Abstract
A study of the austenite nucleation and growth characteristics of bainitic and martensitic 1 Cr–1 Mo–0·75V(Ti,B) steel has been made. Two forms of austenite are observed: these are termed ‘globular’ and ‘acicular’. Globular nucleation and growth leads to grain refinement whereas the acicular mode leads to reconstitution of the original austenite structure. The proportions of acicular to globular austenite are dependent upon heating rate, prior heat treatment, and chemical composition. Acicular nucleation occurs on stabilized retained austenite at bainitic or martensitic lath boundaries. Subsequent growth is restricted by these boundaries. Acicular grains within a given prior austenite grain have a common orientation owing to growth from retained austenite, and they coalesce on impingement, leading to a reconstitution of the original prior austenite grain structure. This effect has been termed the austenite ‘memory’. The presence of titanium carbide in the steel stabilizes the lath boundaries sufficiently for the acicular austenite growth to occur before lath boundary recovery and recrystallization processes dominate the microstructure. Preaustenitizing treatments in the range 650–700°C are effective in decomposing retained austenite and hence in promoting grain refinement on subsequent reaustenitization.
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