Special features of the α → γ transformation in powder carbon steels

The structural state of powder steels after sintering depends on the technological regimes of their production and can markedly affect the transformations in subsequent heat treatment. The effect of the technological regimes on the nature of the transformation is investigated.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 14–16, March, 1996.     

A critical condition for the formation of a coarse columnar γ grain structure in a peritectic solidified carbon steel

The formation of a coarse columnar austenite grain (CCG) structure is a serious problem in continuous casting processes of peritectic solidified carbon steels. In this study, a guiding principle for the avoidance of CCG formation is developed. The critical condition for CCG formation recently put forward based on phase-field simulations, which is given by a balance between the cooling condition and the growth rate of the CCG, is first re-examined and modified by considering the effect of a liquid phase during the CCG formation. The validity of this critical condition is then investigated by three different casting experiments combined with heat conduction analyses. From a comparison between the cooling conditions and the resulting microstructural changes, the validity of the critical condition is successfully demonstrated.     

Eutectic solidification mode of spheroidal graphite cast iron and graphitization

The shrinkage and chilling tendency of spheroidal graphite (abbreviated SG) cast iron is much greater than that of the flake graphite cast iron in spite of its higher amount of C and Si contents. Why? The main reason should be the difference in their graphitization during the eutectic solidification. In this paper, we discuss the difference in the solidification mechanism of both cast irons for solving these problems using unidirectional solidification and the cooling curves of the spheroidal graphite cast iron. The eutectic solidification rate of the SG cast iron is controlled by the diffusion of carbon through the austenite shell, and the final thickness is 1.4 times the radius of the SG, therefore, the reduction of the SG size, namely, the increase in the number, is the main solution of these problems.     

Effect of carbon,chromium, and nickel on the α ⇄ γ transformation and properties of Cr - Ni - Mo - V steels

The aim of the present investigation is to determine some regular features of alloying Cr - Ni - Mo - V steels for critical parts with allowance for the structure, hardenability, level of properties, and semibrittleness temperature. For this purpose the concentration of carbon in Cr - Ni - Mo - V steels was varied from 0.03 to 0.47%, chromium from 0 to 4%, and nickel from 0 to 5% at a constant concentration of the other elements (0.5% Mo, 0.6% Mn, 0.25% Si, -0. 1% V, 0.015% S, 0.012% P), and the cooling rate from the austenitization temperature was varied from 25 to 1000°C/h.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 7 – 9, March, 1996.     

Transition of solidification path in nonequilibrium solidification of Ti–48Al–8Nb alloy

The solidification behavior of Ti-48Al-8Nb alloy under nonequilibrium solidification conditions was studied by electromagnetic levitation technique.The solidification conditions are different undercooling(△T)under the same cooling condition and different cooling methods at the same undercooling condition,respectively.In different undercooling conditions,when the undercooling is above a critical value(△T*≈211K),a remarkable morphological transition from typical hypoperitectic solidification to a sole solidification of the(3 phase resulting in the suppression of the peritectic reaction occurs.For melts with different cooling conditions at the same undercooling(△T≈85 K),the melt was rapidly cooled by quenching them in cooling media.With cooling rate increasing,a transition from β phase to peritectic α phase solidification mode is revealed for Ti-48Al-8Nb alloy.     

Effect of grain refiner and grain size on the susceptibility of Al–Mg die casting alloy to cracking during solidification

The die casting process and its alloys have been developed in recent years for automobile body parts such as B-pillars. However, it is known that die casting alloys with high ductility and fracture elongation often show a higher susceptibility to cracking during solidification than conventional Al–Si alloys. Thus, it is important to estimate and control the susceptibility to cracking during solidification before trial casting or mass-production. In this study, as a representative non-heat treatment type alloy, Al–4.5wt%Mg (JIS AC7A, AA 514) aluminum alloy was used. The effect of the addition of silicon and grain refiner on the reduction of the susceptibility to cracking was examined. In order to evaluate the susceptibility to cracking, both the “I-beam casting cracking test” and the “TIG spot welding cracking test” were carried out. As a result, the addition of Ti + B worked as a grain refiner on both testing methods. The susceptibility to cracking was significantly reduced by the addition of Ti + B in both the I-beam casting and the weld crater. It was found that the finer grain size led to lower susceptibility to cracking. Furthermore, the susceptibility to cracking of the die casting product decreased with the addition of Ti + B.     

