Liqing Chen

Some aspects of high manganese twinning-induced plasticity (TWIP) steel, a review

High manganese twinning-induced plasticity (TWIP) steel is a new kind of structural material and possesses both high strength and superior plasticity and can meet the weight-lightening requirement for manufacturing vehicle body. The excellent formability of the TWIP steel comes from the extraordinary strain hardening effect during plastic deformation. The reduction of specific weight by aluminum alloying and strain hardening effect can lead to an effective weight reduction of the steel components, and provide a better choice for materials in vehicle body design. The TWIP effect in high Mn steels is generally associated with the successive workhardening generated by twins and influenced by some factors, such as Mn content, Al addition revealed by stacking fault energy (SFE), grain size, deformation temperature and strain rate. The present review introduces some aspects of the TWIP steels relating to their physical metallurgy, influencing factors associated with their deformation mechanisms, and a prospect for the future investigation is also described. Moreover, as a potential candidate for replacing Ni-Cr austenitic stainless steel, researches on the oxidation behavior and corrosion resistance of Fe-Mn-Al-C system steels are also reviewed.

Processing, Microstructures, and Mechanical Properties of Magnesium Matrix Composites: A Review

In the last two decades, light-weight magnesium matrix composites have been the hot issue of material field due to their excellent mechanical and physical properties, e.g., high-specific strength and modulus, good wear resistance, and damping capacity. As compared with aluminum matrix composites, magnesium matrix composites have merit in their specific weight and have wide applications in aerospace and aeronautical fields. Generally, the processing techniques for magnesium matrix composites can be categorized as conventional and special processing routes. In recent years, as a special processing route, metal melt infiltration into porous ceramic preform featured by its low cost and availability of high-volume fraction of reinforced ceramics have been receiving much attention. Thus, in this review, one emphasis was put on the description of this processing technique in association with the means to obtain good wettability, the prerequisite for this kind of processing method. Based on the recognized fact that there exist clean interface and bonding ability between ceramics and matrix metal, in-situ reaction synthesis is usually utilized to fabricate magnesium matrix composites. Therefore, the interfacial feature was also reviewed for the in-situ reaction synthesis. Characterizations of microstructures and various mechanical–physical properties were finally summarized for magnesium matrix composites including tensile response, wear resistance, creep behavior, and damping capacity.

Application of FEM to Hot Continuous Rolling Process for Inconel 718 Alloy Round Rod

A finite element model for coupled thermo-mechanical analysis has been developed in hot continuous rolling process for Inconel 718 alloy round rod with diameter of 45 mm. The stability of this alloy is discussed by integration of FEM and processing map reported in literatures. The result shows that the stability of Inconel 718 alloy is analyzed effectively during that process and good stability appears as the initial temperature is 960 °C and the initial velocity is from 0.15 to 0.45 m • s−1 or the initial temperature is 980 °C and the initial velocity is from 0.15 to 0.25 m • s−1.

Research on Behavior of Slab Surface Defects in Forward Slip Zone during V-H Rolling Process

Behavior of the transversal crack and the longitudinal crack on slab surface during V-H rolling was simulated by the FEM. The contact pressure on crack surfaces and the crack-tip stress change rules during rolling were analyzed. Results show that the contact pressure on crack surfaces decreases and the tensile stress appears at crack tip in the zone of slippage on the delivery side, which may make the cracks propagation. For the phenomenon, the stress distribution along rolling direction and along width direction in rolling are analyzed, and the influence of forward slip on the closure and growth of the surface transversal crack and the surface longitudinal crack are discussed. Results support some significant information for researching the behavior of the slab surface defects in rolling process.

Microstructures and Properties of Thermal Barrier Coatings Plasma-Sprayed by Nanostructured Zirconia

Abstract In gas turbine engine, the study of thermal barrier coatings (TBCs) has always been paid more attention because it can effectively reduce metal interface temperatures, reduce oxidation and extend life. In this paper, the microstructures and properties of TBCs plasma-sprayed by nanostructured zirconia were investigated. The TBCs had higher adhesive strength, better thermal shock resistance and micro-cracking resistance to compare with that of conventional TBCs of magnesia and yttria stabilized zirconia, which was due to closed packed structures with a small amount of laminar structures and cavities. The increase of quantities of thermal growth oxide (TGO) was mainly responsible for deteriorating the properties of TBCs.

