Showing papers on "Austenite published in 2001"

Bainite in Steels

Abstract: The mechanism of the bainite transformation in steels is reviewed, beginning with a summary of the early research and finishing with an assessment of the transformation in the context of the other reactions which occur as austenite is cooled to temperatures where it is no longer the stable phase. The review includes a detailed account of the microstructure, chemistry, and crystallography of bainitic ferrite and of the variety of carbide precipitation reactions associated with the bainite transformation. This is followed by an assessment of the thermodynamic and kinetic characteristics of the reaction and by a consideration of the reverse transformation from bainite to austenite. It is argued that there are useful mechanistic distinctions to be made between the coherent growth of ferrite initially supersaturated with carbon (bainite), coherent growth of Widmanstatten ferrite under paraequilibrium conditions, and incoherent growth of ferrite under local equilibrium or paraequilibrium conditions. The nature of the so-called acicular ferrite is also discussed.

On the Influence of Interactions between Phases on the Mechanical Stability of Retained Austenite in Transformation-Induced Plasticity Multiphase Steels

Abstract: The mechanical stability of dispersed retained austenite, i.e., the resistance of this austenite to mechanically induced martensitic transformation, was characterized at room temperature on two steels which differed by their silicon content. The steels had been heat treated in such a way that each specimen presented the same initial volume fraction of austenite and the same austenite grain size. Nevertheless, depending on the specimen, the retained austenite contained different amounts of carbon and was surrounded by different phases. Measurements of the variation of the volume fraction of untransformed austenite as a function of uniaxial plastic strain revealed that, besides the carbon content of retained austenite, the strength of the other phases surrounding austenite grains also influences the austenite resistance to martensitic transformation. The presence of thermal martensite together with the silicon solid-solution strengthening of the intercritical ferrite matrix can “shield” austenite from the externally applied load. As a consequence, the increase of the mechanical stability of retained austenite is not solely related to the decrease of the M s temperature induced by carbon enrichment.

Alloying: Understanding the Basics

Abstract: Alloying: Understanding the Basics, is a comprehensive guide to the influence of alloy additions on mechanical properties, physical properties, corrosion and chemical behavior, and processing and manufacturing characteristics. The coverage considers alloying, to include any addition of an element or compound that interacts with a base metal to influence properties. Thus, the book addresses the beneficial effects of major alloy additions, inoculants, dopants, grain refiners, and other elements that have been deliberately added to improve performance, as well as the detrimental effects of minor elements or residual (tramp) elements included in charge materials, or that result from improper melting or refining techniques. The content is presented in a concise, user-friendly format. Numerous figures and tables are provided. The coverage has been weighted to provide the most detailed information on the most industrially important materials. Contents include: Principles of alloying; Cast irons; Carbon and alloy steels; High-strength low-alloy steels; Tool steels; Maraging and high-fracture-toughness steels; Austenitic manganese steels; Stainless steels; Superalloys; Refractory metal alloys; Ordered intermetallics (nickel, iron, and titanium aluminides); Aluminum alloys; Titanium alloys; Magnesium alloys; Copper alloys; Nickel alloys; Zinc alloys; Tin alloys; Lead alloys; Cobalt alloys; Noble metal alloys; Special-purpose materials (cemented carbides, cermets, low-expansion alloys, electrical contact alloys, magnetic alloys); Index.

Design of novel high strength bainitic steels: Part 1

Abstract: Mixed microstructures consisting of fine plates of upper bainitic ferrite separated by thin films of stable retained austenite have seen many applications in recent years. There may also be some martensite present, although carbides are avoided by the judicious use of silicon as an alloying element. The essential principles governing the optimisation of such microstructures are well established, particularly that large regions of unstable high carbon retained austenite must be avoided. With careful design, impressive combinations of strength and toughness have been reported for high silicon bainitic steels. The aim of the present work was to ascertain how far these concepts could be extended to achieve unprecedented combinations of strength and toughness in bulk samples subjected to continuous cooling transformation, consistent with certain hardenability and processing requirements. Thus, this paper (part 1 of a two part study) deals with the design, using phase transformation theory, of a series ...

