Influence of Composition on the Post-Processing Response of Additively Manufactured Precipitation Hardening Stainless Steels
Restricted (Penn State Only)
- Author:
- Shaffer, Derek
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 24, 2023
- Committee Members:
- John Mauro, Program Head/Chair
Jay Keist, Outside Field Member
Amrita Basak, Outside Unit & Field Member
Tarasankar Debroy, Major Field Member
Allison Beese, Major Field Member
Todd Palmer, Chair & Dissertation Advisor - Keywords:
- Additive Manufacturing
Precipitation Hardening
17-4 PH
Martensitic Stainless Steel
Solution Heat Treatment
Aging - Abstract:
- Moving from traditional thermomechanical processing to additive manufacturing (AM) of precipitation hardening (PH) stainless steels has introduced unexpected differences in the heat treatment response that produce wide variations mechanical properties. Changes in powder compositions and AM processing conditions have been identified as the primary causes since they produce different as-deposited microstructures within the same alloy system. In PH grade stainless steels, differences in nitrogen composition have been identified as the major contributor. For example, with low nitrogen levels on the order of 0.01 wt%, large ferritic grain structures with small amounts of a high temperature delta-ferrite existing along the grain boundaries are produced. On the other hand, high nitrogen levels on the order of 0.1 wt% or greater result in large amounts of retained austenite (81 vol.%). In order to form a more consistent microstructure and evenly disperse critical elemental constituents, such as copper, high temperature heat treatments precede aging in the typical two-step heat treatment. During these high temperature heat treatments, which are on the order of 1040 °C, the goal is to form a complete solid solution in a stable austenite phase, but often overlooked, high temperature carbonitrides also form. The high temperature M(C,N) consists primarily of Nb and Cr which favors CrN for the high N compositions and NbC for the low N compositions. Changing the composition and volume fraction of the M(C,N) and therefore the Cr, Nb, C, and N in the steel by changing the overall nitrogen content can cause differences in the high temperature heat treatment response. The influence of the M(C,N) has not been evaluated with respect to its impact on mechanical properties or aging response. To evaluate the impact of composition on high temperature heat treatment responses, multiple 17-4 PH stainless steel compositions spanning ranges of Cr (15.2-16.5 wt%), Mn (0.13-0.72 wt%), and N (0.025-0.142 wt%) contents were characterized. In the low nitrogen compositions, the hardness remained consistent across all heat treatment times and temperatures, but with higher nitrogen composition, hardness varied with solutionizing conditions. The main drivers for hardness fluctuations in high nitrogen materials during high temperature heat treatments are retained austenite fractions and solid solution strengthening. Decreasing the austenite fraction relative to the as-deposited condition causes an initial increase in the hardness (about 100 HV). Solid solution strengthening then results in continued hardening (variations on the order of 70 HV) which varies with nitrogen content in the martensite as a function of M(C,N) growth and dissolution with changes in solutionizing conditions. Microstructures formed through solutionizing also impact the aging response. While there is no noticeable change in the copper precipitates, there is still rather wide variations in aged properties and aging response. Evaluating tensile properties added insights into the changing microstructures that are not captured in the typical hardness and retained austenite measurements. After solutionizing and aging, the breakdown of the ferritic structures improved elongation in comparison to the aged ferrite that was characteristic of the directly aged material. With respect to the impact of composition, higher required aging temperatures for the high nitrogen alloys has been identified and evaluated so specific solutionizing conditions as well as aging heat treatments can be selected based on the material composition and targeted properties. It is clear that solutionizing and aging heat treatments must be adapted to incoming material composition. To adequately adjust the heat treatments, a composition-based metric to guide appropriate solutionizing and aging is developed based on thermodynamic and empirical results. While nitrogen content varies significantly and has been the primary focus, this metric and heat treatments are based on the entire composition. While solutionizing and aging have been investigated in detail, hot isostatic pressing (HIP) presents a separate issue. HIP is commonly used in AM to heal defects (at 1185 °C for PH stainless steel) and is typically used in addition to solutionizing in PH grade stainless steel, but it has been proposed as a replacement for solutionizing. HIP provided no clear trend or apparent improvement over solutionizing, but the formation of unexpected copper precipitates in both the low nitrogen and higher nitrogen alloys was observed due to slower cooling (10 °C/min compared to 10 °C/sec). Solution heat treatments after standard HIP were found to result in properties that align with the materials that were only solutionized. HIP with uniform rapid cooling (URC) was used in an attempt to increase the cooling rates directly from HIP to avoid copper precipitation, but similar properties to HIP were observed after HIP with URC. Again, solutionizing following HIP with URC results in properties that also align well with the other solutionized materials. While HIP and HIP with URC present interesting options for microstructural development, it is recommended that solutionizing is still performed after any HIP heat treatment.