抄録
In order to examine the applicability of the 25Mn-5Cr-1Ni austenitic steel at cryogenic services, the tensile and impact properties of the steel have been investigated at liquid helium temperature, and compared with those of a typical austenitic stainless steel, SUS 304. The steel exhibited, 0.2% proof strength (PS) of 1 015MPa, tensile strength (TS) of 1 798MPa and elongation (El) of 31.4% in tensile test, and impact energy absorption (vE) of 120J and lateral expansion (LE) of 1.0mm in Charpy impact test. A prestrained and aged specimen also has shown good toughness. The austenite phase in the steel is found to be stable through the testing carried out in this work. 0.2% PS of the steel is approximately twice as high as that of SUS 304 and depends extremely on temperature: it increases with lowering temperatures. This can be considered due to the solid solution hardening of matrix by carbon, nitrogen, etc., and also the grain refinement resulting from niobium addition. The discontinuity of the first stage in the plastic deformation region of the load-displacement curve is considered to be resulted from slip by extended dislocations with stacking faults. It is suggested that serration of the following stage results from the formation and disappearance of the deformation twin and e phase. Serration phenomenon must be considered to result from both deformation mode and adiabatic heating. The rate of deformation hardening is not large in this case. High TS of SUS 304 is considered due to the deformation hardening followed by the deformation induced martensite transformation. Both steels show equally high dependence of TS on temperature. vE and LE decrease with lowering temperature. Nevertheless, the toughness of the 25Mn-5Cr-1Ni steel was maintained high at liquid helium temperature due to the stability of austenite phase without any sign of martensite transformation. From the above results, it is shown that the 25Mn-5Cr-1Ni steel is applicable to cryogenic service.
