Microstructural Evolution in Additively Manufactured Duplex Stainless Steel Alloys
Restricted (Penn State Only)
- Author:
- Iams, Andrew
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 01, 2021
- Committee Members:
- Tarasankar Debroy, Major Field Member
Allison Beese, Major Field Member
Todd Palmer, Chair & Dissertation Advisor
Lucille Giannuzzi, Special Member
Robert Voigt, Outside Unit & Field Member
John Mauro, Program Head/Chair - Keywords:
- additive manufacturing
duplex stainless steel
microstructure
mechanical properties - Abstract:
- Increasing demand for high performance additively manufactured (AM) components is driving demand for more alloy systems beyond the rather limited number currently in wide use. Duplex stainless steels, which offer high strength and superior corrosion resistance at a lower cost than equivalent austenitic stainless steels, are one of these emerging alloy systems. Their attractive properties are achieved in the wrought condition through a series of highly controlled thermo-mechanical and heat treatment steps designed to obtain a balanced ferrite/austenite microstructure. Although duplex stainless steels are used in petroleum, marine, and nuclear power applications, they can fail when subjected to high temperature service environments, which disrupt the balanced ferrite/austenite microstructure and lead to the precipitation of deleterious secondary phases. This balanced ferrite/austenite microstructure is especially susceptible to disruption in fusion-based AM processes, such as powder bed fusion and directed energy deposition. During each of these AM processing routes, the layer-by-layer deposition methodology produces a complex series of rapid heating, melting, solidification, and cooling cycles. Since the duplex stainless steel compositions were originally designed for wrought processing routes, the complex thermal histories characteristic of AM processes make it difficult to obtain the balanced ferrite/austenite microstructure and impact the resulting microstructure and properties. The impact of the complex thermal histories on microstructural formation across different grades must be understood for wide implementation of duplex stainless steel components fabricated through AM processing. In order to explore the feasibility of processing these alloys in this manner, a laser-based directed energy deposition process was used to fabricate lean (UNS S32101), standard (UNS S32205), and super (UNS S32507) duplex stainless steel structures, and their microstructural formation was investigated. The overall austenite levels were comparable to those found in wrought alloys, with austenite phase fractions ranging from 16.1% ± 1.1% in the lean, to 38.5% ± 1.6% in the standard, and 58.3% ± 0.1% in the super duplex stainless steel grades, respectively. Unlike the wrought condition, the austenite present in the as-built structures displayed a range of different morphologies, with intragranular austenite comprising between 55% and 76% of the austenite present within each build. This intragranular austenite primarily nucleated from oxygen-rich inclusions that originated from the powder feedstock and served as heterogenous nucleation sites during the deposition process. Oxygen levels, on the order of five times higher than that present in comparable wrought forms, led to the formation of these oxide phases in the powders as well as the as-built and hot isostatically pressed (HIP) conditions. These oxide inclusions exhibited complex structures with a combination of amorphous, metastable, and stable phases across these different conditions. In the powder particles, which experienced rapid cooling rates during the gas atomization process, amorphous inclusions that were rich in Mn, Cr, Si, and oxygen were observed and surrounded by small crystalline MnS particles. After additive manufacturing, these inclusions transformed to a combination of rhodonite (MnSiO3) and spinel (MnCr2O4) with amorphous regions around the exterior. Post-process hot isostatic pressing treatments resulted in the formation of a stable spinel oxide with MnS particles around the exterior, matching the results predicted by computational thermodynamic calculations. Even with these different microstructural features, room temperature tensile properties of additively manufactured duplex stainless steels are often similar to and even exceed those observed in the wrought condition. In the AM material, the finely dispersed ferrite and austenite phases produced a ductile failure mode and appear to be unaffected by the presence of the oxide inclusions. However, these testing conditions do not capture the performance of these alloys at high strain rates and low temperatures. At these low temperatures, impact energies for the AM fabricated materials were significantly degraded, falling well short of the wrought material requirements in both the as deposited and HIP conditions. This loss of properties is connected to the fine distribution of austenite and the presence of the oxide inclusions in the as deposited and post-processed microstructures. Significant changes in the fracture appearance were evident, with the largely ductile surface present under room temperature conditions becoming one with a combination of ductile and brittle features at the low temperatures. In both the standard and super grades, the brittle fracture features appeared largely in the ferrite phase, while the austenite phase retained a ductile failure appearance.