Grain refinement under multiple warm deformation in 304 type austenitic stainless steel

The dynamic process of fine grain evolution as well as deformation behaviou r under warm working conditions was studied in compression of a 304 type au stenitic stainless steel. Multiple compression tests were carried out at a strain rate of 10(-3)s(-1) to produce high cumulative strains, with changin g of the loading direction in 90 degrees and decreasing temperature from 12 23 to 873 K (0.7-0.5T(m),) in each pass. The steel exhibits two types of de formation behaviours with different mechanical and structural characteristi cs. In the deformation region where flow stresses are below about 400 MPa, conventional dynamic recrystallization takes place accompanied mainly by bu lging of serrated grain boundaries. The dynamic grain size evolved can be r elated to the high temperature flow stress through a power law function wit h a grain size exponent of -0.72. On the other hand, in the region of highe r stresses above 400 MPa the flow stresses show small strain rate and tempe rature dependence, and so it is suggested to be in an athermal deformation region. The stress-strain curves show a steady stale like flow without any strain softening, while the multiple deformation to high cumulative strains brings about the evolution of fine grained structures with grain sizes les s than one micron. The relationship between the warm temperature flow stres ses and the grain sizes evolved also can be expressed by a unique power law function of grain size with an exponent of -0.42. The interrelations betwe en the mechanisms of plastic deformation and microstructure evolution at wa rm and high temperatures are analysed in detail and also the multiple compr ession method for obtaining ultra fine grained structure is discussed as a simple thermomechanical processing.