Cheng-Bin Shi

Crystallization Behaviors of CaO-SiO2-Al2O3-Na2O-CaF2-(Li2O-B2O3) Mold Fluxes

The effects of basicity (CaO/SiO2), B2O3, and Li2O addition on the crystallization behaviors of lime-silica-based mold fluxes have been investigated by non-isothermal differential scanning calorimetry (DSC), field emission scanning electron microscopy, X-ray diffraction (XRD), and single hot thermocouple technique. It was found that the crystallization temperature of cuspidine increased with increasing the basicity of mold fluxes. The crystallization of wollastonite was suppressed with increasing the mold flux basicity due to the enhancement of cuspidine crystallization. The addition of B2O3 suppresses the crystallization of mold flux. The crystallization temperature of mold flux decreases with Li2O addition. The size of cuspidine increases, while the number of cuspidine decreases with increasing mold flux basicity. The morphology of cuspidine in mold fluxes with lower basicity is largely dendritic. The dendritic cuspidine in mold fluxes is composed of many fine cuspidine crystals. On the contrary, in mold fluxes with higher basicity, the cuspidine crystals are larger in size with mainly faceted morphology. The crystalline phase evolution was also calculated using a thermodynamic database, and compared with the experimental results determined by DSC and XRD. The results of thermodynamic calculation of crystalline phase formation are in accordance with the results determined by DSC and XRD.

Control of MgO·Al2O3 Spinel Inclusions during Protective Gas Electroslag Remelting of Die Steel

The effect of calcium treatment and/or aluminum-based deoxidant addition on the oxygen control and modification of MgO·Al2O3 spinel inclusions during protective gas electroslag remelting (P-ESR) of H13 die steel with low oxygen content was experimentally studied. It is found that all the inclusions in the consumable electrode are MgO·Al2O3 spinels, besides a few MgO·Al2O3 spinels surrounded by an outer (Ti,V)N or MnS layer. After P-ESR refining combined with proper calcium treatment, all the original MgO·Al2O3 spinels in the electrode (except for the original MgO·Al2O3 spinels having been removed in the P-ESR process) were modified to mainly CaO-MgO-Al2O3 and some CaO-Al2O3 inclusions, both of which have a low melting point and homogeneous compositions. In the case of only Al-based deoxidant addition, all the oxide inclusions remaining in ESR ingots are MgO·Al2O3 spinels. The operation of Al-based deoxidant addition and/or calcium treatment during P-ESR of electrode steel containing low oxygen content is invalid to further reduce the oxygen content and oxide inclusions amount compared with remelting only under protective gas atmosphere. All the original sulfide inclusions were removed after the P-ESR process. Most of the inclusions in ESR ingots are about 2 μm in size. The mechanisms of non-metallic inclusions evolution and modification of MgO·Al2O3 spinels by calcium treatment during the P-ESR process were proposed.

Investigation of Oxide Inclusions and Primary Carbonitrides in Inconel 718 Superalloy Refined through Electroslag Remelting Process

The effect of remelting atmosphere and calcium treatment during electroslag remelting (ESR) of Inconel 718 superalloy on the oxide inclusions and primary carbonitrides was investigated. The results show that after ESR refining combined with calcium treatment, the original oxide inclusions in the electrode, mainly MgO·Al2O3 spinels and some MgO inclusions, were modified to CaO-Al2O3 system inclusions or the inclusions of MgO·Al2O3 spinel core surrounded by CaO-MgO-Al2O3 system inclusion layer. Without the calcium treatment in ESR process, all the oxide inclusions in superalloy ingots are MgO·Al2O3 spinels. All the oxide inclusions in ESR ingots act as the nucleation site for carbonitride (Nb,Ti)CN with two-layer structure precipitation, except for those with a single (Nb,Ti)CN layer containing a small amount of Ti and N in the ingot refined by a proper amount of calcium addition in ESR process. The carbonitrides (Nb,Ti)CN formed directly on the oxide inclusion have a small amount of Nb and C as well as a relatively fixed atomic ratio of Nb/Ti (about 0.6:1). There is a Nb-rich and C-rich (Nb,Ti)CN layer on the pre-existing (Nb,Ti)CN formed on the oxide inclusion. The size of the observed carbonitrides is in the range of 5 μm to 15 μm. The calcium treatment in the ESR process has a significant effect on the morphology of carbonitrides in superalloy ingot due to modification of oxide inclusions by Ca-treatment resulting in the change of precipitation and growth conditions for carbonitrides. The morphologies of carbonitrides were changed from clustered block or single octahedral to skeleton-like after calcium treatment.

