Comparison of Single and Multiple Pass Compression Tests Used to Simulate Microstructural Evolution During Hot Working of Alloys 718 and 304L

Abstract

Microstructural evolution during primary breakdown of alloy 718 by radial forging was studied via axial compression testing of cylindrical specimens taken from a wrought and solution heat treated bar with an average grain diameter of 254 pm. Tests were performed at a constant true strain rate of 1 s-l and at temperatures of 950°C 105O”C, and 1150°C. Total plastic strains of 0.56 and 0.92 were applied either in one pass or by four consecutive passes. The strain per pass for each four-pass sequence was either 0.14 or 0.23. Dwell periods between passes and after the final pass were utilized to simulate the deformation-time history of a typical production workpiece. The dwell time applied after compression and before the quench in the single-pass test equalled the sum of the individual dwell periods of the four-pass test (240 s). The single-pass tests were found to underestimate the grain refinement at all deformation temperatures and overestimate the volume recrystallized at the lowest temperature. The relatively fine grain size of the four-pass material is attributed to the repetitive static recrystallization that occurs during back to back dwell periods. Softening in the stress-strain curve at high strain and extensive static recrystallization following deformation of single-pass material, deformed at 95o”C, was observed and attributed to deformational heating (approx. 60°C). Deformational heating and its effects were moderated during the four-pass tests by heat dissipation during the dwell periods (70 s) between passes. Alloy 304L was also tested and its behavior was found to be generally similar to that of alloy 718 at or above 1050°C. It is concluded that the multiple-pass compression test provides a better technique for simulating microstructural evolution during primary breakdown of ingot material by radial forging. Superalloys 718,625,706 and Various Derivatives Edited by E.A. Loria The Minerals, Metals & Materials Society, 1994