Present paper shows a simple approach for obtaining hardness data by predicting the hardness values in carburizedquenched steel parts using quench simulations. Conventionally, the relationship between cooling rate and hardness values is evaluated by preparing test pieces of varying carbon concentrations and performing quench tests. In this method, Jominy end quench test method is used with only one test piece to obtain carburized-quenching hardness data that reflects the effect of carbon concentration. Finally, a comparison is made between simulated results and measured values of the effective case depth after carburizing-quenching of a cylindrical steel specimen. A good agreement was observed between experimental and simulated results.

Alloy 625 has been used as a structural material for monopropellant thruster of artificial satellite, because Alloy 625 has good nitriding resistance and low cost than those of L-605 alloy as a conventional material for satellite use. The microstructural observation and investigation of tensile properties of alloy 625 before and after aging at 1173 K in N₂ gas were conducted in order to evaluate the durability after using as a structural material of artificial satellite. Formation of a thin compound layer which consist of chromium nitride was observed on the surface of alloy 625 after aging at 1173 K in N₂ gas. Precipitation of δ-phase (Ni₃Nb) and carbides were detected at a grain boundary in aged alloy 625. Tensile strength and fracture elongation obtained in alloy 625 after aged at 1173 K for 1036.8 ks in N₂ gas were 952 MPa and 36%, respectively. Fully satisfied tensile properties were maintained in alloy 625 after aged at 1173 K for 1036.8 ks in N₂ gas, comparing with those of L-605 alloy.