Changjiang Song

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.

Columnar to equiaxed transition during solidification of small ingot by using electric current pulse

A new approach to applying the electric current pulse (ECP) with parallel electrodes to the promotion of the transition from columnar crystal to equiaxed crystal and the improvement of macrosegregation was introduced. The ECP was applied to different stages of the solidification. The results showed that the application of the ECP in both the initial stage (the thickness of solidified shell reached 2 mm approximately) and the late stage (the thickness of solidified shell reached 14 mm approximately) of solidification can promote the columnar to equiaxed transition (CET). The analysis showed that during solidification, a large number of nuclei around the upper surface fell off due to ECP, which subsequently showered on the melt and impinged the growth front of the columnar crystal. Therefore, the CET occurred. In addition, this method was also employed to influence the solidification process of bearing steel, and the results showed that the structure was changed from columnar crystal to equiaxed crystal, indicating that ECP can enhance the homogeneity of structure and composition of bearing steel.

Microstructural Evolution of Gas Atomized Fe-25Cr-3. 2C Alloy Powders

The microstructural evolution of the gas atomized Fe-25Cr-3. 2C powders was investigated by using optical microscope, scanning electron microscope, and X-ray diffraction. The experimental results showed that the atomized Fe-25Cr-3. 2C powders were mainly composed of austenite and (Fe,Cr)7C3 carbide. Eutectic microstructure was developed in the larger particles, whereas dendritic microstructure was obtained in the particles with diameter less than 38 μm. The reason for microstructure change should be the difference of nucleation undercooling for particles.

Structure evolution of Fe–15 wt-%Mn sub-rapidly solidified strips under different cooling rates

Abstract Owing to the effect of the transformation-induced plasticity (TRIP) or the twinning-induced plasticity (TWIP), Fe–Mn-based alloys have excellent mechanical properties and attract more attention as a promising structural material. Thin strip continuous casting was considered as an effective method to produce high-Mn steels, which were hard to produce by traditional methods. In this paper, the effect of different cooling rates by changing the trip thickness on the microstructure of Fe–15 wt-%Mn alloy prepared under sub-rapid solidification was investigated. The experimental results showed that α′-martensite and ϵ-martensite directly formed in the sub-rapidly solidified strips. The formation of ϵ-martensite in the 1·5 mm and 1 mm thickness strips suppressed with increasing cooling rate, while some nanocrystallines were obtained in these strips. The size of martensite phases was very fine in all strips, and the ϵ-martensite still existed after the heat treatment at 1000°C for 2 h.

Relevance of electrical current distribution to the forced flow and grain refinement in solidified Al-Si hypoeutectic alloy

Significant grain refinement in cast metals can be achieved through the application of electric currents during the solidification process. The present paper investigates the distribution of electric currents on the grain size of solidified Al-7wt.%Si alloy under the application of electric current with constant parameters flowing through two parallel electrodes into the melt within a cylindrical mould. The distribution of electric current was controlled by applying an electrical insulation material coating, boron nitride (NB), to the sidewall of the electrodes. Experimental results showed that the employment of these insulated electrodes can reduce grain size in comparison with the reference case of electrodes without BN coating. Flow measurements were performed in Ga-20wt.%In-12wt.%Sn liquid metal. Higher intensity forced flow occurred when the sidewall of the electrodes was insulated. In order to understand the underlying mechanism behind the stronger forced flow, corresponding numerical simulations were performed to reveal the distributions of the electric current, magnetic field, Lorentz force, and the resultant forced flow. The results achieved indicate that the mechanism of grain refinement driven by electric current is dendrite fragmentation induced by forced flow. In addition, a novel approach to enhance the grain refinement without additional input of current energy was developed.

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.

