Experimental modal analysis of distinguishing microstructural variations in carbon steel SA516 by applied heat treatments, natural frequencies, and damping coefficients

Abstract

The life assessment of materials that structurally shifts, creating mechanical corrosion and damage, during the operation at high temperatures is one of the most critical areas in the gas turbine power plants. This study investigates the widely used carbon steel grade 55 SA516 in the gas turbine blades at metallographic microscopic level and relating it to natural frequency and damping coefficients. In which heat treatments (aging tests) were applied and compared between multiple samples. The results show pearlite is broken up and converted into the ferrite and spherical carbides at grain boundaries. With an increase in microstructural variations in samples due to heat treatments, the first mode of natural frequency slightly decreases but damping ratio increases significantly. In addition, the experimental results show that by increasing the heat treatment time, the Young's modulus decreases by 10.74% and the natural frequency of the second to sixth states of carbon steel also decreases between 4.14 and 4.59%, respectively. As such, the damping coefficients of the second to sixth states increased between 5,609 and 6391 times than their original values, and no connection was obtained between the vibration number and the damping coefficient.