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The effects of nitrogen (N) and niobium (Nb) contents on the precipitation behaviors of AlN, Nb(C,N), and AlN‐Nb(C,N) precipitates and austenite grain growth during the pseudo‐carburizing process are investigated for the carburized steel SCr420H. The results indicate that an increase of N content from 0.011% to 0.019% leads to the emergence of fine and uniform austenite grain structure in the sample while being held at 930 °C for 6 h, which is attributed to a notable rise in the number density of AlN and a decrease in the coarsening rate of AlN. In the steel containing 0.023% Nb, the composite precipitates AlN‐Nb(C,N) play a primary role in the pinning effect on austenite grains growth. The isolated Nb(C,N) and AlN precipitates have minor pinning effects due to their very low volume fractions. Nb(C,N) cap forming on AlN substrate obeys the Shoji–Nishiyama orientation relationship. The favorable lattice matching between AlN and Nb(C,N) and accelerated coarsening rate of Nb(C,N) cap decrease hindrances to the nucleation, growth, and coarsening of Nb(C,N) on AlN substrate. Meanwhile, these simultaneous effects also suppress the same processes for the isolated Nb(C,N) in the matrix. [ABSTRACT FROM AUTHOR]

The onset of abnormal austenite grain growth and precipitation behaviors of aluminum nitride(AlN) during pseudo-carburizing process after solution treatment were investigated for the carburizing steel SCr420H. Thermo-Calc software package was used to calculate the solution temperature and determine the types of equilibrium precipitated phases. The coarsening kinetics of AlN precipitates were simulated based on DICTRA. The critical coarsening temperature of AlN precipitates was 1050 °C and the coarsening behavior of precipitate resulted in abnormal grain growth in this steel. Meanwhile, an approach to predict the starting time of abnormal austenite grain growth occurrence was developed by combining the precipitation thermodynamics, the coarsening kinetics of AlN and the grain growth theory. Microstructure characterization for nitride precipitation behavior during carburizing process indicated that the secondary phases in austenite were mainly hexagonal AlN and a small number of composite precipitates TiN-AlN. The experimental and simulation results of this work can offer a useful reference for avoiding abnormal austenite grain growth in high-temperature carburizing process. [ABSTRACT FROM AUTHOR]

The multifunctional polyethylene terephthalate (PET) fabrics were successfully prepared through a dip-coating technology to endow the flame retardant and antibacterial properties of PET fabrics, which are extensively used in many fields. The flame retardant and antibacterial agent was synthesized by a double drop-reverse precipitation method and surface-modified by the mixtures of titanate coupling agents and stearic acid to result in a good compatibility of the hydrophilic nano-Mg(OH)2 and the hydrophobic PET fabrics. The results indicated that the suitable synthesis conditions of nano-Mg(OH)2 are: Mg2+ concentration 1.5 mg/mL, reaction temperature 50 °C and reaction time 50 min, and the optimal modification conditions of nano-Mg(OH)2 are: modifier ratio 5/5, modification temperature 70 °C and modification time 40 min. The flame retardant test and the antibacterial test showed that the multifunctional PET fabrics had excellent flame retardant and antibacterial properties. [ABSTRACT FROM AUTHOR]

To improve the performance of the steel for brake discs, like mechanical properties and thermal fatigue resistance, Nb is added to the steel. In this paper, the effects of Nb and tempering time on the microstructure and mechanical properties of the steel have been studied. The investigation of precipitation behaviors is based on Thermo‐Calc software and transmission electron microscopy. The results show there is a negative linear relationship between the yield strength and the logarithm of time, and tempering time should not exceed 2 h. V‐rich M8C7 and NbC occur during austenitizing, while (Mo,V)C, M23C6, and M7C3 precipitate during tempering. (Mo,V)C has a strong strengthening effect at the early stage of tempering, but it grows faster than M8C7. With 0.025% Nb addition, the yield strength and impact energy increase simultaneously due to the refinement of martensite; the addition of 0.045% Nb promotes the tempering stability due to the suppression of precipitation and coarsening of Cr carbides and the precipitation strengthening of M8C7 and NbC, therefore, the addition of 0.025–0.045% Nb is conducive to improving the performance of the steel. Moreover, the suppression of Cr carbides with 0.045% Nb addition makes up for the damage of large NbC to toughness. [ABSTRACT FROM AUTHOR]

In order to optimize the tempering process of boron bearing steel 25CrMoNbB, the effect of tempering temperature and holding time on martensitic microstructure, precipitation and mechanical properties has been studied. Thermo-Calc software was used to carry out thermodynamic calculation of the equilibrium precipitation. The precipitates growth behaviors were simulated using the program package diffusion-controlled transformations (DICTRA). Field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAD) and energy dispersive spectrometer (EDS) were used to analyze microstructure and precipitates. B distribution was observed by secondary ion mass spectroscopy (SIMS). Electron back-scattered diffraction (EBSD) and TEM were used to analyze the martensitic microstructure evolution during tempering. The results showed that the B-bearing precipitates had a significant effect on the impact toughness and little effect on the tensile strength. The B atoms segregating at prior austenite grain boundaries (PAG) during austenitizing process promoted precipitation of M 2 B during tempering. The M 2 B precipitates mainly contained Fe, Mo, Cr and B elements and significantly reduced the toughness. Therefore, B content in 25CrMoNbB steel should be controlled below 20 ppm. In addition, the tempering temperature and holding time should also be optimized to reduce the detrimental effect of M 2 B on the impact toughness. In as-tempered condition, besides M 2 B precipitates, M 23 C 6 , M 7 C 3 , and MC carbides were identified. M 7 C 3 and MC carbides tended to precipitate in the matrix, whereas M 23 C 6 tended to precipitate at PAG and sub-grain boundaries. Although carbides were coarsening with increasing tempering temperature or holding time, the coarsening of martensitic blocks and laths has reduced their detrimental effect on the toughness. [ABSTRACT FROM AUTHOR]

In order to improve the hardenability and toughness of train axles for high-speed railway, Nb (0.04 wt%) and B (20 ppm and 36 ppm) were added in 25CrMo steel. The dissolution and the precipitation behaviors of the B bearing phase in the different austenitizing processes and their effects on hardenability and toughness have been studied to clarify the influence mechanisms. Thermo-Calc was used to calculate the precipitation temperature, the quantity and the composition of the B bearing phase; SEM, TEM and EDS were used to investigate the morphology and the type of the B bearing phase; SIMS was used to analyze the distribution of B in steel. The results showed that the B bearing precipitates were found in the forms of different phases such as BN and M 2 B mainly containing Fe, Mo, Cr and B elements due to high Cr and Mo contents in steels. The excessive B atoms segregating at the austenite grain boundaries promote M 2 B precipitation not only during austenitizing to deteriorate the hardenability significantly, but also during tempering to deteriorate toughness obviously. Therefore, the B content in steels should be controlled below 20 ppm, and at the same time, the austenitizing temperature and the holding time should also be optimized to avoid the adverse effect of M 2 B on hardenability and impact toughness. In addition, the hardenability and the impact toughness can be improved by a double quenching process in which both the austenitizing temperatures are low. [ABSTRACT FROM AUTHOR]