Dantian Zhang

Precipitation behavior and martensite lath coarsening during tempering of T/P92 ferritic heat-resistant steel

Tempering is an important process for T/P92 ferritic heat-resistant steel from the viewpoint of microstructure control, as it facilitates the formation of final tempered martensite under serving conditions. In this study, we have gained deeper insights on the mechanism underlying the microstructural evolution during tempering treatment, including the precipitation of carbides and the coarsening of martensite laths, as systematically analyzed by optical microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy. The chemical composition of the precipitates was analyzed using energy dispersive X-ray spectroscopy. Results indicate the formation of M3C (cementite) precipitates under normalized conditions. However, they tend to dissolve within a short time of tempering, owing to their low thermal stability. This phenomenon was substantiated by X-ray diffraction analysis. Besides, we could observe the precipitation of fine carbonitrides (MX) along the dislocations. The mechanism of carbon diffusion controlled growth of M23C6 can be expressed by the Zener’s equation. The movement of Y-junctions was determined to be the fundamental mechanism underlying the martensite lath coarsening process. Vickers hardness was estimated to determine their mechanical properties. Based on the comprehensive analysis of both the micro-structural evolution and hardness variation, the process of tempering can be separated into three steps.

Research on splitting phenomenon of isochronal martensitic transformation in T91 ferritic steel

T91 steel is a representative type of ferritic heat-resistant steel currently used in power plant components, and is a potential candidate for structural steel in nuclear reactors. The isochronal martensitic transformation behaviors during continuous cooling after austenitization in T91 ferritic steel were systematically investigated by high-resolution dilatometry and microstructure observation. The splitting phenomenon of martensitic transformation is accompanied with the precipitation of needle-like M3C particles, which is suppressed by rapid cooling after austenitization. The appearance of this splitting is ascribed to the concentration gradient caused by the consumption of alloy element in process of the formation of M3C. This concentration gradient results in the appearance of wide martensitic laths ahead of the generation of normally narrow laths. These two types of martensitic laths possess different M s (martensitic start transformation) temperatures, which are attributed to the splitting transformation phenomenon.

Effect of austenisation temperature on phase transformation in low carbon microalloyed pipeline steel

Abstract A detailed investigation was carried out on the effect of austenisation temperature on the phase transformation in X65 microalloyed pipeline steel during continuous cooling at a rate of 200°C min−1 by means of high resolution dilatometric measurement and microstructure observation. The results showed that with austenisation temperature increasing in the range of 850–1000°C, phase transformation temperature range shifts to lower temperatures and the formation of acicular ferrite phase is promoted. Furthermore, the increase in austenisation temperature results in refined grains after continuous cooling to room temperature, associating with the appearance of acicular ferrite structure in steel. The present investigation is helpful to optimise the parameters of rolling or heat treatment technology in industrial production of X65 steel.

Microstructure Evolution of HSLA Pipeline Steels after Hot Uniaxial Compression

The mechanical properties of the high-strength low-alloy pipeline steels were mainly controlled by the subsequent phase transformations after rolling. The influence of hot uniaxial compression on the phase transformation of acicular ferrite was explored by viewing of the deformation degree, the deformation temperature, and the strain rate. The results show that the increase of deformation amounts raises the transformation starting and finishing temperature during the subsequent cooling and also promotes the polygonal ferrite transformation, which leads to the decrease of Vickers hardness accordingly. With the increasing of the deformation temperature, the achieved microstructure becomes coarsened and thus decreases the hardness. As the strain rate increases, the microstructure is refined and thus the hardness increases gradually; increasing the strain rate appropriately is beneficial to the refinement of the microstructure.

Bainite Formation Kinetics During Isothermal Holding in Modified High Cr Ferritic Steel

Microstructural observation and high-resolution dilatometry has been employed to investigate the course of isothermal holding at various temperatures during cooling after austenization in a modified high Cr ferritic steel. The formation of bainite during isothermal holding was identified. The amount of bainite increases as transformation temperature decreases. The phenomenon of thermal stabilization of austenite disappears after isothermal holding due to the formation of bainite. The kinetics of incomplete bainite transformation was described by a displacive model in view of autocatalytic nucleation. Kinetics analysis suggests that reduction of holding temperature promotes to bainite transformation by means of an increase in the number of embryos for autocatalytic nucleation and a decrease in activation energy.

Phase transformation mechanism of low-carbon high strength low alloy steel upon continuous cooling

Abstract The transformation characteristics of low-carbon high strength low alloy steel for various cooling rates were systematically investigated by means of dilatometric measurements and microstructure observations. According to the results, it is recognised that the increase of the cooling rate could lead to microstructure evolution from a mixture of polygonal ferrite, acicular ferrite and bainite ferrite to the dual phase of acicular ferrite and bainite ferrite. The kinetics mechanism of the phase transformation was further studied by a modified analytical phase transformation model, which involves site saturation, diffusion/interface-controlled growth, impingement correction for randomly distributed growing particles. It is demonstrated that diffusion-controlled polygonal ferrite and acicular ferrite phase transformation precedes the interface-controlled bainite ferrite phase transformation. For the diffusion-controlled growth, the transformation is slowed down with the increase of the cooling rate, which prevents the diffusion process to some degree and increases the diffusion activation energy QD. For the interface-controlled growth, the interface migration activation energy shows a declining trend with the increase of cooling rate, thus promoting the transformation.

Phase transformations in WB36 heat resistant steel under continuous cooling conditions

Abstract Aiming to optimise the thermal treatment technology of WB36 heat resistant steel, the characteristics of phase transformations during continuous cooling at 5–2000°C min−1 were investigated by means of microstructure observation and dilatometric measurements. The continuous cooling transformation diagram of WB36 heat resistant steel was obtained, as well as the fractions of different transformation products obtained under various cooling conditions. The results showed that the transformation products in continuously cooled WB36 heat resistant steel involve polygonal ferrite, pearlite, granular bainite and lath martensite, with increasing cooling rates from 5 to 2000°C min−1. Owing to the Mo addition to the steel, the super cooled austenite in WB36 heat resistant steel was stabilised significantly, and granular bainite can be obtained in a wide range of cooling rate. However, the substructures of granular bainite vary significantly with the increase in cooling rate, including matrix morphology and the distribution of M/A islands in the matrix.