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Step quenching and tempering (SQT) treatment was applied on HSLA steels to achieve a multi-phase microstructure with a superior deformation performance. Intercritical quenching and tempering (IQT) as well as direct quenching and tempering (DQT) were also processed for comparison. Moreover, effects of intercritical temperatures before quench for different treatment methods on microstructure evolution and mechanical property were also investigated. Compared with IQT and DQT, SQT treatment produces a fine microstructure composited of soft ferrite/pearlite and hard martensite, with precipitates distributed along boundaries to maintain a large quantity of dislocations on deformation. Experiments involving tension and impact tests show that SQT samples with such multi-phase microstructure exhibit a desirable combination of mechanical properties. With increasing the intercritical temperature, the phase fraction of martensite is increased, which improves the strength but weakens the impact toughness in regardless of heat treatment type. Meanwhile, the yield ratio (YR) is found to be increased by higher intercritical temperature in all samples, due to the larger phase fraction of martensite with a large dislocation density and a finer grain structure. Such scenario is elucidated by using a model proposed through Swift's equation. That indicates that the content of multi-phase microstructure can be efficiently controlled via adjusting the intercritical temperature using SQT, which enables the optimization of phase composition according to the requirements of actual productions in engineering. [ABSTRACT FROM AUTHOR]

In this paper, post bonding heat treatments to transient liquid phase bond reduced activation ferrite/martensitic steel were conducted, with a Fe–Si–B amorphous foil as the interlayer. The results show that, the post bonding heat treatments decreased the strength difference between different regions of the transient liquid phase bonded joint and altered the shape and the amount of Cr 2 B along the interface between the diffusion affected zone and base metal. In particular, it is found that the tensile fracture position of the as-bonded joint has been altered by post bonding heat treatment, from the interface between the diffusion affected zone and the base metal to where inside the base metal. In order to unveil such deformation differences between transient liquid phase bonded joints with and without post bonding heat treatment, digital image correlation method was adopted to study the influence of post bonding heat treatment on the local strain evolution of transient liquid phase bonded joint from both macro- and micro-scale. The non-uniform strain distribution upon the nominal surface of the tensile samples during deformation showed that, the strain of the as-bonded joint was always localized at the interface of the diffusion affected zone, resulting brittle fracture eventually. While for the joint after post bonding heat treatment, the strain localization gradually transferred from the interface to the base metal during the tensile test, which is attributed to the changing deformation resistance of these regions during straining. [ABSTRACT FROM AUTHOR]

In this study, the microstructure and high-temperature mechanical properties of a 9Cr-1.5W reduced activation ferritic/martensitic (RAFM) steel processed by friction stir processing (FSP) combined with a tempering treatment were investigated. During the FSP, lath martensite microstructures containing needle-like M 3 C and nano-sized MX precipitates were formed in the steel owing to the severe plastic deformation and high cooling rate. The steel subjected to FSP and the tempering treatment at 700 °C for 60 min (FSP-T60) had a fine-grained microstructure consisting of ferrite and martensite, and a large number of M 23 C 6 precipitates formed at the grain boundaries and within the grains. The FSP-T60 steel showed an increase of ∼100 MPa in the tensile strength at 600 °C and a high elongation of 26% as compared to the steel prepared via traditional hot rolling, normalizing, and tempering. The FSP-T60 steel also exhibited improved creep resistance with a creep rupture time of 4523 h at 600 °C/210 MPa. The increase in the strength and ductility of the steel can be attributed to its refined grain structure and the high density and dispersion of the M 23 C 6 precipitates. Meanwhile, the homogeneous distribution and refinement of M 23 C 6 carbides could reduce the growth rate of the Laves phase during creep, thus extending the formation time of the creep cavities and cracks and further increasing the creep rupture time of the steel. [ABSTRACT FROM AUTHOR]

Aimed at evaluating the effectiveness of Zr and Hf additions on oxide particle refinement and tensile strength improvement of transformable oxide dispersion strengthened (ODS) steels, 9Cr–Zr and 9Cr–Hf ODS martensitic steels were fabricated by mechanical milling, spark plasma sintering and tempering heat treatment at 800 °C. The as-sintered 9Cr–Zr steel consisted of martensite and certain amount of transformed ferrite with M 23 C 6 (M = Fe, Cr) precipites within ferrite interior. With respect to the as-sintered 9Cr–Hf steel, the microstructure consisted of martensite and small amount of residual ferrite. The residual ferrite is characteristics of higher number density of oxide nanoparticles than those in martensite. The added Zr or Hf cannot transform all Y 2 O 3 into Y–Zr–O or Y-Hf-O complex oxides. Except for Y 2 O 3 , ZrO 2 and Y 4 Zr 3 O 12 were identified in 9Cr–Zr steel, and Y x Hf y O z and Y 2 H 2 O 7 were found in 9Cr–Hf steel. The orientation relationship and interface structure between Y–Zr–O/Y-Hf-O and the matrix were studied in detail, as well as the size distributions of oxide nanoparticles in both as-sintered and tempered conditions. Tempering is mandatory to restore ductility of martensitic steels. The tensile strength of tempered 9Cr–Hf steel is superior to most reported results, while the tempered 9Cr–Zr steel exhibits a good combination of tensile strength and ductility. The ferrite phase in both tempered 9Cr–Zr and 9Cr–Hf steels has negligible effects on ductility degradation. The inferior strength of high-angle boundaries to tempered martensite itself is suggested to be responsible for limited ductility of steels. Compared to Al and Ti constituents, Zr and Hf can be considered as potential microalloying elements to produce ODS steels with favorable mechanical performance. [ABSTRACT FROM AUTHOR]