Toshinari Hirayama1 and Masahiko Ogirima1
1Central Research Laboratory, Hitachi Ltd., Kokubunji
In order to improve both Federbiege Grenze and formability of Fe-Cr-Ni stainless steel for spring, the influence of Nb and Al additions on the mechanical properties and on the amount of generated martensite of 17-7 stainless steel were investigated. Furthermore, the influence of Nb additions on the martensitic transformation in Fe-Cr-Ni stainless steel were also studied. Hardness, tensile strength, Federbiege Grenze, Young's modulus, elongation and bendability were measured as mechanical properties, and the amount of generated martensite was estimated by means of the saturation flux density measurement. The Nb content was 0.1∼1.9 wt% in simple additions and about 1 wt% in complex additions with Al. The amounts of Al additions were 0.4∼1.4 wt%.
The results obtained are summarized as follws:
(1) By the addition about 1 wt%Nb to 17-7 stainless steel containing 0.4 wt%Al or more, the formability before age-hardening is remarkably improved and the W type bending radius becomes less than 1.2 times as large as the specimen thickness, and Federbiege Grenze after age-hardening considerably increase to about 180 kg/mm2. The effect of Nb additions seems to appear in specimens containing only 0.3%Nb.
(2) By the addition of about 1%Nb, athermal martensite easily generates because of the substantial decrease of the Ni equivalent due to the fixing of most solute carbon atoms and the resulting decrease of flow stress. On the other hand, the gap between Ni equivalents of chemical compositions appropriate for the generation of athermal martensite and for the generation of strain-induced martensite is lessened.
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