Influence of alloy elements on physical and mechanical properties of single crystalline austenitic stainless steels

The single crystalline austenitic stainless steels based on 316L were developed to improve their resistance to intergranular corrosion and stress corrosion cracking. However the mechanical properties of the single crystals were lower than those of polycrystalline. The precipitation hardening methods were applied to the single crystal for the increase of their mechanical strength by addition of niobium and heat treatments. In this paper, the influences of niobium concentration on the several physical and mechanical properties of these single crystalline austenitic stainless steels were studied. The thermal conductivity, coefficients of thermal expansion and elastic constants of the single crystals were almost the same as those of polycrystalline independently of the niobium concentration. The mechanical properties of the single crystals strongly depended on the niobium concentration and the orientation. In the specimen which contains 1.0mass% niobium, 0.2% proof stress were remarkably improved; 370 MPa, 337 MPa and 403 MPa were obtained in 〈100〉, 〈110〉 and 〈111〉 orientations at the room temperature. The creep rupture strength and the high cycle fatigue strength were also improved by addition of niobium. In the 〈100〉-orientated specimen which contains 1.0mass% niobium, the creep rupture strength at 873 K for 103 hours, 245 MPa and the high cycle fatigue strength at 773 K for 107 cycles, 220 MPa were obtained. Furthermore, the single crystalline pipe, bolts and nuts were successfully manufactured for the application of these single crystals.