Transition of solidification mode and the as-cast γ grain structure in hyperperitectic carbon steels

Formation processes of as-cast γ grain structures during casting of hyperperitectic carbon steels with 0.15–0.45 mass% carbon concentrations have been studied by means of a rapid unidirectional solidification technique. In steels with 0.15–0.41 mass% carbon concentrations, coarse columnar γ grains (CCGs) with a minor axis diameter of 1–3 mm developed along the direction of temperature gradient. In a steel with 0.38 mass% carbon, importantly, columnar γ grains (CGs) whose minor axis diameter is less than 500 μm form before the formation of CCGs and the grain structure changes discontinuously from CG to CCG. The fraction of the CG region increases with an increase in the carbon concentration. In the samples with a carbon concentration higher than 0.43 mass%, the as-cast structure consists of CGs over almost the entire ingots. Analyses of the relation between γ grain and dendrite structures and their crystallographic orientations indicate that the formation of CGs originates from the primary solidification of γ phase instead of δ phase. This is supported by numerical analysis of the dendrite growths.     

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.     

Effect of alloying elements on the γ to α transformation in steel. I

A newly constructed computer program was used to simulate partitionless growth of α in γ of Fe-Ni-C alloys, taking into account so-called solute drag by evaluating the dissipation of Gibbs energy due to diffusion inside the interface and in the nickel spike being pushed in front of it. It could be shown how the conditions at the α/γ interface vary with the velocity. A continuous change from paraequilibrium to quasi-paraconditions could be illustrated in the phase diagram. By combination with an approximate analytical growth equation, it was possible to derive the thickness of α as function of time. The growth velocity was assumed to start at very high values but decreased due to the pile-up of carbon. For alloy compositions outside the limit for quasi-paraconditions and just inside it the growth was predicted to stop suddenly when critical conditions are approached during the reaction. For alloy compositions further inside the limit, there was no such stop, except by the action of impingement of diffusion fields for carbon. Somewhere between the lines for paraequilibrium and quasi-paraconditions there is a rather rapid drop of the final thickness of α. In order to explain experimental information on this drop one must accept an appreciable tendency of segregation of nickel to the α/γ interface.     

The phase-field approach and solute drag modeling of the transition to massive γ → α transformation in binary Fe-C alloys

The transition between diffusion controlled and massive transformation γ →α in Fe–C alloys is investigated by means of phase-field simulations and thermodynamic functions assessed by the Calphad technique as well as diffusional mobilities available in the literature. A gradual variation in properties over an incoherent interface, having a thickness around 1 nm, is assumed. The phase-field simulations are compared with a newly developed technique to model solute drag during phase transformations. Both approaches show qualitatively the same behavior and predict a transition to a massive transformation at a critical temperature below the T₀ line and close to the α/α + γ phase boundary. It is concluded that the quantitative difference between the two predictions stems from different assumptions on how the properties vary across the phase interface yielding a lower dissipation of Gibbs energy by diffusion in the phase-field simulations. The need for more detailed information about the actual variation in interfacial properties is emphasized.     

Study of the structure of cobalt single crystals during the β → α transformation

Methods of metallography, transmission electron microscopy, and X-ray diffraction have been used to study the specific features of the structure formation during the fcc → hcp transformation occurring under different thermokinetic conditions in a single crystal of cobalt. It is shown that only one of four possible versions of hcp-phase crystal orientations is predominantly realized upon the β → α transformation occurring in the single crystal at a low cooling rate (V _cool ～ 1–2 K/min). It has been established that several cycles of slow heating to temperatures of 600, 800 and 1000°C and subsequent cooling of the single crystal do not increase the number of α-phase orientations. The restoration of the initial fcc-phase single-crystal orientation was observed after each heating cycle of the oriented sample, while after cooling the restoration of the preferred hcp-martensite orientation was observed; in this case, the quantity of the retained β phase fixed in the structure at room temperature increases with increasing number of cycles. After rapid quenching from 530°C into salt water (V _cool ～ 600–700 K/min), α-phase crystals of all four possible orientations are formed in the structure. Upon high-temperature quenching from 1000°C, the volume of crystal is divided into packets each containing martensite plates of predominantly one orientation. The transformation-induced recrystallization of the cobalt pseudo-single crystal quenched in salt water has been observed during repeated heating to temperatures above the β → α transformation temperature.     

