Honggang Zhong

Structure of slowly solidified 30Cr2Ni4MoV casting with surface pulsed magneto-oscillation

Abstract Large scale 30Cr2Ni4MoV ingots play a crucial role in nuclear power plants. Shrinkage and carbon segregation are the most common defects in the manufacture of these large scale ingots. Large scale ingots have very low cooling rates. In the present work, an experimental method was employed to achieve similarly low cooling rates with the aim of simulating the solidification process of large ingots in smaller 30Cr2Ni4MoV ingots.Thus, we examined the effect of surface pulsed magneto-oscillation (SPMO)on solidification structure in a laboratory setting. Our experimental results showed an SPMO treated ingot with less carbon segregation and a smaller shrinkage cavity than in an untreated one. Finally, the action mechanism was analysed by numerical simulation.

Hot tearing susceptibility of Fe-20. 96Cr-2. 13Ni-0. 15N-4. 76Mn-0. 01Mo duplex stainless steel

The hot tearing susceptibility of a Fe-20. 96Cr-2. 13Ni-0. 15N-4. 76Mn-0. 01Mo duplex stainless steel was investigated using method of constrained solidification shrinkage in one dimension. An apparatus for realtime measuring the contraction stress and temperature during solidification was developed, which can achieve the in-situ observation of melting and solidification and avoid the large temperature gradient of casting under the condition of pouring. The results show that the contraction stress increases significantly when the core temperature of casting reaches the liquidus temperature. The contraction stress is released when the core temperature of casting reaches 1456 °C. At this temperature, the hot tearing susceptibility of duplex stainless steel is the largest. With decreasing the core temperature to 1363 °C, the slope of contraction stress increases, which is related to the ferrite-to-austenite transformation.

Electric current pulse induced grain refinement in pure aluminium

Abstract In the present work, pure aluminium melt was treated using electric current pulse. The effect of current density on the solidification microstructure was investigated by means of wire netting technology. It was found that wire netting could not only effectively stop crystal nucleus to enter other zones but also has relatively small effects on the heat transfer and convection in melts. Theoretical analysis and ANSYS simulation suggested that variation of current density in different localised zones is a key factor influencing the microstructural features. Increasing the current density is favourable for the formation of equiaxed grains.

Effect of melt superheat on structure of unsteady state unidirectionally solidified duplex stainless steel

Abstract An experimental study with respect to the effect of melt superheat on the unidirectional solidified structure of duplex stainless steel (DSS) is reported. The results show that the melt superheat causes distinct modifications of both macrostructure and microstructure of DSS. For macrostructure of DSS, the columnar to equiaxed transition is observed at different melt superheats. The length of equiaxed zone increases and the equiaxed grain size decrease with melt superheat decreasing. For microstructure of DSS, there are significant differences in the morphology of the austenite at intragranular and boundary. In the interior of ferrite grains, the austenite appears primarily as acicular morphology. Nevertheless, the austenite takes on a widmänstatten side plate appearance at the ferrite grain boundaries. In addition, the austenite content increases with melt superheat decreasing.

Heterogeneous nucleation of pure gold on highly smooth ceramic substrates and the influence of lattice misfit and cooling rate

Abstract The present paper considers the heterogeneous nucleation of pure Au droplet solidified on a single crystal substrate plate under the influence of lattice misfit and cooling rate. The undercooling of droplet was measured by DSC. The experimental results present that the undercooling of Au droplet is directly proportional to both the lattice misfit and the cooling rate. It means that the heterogeneous nucleation can be significantly driven by lattice misfit and cooling rate. In addition, the related results show that the relationship between the undercooling and the cooling rate of Au droplet is independent on the lattice misfit. The results with respect to the interface of solidified Au and substrate analyzed by HRTEM prove that the orientation relationship between the substrate and the solidified Au matches with the minimum lattice misfit.

