Phase transformations and mechanical properties of thermomechanically processed 34CrMo4 steel

Precipitation hardening ferritic pearlitic (PHFP) grade 34CrMo4 steel is subjected to thermomechanical processing (TMP) for e = 0.2, e = 0.4 and e = 0.6 followed by furnace-, natural air- and forced air- cooling. Optical microscopy revealed the ASTM grain size number to change from 6 for the starting sample to 5–6 for the furnace cooled, 7–8 for natural air cooled and 8–9 for forced air cooled samples. A transformation from initial ferrito-pearlitic banded microstructure to almost equiaxed coarse grained ferrito-pearlitic microstructure on furnace cooling, finely distributed ferrito-bainitic microstructure on natural and forced air cooling respectively was noted. For natural and forced air cooling, the samples showed fine grained microstructure comprising of ferrite along with bainite, which became finer with increasing strain as well as cooling rate. The occurrence of ferrito-bainitic microstructures got clearly resolved in scanning electron microscopy (SEM). The microstructure of as received annealed sample in SEM revealed proeutectoid ferrite, almost linear uniformly spaced ferrite and cementite lamellae inside pearlitic nodule and uniformly distributed carbides throughout the matrix. The carbide size, otherwise remaining unchanged around 40 nm, got reduced to 23 nm only in case of forced air cooling. Extreme TMP conditions produced ferrite and broken fine colonies of bainite. The pearlite and bainite morphology assumed various forms of interlamellar spacing based on TMP. The forced air cooled (e = 0.6) samples demonstrated the maximum improvement in yield strength (YS = 700 MPa) by almost more than 2.5 times and in ultimate tensile strength (UTS = 790 MPa) by almost 40%. A maximum improvement in toughness (= 54 Joule) by almost 10% without loss of any other properties was observed for the TMP comprising of e = 0.4 and furnace cooling.