RECRYSTALLIZATION STUDIES OF A LOW ALLOY STEEL IN THE (a +y) PHASE REGION

Abstract

An investigation of the technical viability of alloy development via thermomechanical processing, in steels, has been undertaken; by: a systematic study of the recrystallization kinetics in a low alloy steel in the (a+y) phase transformation region. Two starting point microstructures (pearlite and tempered martensite) were prepared from as-received samples by the well known normalisation and tempering heat treatment processes. The initial microstructures, were variously deformed by rolling and then subjected to recrystallization annealing at 740°C and 760°C. The soaking times were so chosen that the formation of the following microstructures was possible: recovered microstructures; partially recrystallized microstructures; and fully recrystallized microstructures. For each of the specimens, the start and progress of recrystallization and austenite precipitation were followed up by means of optical microscope as well as hardness measurement. The results obtained show that varied microstructures can be produced and characterised, by manipulation of the test parameters (initial microstructure, degree of cold deformation, recrystallization annealing temperature, recrystallization annealing time), which were observed to influence the kinetics of the recrystallization/austenite precipitation process. In all, well defined duplex structures of recrystallized ferrite and martensite, having average grain sizes of 27.5Jlm and IO.6Jlm, were obtained from the specimens having the pearlite/ferrite structure, deformed to a degree of eighty percent (80%), and subjected to annealing at 760°C for 30 minutes and 1 hour respectively, before quenching in water.