Strengthening of Cr?Ni steels with unstable austenite

Abstract

1. Deformation of austenite without the formation of martensite causes an increase of strength by 0.7–0.9 kg/mm2 for each 1% deformation. The highest yield and ultimate strengths obtained for steel Kh18-N9 (30% deformation) do not exceed 45 and 85 kg/mm2 respectively (original values 25 and 60 kg/mm2). 2. The defects in deformed austenite are inherited by the martensite formed during subsequent cooling to low temperatures at degrees of deformation up to 8–10%, corresponding to the initial formation of cellular structure. The determining factor in strengthening at these degrees of deformation is the stabilization of austenite, i.e., the small tendency to form martensite. Deformation by rolling stabilizes austenite at small and large degrees of deformation. 3. The martensitic transformation in steel Kh16N6 (cooled to −196°) causes an increase in the amount of martensite from 10 to 70%, leading to an increase of the ultimate strength by 30 kg/mm2 and yield strength by 55 kg/mm2, i.e., 0.5 and 1 kg/mm2 respectively for each 1% martensite formed. 4. Up to 4–6% deformation by elongation and up to 40% deformation by rolling of steel Kh16N6 with a primarily martensitic structure leads to the formation of additional martensite (to 20–25%) and to an increase of the ultimate strength by 10 kg/mm2 (elongation) and 40 kg/mm2 (rolling), and increase of the yield strength by 80–90 kg/mm2. Calculations of the increase in yield strength from the amount of strain martensite on the basis of the section of the curve where there is almost no increase in the amount of martensite gave the following results: 10–13 kg/mm2 with deformation by elongation and 1–2 kg/mm2 for each 1% deformation by rolling. The same values of the strength can be obtained with 3–4% deformation by elongation and 20% deformation by rolling. 5. The martensitic transformation and the subsequent strain hardening of martensite are the determining factors in the high strength of Cr-Ni steels with unstable austenite.