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| Effect of Deep Cryogenic Treatment on Microstructure
and Mechanical Properties of 18Ni(200) Maraging Steel |
| LI, Xiao-Lin |
| State Key Laboratory of Solidification Processing,
Northwestern Polytechnical University, Xi’an 710072, China |
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Cite this article:
LI, Xiao-Lin. Effect of Deep Cryogenic Treatment on Microstructure
and Mechanical Properties of 18Ni(200) Maraging Steel. Acta Metall Sin, 0, (): 0-.
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Abstract With advances in
cryogenic engineering, increasingly stringent performance requirements are
imposed on cryogenic structural alloys. Conventional heat treatments are often
inadequate to satisfy the demanding performance requirements of 18Ni maraging
steel used in aerospace and other high-end applications, creating a need for
alternative heat-treatment strategies. Although deep cryogenic treatment has
been reported to enhance the properties of various steels, its effects on
maraging steels remain insufficiently understood. Accordingly, this study
investigates the influence of cryogenic treatment temperature and duration on
the microstructure and mechanical properties of 18Ni(200) maraging steel. The
material was subjected to a combined process comprising solution treatment,
cryogenic treatment, and aging. After solution treatment at 800 °C for 1 h,
cryogenic treatments were performed at −78 °C for 4, 8, and 12 h, and at −196°C
for 12 h, followed by aging at 550 °C for 4 h. Mechanical properties were
evaluated using tensile testing and hardness measurements, while
microstructural evolution was characterized by XRD, EBSD, and TEM. The results
indicate that deep cryogenic treatment not only promotes the transformation of
reversed austenite into martensite and refines martensitic laths, but also
induces lattice distortion in martensite. This distortion is manifested by high
stress concentrations and a high dislocation density, which increase the
internal energy of the material. During subsequent aging, the release of this
stored energy enhances the diffusion driving force of alloying elements,
thereby promoting the formation of fine precipitates with a high number
density. Among the investigated conditions, the specimen cryogenically treated
at −78 °C for 12 h exhibited the best overall performance, achieving a yield
strength of 1778.9 MPa, a hardness of 484.9 HV, and an elongation of 7.2%. In
comparison, specimens treated at −196 °C showed a lower content of reversed
austenite after aging. The lower cryogenic temperature induced greater lattice
strain in the martensite, leading to the accumulation of a large amount of
internal (potential) energy within the material. During subsequent aging, this
stored energy was rapidly released, leading to strain recovery, a reduction in
dislocation density, and a decrease in both the size and number density of
precipitates. These effects ultimately resulted in lower strength and ductility
compared with specimens treated at −78 °C. At −78 °C, increasing the cryogenic
treatment duration progressively reduced the content of reversed austenite and
refined its morphology. Concurrently, the fraction of low-angle grain
boundaries and the precipitate size decreased. These microstructural evolutions
enhanced grain-refinement and dispersion strengthening, resulting in
significant increases in the strength and hardness of the steel. Overall, deep
cryogenic treatment effectively optimizes the microstructure and mechanical
properties of 18Ni(200) maraging steel by promoting martensite refinement and a
favorable precipitate distribution. However, excessively low cryogenic
temperatures may accelerate strain recovery during aging and diminish
strengthening effects. This study provides practical guidance for designing
cryogenic treatment protocols for high-performance maraging steels intended for
cryogenic applications.
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Received: 08 May 2025
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