Strain induced martensite formation in stainless steel
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INTRODUCTION
STAINLESS steels form a group of alloys which, because of their corrosion and oxidation resisting qualities, are commonly used in environments not suitable to carbon and low alloy steels. The inclusion of a chromium content, generally greater than 11 pct, is responsible for their corrosion resistance due to the formation of a chromium oxide layer on the surface, i Austenitic stainless steels (300 series) contain, in addition, a large amount of nickel which stabilizes the alloy into a face-centered cubic (fcc) structure and thus suppresses the formation of the body-centered cubic (bcc) martensite. Type 316 stainless steel is particularly suitable for use in the marine environment due to a small amount of molybdenum which increases its resistance to pitting. In general, all nickel-chromium alloys show an increased corrosion resistance when compared with ferrous alloys. A common observation in austenitic alloys is their transformation to a martensitic phase due to plastic deformation. This latter structure is known to exhibit inferior qualities with regard to corrosion resistance. The amount of martensite formed due to cold work depends not only on the amount of strain but also on the composition of the alloy and the temperature of the environment. As a result, many studies have been previously undertaken on different alloys, primarily binary, which have been subjected to various strain inducing conditions over a wide range of temperature. 2-13Two important conclusions from past studies show that the austenite-marsensite transformation is partially reversible and the structure of martensite may be bcc or body-centered tetragonal (bct), depending on the amount of interstitial elements in the alloy. The use of the Mrssbauer effect to study the paramagnetic austenite and the ferromagnetic martensite is well established since each structure exhibits different magnetic properties. This paper presents the results of such a study on Type 316 stainless steel, an alloy which contains a very low concentration of interstitial elements. It is the first in a series of studies on the surface changes in stainless steel which have D.C. Cook is Assistant Professor, Department of Physics, Old Dominion University, Norfolk, VA 23508. Manuscript submitted February 24, 1986.
METALLURGICALTRANSACTIONS A
been subjected to different marine environments. The resuits in this paper relate to the structural changes which occur at the surface of many samples, as a result of sample preparation, before they are exposed to the environment. Both the austenitic and martensitic structures have been studied as a function of temperature using the surface sensitive techniques of Conversion Electron (CEMS) and X-ray (XMS) Mrssbauer spectroscopy. The martensite was formed as a thin layer on the surface of the austenite as a result of plastic deformation due to brushing. The results show that the concentration and type of alloying element affect the local environment at the iron site differently. They also show that previously developed mo
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