Microstructure Evolution Control and Performance Degradation of SA-178 Grade C Boiler Tubes Driven by Pearlite Spheroidization

SA 178 grade C carbon steel is a material commonly used in boiler tubes. Boilers are crucial in the energy industry; however, their service life degrades over time. If a boiler malfunctions, processing operations must be halted, resulting in financial losses for the company. The aim of this study is to examine the effect of microstructural evolution, especially the 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 involves the use of Small-Angle X-ray Scattering (SAXS) supported by metallographic analysis, Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray (EDX) Spectroscopy, and mechanical testing. The SAXS results indicate that the microstructure of SA 178, which initially consisted of ferrite and lamellar 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, as well as a reduction in hardness from 175 HV to 89 HV, ultimately decreasing the service life of the boiler tube.