Papers by Keyword: Hot-Forging

Abstract: Recently developed ultra high-strength low alloy transformation-induced plasticity (TRIP)-aided steel with martensitic lath structure matrix or "TRIP-aided Martensitic steel; TM steel" possesses a high impact toughness. In this study, to apply the TM steel to some hot-forging parts, the effects of hot-forging on microstructure, retained austenite characteristics, tensile properties and toughness in the TM steels with chemical composition of 0.3-0.4%C, 1.5%Si, 1.5%Mn, 0.002%B, 0.02Ti, 0.05Nb (mass%) were investigated. The hot forging brought on an excellent combinations of tensile strength of 1500-2000 MPa or 0.2% offset proof stress of 1200-1560 MPa and Charpy impact absorbed value of 35-80 J/cm² when partitioned at 250-350°C after quenching in oil. The combinations exceeded so much those of the conventional quench and tempering structural steels. From examinations of microstructure and retained austenite characteristics, it was found that the excellent combinations are mainly caused by (i) refined and uniform martensitic lath structure matrix with a small amount of carbide, (ii) increasing narrow martensite with high dislocation density and (iii) the increased stability of retained austenite, resulting from the FQP process.

Abstract: This study investigates the formability of AZ31 magnesium alloy for bearing cover with inner cavity under hot forging. Firstly high speed metal test machine of China steel Co. Ltd. is used to carry out the compression tests under different forming temperatures and strain rates to obtain the stress-strain curves. Then, the stress-strain data obtained from compression test under different dies are applied to analyze the formability of magnesium alloy for bearing cover of the car under forging by commercial package DEFORM. Besides, hot forgings of magnesium alloy for bearing cover are carried out to study the formability of magnesium alloy, and to find the best forging condition. Meanwhile, from the measured result of hardness and metallographic observation of forged part, the influence of forming temperatures on the strength and microstructure of magnesium alloy under forging of bearing cover are evaluated. Finally, the Artificial Neural Network (ANN) is applied to learn the data obtained from experiments and to predict the experimental result under new combination of process parameters. Also, confirmatory experiment is carried out to prove the usefulness of the ANN model.

Abstract: The paper focuses on the research of using the coupled thermo mechanical analysis method of rigid viscoplastic finite element and the evolution of dynamic recrystallization to establish the calculation model coupling factors of deformation, heat conduction, dynamic recrystallization in the DEFORM, and on the hot-forging simulation of AISI4140. Through the simulation, it makes a valid prediction of dynamic recrystallization volume fraction and the grain size of recrystallized during the metal hot-forging process.

Abstract: Ultra high-strength TRIP-aided steel consisting of bainitic ferrite matrix and interlath retained austenite films (TBF steel) possesses high toughness and fatigue strength, as well as high resistance against hydrogen embrittlement. In this study, to improve further these mechanical properties, the effects of hot forging and subsequent isothermal transformation holding process (FIT process) on microstructure, retained austenite characteristics, tensile properties and toughness of the TBF steel with chemical composition of 0.4%C, 1.5%Si, 1.5%Mn, 0.5%Cr, 0.2%Mo, 0.05%Nb and 0.5%Al (mass%) were investigated. The FIT process brought on an excellent combination of tensile strength of 1350-1550 MPa and Charpy impact absorbed value of 100-110 J/cm2 in the developed TBF steel, exceeding so much that of SCM440 steel. The excellent combination was mainly caused by (i) refined mixed structure of bainitic ferrite and retained austenite and (ii) the increased mechanical stability of retained austenite due to the FIT process.

Abstract: A model hot forging test was carried out using tools of matrix high-speed steel. Dynamic thermal loads were applied to tools subjected to ion nitriding on the surface layer of the tool, and the conditions of deterioration and thermal softening were examined by measuring Vickers hardness in the vicinity of the tool surface after the thermal load test. In addition, static tempering softening tests were carried out, and thermal softening equations are obtained using tempering parameter on the basis of the obtained master tempering curves. When the thermal load was applied under the same conditions to nitrided and heat-treated tools, the degree of surface deterioration and thermal softening was small for nitrided tools. Equations of softening for two cases, i.e., specimen subjected only to heat treatment and specimen subjected to both heat treatment and nitriding, were expressed using tempering parameter.

