Study of Cryogenic Treatment on the Microstructure and Mechanical Properties of Marine 10Ni5CrMoV Steel

In this study, the multi-stage heat treatment involving quenching (Q), lamellarizing (L), cryogenic (C) and tempering (T) is applied in marine 10Ni5CrMoV steel to study the microstructure and mechanical properties by multi-scale characterizations. Strengthening mechanism, strain hardening behavior and cryogenic toughening mechanism are further investigated. The results indicate that cryogenic treatment induces lattice distortion in martensite, resulting in significant internal stress. This process leads to the shrinkage of the laths, thereby reducing the equivalent grain size of the specimen. The combined cryogenic and tempering process induces continuous martensitic transformation, resulting in a reduction of the austenite volume fraction and dislocation density. The strong internal stress accelerates the movement of defects (dislocations) to grain boundaries. It promotes the precipitation of numerous alloy elements in the form of carbides at these defects, causing a significant decrease in solution strengthening. This is the primary reason why the yield strength of the QLCT specimen is reduced from 889.5 ± 7.8 MPa to 838.5 ± 7.4 MPa compared to the QLT specimen. The QLCT specimen exhibits a single stage strain hardening behavior during plastic deformation and possesses strong strain hardening ability and good machinability. This is attributed to the interaction between fine precipitates and dislocations. During the cryogenic impact process, secondary lattice distortion occurs in the QLCT specimen, further refining the precipitates. This enhances the pinning effect on grain boundaries and dislocations, improving the ability to hinder crack propagation. Consequently, the impact energy of the QLCT specimen increases from 264.3 ± 6.4 J to 278.7 ± 6.3 J at -84 °C, compared to the QLT specimen.

Graphical Abstract