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 were investigated at liquid helium temperature, and were compared with those of a typical austenitic stainless steel, SUS 304. The 25Mn-5Cr-1Ni steel exhibited, 0.2%proof stress (PS) of 103.5 kgf/mm2, tensile strength (TS) of 183.3 kgf/mm2 and elongation (El) of 31.4% in tensile test, and impact energy absorption (vE) of 12 kg-m and lateral expansion (LE) of 1.0 mm in Charpy impact test. Pestrained and aged specimen also showed good toughness. The austenite phase in the steel was stable through the testing carried out in this work. 0.2% PS of the steel was approximately twice as high as that of SUS 304 and depended extremely on temperature; it increased 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 was considered to be resulted from slip by extended dislocation with stacking faults. It was suggested that the serration of the following stage resulted from the formation and disappearance of the deformation twin and ε phase. The rate of deformation hardening was not large in this case. High TS of SUS 304 was considered to be due to the deformation hardening followed by the deformation induced martensite transformation.Both steels showed equally high dependence of TS on temperature. vE and LE decreased with lowering temperature. Nevertheless, the toughness of the 25Mn-5Cr-1Ni steel was kept high at liquid helium temperature, due to the stable austenite phase without any sign of martensite transformation. From the above results, it was shown that the 25Mn-5Cr-1Ni steel was applicable to cryogenic service.