Magnetic and X-ray diffraction measurements for the determination of retained austenite in TRIP steels

Abstract: The accurate determination of the volume fraction of retained austenite is of great importance for the optimization of transformation induced plasticity (TRIP) steels. In this work, two aluminium-containing TRIP steels are studied by means of magnetization and X-ray diffraction (XRD) measurements. By fitting the field dependence of the approach to saturation in the magnetization curves, the saturation magnetization is determined, which is linearly related to the volume fraction of retained austenite. Moreover, information with respect to the microstructure can be obtained from the fitting parameters and the demagnetizing factor for the magnetization curve. The volume fractions obtained from the magnetization measurements are compared with data from XRD measurements. A discussion of the data suggests that magnetization measurements lead to more reliable results and a more sensitive detection of the retained austenite than XRD measurements.

Stabilization mechanisms of retained austenite in transformation-induced plasticity steel

Abstract: Three stabilization mechanisms—the shortage of nuclei, the partitioning of alloying elements, and the fine grain size—of the remaining metastable austenite in transformation-induced plasticity (TRIP) steels have been studied by choosing a model alloy Fe-0.2C-1.5Mn-1.5Si. An examination of the nucleus density required for an athermal nucleation mechanism indicates that such a mechanism needs a nucleus density as large as 2.5 · 1017 m−3 when the dispersed austenite grain size is down to 1 µm. Whether the random nucleation on various heterogeneities is likely to dominate the reaction kinetics depends on the heterogeneous embryo density. Chemical stabilization due to the enrichment of carbon in the retained austenite is the most important operational mechanism for the austenite retention. Based on the analysis of 57 engineering steels and some systematic experimental results, an exponential equation describing the influence of carbon concentration on the martensite start (Ms) temperature has been determined to be Ms (K)=273+545.8 · e−1.362wc(mass pct). A function describing the Ms temperature and the energy change of the system has been found, which has been used to study the influence of the grain size on the Ms temperature. The decrease in the grain size of the dispersed residual austenite gives rise to a significant decrease in the Ms temperature when the grain size is as small as 0.1 µm. It is concluded that the influence of the grain size of the retained austenite can become an important factor in decreasing the Ms temperature with respect to the TRIP steels.

The Developments of Cold-rolled TRIP-assisted Multiphase Steels. Al-alloyed TRIP-assisted Multiphase Steels

Abstract: The influence of heat-treating conditions on the retention of carbon-enriched austenite of TRIP-assisted multiphase steel grades containing different amounts of silicon and/or aluminium is investigated. The ensuing mechanical properties resulting from the TRIP effect are also scrutinised. The bainite transformation kinetics was followed by dilatometry whereas a detailed characterisation of the microstructures led to the construction of transformation maps giving the volume fractions of the different phases and the carbon content of austenite. The role of silicon and aluminium additions (i) on the retention of austenite by partial bainite transformation and (ii) on the mechanical properties is enlightened. A strong influence of the solid-solution strengthening effect of silicon is highlighted. Aluminium seems to be an effective alloying element for the retention of austenite in TRIP-aided steels even if lower strength levels can be attained. A mixed Al-Si TRIP-aided steel seems to be a very good compromise between the processing needs, the required mechanical properties and the industrial constraints.

Fatigue-crack growth behavior in the superelastic and shape-memory alloy nitinol

Abstract: This article presents a study of fatigue-crack propagation behavior in Nitinol, a 50Ni-50Ti (at. pct) superelastic/shape-memory alloy, with particular emphasis on the effect of the stress-induced martensitic transformation on crack-growth resistance. Specifically, fatigue-crack growth was characterized in stable austenite (at 120 °C), superelastic austenite (at 37 °C), and martensite (at −65 °C and − 196 °C). In general, fatigue-crack growth resistance was found to increase with decreasing temperature, such that fatigue thresholds were higher and crack-growth rates slower in martensite compared to stable austenite and superelastic austenite. Of note was the observation that the stress-induced transformation of the superelastic austenite structure, which occurs readily at 37 °C during uniaxial tensile testing, could be suppressed during fatigue-crack propagation by the tensile hydrostatic stress state ahead of a crack tip in plane strain; this effect, however, was not seen in thinner specimens, where the constraint was relaxed due to prevailing plane-stress conditions.

