JOURNAL OF MATERIALS SCIENCE LETTERS 21, 2 0 0 2, 751 – 753 Development of ultra fine grain structure by martensitic reversion in stainless steel A. DI SCHINO Materials Engineering Center, University of Perugia, Terni, Italy M. BARTERI Centro Sviluppo Materiali, Italy J. M. KENNY ∗ Materials Engineering Center, University of Perugia, Terni, Italy E-mail: Kenny@unipg.it Austenitic stainless steels have good corrosion resis- tance and good formability but they have also relative low yield strength. It is well known that the mechanical properties of austenitic stainless steels are very sensible to the chemical composition (which can induce harden- ing by both substitutional and interstitial solid solution) and to microstructural features (such as grain size and δ-ferrite content). Recently there have been commercial developments to exploit the effect of these variables in stainless steel taking advantage of changes in the chem- ical composition induced by nitrogen addition [1, 2]. Another effective way to increase yield strength with- out impairing good ductility is grain refining. Although this approach has induced the development of ultrafine grain carbon steels (e.g. [3]), no attempts have been still reported on this approach for austenitic stainless steels. In fact, austenitic stainless steels do not undergo phase transformation at typical annealing temperatures and then the only way to refine the grain is recrystal- lization after cold rolling. However, the strengthening by grain refining is limited, due to the high recrystal- lization temperature of this stainless steel grade. For instance, the recrystallization temperature of the AISI 301 steel is above 900 ◦ C and the minimum grain size obtained is in the range 10–30 μm [4]. In austenitic stainless steels, plastic deformation of austenite cre- ates the proper defect structure which acts as embryo for martensite deformation: the successive reversion of deformation-induced martensite (α ′ ) enables a marked grain refining [5, 6]. In this letter the production of an ultra fine microstructure in an AISI 301 stainless steel by martensitic reversion is reported. The chemical composition of the steel used is shown in Table I. The procedure used to refine the grain is the following (see Fig. 1): • Metastable γ is almost entirely transformed to α ′ by heavy cold rolling: in fact the retained γ cannot be refined during the subsequent annealing. • α ′ reverts to recrystallized austenite γ R during an- nealing at low temperature. ∗ Author to whom all correspondence should be addressed. The requirement of entirely transforming γ to α ′ im- plies cold rolling at temperatures below M d 30. In Fig. 2 the percentage of magnetic phase, formed in the AISI 301 steel by cold rolling both in standard conditions (no prior cooling of the steel) and after cooling in liq- uid nitrogen, is shown. Fig. 2 clearly shows the effect of the cooling on the formation of the magnetic phase (measured by ferritoscope). This phase is constituted by residual δ-ferrite showing a constant value (estimated by automatic image analysis as approximatively equal to 1.5%) and by α ′ martensite, which increases with cold reduction. In the cooled steel γ is almost com- pletely transformed into α ′ , while in the rolled steel without any cooling treatment only 35% martensite is produced with 90% cold reduction. Furthermore, in the cooled steel, martensite volume fraction increases linearly to 60% reduction and then saturates. These data are in agreement with those from analogue mea- surements performed by Takaki [7] who investigated the effect of cold working on the microstructure of T A B L E I Chemical composition of the studied AISI 301 stainless steel C Si Mn Cr Ni Mo Cu N 0.058 0.42 1.14 17.61 7.90 0.25 0.28 0.045 Figure 1 Thermo mechanical treatment adopted to obtain an ultra fine structure by martensitic reversion. 0261–8028 C  2002 Kluwer Academic Publishers 751