Effects of post-heat treatment on the mechanical properties of additive manufactured 316L stainless steel and Inconel 625 super alloy

In this work, the laser powder bed fusion technique was utilized to fabricate samples from two different materials: stainless steel 316L (SS 316L) and Inconel 625 (IN625). The main aim of this study is to explore how the mechanical properties, specifically tensile strength and hardness, of these materials are affected by post-heat treatment. Annealing was chosen as the post-heat treatment method for this research. The investigation primarily focused on evaluating tensile strength and Brinell hardness (HRB) properties. Following post-heat treatment, both SS 316L and IN625 displayed a decrease in elongation and an increase in ultimate tensile strength (UTS) at lower temperatures, approximately 700°C and 900°C. As the annealing temperatures increased, elongation gradually improved while UTS decreased. Higher annealing temperatures, reaching up to 1100°C, tended to restore elongation to around 78% to 95% of the as-built conditions. In terms of hardness properties, both low and high-temperature annealing resulted in a reduction in hardness, with higher temperatures leading to a more significant decrease. The decline in HRB values ranged from 5 to 10 HRB across the lower and higher annealing temperature range. Moreover, elevating printing variables like layer thickness and scanning speed, in addition to post-heat treatment, led to a progressive decrease in mechanical characteristics. The most significant degradation is noted with an 80 µm layer thickness and 2000 mm/s scanning speed, resulting in a 92-94% decrease in elongation compared to as-built samples, and a reduction in UTS by 300-350 MPa for IN625 and 200-250 MPa for SS 316L.