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Damping Properties of Austenitic Stainless Steels Containing Strain-Induced Martensite JUHO TALONEN and HANNU HÄNNINEN Damping properties of two austenitic stainless steel grades, EN 1.4318 and EN 1.4301, were investigated. The test materials were cold rolled to different reductions and damping capacity was measured as a function of temperature with an internal friction method. Microstructures of the test materials were studied by means of X-ray diffraction (XRD) and magnetic measurements. The results showed that damping capacity of the studied materials depended on the amounts of strain-induced - and martensite phases. At temperatures around 0 °C, highest damping capacity was achieved with cold rolling reduction of 10 to 15 pct. This behavior is related to the existence of -martensite and stacking faults. Internal friction peak due to -martensite phase was present at the temperature of 130 °C. Strain aging heat treatment at 200 °C for 20 minutes decreased the damping capacity in the entire studied temperature range.

I. INTRODUCTION

AUSTENITIC stainless steels exhibit a unique combination of strength and ductility, which makes them an attractive material for the applications where high strength and good formability are needed together with a good corrosion resistance. The yield strength of austenitic stainless steels can be increased by cold working. The microstructure of the austenitic stainless steels consists of metastable austenite phase, which may be transformed to strain-induced martensite phase during deformation of the steel. Two forms of martensite exist: hexagonal close-packed -martensite and body-centered tetragonal -martensite. During the early stages of deformation, shear bands consisting of stacking faults and -martensite form on the {111} planes of austenite. The -martensite nucleates at the intersections of the shear bands. As the deformation level increases, the -martensite grows at the expense of the -martensite.[1,2,3] The amount of - and -martensites is controlled by two important parameters: Md30-temperature and stacking fault energy (SFE) of the steel. The Md30-temperature is determined by the composition and the grain size of the steel. The SFE of the steel depends on the composition of the steel and increases with increasing temperature. Several empirical equations have been determined to calculate the values of Md-temperature and SFE. Nohara et al.[4] determined the following equation to calculate the Md30-temperature: Md30 (ºC)  551  462(Pct C  Pct N)  9.2 Pct Si  8.1 Pct Mn  13.7 Pct Cr  29(Pct Ni [1]  Pct Cu)  18.5 Pct Mo  68 Pct Nb  1.42(GS  8) where GS is ASTM grain size number.

JUHO TALONEN, Research Scientist, and HANNU HÄNNINEN, Professor, are with the Laboratory of Engineering Materials, Department of Mechanical Engineering, Helsinki University of Technology, FIN-02015 HUT, Finland. Contact e-mail: [email protected] Manuscript submitted June 10, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A

Schramm and Reed[5] deriv