Nitrogen and carbon expanded austenite produced by PI3

Abstract: Expanded austenite can be formed either by nitrogen or carbon plasma immersion ion implantation (PI3 ™) from a nitrogen or methane plasma at elevated temperatures. The structure and properties of nitrogen and carbon expanded austenite layers produced on austenitic stainless steel X5CrNi189 are compared. A new structural model of expanded austenite based on a defect rich face centred cubic (fcc) lattice is proposed. Although the structure of the two expanded austenite layers is similar, there is a remarkable difference in the uptake of nitrogen or carbon, despite the use of similar treatment conditions. The modified surfaces have different hardness, corrosion and wear properties.

On the sources of work hardening in multiphase steels assisted by transformation-induced plasticity

Abstract: The mechanisms effectively responsible for the enhancement of the work-hardening capabilities of multiphase steels assisted by transformation-induced plasticity are highlighted. Different microstructures, some containing a proportion of retained austenite with various mechanical stabilities, are studied. The dislocation density generated within ferrite by the mechanically induced martensitic transformation of retained austenite is shown to scale with the incremental work-hardening exponent. The acoustic emission generated during tensile straining was also measured. The acoustic emission was revealed to result mainly from dislocation motion, especially from the motion of the additional dislocation density generated in intercritical ferrite by the strain-induced martensitic transformation.

Heat and corrosion resistant cast stainless steels with improved high temperature strength and ductility

Abstract: A cast stainless steel alloy and articles formed therefrom containing about 0.5 wt. % to about 10 wt. % manganese, 0.02 wt. % to 0.50 wt. % N, and less than 0.15 wt. % sulfur provides high temperature strength both in the matrix and at the grain boundaries without reducing ductility due to cracking along boundaries with continuous or nearly-continuous carbides. Alloys of the present invention also have increased nitrogen solubility thereby enhancing strength at all temperatures because nitride precipitates or nitrogen porosity during casting are not observed. The solubility of nitrogen is dramatically enhanced by the presence of manganese, which also retains or improves the solubility of carbon thereby providing additional solid solution strengthening due to the presence of manganese and nitrogen, and combined carbon. Such solution strengthening enhances the high temperature precipitation-strengthening benefits of fine dispersions of NbC. Such solid solution effects also enhance the stability of the austenite matrix from resistance to excess sigma phase or chrome carbide formation at higher service temperatures. The presence of sulfides is substantially eliminated.

Dilatometric analysis of phase transformations in hypo-eutectoid steels

Abstract: Dilatometry is a useful technique to obtain experimental data concerning transformation kinetics in ferrous alloys. This technique is commonly used in cooling experiments to study the austenite decomposition of hypo-eutectoid steel grades. In the standard analysis of the dilatation signal there are two factors that are normally neglected. During the pro-eutectoid ferrite formation the austenite enriches in carbon, resulting in a non-linear temperature dependence of the specific austenitic volume. Furthermore, the specific volume of the formed ferrite is considerably different from that of the formed pearlite. In total not taking into account these two effects can lead to an error in the determined fraction ferrite of up to 25%. A method is presented that takes into account the two above-mentioned factors. In order to determine both the fraction ferrite and the fraction pearlite, in the analysis the temperature range of the transformation is divided into a ferrite-formation range and a pearlite-formation range. Two possible criteria for this division are discussed, and it is shown that the choice does not have an essential influence on the results.

Characteristics of a plasma electrolytic nitrocarburising treatment for stainless steels

Abstract: In this work, we investigate the feasibility of a plasma electrolytic nitrocarburising (PEN/C) treatment applied to AISI 316 stainless steel using a modified aqueous solution of urea as the treatment electrolyte. The substrate samples were connected cathodically to a high-current DC power supply and biased with a negative voltage in the range 220–260 V. The treatment time was typically in the range of 30–60 s. Investigations of the characteristics of the treated component show that the friction coefficient against a WC–Co ball counterface can be slightly reduced, and that the wear rate decreases by several decades. The microstructure of the treated layers depends strongly on the electrical parameters (e.g. the applied voltage, which controls the treatment temperature) and can be adjusted from a single expanded austenite phase (γ N ) to multi-phase combinations, including mixtures of Fe(Fe,Cr) 2 O 4 , γ N and (Cr,Fe)N x . The corrosion properties of the treated layers are closely correlated to the microstructure and composition and can be significantly improved, particularly if a thin, but dense, magnetite-based iron–chromium oxide layer is produced at the surface. For improvement of the tribological properties of substrates treated at low temperature, a diamond-like-carbon coating was deposited on the PEN/C pre-treated substrate using a plasma-immersion ion-assisted deposition (PIAD) process. Such duplex treatments show great potential for surface modification of stainless steels for applications in aggressive corrosive-wear environments.

