Creep deformation should be considered in the thermal deformation analysis of castings because the casting process includes a high-temperature region. For this reason, the thermal deformation of castings should be predicted by the so-called “elasto-plastic-creep FEM analysis.” However, this method is rarely applied for prediction as the pre-experiments take considerable time because the material constants for the analysis must be determined through long creep tests, under at least three stress conditions and at more than three temperatures, in addition to tensile tests.
  To solve this problem, this paper describes an experimental method called “tension-strain maintenance test” that can estimate the material constants for elasto-plastic-creep analysis efficiently. The method was applied to SCS13 stainless cast steel, and the characteristics of its deformation at five temperatures were evaluated. Based on the evaluation results, the material constants of an elasto-plastic-creep model used for thermal deformation FEM analysis were determined, and using these material constants, elasto-plastic-creep FEM analysis was conducted to predict the thermal deformation of SCS13 generated in the cooling process of casting. The analysis results were compared with the results of ordinary elasto-plastic FEM analysis, and it was found that elasto-plastic FEM analysis may not be able to predict the thermal deformation of castings precisely. These results suggest that elasto-plastic-creep FEM analysis is better for predicting the thermal deformation of castings.

View full abstract

  In this study, the influence of Mo and W addition on the high temperature hardness of M₇C₃ carbide was investigated using unidirectionally solidified hypereutectic cast iron with 25mass%Cr. Concentrations of alloying elements in primary M₇C₃ carbide were measured by EDS. As Mo or W content of the cast iron increased, the concentration of Mo and W in the carbide increased and that of Fe decreased. However, the Cr content was more or less the same as about 60mass% in spite of the increasing Mo and W contents. The XRD results showed that the lattice constant of the M₇C₃ carbide changed and the volume of the unit cell increased with the increase in the Mo or W content. It is thought that the Fe atoms in the M₇C₃ carbide were substituted by Mo or W which has a larger atomic radius than Fe. In all the specimens, the hardness of the M₇C₃ carbide was about 1600HV0.3 at room temperature, and it decreased gradually with increasing test temperature. The decreasing rate of the carbide hardness in the cast iron with Mo or W was small at high temperature ranges in comparison with that of alloy-free cast iron. In addition, this decreasing rate at high temperatures decreases with increasing Mo or W concentration in the M₇C₃ carbide. Therefore, it can be said that the dissolution of Mo or W atom in the M₇C₃ carbide is very effective for maintaining the high temperature hardness of carbide. It was also found that an excess concentration of both elements in the M₇C₃ carbide caused the increasing rate of hardness to decrease. The fracture toughness of the M₇C₃ carbide at room temperature was measured using an indentation fracture method, and the values were very similar among the carbides regardless of Mo and W concentration in the M₇C₃ carbide.

View full abstract