Yuqing Weng

Effect of annealing temperature and time on microstructure evolution of 0·2C–5Mn steel during intercritical annealing process

Abstract Microstructure evolution of 0·2C–5Mn steel during the intercritical annealing at different temperatures was studied. It was found that the microstructure gradually evolved from martensite structure into a structure consisted of austenite and ferrite during the intercritical annealing process. The retained austenite volume fraction reached the maximum value of 36·5% after about 10 min annealing at 680°C. It was also found that carbides precipitated during initial annealing stage and gradually dissolved during following annealing process for all annealing temperatures; and interestingly, the fresh martensite was found, indicating that part of the newly forming austenite was unstable and would transform into martensite when quenching. Based on the microstructural analysis and the calculation by Thermo-Calc software, it was proposed that the different evolution behaviours of the microstructure during annealing were not only controlled by the thermal kinetics of the austenite, but also affected significantly by the thermal stability of the austenite.

Hot Deformation Behavior of a New 9%Cr Heat Resistant Steel G115

The hot deformation behavior of a new heat resistant steel G115 designed for 650 °C ultra-supercritical (USC) power plants was experimentaly studied. Hot compression test was carried out in the temperature range of 900−1200°C and the strain rate range of 0.1–20 s−1 by using Gleeble-3800 thermal-mechanical simulator, and the corresponding flow curves were obtained. Experimental results show that the flow stress increases with the decrease of deformation temperature and the increase of strain rate. The hot deformation activation energy of G115 steel was determined to be 494 kJ/mol and the constitutive equation was also obtained. For convenience of the practical application, a good approximate equation was obtained for calculating the peak stress values of G115 steel under different deformation conditions. At the strain value of 0.9, natural logarithm of the critical Zener-Holomon parameter Zc of G115 steel was determined to be in the scope of 0.9, natature and 50.65, above which there wil be no dynamic recrystaliza-tion (DRX). And natural logarithm of the critical Zener-Holomon parameter Zs of G115 steel was determined to be in the scopes of 45.58 and 46.27, below which ful DRX may occur. Then, the status diagram of dynamic micro-structures of G115 steel was established. In addition, the strain rate sensitivity of G115 steel is not constant during the test temperature range and it increases linearly from 900 to 1200 °C. Therefore, hot deformation at higher temperatures would obtain beter workability.

Detrimental effect of cellular precipitation on the creep strength of Inconel740H

Cellular precipitation (also known as discontinuous precipitation) has been observed at the grain boundaries of a newly developed nickel-based Inconel740H alloy designed for use at 700 °C in advanced ultrasupercritical coal-fired power plants. By means of element mapping and selected area diffraction, the cellular precipitates were identified as Cr-rich M23C6 carbides. The onset of cellular precipitation was found to follow a pucker mechanism in Inconel740H. The cellular precipitates at the grain boundaries, even at low volume fractions, were severely detrimental to the creep strength at 750 °C. The creep rupture life of Inconel740H containing cellular precipitates at grain boundaries was only one-tenth of that for the alloy without cellular precipitates. The reason for the drastically decreased creep rupture life is attributed to the poor resistance of cellular precipitates to crack propagation during creep.

Heat Treatment of a Candidate Material for 700 °C A-USC Power Plants

The preliminary chemical composition optimization and heat treatment parameters of Inconel 740H were investigated systematically. Six heats of experimental alloys (designated as heat 401 to heat 406) with varied chemical compositions were subjected to standard heat treatment at four different solution temperatures. Due to its superior combination of strength, ductility, and toughness together with its minor mechanical property changes at different solution temperatures, heat 4 05 was chosen as an optimized one to conduct further heat treatment investigations. The evolution of grain size with solution temperature for heat 405 was studied and the optimal solution temperature was determined. After solution treatment at the optimal temperature, four different cooling rates were applied to investigate the effect of cooling rate on the size distribution modal and mean radius of the γ’ precipitates. No bi-modal size distribution of γ’ precipitates was found for all cooling rates and water cooling was recommended for Inconel 740H. In addition, the effect of pre-aging at 800 °C for 1 6 h on the microstructure of Inconel 740 H aged for a long time was studied. It is found that prc-aging at 800 °C would result in bi-modal size distribution of γ’ precipitates after long time aging at 750 °C compared with mono-modal size distribution in samples without pre-aging, which might be caused by the difference in initial size distribution modal before Ostwald ripening of γ’ precipitates in samples with and without prc-aging at 800 °C.

Variation of microstructure and mechanical properties of medium Mn steels with multiphase microstructure

Specimens of medium manganese steels with different manganese contents were austenised at 850°C for 30 min followed by intercritical annealing and water quenching at three different temperatures with different times. After heat treatments, mechanical properties were measured through tensile test. X-ray diffraction, electron back scattered diffraction and scanning electron microscopy examinations were also performed. Results indicate that annealing time, annealing temperature and manganese content all affect the final microstructure, which may consist of ferrite, retained austenite and fresh lath martensite, and variation of microstructure leads to different mechanical properties of steel.

Effect of Preferential Heat Treatment on Microstructure of New Martensitic Heat Resistant Steel G115

Intermediate heat treatment and thermo-mechanical treatment are the two possible ways of preferential heat treatment. Microstructure of G115 martensitic heat resistant steel after traditional heat treatment and preferential heat treatment was tested by field emission scanning electron microscope (FESEM) and electron back-scattered diffraction (EBSD). The results show that intermediate heat treatment can hardly change the size of precipitates while the precipitates can be obviously refined after thermo-mechanical treatment. Additionally, there is no obvious difference in precipitate size after deforming at different thermo-mechanical treatment temperatures. The reason why intermediate heat treatment failed in G115 steel can be attributed to that the content of Nb is low and most of V is in solution. The reason for the success of thermo-mechanical treatment is that after thermo-mechanical treatment but without tempering, the amount of large angle boundaries has a three-fold increase compared with that after traditional heat treatment. Since M23C6 particles are mainly precipitated along large angle boundaries, the increase of large angle boundaries can induce the increase of the precipitation site of M23C6 particles, resulting in the refinement of M23C6 particles. After deforming at different temperatures, the amount of large angle boundaries is close, so the size of M23C6 is nearly the same.

A Novel Low-cost Hot Rolled High Strength Steel for an Automatic Teller Machine

A novel hot rolled steel LG600A with the tensile strength exceeding 700 MPa was developed for automatic teller machine application. The low-cost C-Mn steel was microalloyed with 0.08 mass%–0.12 mass% Ti rather than noble alloying elements, such as Nb, V, Mo, and Cu, etc. The novel steel had a good surface quality and welding property. After the hot rolled steel coils were leveled, the steel plates, the length of which was even down to 1500 mm, had an excellent flatness. The effects of hot rolling parameters on mechanical performance, microstructure and recrystallization behavior were studied. The metallurgical concept for the steel production was also discussed. The result shows that decreasing the finish rolling temperature, increasing cooling rate in the first cooling stage and decreasing the cooling rate in the last cooling stage, together with coiling at a modestly high coiling temperature all resulted in the refined grains and TiC precipitates, thereby improving the strength and toughness of this new steel greatly.