Abstract: The composite used in this paper was prepared by hot-pressing ball-milled Mg alloy powders, in which NiTi shape memory alloy fibers in a row were sandwiched. The microstructure were examined by an optical microscope, scanning electron microscope, X-ray diffraction, and transmission electron microscope to measure its microhardness and density. It is shown that the composite consisted of a homogenous matrix with uniformly distributed NiTi shape memory alloy fibers, where recrystallization took place in the Mg alloy matrix that was subjected to plastic deformation. It is known that plastic deformation is beneficial to the refinement of the grains, that is an adequate bonding forms between the matrix and fibers, the density of the composite increases after the hot-forging; its tensile strength increases significantly because of the grain refinement; the hot-forging process improves the properties of the NiTi shape memory alloy fibers reinforced Mg matrix composite.

Abstract: The aim of this study is to design flash geometry of bearing hub to apply the automatic transfer system in hot forging process. The flash geometry is very important in hot forging process because the flash geometry effects on the metal flow, material losses, forging load, die pressure and so on. In this study, the problem of designing the flash geometry is studied with flash thickness and width considering the maximum die pressure to apply an automatic transfer system in hot forging process for bearing hub. The numerical analysis was conducted by means of the commercial S/W DEFORM. On the basis of numerical analysis the flash geometry of hot forging die was redesigned, and experiment was conducted. From the experimental results, it was possible to produce bearing hub with an automatic transfer system without any deterioration of die lifetime.

Abstract: The kinetics of hot pressing and the resulting microstructure of two types of polycrystalline spherical powders (beads) were studied. The first were spherical ZrO2 beads synthesized by sintering TOSOH spray dried ZrO2 powder loose in a crucible. The average bead diameter was 51 μm and contained 0.3 μm grains. The second were Al2O3-ZrO2(1%Y2O3) synthesized by plasma spraying spray dried powders into water to quench-in a partially amorphous, partially nanocrystalline structure. The particle size was <25μm. Densification in both cases depended on creep of the beads and densification rates at 1350°C were predicted well both by equations from Helle et al.[1] for hot isostatic pressing and a finite element analysis method. The microstructure of the Al2O3-ZrO2(1%Y2O3) was much more complex but densification kinetic also followed the Helle et al.[1]equations if creep data from hot pressed specimens of the same powder were substituted into the equations rather than literature values.

Abstract: The piezoelectric properties of Nd and V cosubstituted bismuth titanate ceramics, (Bi4-y,Ndy)1-(x/12)(Ti3-x,Vx)O12 [BNTV-y (x=0.01, y=0.00~1.00)] , are investigated. Furthermore, the grain orientation effects of BNTV-y ceramics on their piezoelectric properties are studied by the hot forging (HF) method. The electromechanical coupling factor, k33, of ordinarily fired (OF) BNTV-y0.75 ceramic was 0.21, which is one of the highest k33 values for bismuth layer-structured ferroelectrics (BLSFs). On the other hand, the k33 of the grain oriented (HF) BNTV-y0.25 ceramic was 0.30.

Abstract: A high speed hot forging process was applied to Nd-Fe-B and Nd-Fe-V as cast alloys in order to develop both the microstructure and the crystallographic texture appropriate for permanent anisotropic magnet properties. Neutron diffraction texture analyses are used to demonstrate the effect of the hot forging process on both kind of alloys. Microstructural changes are an important feature on forging in both cases. Coercivity is developed in the Nd-Fe-B alloy mainly from grain size reduction and disappearance of free iron. Stabilisation of the Nd(Fe,V)12 hard magnetic phase is achieved from the iron and Nd-rich microstructure of the starting Nd-Fe-V material. A comparison of the crystallographic textures of Nd2Fe14B and Nd(Fe,V)12 phases is done to account for the development of extrinsic magnetic anisotropy. In both cases, a nearly fibre texture is obtained in correlation with the symmetry of the deformation. However, the orientations are quite different in both alloys and the consequences on the magnetic properties are evidenced.