The Mechanism of High-Strength Quenching-Partitioning-Tempering Martensitic Steel at Elevated Temperatures

Abstract

High-strength medium-carbon martensitic steel was heat treated through a quenching-partitioning-tempering (Q-P-T) treatment. Both the mechanism for improved ductility and the high temperature stability of austenite were investigated. The Q-P-T martensitic steel showed good products of strength and elongation (PSE) at various deformation temperatures ranging within 25–350 °C. The optimum PSE value (>57,738 MPa%) was achieved at 200 °C. The microstructure of the Q-P-T steel is constituted of laths martensite with dislocations, retained austenite located within lath martensite and small niobium carbides (NbC), and/or transitional ε-carbides that precipitated in the lath martensite. The good ductility can be mainly attributed to the laminar-like austenite that remained within the lath-martensite. The austenite can effectively enhance ductility through the effect of dislocation absorption by the retained austenite and through transformation-induced plasticity. The relationship between the microstructures and mechanical properties was investigated at high deformation temperatures.

Effect of 1.5 wt% Copper Addition and Various Contents of Silicon on Mechanical Properties of 1.7102 Medium Carbon Steel

“…This steel reaches both high strength and toughness and thus satisfies the requirements for spring materials or for other heavy-duty structural parts [ 1 , 2 ]. The heat treatment of medium carbon steel includes quenching followed by tempering or partitioning [ 3 ]. The typical as-quenched structure of medium carbon steels is lath martensite.…”

“…However, toughness and elongation increase with increasing tempering temperature [ 10 , 11 , 12 , 13 ]. In medium carbon steels alloyed with chrome and silicon, increasing strength was found for tempering at 350 °C [ 14 ], whereas alloying with manganese and silicon causes this effect in the tensile test temperature range from 200 to 250 °C [ 3 ]. In both studies [ 3 , 14 ], the increase in strength during tempering results from ε-carbides dispersed in the martensite matrix.…”

“…In medium carbon steels alloyed with chrome and silicon, increasing strength was found for tempering at 350 °C [ 14 ], whereas alloying with manganese and silicon causes this effect in the tensile test temperature range from 200 to 250 °C [ 3 ]. In both studies [ 3 , 14 ], the increase in strength during tempering results from ε-carbides dispersed in the martensite matrix. The latter is provided by silicon alloying, which restricts cementite precipitation by forming a Si-rich layer around the growing cementite particles in the martensite matrix [ 15 , 16 , 17 ].…”

Microstructures and Properties of Martensitic Materials

“…Martensite is the product of a diffusionless shear-like transformation. The papers published in this Special Issue [1][2][3][4][5][6][7][8][9] confirm that research in martensitic materials is very active. Even the old 'simple' Fe-C alloys keep many secrets that are still beyond the capabilities of modern computers.…”

“…The effect of carbon is of prime importance in steels; most of their complexity and mechanical properties come from the stable and metastable phases. Zhang et al [2] have measured the mechanical properties of steels elaborated by quenching-partitioning-tempering in the temperature range 20 • C to 350 • C, and they have systematically investigated the microstructure by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). They show that the high strength x elongation product values (that are useful for shock absorption) are the consequences of the complex microstructures made of metastable retained austenite, that transforms into martensite under stress (TRIP effect) and fine dispersion of niobium carbides and iron carbides.…”