Fatigue and Environmentally Assisted Cracking Behavior of Wrought and Additively Manufactured 17-4PH Stainless Steel

Additive manufacturing (AM) is an exciting technology that enables production of complex component geometries with just an AM machine and feedstock material. The potential logistical benefits and design freedoms offered by this technology have resulted in a recent explosion of AM research, investment, and utilization. For demanding applications, AM can produce high-strength components by incrementally melting/solidifying metal feedstock to build-up material layer-by-layer. One alloy of interest for metal AM is 17-4PH, a precipitation-hardening martensitic stainless steel. This alloy has been widely used in its traditional, wrought form since the 1940’s due to its combination of high-strength and moderate corrosion resistance. These properties have motivated significant microstructural and mechanical properties focused research on “AM” 17-4PH built using the laser powder bed fusion (LPBF) process. However, applications requiring an alloy like 17-4PH typically involve long service lives in aggressive environments. In these situations, failure can be governed by sub-critical environmentally assisted cracking (EAC) modes like corrosion fatigue and stress corrosion cracking (SCC); neither of which have received significant attention in AM 17-4PH research. Critically, EAC is notoriously sensitive to subtle microstructural variations, so AM-induced microstructural features like pores, secondary phases, solidification structures, etc. could be highly detrimental to AM 17-4PH EAC performance. The (i) lack of AM 17-4PH EAC research and (ii) unknown impact of AM-induced microstructural features on EAC performance serve as key knowledge gaps motivating this work.

Wrought 17-4PH can serve as both a benchmark and a baseline for AM 17-4PH. If wrought 17-4PH performance benchmarks can be met or exceeded, then a wide-range of opportunities are opened for AM 17-4PH. Further, careful processing of wrought and AM 17-4PH with similar compositions can allow direct comparison of the two material forms. These assertions and the aforementioned knowledge gaps led to five research objectives guiding this work. (1) Identify processing strategies to strength-match wrought and AM 17-4PH for comparison. (2) Determine the persistent microstructural differences between AM and wrought 17-4PH. (3) Quantify and compare wrought and AM 17-4PH SCC behaviors. (4) Quantify and compare wrought and AM 17-4PH corrosion fatigue behaviors. (5) Link differences in wrought and AM 17-4PH EAC behaviors to their persistent microstructural differences. To meet these objectives, the effects of various heat treatments (i.e., solution anneal, homogenization, hot isostatic press (HIP), peak age, and overage) on the microstructure and mechanical performance of LPBF AM 17-4PH were evaluated. With appropriate heat treatments determined, strength-matched AM and wrought 17-4PH SCC and corrosion fatigue performance was compared via fracture mechanics based crack growth rate testing. Test environments included humidified air (fatigue only) and 0.6M NaCl at various electrochemical potentials. Additionally, a study on the effect of residual stress and its effect on wrought 17-4PH fatigue crack growth behavior was performed as a supplement. Each of the performance evaluations were complemented by a variety of fractographic and/or crack path analyses to identify mechanisms and microstructural features responsible for the observed fracture behaviors.

Some key conclusions from this work are as follows. AM 17-4PH can be successfully strength-matched to wrought 17-4PH with a range of heat treatments. Of these, the most effective treatment for matching wrought microstructure and strength is two stages of >1000°C treatments followed by an aging heat treatment. Unfortunately, sub-micrometer porosity was persistently present in AM 17-4PH (i.e., in spite of HIP) and plays a critical role in its fracture behavior. This porosity, along with similarly sized oxide-inclusions, was held responsible for AM 17-4PH ductility reductions, degraded SCC performance, and degraded corrosion fatigue performance. These pore-fracture interactions were verified via qualitative morphological examinations, quantitative analysis of pore expansion and spacing, and quantification of microstructural deformation proximate to porosity. In addition to porosity concerns, prior austenite grain (PAG) boundaries in AM 17-4PH were much more susceptible to cathodic electrochemical potentials than wrought 17-4PH. The linkage of degraded AM 17-4PH performance with (1) sub-micrometer scale porosity/inclusions and (2) PAG boundaries constitutes the primary contribution of this work. These key microstructural features in AM 17-4PH can serve to either limit its utilization or act as targets for study/amelioration in future research.