Kinetics of the δ to γ zirconium hydride transformation in Zr-2.5Nb

When the concentration of hydrogen exceeds the solubility limit in a metal matrix, metal hydrides may appear as precipitates that degrade the performance of the material. Neutron diffraction was combined with microscopy to study the δ to γ phase transformation of zirconium hydride precipitates in Zr-2.5 wt.% Nb. Specimens were heated to dissolve all hydrides, then cooled to holding temperatures ranging from 17–100 °C, to investigate the kinetics of transformation from the high-temperature δ-hydride to the low-temperature γ-hydride. The δ to γ transformation proceeds over a period of many hours, with a rate that increases as the holding temperature is decreased. Transmission Electron Microscopy images indicate that the boundary regions of hydride precipitates transform to the γ-phase, leaving a shrinking core of the δ-phase. The crystallographic orientations of the hydrides appear to be determined by the texture of the α-Zr matrix, even after complete dissolution and re-precipitation.     

Active Eutectoid Decomposition of α → γ + τ _1 and the Morphological Evolution in a Ru-Containing TiAl Alloy

The active eutectoid decomposition and its morphological evolution during slow cooling and isothermal holding were investigated in a Ru-containing Ti Al alloy. The fine τ 1/γ active eutectoid nodules precipitated at α grain boundary and interior during water quenching. The active eutectoid microstructure evolved from sheaf-like, granular to bulky net-like sluggish eutectoid morphology gradually with the decrease in quenching/holding temperatures. The active eutectoid reaction occurs from 1220 to 1290 ℃, while the beginning temperature of sluggish eutectoid locates at 1305 ± 5 ℃. The combination of the intact τ_1 phase and immature nano-sized γ laths suggests a short incubation period of τ_1 phase in the active type. Furtherly, the semiquantitative estimation shows that the growth velocity of active eutectoid is about ninety times higher than sluggish type. In addition, a representative feature of γ phase abruptly ripening along {111} crystallographic plane was also observed in the active eutectoid.     

Special features of the formation of wear-resisting coatings in deposition of alloys of the aluminium–magnesium system on steel

Argon-arc facing processes of the wear-resistant iron-aluminide-based coatings using filler wire of the aluminium–magnum system are studied. Conditions effect on the geometries, chemical composition, and properties of the deposited bead are established.     

Special features of the distribution of interstitial elements in cast and deformed titanium pseudo-α-alloy

Conclusions  

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.     

Elasticity-based model of the variant selection observed in the β to α phase transformation of a Zircalloy-4 sample

The β→α texture inheritance of a Zircalloy-4 sample has been investigated after an α→β→α transformation cycle. The final inherited α texture has been determined from a crystal orientation map determined by electron back-scattering diffraction, whereas the texture of the high temperature β phase has been reconstructed by a method analysing the orientations and misorientations of α variants. The comparison of the α texture calculated from the parent β texture without variant selection with the experimental sharp α texture shows differences due to a strong variant selection mechanism occurring during the phase transformation at cooling.A model of a variant selection mechanism based on the elastic anisotropy of the parent β phase leads to a simulated inherited α texture with the main characteristics of the experimental texture.     

Recrystallization and the possibility of the realization of the α-γ diffusionless transformation during the ultrarapid laser heating of steels

Analysis is given of phase and structural transformations occurring upon ultrarapid laser heating in steels with different initial structures, namely, after annealing, after preliminary quenching, quenching and tempering, and after quenching with subsequent deformation and tempering. It is shown that a significant suppression of diffusion processes occurs during laser heating; this circumstance substantially affects the nature of the phase and structural transformations proceeding during laser processing. Special attention is given to studying the process of recrystallization and to the phenomenon of structural heredity during laser heating. The process of recrystallization during laser heating is considered as consisting of two stages, namely, an ordered lattice rearrangement (α-γ transformation) and the recrystallization of austenite that suffered phase-transformation-induced hardening (“phase naklep”). The effect of tempering and plastic deformation on the recrystallization of a preliminarily quenched steel consists in the intensification of the second stage, i.e., of the recrystallization of the transformation-hardened austenite. It is shown that the α-γ transformation during the laser heating of steels with the initial structure of lath martensite occurs by the “mechanism of recovery,” i.e., via the formation and growth of austenite nuclei. In steels with the initial structure of pearlite, the nucleation of austenite during laser heating can occur by a shear martensite-like diffusionless mechanism with the observance of characteristic orientation relationships between the initial ferrite and the newly formed austenite.     