Effect of Mo and Cu on stress corrosion cracking of ferritic stainless steel in chloride media

Stress corrosion cracking behaviour of ferritic stainless steels with copper and molybdenum additions in 42 wt-% boiling magnesium chloride at 143 ± 1°C has been determined. The nature of the corrosion products was analysed by X-ray photoelectron spectroscopy (XPS). XPS results show that the presence of Fe(0), Cr(0) and Mo(0) unoxidised states on the crack tips of (copper+molybdenum) addition ferritic stainless steel cannot form the stable passive film and causes the further corrosion in the chloride solution. The addition of both copper and molybdenum to 19% Cr ferritic stainless steel causes stress corrosion cracking. The susceptibility to stress corrosion cracking increases with the growth of ε-copper precipitates, and the fracture mode changes from transgranular to intergranular with the increasing aging time. Stress corrosion cracking initiates from pitting of ε-copper phases, then propagates to molybdenum atoms, and finally propagates to the other ε-copper precipitations perpendicular to the direction of maximum strain.

Grain Growth Behavior of a Ni‐Cr Based Superalloy GH4033 in Reheating Process Prior to Hot Rolling

In order to overcome the difficulties in multi-stand tandem hot rolling Ni-Cr based superalloy caused by their narrower processing temperature range, a preliminary study on grain growth behavior of superalloy GH4033 has been conducted in a wide reheating temperature range. The results show that carbides Cr23C6, Cr7C3 and TiC can restrain grains from growing when reheating temperature is lower than 1110 °C. With dissolution of the second phase particles when the reheating temperature is above 1150 °C, the austenite grain grows rapidly. A discussion and kinetic analysis of grain growth have been made in association of grain growth exponent with the reheating temperature. The grain growth models were finally established for this alloy upon the reheating temperature range.

Weldability of 1000 MPa Grade Ultra-low Carbon Bainitic Steel

Maximum hardness test in weld heat-affected zone (HAZ), oblique Y-groove cracking test and mechanical property test of welding joint of 1000 MPa grade ultra-low carbon bainitic steel were carried out, so as to research the weldability of the steel. The results show that the steel has lower cold cracking sensitivity, and preheating temperature of 100 °C can help completely eliminate cold cracks, generating good process weldability. The increase of preheating temperature can reduce the hardening degree of heat-affected zone. The strength of welding joint decreases and hardness reduces when heat inputs increase, and excellent mechanical properties can be obtained when low welding heat inputs are used. Fine lath bainites of different orientations combined with a few granular bainites that effectively split the original coarse austenite grains are the foundation of good properties.

Temperature field analysis and its application in hot continuous rolling of Inconel 718 superalloy

A coupled thermo-mechanical model containing metal flow and temperature field for calculating temperature variation has been developed on fourteen-pass hot continuous rolling of round rod for Inconel 718 alloy using 3D elastic-plastic finite element method (FEM). The temperature of characteristic analysis points in the intermediate cross-section of the workpiece has been simulated at initial temperature ranging from 960 to 1000 °C and initial velocity in range of 0.15–0.55 m·s−1. Based on finite element analysis and microstructural observation in cylindrical hot compression experiments, the appropriate hot continuous rolling technologies have been designed for rod products with diffierent diameters. For a real rolling practice, the simulated surface temperature was examined and is in good agreement with the measured one.

Effect of W and Ce additions on the electrochemical corrosion behaviour of 444-type ferritic stainless steel

ABSTRACT In order to achieve excellent heat resistance and corrosion resistance, high melting-point and rare earth elements were added to ferritic stainless steel. The effect of W and Ce addition in ferritic stainless steel on the corrosion resistance in 0.1 M NaOH solution was studied. Potentiodynamic polarisation curves indicated that W- and Ce-containing samples had lower corrosion rate and lower passive current density. The electrochemical impedance spectroscopy measurements revealed that W–Ce-containing samples had higher polarisation resistance values and thicker passive films. Capacitance analysis indicated that the semiconducting behaviour and the properties of passive film remain unchanged. The addition of W and Ce led to a decrease in donor and acceptor density, which improved the passive film stability.