Based on the Thermomechanical Control Process–Ultrafast Cooling–Intercritical Annealing and Tempering Process: Effect of Intercritical Annealing Temperature on the Microstructure and Mechanical Properties of Cr‐Alloyed 5.5% Ni Steels

Abstract

An innovative process (thermomechanical control process–ultrafast cooling–intercritical annealing and tempering, TMCP‐UFC‐IAT) is used to prepare 5.5% Ni steel. The evaluation of microstructure and mechanical properties of Cr‐alloyed 5.5% Ni steels at various intercritical annealing temperatures is studied. Higher intercritical annealing temperatures enhance lath fresh martensite formation, refine grain size, and facilitate granular reversed austenite during tempering. During the intercritical annealing, alloying elements concentrate along the γ/α interface, resulting in most alloy‐rich austenite transforming into fresh martensite and stabilized reversed austenite after quenching. After tempering, alloying elements diffuse into the reversed austenite, enhancing its stability. Heat treatment and optimized composition regulate the ratio of soft/hard phases, grain size, and boundary angles, further enhancing reversed austenite stability. This approach synergistically combines grain refinement strengthening, dislocation strengthening, and grain boundary strengthening to achieve an excellent balance of strength and cryogenic toughness. After intercritical annealing at 700 °C, the tensile strength exceeds 860 MPa, and the impact energy at −196 °C is greater than 180 J, resulting in excellent mechanical properties.