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A model for interphase precipitation has been developed based on the ledge mechanism of austenite-to-ferrite transformation. Carbide precipitation is considered on the migrating ferrite/austenite interface as an interaction of transformation and precipitation kinetics. The derived equations describe sheet spacing and particle spacing of interphase-precipitated carbides as well as the overall interface velocity which are related to the nucleation rates of carbides and ferrite ledges, respectively. The microstructure characteristics of interphase precipitation are predicted as a function of transformation temperature and steel composition and replicate trends observed experimentally. [ABSTRACT FROM AUTHOR]

The objective of the study described here is to elucidate structure‐property relationship in a 460 MPa multiphase steel with particular emphasis on the influence of intercritical annealing process and retained austenite on mechanical properties. The study suggests that there is significant effect on comprehensive mechanical properties of heavy gauge plate (60 mm thickness) on intercritical annealing, especially the low temperature toughness is increased from 135 to 254 J at the test temperature of −40 °C. Fine retained austenite can be obtained by optimization of interrupted on‐line cooling and partitioning during intercritical annealing heat treatment. The retained austenite stabilized by enrichment with alloying elements during heat treatment replacing coarse martensite and austenite (M/A) constituent is the underlying reason for superior toughness. The study underscores that a practical approach to enhance the toughness of heavy plate is via multiphase control during thermo‐mechanical control processing (TMCP) and off line intercritical annealing. Large M/A constituents in as‐rolled specimen are replaced by fine retained austenite (RA) after intercritical annealing heat treatment. Compared to as‐rolled specimen, both crack initiation energy (Ei) and crack propagation energy (Ep) are improved for intercritically annealed specimen, which leads to significant improvement of low temperature toughness and a low yield to tensile strength ratio in intercritically annealed specimen. [ABSTRACT FROM AUTHOR]

Nanostructured bainitic steels exhibit high strength and toughness. A potential approach to improving their toughness is enhancing the chemical or mechanical stability of blocky austenite while maintaining the volume fractions of austenite and bainitic ferrite. This study investigates the effects of ausforming at 250 °C with a 20 % strain on the microstructure and stability of blocky austenite and contrasts these effects with non-ausformed bainite. The stability of austenite is assessed by cryogenic treatments. Two types of bainitic ferrite are observed in ausformed bainite. The fine bainitic ferrite forms around austenite twins and has the Kurdjumov-Sachs (K-S) orientation relationship with austenite. In contrast, the coarse bainitic ferrite, which has the Nishiyama-Wassermann (N-W) orientation relationship, creates an interlocking microstructure where blocky austenite is refined and has a high dislocation density. The blocky austenite in the ausformed bainite remains untransformed after the cryogenic treatment, while some blocky austenite in non-ausformed bainite transforms into martensite. These results suggest that two types of bainitic ferrite may form via different mechanisms, and that the interlocking microstructure enhances mechanical stability of blocky austenite by dislocations and block size refinement. • This study examines rolling at 250 °C as an ausforming process affecting microstructure and stability of nano-bainite. • Ausformed bainite contains two bainitic ferrite types with distinct morphologies and orientation relationships to austenite. • Rolling at 250 °C causes strong variant selection and produce more variants than uniaxial deformation in ausformed bainite. • Blocky austenite remains untransformed during cryo-treatment due to refined austenite block and high dislocation density. [ABSTRACT FROM AUTHOR]

Abstract: The cooling rate and large undercooling significantly affect the fusion zone microstructure in pulsed GTAW weldment under the same heat input condition. The weld pool solidified at fast cooling rate about 139°C/s superimposed a relative amount of undercooling has a desired higher γ content of about 37vol.% without tradition nitrogen addition or post-weld heat treatment. The final structure of the pulsed weld metal at 7°C plate consists of a great amount of desirable intra-granular austenite (IGA) inside the α grain matrix, besides Widmannstätten austenite (W) and grain boundary austenite γ 2 (GBA). It results in the weldment with an uniform microhardness distribution and a homogeneous mechanical property. [Copyright &y& Elsevier]

Abstract: Interphase carbide precipitation due to austenite decomposition was investigated by high resolution transmission electron microscopy and atom probe tomography in tempered nanostructured bainitic steels. Results showed that cementite (θ) forms by a paraequilibrium transformation mechanism at the bainitic ferrite–austenite interface with a simultaneous three phase crystallographic orientation relationship. [Copyright &y& Elsevier]

As-received ultrahigh 300M strength steel was impacted at strain rates of 1×103 and 4×103 s–1. At the strain rate of 1×103 s–1, the retained austenite transformed into upper bainite and M7C3 precipitates. Fine Mo2C carbides precipitated in the bainitic ferrite matrix in addition to upper bainite during deformation contribute to the strengthening effect at a relatively high strain rate of 4×103 s–1. [Copyright &y& Elsevier]