Abstract
Different microstructures were obtained under various thermal conditions by adjusting the heat treatment parameters of the Cr-Co-Ni-Mo series of ultra-high strength stainless steel. The effect of organizational evolution on the stress corrosion cracking (SCC) of the Cr-Co-Ni-Mo series of ultra-high strength stainless steel was investigated using potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and other test methods in combination with slow strain rate tensile tests (SSRTs). The results show that the Mo- and Cr-rich clusters and precipitation of the Laves phase reduce the corrosion resistance, while increasing the austenite content can improve the corrosion resistance. The Cr-Co-Ni-Mo series of ultra-high strength stainless steel has a high SCC resistance after quenching at 1080 °C and undergoing deep cooling (DC) treatment at −73 °C. With increasing holding time, the strength of the underaged and peak-aged specimens increases, but the passivation and SCC resistance decreases. At the overaged temperature, the specimen has good SCC resistance after a short holding time, which is attributed to its higher austenite content and lower dislocation density. As a stable hydrogen trap in steel, austenite effectively improves the SCC resistance of steel. However, under the coupled action of hydrogen and stress, martensitic transformation occurs due to the decrease in the lamination energy of austenite, and the weak martensitic interface becomes the preferred location for crack initiation and propagation.
Highlights
Ultra-high strength stainless steel is widely used in aviation marine and other fields such as aircraft landing gear, wing girder and bolts due to its high strength, high toughness and good corrosion resistance [1,2,3]
When the potential is higher than 0.193 VSCE, a large increase in the current density indicates that the rupture of the local passivation film causes anodic dissolution and pitting
C, the electrochemical behaviour of the specimen changes significantly with the increasing holding time. It can be seen from the curve that the samples with holding times of 0.5 h and 4 h are passivated, while the specimens with holding times of 80 h are in a state of activation and dissolution, indicating that the corrosion resistance of the matrix is reduced by the long ageing treatment
Summary
Ultra-high strength stainless steel is widely used in aviation marine and other fields such as aircraft landing gear, wing girder and bolts due to its high strength, high toughness and good corrosion resistance [1,2,3]. The excellent mechanical properties of ultra-high strength stainless steel is attributed to martensitic transformation strengthening [4] and precipitation strengthening [5]. To obtain a higher strength, researchers have carried out considerable research, especially on the precipitation strengthening behaviour of ultra-high strength stainless steel with different strength grades. Cu-rich precipitates had a significant precipitation strengthening effect and that the tensile strength could reach 1240 MPa, while the overaging treatment coarsened the Cu-rich precipitates, which in turn led to a decrease in the strength of the steel. It is of great practical significance to study the SCC of ultra-high strength stainless steel
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