Characteristics of inclusions in high-Al steel during electroslag remelting process

The characteristics of inclusions in high-Al steel refined by electroslag remelting (ESR) were investigated by image analysis, scanning electron microscopy (SEM), and energy-dispersive spectrometry (EDS). The results show that the size of almost all the inclusions observed in ESR ingots is less than 5 μm. Inclusions smaller than 3 μm take nearly 75% of the total inclusions observed in each ingot. Inclusions observed in ESR ingots are pure AlN as dominating precipitates and some fine spherical Al2O3 inclusions with a size of 1 μm or less. It is also found that protective gas operation and slag deoxidation treatment during ESR process have significant effects on the number of inclusions smaller than 2 μm but little effects on that of inclusions larger than 2 μm. Thermodynamic calculations show that AlN inclusions are unable to precipitate in the liquid metal pool under the present experimental conditions, while the precipitation of AlN inclusions could take place at the solidifying front due to the microsegregation of Al and N in liquid steel during solidification.

Effect of magnesium addition on inclusions in H13 die steel

The effect of magnesium addition on the number, morphology, composition, size, and density of inclusions in H13 die steel was studied. The results show that the total oxygen content in the steel can be significantly decreased to 0.0008wt%. Al2O3 and MnS inclusions are changed into nearly spherical MgO·Al2O3 spinel and spherical MgO·MgS inclusions, respectively. The number of inclusions larger than 1 μm decreases and the number of inclusions smaller than 1 μm increases with increasing magnesium content. V(N,C) precipitates around MgO·Al2O3 and MgO·MgS inclusions during solidification of liquid steel. The densities of MgO·Al2O3 spinel inclusions are lower than that of alumina inclusions. With increasing magnesium content in the Mg-containing inclusions, the density of inclusions decreases, leading to the improvement of inclusion removal efficiency.

Effect of SiO2 on the Crystallization Behaviors and In-Mold Performance of CaF2-CaO-Al2O3 Slags for Drawing-Ingot-Type Electroslag Remelting

The crystallization characteristics of CaF2-CaO-Al2O3 slags with varying amounts of SiO2 were experimentally studied. The effects of slag crystallization behaviors on the horizontal heat transfer and lubrication performance in drawing-ingot-type electroslag remelting (ESR) were also evaluated in terms of as-cast ingots surface quality and drawing-ingot operation. The results show that increasing SiO2 addition from 0 to 6.8 mass pct strongly suppresses the crystallization of ESR type CaF2-CaO-Al2O3 slags. The crystallization temperature of the studied slags decreases with the increase in SiO2 addition. The liquidus temperatures of the slags also show a decreasing trend with increasing SiO2 content. In CaF2-CaO-Al2O3-(SiO2) slags, faceted 11CaO·7Al2O3·CaF2 crystals precipitate first during continuous cooling of the slag melts, followed by the formation of CaF2 at lower temperatures. 11CaO·7Al2O3·CaF2 was confirmed to be the dominant crystalline phase in the studied slags. CaF2-CaO-Al2O3 slags with a small amount of SiO2 addition are favorable for providing sound lubrication and horizontal heat transfer in mold for drawing-ingot-type ESR, which consequently bring the improvement in the surface quality of ESR ingot and drawing-ingot operating practice as demonstrated by plant trials.

