Effect of intrinsic heat treatment in wire arc additive manufacturing on grade 91 steel: microstructure and mechanical properties

This study investigates the impact of intrinsic heat treatment (IHT) on the microstructure, mechanical properties, fatigue performance, and impact toughness of grade 91 steel fabricated using wire arc additive manufacturing (WAAM). WAAM’s layer-by-layer construction inherently subjects materials to repeated thermal cycles akin to heat treatments, significantly affecting the microstructure and mechanical performance. Through detailed analysis, including microstructural examination with electron microscopy, residual stress measurement via X-ray diffraction, fatigue testing, and impact toughness evaluation, the research identifies a notable refinement in microstructure and distribution of strengthening carbides (M₂₃C₆) across the deposited layers. Mechanical testing reveals variations in hardness, tensile strength, and fatigue resistance across different layers, demonstrating how IHT enhances material performance, particularly in the bottom layers where toughness and fatigue resistance are maximized.