Thermal Cycling Treatment with Pre-Precipitation to Improve Aging Kinetics of Maraging Steel C-250

Applications of maraging steels are limited by their lower wear resistance and lower red hardness. Further, it took a substantial long period of time during precipitation hardening process so as to achieve peak mechanical properties. In this work, a new heat treatment cycle is developed to address the later issue. A new heat treatment cycle is designed to substantially reduce precipitation hardening cycle time without degrading mechanical properties of maraging steel. The heat treatment involves modification in thermal cycling treatment given to maraging steel by addition of a pre-precipitation step. The objective of this modification was to achieve a refined martensitic structure with high defect density. Results showed that the new heat treatment cycle reduced the effective time required to achieve peak properties of around 90% compared to conventional precipitation hardening heat treatment given to maraging steel. Prior austenite grain size (PAGs), packet size and width of blocks of martensitic structure significantly reduced due to the modification in heat treatment cycle. Results showed reduction of 69, 37 and 82% in PAG, packet size and width of blocks of martensitic structure, respectively, after second thermal cycle compared to solution annealed microstructure. Crystallized size reduced by 31%, whereas dislocation density increased by 123% after second thermal cycle. Peak hardness value increases by 102% after aging cycle followed by first thermal cycle and by 98% after aging cycle followed by second thermal cycle compared to solution annealed specimen. New heat treatment cycle also yields an appreciable amount of retained austenite (volume fraction of 6 to 9%). The presence of retained austenite which is formed without austenite reversion phenomenon may lead to improved ductility and toughness without deterioration of mechanical strength.