Kinetics of Isothermal Melt Crystallization in CaO-SiO2-CaF2-Based Mold Fluxes

A kinetic study for isothermal melt crystallization of CaO-SiO2-CaF2-based mold fluxes with different basicity of 0.94 and 1.34 has been carried out systematically by DSC measurements. The kinetic parameters were determined by Johnson–Mehl–Avrami equation. The average Avrami exponent of cuspidine (3CaO·2SiO2·CaF2) crystallization for mold flux of lower basicity (0.94) is calculated to be 3.1, implying that the crystallization mode is instantaneous nucleation followed by 3-dimensional growth. For the mold flux of higher basicity (1.34), the average Avrami exponent of cuspidine equals to 3.4, strongly suggesting that the growth is still 3 dimensional but the nucleation should be continuous. It was found that the effective crystallization rate constant for both mold fluxes increases as the crystallization temperature decreases, showing that the crystallization rate could be governed by nucleation rate. The negative effective activation energy indicates an anti-Arrhenius behavior for crystallization of the mold fluxes studied. Therefore, it is concluded that the melt crystallization for the commercial mold fluxes will be determined by thermodynamics of nucleation which is relevant to degree of undercooling. The morphology of cuspidine crystals observed by SEM agreeds well with the isothermal crystallization kinetics results.

Effect of trace magnesium on carbide improvement in H13 steel

In the present work, the carbides and inclusions in H13 steel with different magnesium contents were investigated with the scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and were calculated by the ThermoCalc and FactSage softwares, respectively. The growth of the MgAl2O4 spinel and Al2O3 at 1500°C in molten H13 was analysed by the confocal laser scanning microscopy (CLSM). The disregistries between the carbides and other phases were computed. The results are as follows. The primary carbides formed in H13 ingots were M(CN) and M6C, which existed in the segregation areas only. The MgAl2O4 particles, acting as more effective heterogeneous nuclei and providing more nucleation sites than Al2O3 particles, induced the formation of more developed austenite dendrites, and a greater number, smaller size and more uniform distribution of primary carbides, which finally relieved carbide segregation.

Effect of electroslag remelting on carbides in 8Cr13MoV martensitic stainless steel

The effect of electroslag remelting (ESR) on carbides in 8Cr13MoV martensitic stainless steel was experimentally studied. Phases precipitated from liquid steel during solidification were calculated using the Thermo-Calc software. The carbon segregation was analyzed by original position analysis (OPA), and the carbides were analyzed by optical microscopy (OM), scanning electron microscopy (SEM), energy- dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The results indicated that more uniform carbon distribution and less segregation were obtained in the case of samples subjected to the ESR process. After ESR, the amount of netty carbides decreased significantly, and the chromium and vanadium contents in the grain-boundary carbides was reduced. The total area and average size of carbides were obviously smaller after the ESR process. In the sample subjected to ESR, the morphology of carbides changed from lamellar and angular to globular or lump, whereas the types of carbides did not change; both M23C6 and M7C3 were present before and after the ESR process.

Evolution of Al2O3 inclusions by magnesium treatment in H13 hot work die steel

To investigate the evolution mechanism of Al2O3 inclusions after Mg treatment in H13 die steel, the determination of inclusions characteristics and thermodynamic calculation were carried out in the present study. The results showed that irregular and cluster Al2O3 inclusions in H13 die steel were modified to MgO·Al2O3 and MgO after Mg treatment. Two types of MgO·Al2O3 were detected after Mg treatment. The long-range gravity between the MgO-containing inclusions is weaker than that of the MgO-free inclusions. The long-range gravity between the inclusions decreases with the increasing of Mg content of the steel. The evolution mechanisms of inclusion in low Mg-containing and high Mg-containing H13 die steel were comprehensive discussed. The different reaction mechanisms result in the different of the core of the MgO·Al2O3 inclusion.