Thermal cycle simulation of a Ni-base superalloy

Abstract

Thermal cycle simulation techniques have been investigated as a method of studying the effect of cooling rate, from sub-solvus solution heat treatment temperatures, on the microstructure and fatigue properties of Ni-based superalloy specimens. A Smitweld thermal cycle simulator (TCS) and a Gleeble 1500 TCS were used during the investigation. Grain growth was observed, predominantly in the sample centre, in a number of specimens simulated using the Smitweld TCS as a result of severe temperature gradients set up across the specimen. It is important both to quantify and minimise this temperature gradient, to allow accurate heat treatment at the desired temperature. The discrepancy between surface and core temperatures, under different simulation conditions, was determined using three different specimen geometries for internal thermocouple placement. The optimum conditions to minimise the temperature overheat were found to be a highly polished specimen surface tested under high vacuum using a small (0.15mm) diameter K-type thermocouple for temperature control. The radial temperature distribution in the simulated zone was further investigated by microstructural examination of the supersolvus grain growth region in a series of samples simulated to a range of peak temperatures.