Temperature induced microstructural changes in the nitride layers of nitrocarburized austenitic valves

Engine valves are subject to the effects of high temperatures during operation. In order to simulate the behavior of nitrocarburized valves, annealing trials were carried out on nitrocarburized valves made of X 50 CrMnNiNbN 21 9 and X 33 CrNiMnN 23 8. The structure of the nitride layers were found to change as a function of time and temperature. The microstructural transformations taking place in the nitride layers are illustrated by the results of metallographical investigations. They are confirmed through element depth profile analyses carried out by Glow Discharge Optical Spectroscopy (GDOS). The changes occurring in the nitride layers were also detectable through hardness measurements. The changes found to take place in the nitride layers as a result of exposure to high temperatures are now used as guidelines to estimate the temperatures to which valves have been exposed during operation.