Determining role of heterogeneous microstructure in lowering yield ratio and enhancing impact toughness in high-strength low-alloy steel

Abstract

Here we present a novel approach of intercritical heat treatment for microstructure tailoring, in which intercritical annealing is introduced between conventional quenching and tempering. This induced a heterogeneous microstructure consisting of soft intercritical ferrite and hard tempered martensite, resulting in a low yield ratio (YR) and high impact toughness in a high-strength low-alloy steel. The initial yielding and subsequent work hardening behavior of the steel during tensile deformation were modified by the presence of soft intercritical ferrite after intercritical annealing, in comparison to the steel with full martensitic microstructure. The increase in YR was related to the reduction in hardness difference between the soft and hard phases due to the precipitation of nano-carbides and the recovery of dislocations during tempering. The excellent low-temperature toughness was ascribed not only to the decrease in probability of microcrack initiation for the reduction of hardness difference between two phases, but also to the increase in resistance of microcrack propagation caused by the high density of high angle grain boundaries.

Tailoring Heterogeneous Microstructure in a High-Strength Low-Alloy Steel for Enhanced Strength-Toughness Balance

“…After holding at 740, 760, 780, and 800 • C for a long time, 36%, 48%, 65%, and 85% reverse austenite was obtained, respectively. Heterogeneous microstructure with hard and soft phases is characterized by microstructural heterogeneity and/or compositional heterogeneity, which lead to a dramatic difference in strength between different structures [4,[6][7][8]. The strength mismatch between different structures will cause the inhomogeneous deformation in local areas, by which a strain gradient around the interfaces could be triggered, and geometrically necessary dislocations (GNDs) could build up.…”

“…In general, HSLA steels are treated by quenching and tempering (Q&T) to obtain tempered martensite/bainite and nano precipitates so as to ensure an excellent combination of strength and toughness [2,3]. However, the single martensitic/bainitic structure obtained by the Q&T processing often leads to a high yield ratio, which cannot meet specific service requirements [4]. In recent years, intercritical heat treatment has proven to be an effective approach to obtain low yield ratio and balanced strength and toughness, by which a heterogeneous microstructure can be achieved through introducing a soft phase to the hard matrix [5,6].…”

“…Accordingly, numerous efforts have been made for high-strength steels with heterogeneous structures, mainly focusing on the steels containing retained austenite and aiming at the stability control of retained austenite and its influence on ductility and toughness [5,[11][12][13][14][15][16][17][18][19][20][21]. For HSLA steels without retained austenite, the mechanical properties are affected by the features of heterogeneous microstructure, i.e., volume fraction and hardness of soft and hard phases and the microstructure refinement, showing different characteristics of mechanical behavior [4,22,23]. However, the morphological and crystallographic features of the heterogeneous microstructure without retained austenite in HSLA steels are poorly understood to date and need further investigation.…”

Yield Load Solutions for SE(B) Fracture Toughness Specimen with I-Shaped Heterogeneous Weld

“…The application of the SINTAP procedure is based on the implementation of the yield load solution in the failure assessment diagram (FAD) to determine the safe operation of the assessed structure. There are a number of studies dealing with various aspects of the fracture behavior of homogeneous welds with strength mismatch compared to the base metal, including recent ones [ 4 , 5 , 6 , 7 , 8 , 9 ], as well as a number of studies dealing with heterogeneity in welds with strength mismatch [ 10 , 11 , 12 , 13 , 14 , 15 , 16 ].…”

Morphology and Crystallography Analyses of HSLA Steels with Hardenability Enhanced by Tailored C–Ni Collocation

“…For conventional high-strength low-alloy ultra-heavy plates, a matrix structure with sufficiently high strength, such as lath martensite/bainite is obtained by quenching, which is subsequently tempered to improve toughness [1][2][3][4]. However, it is usually impossible to avoid the low cooling rate in the center of the ultra-heavy plates during cooling, causing inhomogeneous microstructure and mechanical properties across the thickness direction, which highlights the need to enhance the hardenability of the alloy [5][6][7].…”