Mechanism of Service Life Degradation on SA 178 Boiler Tubes Caused by Microstructure Evolution

Carbon Steel SA 178 grade C is a common material used for boiler tubes. A boiler is a crucial unit in the energy industry, meanwhile its services life can be degraded after long term operation. If it malfunctions, the processing operations must be halted, resulting in financial losses for the company. The aim of this study is to examines the effect of microstructural evolution especially transformation of lamellar pearlite into spheroidized pearlite on the service life degradation of boiler tubes. Understanding these changes is essential for preventing catastrophic system failures. The methodology of this study involves the use of Small-Angle X-ray Scattering (SAXS), supported by metallographic analysis, Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDX), and mechanical testing. The SAXS results indicate that the microstructure of SA-178, initially consisting of lamellar ferrite and pearlite, gradually transforms into spheroidized pearlite. These microstructural changes lead to reductions in tensile strength from 523 MPa for 0% spheroidization to 335 MPa for 100% spheroidization, and hardness from 175HV to 89 HV, ultimately decreasing the service life of the boiler tube.