Microstructure and mechanical properties of Co-28Cr-6Mo-0.22C investment castings by current solution treatment

This study examined the impact of current solution treatment on the microstructure and mechanical properties of the Co-28Cr-6Mo-0.22C alloy investment castings. The findings reveal that the current solution treatment significantly promotes the dissolution of carbides at a lower temperature. The optimal conditions for solution treatment are determined as a solution temperature of 1,125 degrees C and a holding time of 5.0 min. Under these parameters, the size and volume fraction of precipitated phases in the investment castings are measured as 6.2 mu m and 1.1vol.%. The yield strength, ultimate tensile strength, and total elongation of the Co-28Cr-6Mo-0.22C investment castings are 535 MPa, 760 MPa, and 12.6%, respectively. These values exceed those obtained with the conventional solution treatment at 1,200 degrees C for 4.0 h. The findings suggest a phase transformation of M23C6 ->sigma+C following the current solution treatment at 1,125 degrees C for 5.0 min. In comparison, the traditional solution treatment at 1,200 degrees C for 4.0 h leads to the formation of M23C6 and M6C carbides. It is noteworthy that the non-thermal effect of the current during the solution treatment modifies the free energy of both the matrix and precipitation phase. This modification lowers the phase transition temperature of the M23C6 ->sigma+C reaction, thereby facilitating the dissolution of carbides. As a result, the current solution treatment approach achieves carbide dissolution at a lower temperature and within a significantly shorter time when compared to the traditional solution treatment methods.