Simulation of Residual Stresses in Castings

Abstract

Unpredictable residual stresses are in most components considered to be a defect. The stresses are formed during the cooling process, caused by the temperature and visco plastic strains. Performing a heat treatment (HT), the material starts to creep and residual stresses can be reduced with over 90% of the total initial stress. This study has been done in order to examine residual stress relieving in the HT for three cast iron materials, VIG-275/190, GJL-250 and GJS-500-7. The purpose is to find the effect of varying parameters and compare the result against simulation. A stress lattice component, designed to create residual stress are used to investigate the stress relieving. Through a sectioning method the stress is released and can be measured by strain gauges placed on the surface. The stresses are measured on each material both as cast and after HT and trough empirical testing the effect of different parameters during the HT was established. Comparing the practical test against simulations by Magma5 the cooling rate during solidification process is too quick in simulations. This fault is attributed to the non-included latent heat release during phase transformation at 723 °C (austenite to pearlite). By changing the specific heat capacity of sand and the cast iron this error can be corrected. The simulations also predict the stress to be fully relieved when reaching the hold temperature of 610 °C, this have been confirmed to be wrong shown by the significant effect of hold time in all investigated materials. The effects of varying cooling rates and drop temperatures are also difficult for the simulations to predict. Heat treatment experiments on of VIG-275/190 shows that the alloying of Molybdenum and Chromium makes the material more resistant against creep at elevated temperatures. The hold time and time spent over 500 °C are the most significant parameters. The unalloyed GJL-250 creeps more easily which makes all the heat treatment parameters more important, i.e. heating rate, hold time and cooling rates. Lastly the ductile iron GJS-500-7 has the highest residual stress in as cast condition and shows the largest stress relief after the heat treatments.