The Effect of Testing Variables and Microstructure on the Flow Stress and Dynamic Re-Crystallization of SCMV and Aermet 100 in Compression

The inertia welding process by has been recognized to yield very high magnitudes and rates of deformation with a wide temperature profile along the weld leading to near melting point at the weld interface. The elevated temperature properties of SCMV and Aermet 100 steels were studied by means compression testing on a single shot servo-hydraulic test frame. A wide combination of high ram velocities, strain rates and temperatures were used to mimic the conditions in inertia welding and determine true stress – true strain curve. Flow curves were used to calculate the Zener – Hollomon parameter (Z). The second derivative of the flow curves were used to find the critical strain for the onset of dynamic re-crystallization (DRX). The kinetics of dynamic re-crystallization were found by applying the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation and was used to find a relationship between fraction of re-crystallized austenite with respect to strain. Prior austenite grain (PAG) boundaries were revealed and studied by optical microscopy as a tool to study DRX and validate the JMAK equation. Strain, strain rate and temperature were found to be the main determining factors in controlling the flow stress. The austenite grain size was found influence the flow stress (Hall-Petch relationship) but the influence diminished at higher testing temperatures. This could invalidate Z since a constant microstructure cannot be assumed over the range of temperatures. Initial PAG size affected the final PAG size and the kinetics of DRX. Constant strain rate tests were prepared in a manner as to de-couple the PAG size with respect the flow stress. Flow curves showed flow softening by DRX whereas constant velocity curves showed persistent work hardening as the strain rate increased even though DRX was seen in the microstructure.