Effects of process variables on the structure and properties of hot rolled 718 bar

Abstract

An experiment was statistically designed to determine the effects of ten variables on the microstructure and mechanical properties of alloy 718 bar produced on Allvac's state-of-the-art, continuous, 16-stand rolling mill. A series of 32 rolling trials was conducted in which each variable was set at one of two levels. Microstructures and mechanical properties were determined on samples taken after the material exited selected stands. Surface temperatures of the moving metal were measured with pyrometers at six locations along the rolling mill. The grain size and mechanical properties after stand 4 are determined largely by the initial billet temperature. As rolling progresses, the speed of the finishing portion of the mill becomes increasingly important until it finally becomes the dominant factor. Correlations exist between surface temperature, microstructure, and mechanical properties; these include the influence of delta phase distribution on stress-rupture properties. Experimental results agree well with predictions of both surface and internal temperatures made with a steady state, finite element model. The model is now available for optimizing rolling procedures.