Improving the Solidified Structure by Optimization of Coil Configuration in Pulsed Magneto-Oscillation

Using both numerical and experimental methods, we studied the effect of coil configuration of pulsed magneto-oscillation (PMO) on distribution of electromagnetic field, flow field and solidification structure with the same pulse current parameters in Al ingots. We designed and constructed three types of coils: surface pulsed magneto-oscillation, hot-top pulsed magneto-oscillation (HPMO) and combined pulsed magneto-oscillation (CPMO). PMO treatment refined the solidification structure in all the ingots. The configuration of the PMO, however, introduced differences in magnetic field intensity, electromagnetic force, Joule heat, flow field, equiaxed grain zone, grain size and growth direction of columnar grains. The largest equiaxed grain zone was found in CPMO treated ingot, and the smallest grain size was found in both HPMO and CPMO treated ingots. Numerical simulation indicated that difference in electromagnetic field and flow field resulted in differences in solidification structure. HPMO is more advantageous over others for large ingot production.

Microstructure Evolution in Solidified Al–Si Hypereutectic Alloys Under the Impact of an Electric Current Pulse

Investigating the structure evolution of silicon phase in Al–Si alloys in the extra energy field is of high importance to understand and control the growth behavior of faceting phases. The present paper focuses on the influence of an electric current pulse (ECP) on the structure of directionally solidified Al–20.5 wt%Si hypereutectic alloy. Experimental results showed that ECP had a great impact on the structures of silicon phase. The interconnected, porous primary silicon structure was observed in the initial growth period, accompanied by a small quantity of eutectic silicon directly growing from the primary silicon. In the following growth period, it was surprisingly found that the numerous complex regular silicon and eutectic silicon structures appear instead of the primary silicon. On the other hand, the reference sample without ECP showeds that the structures were composed of several coarse plate-like primary silicon and eutectic structures. The variation of silicon structures indicated that the solidification behavior of faceting phases was remarkably modified by ECP, which may be due to the forced melt flow generated by the electromagnetic force.

The Effect of Surface Pulsed Magneto‐Oscillation on Solidification of Low Pressure Rotor 30Cr2Ni4MoV Steel

The influence of surface pulsed magnetic oscillation (SPMO) on the solidification of 30Cr2Ni4MoV steel was tested. The experimental results indicated that the SPMO significantly refined the solidification structure and homogenize composition of this steel. Firstly, the rate of equiaxed grains was increased from 25.3% to 59%. Secondly, the position of CET was changed significantly and the average length of columnar dendrite moved from 21.1mm to 7.9mm. Meanwhile the average secondary dendritic arm spacing (SDAS) decreased from 178.4μm to 144μm. Furthermore, the SPMO also had a positive effect on carbon segregation, ratio of which changed from 0.85~1.2 to 0.95~1.1.

The Study on Vacuum Degassing Process of AlV55 Alloy

Vacuum refining is admitted as an effective means to remove impurities in the process of alloy smelting. In this paper, Orthogonal and single factor experiments were carried out to investigate effects of multiple factors, such as superheat degree of the melt, degassing time and vacuum degree, on the vacuum refining process. The results showed that the best removal rate of oxygen and nitrogen is up to 82.71% and 61.3%, respectively, due to high vacuum degassing efficiency. It is found that the superheat degree is minimal impact on the deoxidization rate, in addition, nitrogen removal is controlled by the degassing time. The inclusions exist in the AlV55 alloy are mainly composed of oxides and vanadium-nitrogen compounds.

Simulation of Temperature Field and Microstructure in Heavy Steel Ingots Solidification

The heavy forging ingots have been widely used in the large nuclear power plant, which play a crucial role as the low-pressure rotors. However the quality of heavy ingots was not satisfied. In this paper, the solidification process of a 25-ton 30Cr2Ni4MoV ingot was simulated using the commercial FEM casting simulation software Procast package. The temperature and solid fraction distribution was calculated. On the basis of temperature field simulation results, nucleation and grain growth in steel ingot solidification process is computed using cellular automaton model to show the solidification microstructure. Effects of casting parameters including heat transfer conditions and casting temperature were investigated. By analyzing the simulation results, the main factors to affect the solidification process were obtained. The statics can give references for industry production to optimize mould design and improve the quality of ingot.