Effect of Heat Treatment on Mechanical/Metallurgical Properties of Direct Metal Laser Sintered 17-4 Precipitate Hardened Stainless Steel

Abstract

Greatbatch Medical, specializing in orthopedic implants and instruments, is currently utilizing direct metal laser sintering (DMLS) technology to develop concept prototypes. Greatbatch uses EOS GP1 Stainless Steel which adheres to the American standard for 17-4 Precipitation Hardened SS. Following DMLS, Greatbatch heat treats its parts to obtain desirable mechanical properties. In this study, three different heat treatment methods were examined: heat treatment and stress relief (HT+SR), stress relief (SR), and solution aging and annealing (SAA). The Vickers Hardness Test and the Tensile Strength Test were used to assess the mechanical properties of each sample. The research also included grain structure analysis using a Scanning Electron Microscope and surface roughness studies via profilometer measurements. For example, the HT+SR sample yielded 456 HV (hardness), an ultimate tensile strength of 1319 MPa, an yield strength of 1120 MPa, and 6.36% elongation. It was found that compared to the untreated sample, HT+SR decreased the total elongation by 73% and SAA decreased total elongation by 17% and additionally decreased hardness by 17%. It was learnt that stress relieving the part after DMLS was the superior method of choice based on its resulting mechanical properties. It was found that the grain structure of the non-treated sample resembled a solution treated sample and the stress-relieved sample actually matched an age-hardened sample. The sample that was precipitation hardened was actually over-aged. Thus it was found that the DMLS process seemed to be acting as an aging process while simultaneously building the part. Further studies in examining specific effects of DMLS and how it impacts what order heat treatments should follow would be appropriate.