Paixian Fu

Inclusion flotation-driven channel segregation in solidifying steels.

Channel segregation, which is featured by the strip-like shape with compositional variation in cast materials due to density contrast-induced flow during solidification, frequently causes the severe destruction of homogeneity and some fatal damage. An investigation of its mechanism sheds light on the understanding and control of the channel segregation formation in solidifying metals, such as steels. Until now, it still remains controversial what composes the density contrasts and, to what extent, how it affects channel segregation. Here we discover a new force of inclusion flotation that drives the occurrence of channel segregation. It originates from oxide-based inclusions (Al2O3/MnS) and their sufficient volume fraction-driven flotation becomes stronger than the traditionally recognized inter-dendritic thermosolutal buoyancy, inducing the destabilization of the mushy zone and dominating the formation of channels. This study uncovers the mystery of oxygen in steels, extends the classical macro-segregation theory and highlights a significant technological breakthrough to control macrosegregation.

Comparison of Channel Segregation Formation in Model Alloys and Steels via Numerical Simulations

In the current study, the evolutions of channel segregations in several alloy systems, such as the typically used model alloys (e.g., Ga-In, Sn-Pb, Sn-Bi, Al-Cu, and Ni-based superalloy) and some special steels, are numerically simulated in a cavity solidified unidirectionally. The simulations are based on a modified continuum macrosegregation model with an extension to the multicomponent systems. The results of model alloys and steels indicate that when the thermosolutal convection is strong enough, flow instability occurs, which in turn destabilizes the mushy zone. Subsequently, the channel segregation forms with the continuous interaction between solidification and flow. The formation behavior and severity of channel segregations in various systems are different owing to their distinct melt convection strengths and solidification natures. In the current simulations, channels are apparent for model alloys with high content of solutes, whereas they are slight in some special steels, such as 27SiMn steel, and totally disappear in carbon steels. These occurrence features of channel segregation in simulations of steels are consistent with the analyses by a modified Rayleigh number associated with alloying elements, and both outcomes are well supported by the fully sectioned steel ingots in experiments.

Process modelings and simulations of heavy castings and forgings

The Materials Process Modeling Division, IMR, CAS has been promoting for more than 10 years research activities on modeling and experimental studies on heavy castings and forgings. In this report, we highlight some selected achievements and impacts in this area: To satisfy domestic strategic requirements, such as nuclear and hydraulic power, marine projects and high speed rail, we have developed a number of casting and forging technologies, which combine advanced computing simulations, X-ray real time observation techniques and industrial-scaled trial experiments. These technologies have been successfully applied in various industrial areas and yielded a series of scientific and technological breakthroughs and innovation. Important examples of this strategic research include the hot-processing technologies of the Three Gorge water turbine runner, marine crankshaft manufacturers, backup rolls for hot rolling mills and the production of hundreds-ton steel ingot.

Improvement of Strength-Toughness-Hardness Balance in Large Cross-Section 718H Pre-Hardened Mold Steel

The strength-toughness combination and hardness uniformity in large cross-section 718H pre-hardened mold steel from a 20 ton ingot were investigated with three different heat treatments for industrial applications. The different microstructures, including tempered martensite, lower bainite, and retained austenite, were obtained at equivalent hardness. The microstructures were characterized by using metallographic observations, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and electron back-scattered diffraction (EBSD). The mechanical properties were compared by tensile, Charpy U-notch impact and hardness uniformity tests at room temperature. The results showed that the test steels after normalizing-quenching-tempering (N-QT) possessed the best strength-toughness combination and hardness uniformity compared with the conventional quenched-tempered (QT) steel. In addition, the test steel after austempering-tempering (A-T) demonstrated the worse hardness uniformity and lower yield strength while possessing relatively higher elongation (17%) compared with the samples after N-QT (14.5%) treatments. The better ductility of A-T steel mainly depended on the amount and morphology of retained austenite and thermal/deformation-induced twined martensite. This work elucidates the mechanisms of microstructure evolution during heat treatments and will highly improve the strength-toughness-hardness trade-off in large cross-section steels.

The Experimental Characterization and Numerical Simulation of A-Segregates in 27SiMn Steel

A 500-kg 27SiMn steel ingot has been produced via low-oxygen purifying technology and then fully dissected in the longitudinal center plane to investigate the typical features and formation mechanism of A-segregates. The macrostructure, microstructure, inclusions, cavity and solute segregation surrounding the A-segregates were characterized and analyzed in detail. It shows that the main solute components such as C, Si, and Mn, and even some large cavities accumulate obviously within the A-type channels. Owing to the low content of oxygen and other impurities, there is no apparent enrichment of the inclusions. Based on the classical theory of macrosegregation, driven by thermo-solutal buoyancy, A-segregates in the body of a 27SiMn steel ingot have been simulated successfully by using the multi-component continuum model. They disappear in a referred 1045 steel ingot. By comparing the evolution processes during solidification of both steels, it is demonstrated that the strong thermo-solutal convection originated from the high content of Si element in 27SiMn steel destabilizes the mushy zone and therefore induces the A-segregates. In terms of the occurrence zones of A-segregates and corresponding distributions of Rayleigh number, its critical value to predict the A-segregates in industrial steel ingots was proposed and then applied successfully.