Laminar plasma quenching-tempering: A rapid surface heat treatment technique for controllable modification of the rail steel

Plasma selective quenching is promising in enhancing the rail's wear resistance by preparing martensitic hardened zones discretely embedded in the substrate on the surface layer. However, severe cracks and spalling occur in the hardened zone during the rail service, which may be related to its microstructure and hardness. To modify the microstructure and hardness of the hardened zone, a novel laminar plasma quenching-tempering (LPQT) technique that consists of performing laminar plasma quenching (LPQ) and laminar plasma tempering (LPT) successively was proposed to obtain the hardened zone with desired microstructure and hardness. LPQ is firstly performed on the rail surface to prepare martensitic hardened zones with high hardness. LPT is then performed on each hardened zone to modify its microstructure and hardness. The numerical simulation of LPT was established to determine the processing parameters of LPT for accurately controlling the tempering temperature of the hardened zone. The effects of the tempering temperature on the microstructure and hardness of the hardened zone were investigated. Results showed that with the increase of the tempering temperature from Ms temperature to austenitizing temperature, the hardness of the hardened zone was decreased from 800 HV to 450 HV as the acicular martensite in the hardened zone precipitated carbides and was transformed into the tempered martensite. Besides, the total processing time of LPQT for treating a hardened zone is normally <2 s. Application of LPQT in modification of the rail steel showed that it helped to eliminate the severe cracks and spalling in the LPQ treated hardened zone while its wear resistance was still improved compared with that of the untreated rail steel. Overall, LPQT is a rapid and controllable surface heat treatment technique for modifying the microstructure and hardness of the rail surface, which is promising to be used to balance the wear and RCF resistances based on the service condition of the rail.