite case thickness. No cracks were observed. Thermal results and microstructures are discussed.