Structures and Properties of Near-Rapidly Solidified Fe–12Mn–9Al–1.2C Dual-Phase Lightweight Steel Containing 3 wt% Ni

Fe–Mn–Al–C lightweight steel possesses not only excellent comprehensive mechanical properties, but also a low density, which is attractive to the automotive industry. In the present paper, an austenite stabilizer Ni element which usually increases the austenite content was added and its effects on the structure and mechanical properties of a near-rapidly solidified Fe–12Mn–9Al–1.2C dual-phase lightweight steel strip were studied. It was found that the addition of 3% Ni did not show the potential to increase the austenite content or enhance the mechanical properties of Fe–12Mn–9Al–1.2C dual-phase lightweight steel strip. Moreover, the addition of 3% Ni deteriorated the thermal stability of Fe–12Mn–9Al–1.2C steel. In the strip with the addition of Ni, the formed metastable austenite almost fully transformed into ferrite + κ-carbide at a lower annealing temperature. The formation process of constituent phase in this near-rapidly solidified dual-phase lightweight steel was analyzed. The related results suggested that the constituent phases of the near-rapidly solidified dual-phase lightweight steel depended on the liquid/solid transition, which was controlled by both thermodynamics and kinetics factors.

Influence of Al and C on Mechanical Properties of Sub-Rapidly Solidified Fe-20Mn-xAl-yC Low-Density Steels

Fe-Mn-Al-C low density steels with high manganese and high aluminum content have received much attention recently because their potential of more than 10wt.% reduction of density, excellent mechanical properties and multiphase microstructures. For present study, sub-rapidly solidified Fe-20Mn-xAl-yC low-density steels with different Al (5, 9,12wt.%) and C(0.4, 0.6, 0.8, 1.0,wt.%) have been produced to understand the influence of Al and C on phases content and mechanical properties. These near-net shaped Fe-20Mn-xA-yC steels all revealed duplex phases (δ+γ) during sub-rapid solidification, and some ordered DO3 phases formed in -ferrite. The changes of aluminum and carbon content lead to the volume fractions changes of -ferrite, and the dissolved aluminum content have a vital influence on tensile properties, but there is no clear relationship between phase proportions and tensile properties. The yield strength, ultimate tensile strength and total elongation of 9Al-0.8C steel are 593MPa, 952 MPa, and 46%, respectively, which shows the best comprehensive performance of these sub-rapidly solidified Fe-20Mn-xA-yC steels.

Study on Tensile Behaviors of Al‐Killed Steel Under Batch‐Annealed and Continuous‐Annealed Conditions

The annealing procedure is crucial to promote the quality of Al-killed steels, which are widely used in the automotive industry. The continuous annealing (CA) has a potential to replace the batch annealing (BA) due to its benefits of higher efficiency and energy saving. Unfortunately, the yield point phenomena would be easily induced during the mechanical process after the treatment of CA. The tensile behavior, the grain size and the interstitial atom quantity of BA or CA treated samples with and without aging treatment were investigated to understand the reason for appearance of yield phenomena. Tensile tests showed the yield point was observed in the CA sample after aging treatment. Atom probe analyses presented that the content of interstitial carbon atom are very low in both BA and CA samples. Additionally, the grain size of BA sample is larger than CA. Finally, the reason of yield point appearance is discussed.

Phase Transition Research on Fe‐2Mn Alloy Powders Prepared by Gas Atomization

Fe-Mn based alloy has the potential as “third generation” advanced high strength steels (AHSS) for automotive due to its excellent comprehensive properties. The present paper considered the structure and phase transition of Fe-2Mn alloy gas-atomized powders. It was found that only bcc structure ferrite obtained when the temperature was at room temperature, then the ferrite ➛hcp-e phase ➛ γ-austenite and ferrite ➛γ-austenite phase transition was triggered with the temperature increasing. Results of Differential Scanning Calorimeter (DSC) measurement showed that the ferrite ➛e and ferrite ➛ austenite phase transition occurred at 858°Cand 910°C, respectively, transition from e phase to γ-austenite occurred at 1158°C. Dynamics analysis was conducted on solid phase transformation, nucleation and the free energy of solid phase transition were calculated, aimed to clarify the solidification process.