Degradation of microstructure and strength properties of heat-resistant steels operating under variable loads

Abstract

The paper presents the results of the structure and mechanical tests of the low-alloy steel 14MoV6-3 which shows internal damage after long-term creep service. The tests of mechanical properties, including: tensile and yield strength at room temperature, yield strength at elevated temperature (+540 °C) and brittle transition temperature, were carried out on longitudinal samples in accordance with the requirements of standards for elements pressure equipment. The internal damage of the tested material was also classified depending on the degree of wear, based on the principles set out in the industry specification. It was shown that the strength properties of the tested material from pipelines operated under primary steam conditions decrease with increasing service time. Microscopic tests (Scanning Electron Microscope SEM) revealed that the cause of damage to steel with a ferrite-bainite structure after long-term operation is the cavitation cracking mechanism. The study of the microstructure was also carried out by means of the backscattered electron diffraction method EBSD using the IQ, KAM and GOS functions. These parameters are very important in terms of analysis of the occurring structural changes in the fatigue and creep processes. They allow to determine the deformed part of the substructure subjected to these processes.