ULTRAFINE MICROSTRUCTURES DEVELOPED DURING TORSIONAL TESTING OF HADFIELD MANGANESE STEELS

Three Hadfield manganese steels were investigated containing 1.15,1.40 , and 1.70 wt-%C, all with about 13 wt-%Mn. The as processed steels co nsisted of austenite grains, 10-12 mum in size, with a dispersion of c arbides. The steels were deformed at temperatures in the range 500-950 -degrees-C in three different phase regions, namely, three phase (aust enite + carbide + ferrite), two phase (austenite + carbide), and singl e phase (austenite). Ultrafine microstructures were developed during l arge strain torsional deformation. The large torsional deformation in the two phase region resulted in grain refinement through dynamic recr ystallisation with the austenite grains pinned by carbide particles, m inimising grain growth. Strain assisted transformation followed by sph eroidisation of carbides was obtained on testing in the three phase re gion. The concurrent deformation and transformation resulted in ultraf ine mirostructures of submicrometre size constituents. Torsional prope rties were evaluated and related to the observed microstructural chang es. Stress exponents of about 7 were obtained. The activation energy f or the deformation process was found to be close to that for lattice s elfdiffusion of iron in austenite.