Current Advances in Mechanical Design & Production, MDP-6, Cairo Uni., 6 th Int. Conf., Cairo, Egypt, Jan. 2-4, 1996 337 The EFFECT OF GRAIN SIZE AND STRAIN RATE ON COLD AND HOT WORKING OF FERRITIC STAINLESS STEEL M.A. Shaker*, T.A.Elsarrag ** and A.E.Nassef*** * Prof., ** Associate Prof. *** Assistant Prof. Faculty of Engineering and Technology, Suez Canal University, Port-Said, Egypt. ABSTRACT The effect of grain size and strain rate on both cold and hot working of ferritic stainless steel have been examined. Upsetting test samples were performed at strain rates of 8.23x 10 -3 , 2.25x 10 -2 , 0.64 and 1.0 s -1 for cold working specimens while the strain rates 3.10 -4 , 3.x10 -3 , 3.10 -2 and 3.10 -1 s -l for hot worked specimens. The results obtained from the present investigation showed clearly that, the variation in strain rate has a significant effect on workability- limits at coarse grain size. The strain rate at fine grain size has no significant effect on cold and hot working limit lines. KEYWORDS : Cold Working, Hot Working, Grain Size (G. S.), Strain Rate, Upsetting, Ferritic Stainless Steel INTRODUCTION Stainless steel cast for subsequent rolling generally has poor mechanical properties at room temperature, since the coarse microstructures and pronounced segregation of impurities to the interdendritic boundaries give rise to poor cold and hot workability. Over the past few years, considerable interest has been shown in the cold and hot ductility of stainless steel particularly ferritic stainless steel, since difficulties have been encountered with them in continuous casting. Grain size is one of the most important parameters determined in quantitative metallography, owing to the importance of this microstructural feature in influencing mechanical properties of metals [1, 2]. However, extensive studies in the field of creep have generally shown that high temperature ductility increased with decreasing grain size. Similarly, the hot ductility of metals and alloys on tensile testing is generally improved with refinement in grain size, the most extreme example being those alloys which give superplastic behavior at ultra-fine grain size [1-4]. Hot ductility is improved by applying higher strain rates for the following reasons [4]: 1)There is in sufficient time for strain induced precipitation, 2) The amount of grain boundary sliding is reduced, i.e. g/t , decreased as the strain rate is increased, where 8g is the strain due to grain boundary and et the total strain to fracture, 3)There is in sufficient time for the formation and diffusion controlled growth of voids next to the precipitates and inclusions present at the grain boundary 4) It has also been suggested, that increasing the strain rate prevents the formation of deformation induced ferrite. Decreases in strain rate (e.g. from 10 -1 to 10 -2 s -1 ) has also been shown to improve the hot ductility when unstable fine precipitates such as the Fe-Mn sulphides are present [5]. In hot working, increasing temperature leads to lower flow stresses via increasing dynamic recovery, which reduce the stress concentrations at the crack nucleation sites [6, 7]. Workability is defined as the amount of deformation that a material will withstand without fracture in a particular metal working process. It is not a unique property of a material but depends on process variables like strain rate, geometrical factors, temperature, and material variables like inclusion content and grain size. Formability can be measured by different techniques. Axial compression of cylindrical specimens provides a deformation test in which the stress pattern is inherently similar to that of bulk deformation processes. The use of the compression test, its limitations, the concept of the forming limit line (FLL), and the practical applications of formability indexes were reviewed recently [8, 9]. Cold and hot deformations of steel and alloy steels are of great importance in industry and construction fields. Therefore, the present study was aimed at the determination of the effect of grain size and strain rate on the formability of ferritic stainless steel.