Evaluation of Mechanical Properties and Microstructures of Direct‐Quenched and Direct‐Quenched and Tempered Microalloyed Ultrahigh‐Strength Steels

• Published in: Steel research international Vol. 92; no. 3
• Main Authors: Hannula, Jaakko, Kaijalainen, Antti, Porter, David A., Somani, Mahesh C., Kömi, Jukka
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.03.2021
• Subjects: direct quenching; Dislocation density; High strength steels; high-strength; Mechanical properties; Microalloying; microstructural characterization; Molybdenum; Niobium; Precipitates; Precipitation hardening; Quenching and tempering; Tempering; thermomechanical processing; Transition temperature; X-ray diffraction; Yield strength; Yield stress
• ISSN: 1611-3683; 1869-344X
• DOI: 10.1002/srin.202000451

Herein the effects of molybdenum and niobium on the microstructures and mechanical properties of laboratory‐rolled and direct‐quenched and direct‐quenched and tempered steels are revealed. The microstructures are martensitic with yield strength of 766–1119 MPa in direct‐quenched condition and 632–1011 MPa in direct‐quenched and tempered condition. Mo and Nb additions lead to a fine martensitic microstructure that imparts a good combination of strength and toughness. Steel with 0.5 wt% molybdenum has a high yield strength of 1119 MPa combined with low 28 J transition temperature of −95 °C in direct‐quenched condition. Molybdenum and niobium increase the strength significantly during tempering due to enhanced solute drag and precipitation hardening. Addition of 0.25 wt% molybdenum increases yield strength from 632 to 813 MPa after tempering. However, the combination of niobium and molybdenum results in even greater increase in yield strength during tempering compared to the nonalloyed version, producing almost 400 MPa increase in yield strength. Transmission electron microscopy reveals that only niobium forms stable precipitates during tempering, indicating that molybdenum largely remains in solution. X‐ray diffraction analysis elucidates that molybdenum and molybdenum–niobium alloying prevents annihilation of dislocations, leading to the presence of high densities of dislocations after tempering. The alloying of molybdenum and niobium in high strength steels has been investigated in both direct‐quenched and direct‐quenched and tempered conditions. Results indicate that molybdenum prevents softening during tempering mainly by solute drag. Addition of niobium combines strong precipitation strengthening and solute drag. Superior mechanical properties can be achieved with studied compositions in both direct‐quenched and tempered conditions.