Strain visualization of DED 316 steel via Digital image correlation and Fourier Transform analysis: effects of hydrogen and heat treatment

This study investigates the influence of heat treatment at different temperatures (600, 800, and 1200 °C) on the microstructure, dislocation stability, and element segregation of 316L stainless steel fabricated by direct energy deposition. To understand how these changes affect hydrogen embrittlement resistance and the mechanical properties of the samples, tensile testing and digital image correlation (DIC) were employed to examine the strain/instability evolution under combined conditions after heat treatment and hydrogen charging. Results show that dislocation densities decreased after heat treatment, which in turn increased the hydrogen embrittlement resistance of samples with emergence of recrystallization and twin grains. DIC revealed that heat-treated samples exhibited more strain hotspots, which initiated and developed more extensively compared to as-received (AR) samples. In comparison, AR samples showed fewer sites of strain accumulation, leading to wider crack propagation and earlier failure. Hydrogen-charged specimens exhibited narrower strain ranges and more localized strain distributions, indicating a shift toward brittle fracture behavior. In contrast, hydrogen-free samples exhibited higher mean magnitude values and spatial variation across the range of spatial frequencies, as confirmed by two-dimensional Fourier Transform analysis. The methods and analyses in this paper provide a new perspective on evaluating hydrogen embrittlement via demystifying strain/instability evolution.