Modeling of solidification grain structure for Ti-45% Al alloy ingot by cellular automaton

A cellular automaton model for simulating grain structure formation during solidification processes of Ti45% Al(mole fraction) alloy ingot was developed, based on finite differential method for macroscopic modeling of heat transfer and a cellular automaton technique for microscopic modeling of nucleation, growth, solute redistribution and solute diffusion. The relation between the growth velocity of a dendrite tip and the local undercooling,which consists of constitutional, thermal, curvature and attachment kinetics undercooling is calculated according to the Kurz-Giovanola-Trivedi model. The effect of solidification contraction is taken into consideration. The influence of process variables upon the resultant grain structures was investigated. Special moving allocation technique was designed to minimize the computation time and memory size associated with a large number of cells. The predicted grain structures are in good agreement with the experimental results.     

Increasing the load‐carrying capacity of spot welded joints in low‐carbon steels

It is shown to be possible to produce a heat-resisting coating for an intermetallic titanium alloy with the hardening ortho phase preventing changes in the surface layer of the alloy as a result of the build-up of oxide corrosion products, which can soften the component at working temperatures of 700–800°C. The results of investigation of the heat resistance properties, evolution of the composition and structure of the alloyed ion-plasma coatings on the titanium alloy with the hardening ortho-phase at working temperatures show that the deposition of the coating greatly reduces the depth in which the surface layer of the alloy changes and the thickness of this layer does not exceed 5 μm.     

Role of Mo in the local configuration and structure stabilization of amorphous steels,a Synchrotron X-ray diffraction and Mössbauer study

Amorphous steels are promising materials with potential structural applications. The glass-forming ability (GFA) and mechanical properties of metallic glasses are intimately related to the local structure. In Fe-based alloys, Cr and Mo content seem to play a key role in stabilizing the amorphous atomic-level structure. Here we present a study on the effects of changing Mo content in Fe_72?xC₇Si_3.3B_5.5P_8.7Cr_2.3Al₂Mo_x amorphous steels. We study the local structure of these alloys by Synchrotron X-ray diffraction and Mössbauer spectroscopy. The results show how the amorphous phase evolves from a ferromagnetic Fe-rich structure to a structure with predominance of paramagnetic environments with the increase of Mo content. The changes in the distribution of magnetic environments cannot be attributed only to the Fe–Mo substitution but to a change of local configuration in the amorphous phase.     

Effect of heat-treatment conditions on the structure and phase transformations in the shape-memory 45Ti–45Ni–10Nb alloy in as-cast and extruded states

The effect of the types and conditions of heat treatment on the element composition, microstructure, phase composition, kinetics and temperatures of phase transformations, crystal lattice and substructure parameters, and mechanical characteristics of the shape-memory 45Ti–45Ni–10Nb (at %) alloy in the ascast and extruded states was studied. The interrelation between the structure features of the alloy and its mechanical characteristics was found. The conditions of heat treatment and the state of the alloy, which assure its high mechanical characteristics, were determined.     

A combined model for the description of austenitization,homogenization and grain growth in hypoeutectoid Fe–C steels during heating

A combined model which allows one to simulate all the steps of the reaustenitization process of ferrito-pearlitic plain carbon steel has been developed. The dissolution of pearlite, the transformation of ferrite into austenite and the homogenization of the carbon distribution is described with a finite volume method. The simulation is performed on a bidimensional domain where ferrite (α), pearlite (P) and austenite (γ) grains are represented. The dissolution of pearlite is described by the growth of spherical grains and simple nucleation and growth laws. The movement of α/γ interfaces is calculated by solving the diffusion equation for carbon in the α and γ phases and accounting for the solute flux balance at the interface using a pseudo-front tracking method. The diffusion model is coupled with a Monte Carlo simulation which describes the grain growth occurring in austenite at a later stage of austenitization. The evolution of the volume fractions of pearlite and ferrite, the maximum and minimum carbon concentrations in the domain and the mean austenite grain size are represented as a function of the temperature for a typical case of constant heating rate. The influence of the different steps of the austenitization process on the global kinetics is discussed.     

