Influence of Heat Treatment on the Microstructure and Hardness of 17-4PH Stainless Steel Fabricated Through Direct Energy Deposition

AISI 630 stainless steel (ASTM A564-89, 17-4PH) is widely used in die-casting molds owing to its excellent wear and heat resistance. Recently, a cooling strengthening technology that densifies the microstructure of the casting through rapid cooling has been developed. Additive manufacturing can be used to fabricate casting molds with complex three-dimensional cooling channels. 17-4PH stainless steel, a martensitic precipitation hardening steel, can be subjected to heat treatment to improve its mechanical properties, which are highly dependent on its microstructure. Specifically, the formation of martensite and δ-ferrite, and the coarsening of Cu-rich precipitates considerably decreases the hardness of 17-4PH stainless steel. In this study, we investigate the microstructural evolution of 17-4PH stainless steel during aging and solution heat treatment processes and determine their effect on the formation of martensite and δ-ferrite. Furthermore, the effect of heat treatment on the microstructure and hardness of the steel is studied experimentally. Accordingly, three specimens—as-built, H-1150-M (aging heat treatment), and SH-1150-M (solution and aging heat treatment)—were analyzed and compared herein. The results revealed that the martensite fraction was the highest in the aged H-1150-M specimen, resulting in a high hardness. In contrast, in the SH-1150-M specimen, the austenite and δ-ferrite fractions were higher than the martensite fraction, resulting in a lower hardness than those of the other two specimens. Therefore, aging heat treatment without solution heat treatment can effectively increase the hardness of additively manufactured 17-4PH stainless steel. Graphic Abstract