October_2021_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 1 2 0 key components of each subsystem with linkages representing a model- lable dependency [3] . The genesis of the Ferrium C64 al- loy design was the establishment of ag- gressive property goals as defined by the U.S. Navy in a Small Business In- novation Research (SBIR) proposal so- licitation in 2005 seeking to replace traditional steels such as AISI 9310 or Alloy X53. Specifically, this called for a surface carburizable steel—up to 64 HRC—with improved core properties and increased temperature resistance. As such, during the design pro- cess, a double vacuum melted second- ary hardening, martensitic steel was identified as the most promising ma- terial class to achieve these lofty goals. Further, the material was designed to be carburized to develop ultrahard sur- face properties. Thermo-Calc software was used to execute the mechanistic, thermodynamics-based approach to calculate key microstructural aspects. These microstructure predictions fed proprietary structure-property models to complete the material design frame- work as follows: Optimal material composition: With the material processes and micro- structural concepts identified, Ques- Tek’s materials design engineers set forth to develop the precise and op- timal material composition. In effect, FerriumC64 comprises twomaterial de- signs: one composition within the core material and a second composition for the case. The variation be- tween the core and case evolves during the carbu- rization process, resulting in a case exhibiting an en- riched carbon content for hardness. Processing: The specif- ic alloying additions were identified and chosen to establish a microstructure consisting of a martensit- ic matrix with an optimized strengthening carbide dis- persion that derives from a solutionize, quench, and age heat treat- ment processing route. Overall alloy content was constrained by the process- structure linkages outlined in Fig. 1. Maximum solution temperature was limited by a combination of process ca- pability within the manufacturing base and the desire to maintain a fine grain structure in the final product. The design called for high tem- perature vacuum carburization as a key processing step that would combine solutionizing and carburization into a single operation. Next, a sufficiently high martensite start (Ms) temperature was necessary to ensure nearly full mar- tensitic transformation upon quench- ing from the solution treatment (the overall alloy content was designed to ensure suitable Ms temperature). Last- ly, the strengthening carbide phase fraction and its nucleation and growth parameters had to be optimized to en- sure a reasonable aging temperature and time. Alloying additions: Each elemental alloying addition was designed to play a crucial role in either the material ma- trix or precipitate dispersion. Ni alloy- ing was incorporated to improve matrix toughness, while Co was included to re- strict dislocation recovery during heat treatment, resulting in a martensit- ic matrix with high dislocation densi- ty. This dislocation network provides heterogeneous carbide nucleation sites that help increase the carbide nu- cleation driving force to ensure a fine strengthening carbide dispersion. While the C content sets the overall phase fraction of the strengthening carbide, the number density and precipitate size within the matrix plays a crucial role re- garding mechanical properties. Carbide forming Mo, Cr, V, and W contents were designed to achieve a 3-nm-sized car- bide volume fraction after aging that optimizes the carbide strengthening ef- fect. Additionally, the carbide forming elements are significantly overbalanced compared to the core carbon content to enable greater carbide formation in the higher carbon containing case region. Further, the aging treatment was selected based on the tradeoff between nucleation driving force, inversely pro- Fig. 1 — Systems design chart for FerriumC64 highlights critical processing-structure-property relationships. Rack of gears undergoing vacuum heat treatment at Solar Atmosphere’s Souderton, Pa., facility. Courtesy of Solar.