Influence of carbon content on the microstructure,martensitic transformation and mechanical properties in austenite/ε-martensite dual-phase Fe–Mn–C steels

We report on the effects of carbon content on the martensitic transformation and its contribution to the work-hardening behavior of Fe–Mn–C steels during tensile deformation based on analysis by X-ray diffraction, electron backscatter diffraction and transmission electron microscopy. Austenite/ε-martensite dual-phase Fe–17Mn–C (wt.%) steels containing different carbon contents (0.01, 0.10, 0.20 wt.%) were investigated before, during and after tensile deformation. Before deformation, the transformation of austenite to thermally induced ε-martensite on cooling was suppressed as the carbon content increases. To precisely monitor microstructural changes during deformation, stepwise loading experiments were carried out in combination with electron backscatter diffraction analysis. This approach revealed that with increasing carbon content, the kinetics of transformation of γ phase to deformation stimulated ε-martensite became faster, while that of ε-martensite to α’-martensite was sluggish. We attribute this controversial effect to an increased γ grain size by the microstructural refinement of thermally induced ε-martensite and the reduction of solid solution strengthening effects by the redistribution of solute carbon. In addition, the dependence of deformation-induced ε-martensite on the loading direction differed from that of α’-martensite, and the evolution of α’ morphology was controlled by achieving appropriate levels of strain during stepwise loading. Based on the observations at the surface and inside the bulk after deformation, insights into various deformation-driven displacive phenomena, such as the formation of α’-martensite at the nonintersecting parts of two ε_initial bands, the presence of nanotwinned bundles inside austenite, cementite precipitation inside α’-martensite, and the origin of the serrated flow in strain–stress curves, were obtained. Therefore, the present study is able assist in identifying whether the deformation-induced martensitic transformation varied as a function of carbon content and the resulting fracture behavior, thereby enabling us to understand the work-hardening behavior of these steels.     

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.     

Advances in the research of nitrogen containing stainless steels

The current status of nitrogen containing stainless steels at home and aboard has been introduced. The function and existing forms of nitrogen in the stainless steels, influence of nitrogen on mechanical properties and anti-corrosion properties as well as the application of nitrogen containing cast stainless steels were discussed in this paper. It is clear that nitrogen will be a potential and important alloying element in stainless steels. And Argon Oxygen Decarbonization (AOD) refining can provide an advanced manufacture process for nitrogen containing stainless steels with ultra-low- carbon and high cleanliness.     

On the fcc→D019 transformation in Co–W alloys

The continuous precipitation of D0₁₉ Co₃W from a highly supersaturated Co-rich fcc matrix has been investigated. A detailed analysis of planar defects generated in the precipitates has been carried out using conventional and high-resolution transmission electron microscopy. The spatial arrangement of stacking faults and antiphase boundaries revealed the actual defect formation mechanisms and disclosed important aspects of the phase transformation at an atomic level. The results point to a displacive/diffusional Widmanstätten type of transformation, which is not simply based on a common ledge growth and, furthermore, follows the path: fcc→hcp→D0₁₉. The α′+Co₇W₆→Co₃W peritectoid transformation has also been studied.     

Model of the B2 → R martensitic transformation in alloys with a B2 superstructure

Various B2 ?? R structural transformations that can occur through an intermediate R ₀ structure belonging to the space group D _3d ³ have been considered. Subgroups with a hexagonal lattice (group D _3d ³ ) related to the symmetry breakdown of the R ₀ structure due to atomic displacements with wave vectors belonging to the two-arm star $K_{13}^H = \left\{ {k_1^H = \frac{1} {3}\left( {b_1^H + b_2^H } \right),k_2^H = - k_1^H } \right\}$ , where b ₁ ^H and b ₂ ^H are the reciprocal-lattice vectors of the hexagonal system, have been calculated. A crystallographic model of the possible transformations of the B2 structure into the R martensite, which has a single (common for all transformations) order parameter, has been suggested.     

Orientation dependence of the γ-α’ martensitic transformation in single crystals of austenitic stainless steels with low stacking-fault energy

The development of the γ-α’ martensitic transformation (MT) upon tensile deformation of single crystals of austenitic stainless steels of compositions (wt %) Fe-17% Cr-12% Ni-2% Mn-0.75% Si (I) and Fe-18% Cr-12% Ni-2% Mo-0.015% C (II) has been studied as a function of the crystal-axis orientation and test temperature by X-ray diffraction and electron microscopy. It has been established that the orientation dependence of the slip deformation preceding the γ-α’ MT is determined by two factors, namely, the orientation dependence of slip deformation preceding the γ-? MT and the orientation dependence of the work U necessary for the formation of the α’-martensite crystals. The orientation dependence of slip deformation preceding the γ-? MT leads to the γ-α’ MT in the [\(\bar 1\)11], [\(\bar 1\)23], [011], and [012] crystals with different defect densities and, correspondingly, at different stress levels. In the [001] crystals, no γ-α’ MT is observed macroscopically since the γ-? MT in these crystals is suppressed. It has been established that the γ-α’ MT in the [\(\bar 1\)11], [011], [\(\bar 1\)23], and [012] crystals can be developed at T = 300 K after preliminary deformation at T = 77 K. The development of the γ-α’ MT at T = 300 K is physically related to the growth of the α’-martensite nuclei formed upon plastic deformation at T = 77 K.