Effect of Tempforming on Strength and Toughness of Medium-Carbon Low-Alloy Steel

Abstract

The effect of tempforming on the strength and fracture toughness of 0.4%C-2%Si-1%Cr-1%Mo-VNb steel was examined. Plate rolling followed by tempering at the same temperature of 600 °C increases yield stress by 25% and the Charpy V-notch impact energy by a factor of ~10. Increasing rolling reduction leads to the reorientation and elongation of grains toward the rolling direction (RD) and the development of a strong {001} <110> (rotated cube) texture component that highly enhances fracture toughness. A lamellar structure with a spacing of 72 nm between boundaries and a lattice dislocation density of ~1015 m−2 evolves after tempforming at 600 °C with a total strain of 1.4. Two types of delamination were found, attributed to crack branching and the propagation of secondary cracks along the rolling plane perpendicular to the propagation direction of the primary crack. Delamination toughness is associated with the nucleation of secondary cracks in RD and their propagation over a large distance. The critical condition for delamination toughness is the propagation of primary cracks by the ductile fracture mechanism and the propagation of secondary cracks by the brittle quasi-cleavage mechanism.

Strengthening Mechanisms in a Medium-Carbon Steel Subjected to Thermo-Mechanical Processing

“…As a result, the dispersion strengthening becomes the major contributor to overall YS in tempformed steel with a high volume fraction of boundary M 23 C 6 carbides. It seems that tempforming provides about 50% increment in YS of medium-carbon low-alloy steels [2][3][4]6] in comparison with low-carbon steels [5,7] due to the frequent M 23 C 6 carbide chains on the lamellar boundaries after tempforming. These chains contribute to YS through dispersion strengthening.…”

“…Some of the microstructural features such as the texture, the distance between HAB, the transverse lath/subgrain size after tempering and tempforming at 600 • C and 650 • C with a total strain of 1.4 were described in previous papers [6,11]. The main structural parameters of the steel in four different conditions are summarized in Table 1.…”

“…This processing consists of austenitizing, water quenching, and high-temperature tempering at T ≥ 500 • C, followed by large-strain rolling at the same temperature. Tempforming increases the impact toughness by a factor of 10 along with a 30 pct increase in YS owing to the formation of a lamellar-type microstructure with the transverse crystallite dimension of ~100 nm [2][3][4][5][6]. Numerous works have dealt with the examination of fracture mechanisms in tempformed steels [2][3][4]7].…”

Quench and Tempered Embrittlement of Ultra-High-Strength Steels with Transition Carbides

“…Crack propagation in the CVN specimens was analyzed on the fractured specimens by EBSD on lateral surfaces [35]. Other details of structural and mechanical characterization were reported in previous works [16,17,20,29,35,36]. The volume fraction of RA was determined by magnetic saturation measurements using Fischer Feritscope FMP30 (Helmut Fischer Gmbh, Sindelfingen, Germany).…”

“…Additions of Si ≥ 1.5 wt.% reduce significantly the precipitation of cementite which enhances the strength and toughness of UHSS [1,4,10,[12][13][14][15][16]. The microstructure of Si-enriched UHSS consists of lath martensite with transition carbides located in the matrix and film-like retained austenite (RA) located at lath and block boundaries [1,14,17].…”