Yanlin Wang

Nanostructured bainite (NB) bearing steel has excellent strength and ductility combinations, which can improve the fatigue life and wear resistance of bearing steel in harsh conditions. However, the phase transformations and the correlation between the microstructure and wear properties of NB bearing steel are still unclear. In this study, bearing steels with different Si contents (GCr15SiMo and GCr15Si1Mo) were prepared to have nano-bainitic structures, and their microstructure transformations and wear mechanisms were investigated. The results show that the Si element can inhibit the precipitation of carbides and can then promote the block-like retained austenite formation and refine the bainitic ferrite lamellar structure. The impact energy of GCr15Si1Mo is larger than that of GCr15SiMo because the nanostructured bainite and retained austenite are the main toughness phase in these steels. The wear results indicate that the steels which possess appropriate strength and toughness are helpful for improving wear resistance properties. Finally, the wear resistance performance of the GCr15Si1Mo austempered at 210 °C and GCr15SiMo austempered at 230 °C was good in this work.

Solid solution and coupling precipitation behavior of multiple microelements in 50CrVA spring steel under different temperatures were analyzed based on thermodynamics. Quantitative relationships between the multiple microelements’ contents and secondary phases, and their effects on fatigue life, were systematically studied in conjunction with the secondary phase microstructure characterization using scanning and transmission electron microscopy, etc. The solid solution contents of different microelements decreased as the temperature decreased, especially N and Ti, but the number of compounds gradually increased when the temperature decreased. Carbonitride constitutional liquation occurred in 50CrVA-S1# spring steel-containing microparticles, and without carbonitrides, constitutional liquation occurred in 50CrVA-S2# spring steel-containing nanoparticles. The experimental results indicate that the fatigue life reduces by about an order of magnitude when the secondary phase size changes from nanometers to microns, and the corresponding relationship among multiple microelements, microstructure of secondary phases, and fatigue life, was established in this spring steel.

Based on the Wagner’s formalism combined with mass conservation, a thermodynamic analysis method has been developed previously. This method enables the calculation of the equilibrium matrix composition, precipitate composition and precipitate total molar fraction for Ti_xO_y(s) in molten metal, which can be determined at any appropriate temperature. In this present study, the Ti₃O₅ phase precipitation and the quantitative relationship between the addition of Ti, O and Ti₃O₅ in the molten steel were studied using the thermodynamic model. Using the combined multipoint dispersion supply method, electromagnetic stirring and well-dispersed 5-nm Ti₃O₅ nanoparticles were fabricated in the ferrite matrix of the as-cast high-strength steel with 0.05 wt % Ti—0.002 wt % O. The as-cast microstructure was improved by the homogeneously dispersed Ti₃O₅ nanoparticles through heterogeneous nucleation and grain refinement.

Thermodynamic analysis and computations of the equilibrium solution for the multivariable secondary phase in steels have been conducted, based on the chemical equilibrium. Solid solution precipitation in the Ti–N–C microalloy steel for the secondary phase was analyzed, and its engineering application in the development of carburizing gear steels was investigated. The complete dissolution temperature increases with more C, N, or Ti in gear steels, and the effects of Ti and N are more significant than C. At a given temperature, the amount of [Ti] increases naturally with more Ti element content, while it decreases with more C or N elements content. It declines rapidly in the complete dissolution temperature range in steels until 1100 °C. The effective activity coefficient k₁ increases with the temperature decreases, and increases with the higher content of Ti or C, while it decreases with the higher N content under the same temperature. Thermodynamic analysis shows that nitrogen has been precipitated largely as TiN micron-particles above 1400 °C in gear steels. Then, titanium precipitated mainly as TiC nano-particles, thus this secondary phase can hinder grain coarsening during heat treatment.