Effect of Isothermal Ageing on Microstructure and Mechanical Behavior of 0.5Cr-0.5Mo-0.2V Low Alloy Steel

Abstract

Cr-Mo-V based low alloy steels are widely used in thermal power plants because of their ability to withstand elevated temperature and high pressure under continuous operation. With ferrite-pearlite and/or ferrite-bainite microstructures, these steels find their applications in boiler tubes, pressure vessels and steam turbines. Being cost effective is one of the important factors, which draws attention of many industrialists towards low alloyed steels over highly alloyed steels with austenitic matrix. In this work, the microstructure and mechanical properties degradation of low alloy steel sample were analyzed. Investigated material was in service for 35 years. During its service life, it was exposed to high temperature and pressure resulting in properties to undergo changes. To figure out these evident changes, sample was solutionized and tempered for different ageing times under laboratory conditions. Changes in the morphology of the steel microstructure such as pearlite decomposition, carbide precipitation and spheroidisation etc. could be observed. Due to its coherent relation with microstructure, mechanical properties were also found to have undergone variations. In order to investigate the behavior of this steel after its exposition to different environments, hardness and tensile tests were conducted. Microstructural evolution of the sample was observed using SEM. TEM analysis was done to figure out the nature of carbides formed during experimentation. Experiments revealed the improvement in mechanical properties of the steel in the initial hours of its service due to the phenomenon of secondary precipitation. But, after reaching its peak condition (over-ageing) deterioration in the properties started.