Corrosion-erosion resistance of Zn-Al co-cementation coatings on carbon steels in aqueous media

A novel Zn-Al co-cementation coating was obtained by a pack cementation method,This coating possesses a two-layered structure,The outer layer is mainly composed of Fe2Al3 and FeAl intermetallics with a small amount of Zn,and the inner layer consists of Zn,Fe and a small amount of Al.The corrosion-erosion resistance of Zn-Al con-cementation coatings on carbon steel was studied by a rotary corrosion method in various NaCl and H2S containing solutions and relevant SiO2 containing media,The experimental results are compared with those of carbon steels and the sherardizing and aluminizing coating ,showing that the Zn-Al co-cementation coating have excellent corrosion-erosion resistance in various aqueous media.     

A role of fractions of mobile grain boundaries in secondary recrystallization of Fe–Si steels

Grain growth during annealing of polycrystalline materials is influenced by the type of grain boundaries a grain encounters as it grows. Simple computer experiments have been performed to ascertain which type of boundaries are responsible for the abnormal grain growth (AGG) in Fe–3% Si steels. In modelling abnormal grain growth, two different assumptions are used, the first one is that the coincidence site lattice (CSL) boundaries have high mobility and the second is that high energy boundaries are more mobile than other boundaries. The results of the computer experiments support the latter model for abnormal grain growth in Fe–3% Si steels. Finally, the importance of fractions of mobile grain boundaries on the development of Goss texture is discussed.     

Solidification structure in a cast γ alloy

Solidification microstructure of a cast Ti–50Al–6Mo alloy has been examined. The as-cast microstructure consists of β-dendrites, lamellar α₂+γ regions, eutectoid β+γ and a thin γ layer at the interface between β dendrite and lamellar structure. The microscopic basis of crystallographic texture present in the material as a consequence of the solidification path in the alloy Ti–50Al–6Mo, which solidifies through a L+β phase field has presented.     

Effect of pulse magnetic field on solidification structure and properties of pure copper

The application of pulse magnetic field to metal solidification is an advanced technique which can remarkably refine solidification structure. In this paper, the effect of pulse magnetic field on solidification structure, mechanical properties and conductivity of pure copper was experimentally investigated. The results showed that the solidification structure transformed from coarse columnar crystal to fine globular crystal with increasing pulse voltage. Increasing pulse voltage also improved the tensile strength. However, with the increase of pulse voltage, the elongation and electrical resistivity firstly decreased, then increased when the pulse voltage beyond a critical value. Moreover, in some conditions, pulse magnetic field can simultaneously improve the conductivity and mechanical property of pure copper.     

Carbon content dependence of grain growth mode in VC-doped WC–Co hardmetals

Carbon content dependency of grain growth mechanism and grain growth inhibition mechanism in VC-doped WC–Co hardmetals is investigated. VC-doped WC–Co hardmetals with three different carbon contents were sintered with liquid phase and then rapidly quenched to freeze up the structure at the sintering temperature. In these samples, spatial distributions and atomic scale structures of V-rich phases are investigated using transmission electron microscopy (TEM) and related techniques. In these measurements, doped V is found in liquid phase as solute, in large (W,V)C_x precipitates and in interface segregations. Further detailed observations and discussions are carried out for the (W,V)C_x segregated at the WC grain/Co phase interfaces. These (W,V)C_x phases change their form from planar films to small islands depending on the carbon content. The WC grain/Co phase interfaces are fully covered by planar (W,V)C_x in the sample of low carbon content. On the other hand, the WC grain/Co phase interfaces are partially covered by (W,V)C_x islands in the material of high carbon content. During sintering, the WC grains in this sample grew much faster than those in the sample of low carbon content. These structural differences are discussed in terms of WC grain/(W,V)C_x interface energy.