Cyclic deformation behavior of single crystal NiTi

Abstract: Single crystals of NiTi (with 50.8 at.% Ni) were subjected to cyclic loading conditions at room temperature which is above the M s (martensite start) temperature of −30°C. The single crystals exhibited remarkable cyclic hardening under zero to compression strain control experiments. The stress range under strain control increased by as much as a factor of 3 in compression. The increase in stress range is primarily due to the increasing strain hardening modulus. In the tension case, loop shape changes occurred but the increase in stress range is rather small. The fatigue cycling was undertaken with a strain range of 3% which is far below the theoretical transformation strains levels exceeding 6%. The maximum stress levels reached in the experiments are below those that cause martensite slip. Therefore, the stress–strain response is governed by transformation from the austenite to the martensitic phases and the dislocation structure evolution in the austenite domains. Two single crystal orientations [148] and [112] were examined during the experiments with single and double CVP (correspondent variant pair) formations respectively. The strain hardening in compression cases is rather substantial with the stress range in the double CVP case surpassing the single CVP case. Two heat treatments were selected to produce coherent and incoherent precipitates in the microstructure respectively. The influence of the coherent precipitates on the stress–strain response is significant as they lower the transformation stress from austenite to martensite, and at the same time, they raise the flow stress of the austenite and martensite domains leading to higher saturation stresses in fatigue.

Estimation of the amount of retained austenite in austempered ductile irons using neural networks

Abstract: Many of the properties of austempered ductile iron depend on the austenite which is retained following the bainite reaction. A neural network model within a Bayesian framework has been created using published data to model the retained austenite content. The model allows the quantity of retained austenite to be estimated as a function of the chemical composition and heat treatment parameters. The computer programs associated with the work have been made freely available ( http //www.msm.cam.ac.uk/map/mapmain.html )

Improved Model for Static Recrystallization Kinetics of Hot Deformed Austenite in Low Alloy and Nb/V Microalloyed Steels

Abstract: Using torsion tests a improved model has been constructed to predict the static recrystallization kinetics of deformed austenite in low alloy and microalloyed steels. The model quantifies the influence of the most common elements (C, Si, Mn, Mo) in low alloy steels and the typical elements (V, Nb) in microalloyed steels, when they are in solution. Activation energy (Q) is the parameter sensitive to the content and nature of each alloying element, and an expression for Q is shown as a function of the percentage of each one. Nb is the element that contributes most to increasing the value of Q, and thus that which most delays recrystallization kinetics. C is seen to be the only alloying element that contributes to lowering the value of Q, and thus to accelerating recrystallization kinetics. Extrapolation of the expression of Q to pure iron in the austenitic phase gives a value of 148 637 J mol-1, which is similar to other values found in the literature for the grain boundary self-diffusion energy of pure Feγ . Static recrystallization kinetics follow Avrami's law and expressions are given for the parameter t0.5 and the exponent n.

Formation of ultra-fine ferrite in hot rolled strip: potential mechanisms for grain refinement

Abstract: A novel single-pass hot strip rolling process has been developed in which ultra-fine (<2 μm) ferrite grains form at the surface of hot rolled strip in two low carbon steels with average austenite grain sizes above 200 μm Two experiments were performed on strip that had been re-heated to 1250°C for 300 s and air-cooled to the rolling temperatures The first involved hot rolling a sample of 009 wt%C–168Mn–022Si–027Mo steel (steel A) at 800°C, which was just above the Ar3 of this sample, while the second involved hot rolling a sample of 011C–168Mn–022Si steel (steel B) at 675°C, which is just below the Ar3 temperature of the sample After air cooling, the surface regions of strip of both steel A and B consisted of ultra-fine ferrite grains which had formed within the large austenite grains, while the central regions consisted of a bainitic microstructure In the case of steel B, a network of allotriomorphic ferrite delineated the prior-austenite grain boundaries throughout the strip cross-section Based on results from optical microscopy and scanning/transmission electron microscopy, as well as bulk X-ray texture analysis and microtextural analysis using Electron Back-Scattered Diffraction (EBSD), it is shown that the ultra-fine ferrite most likely forms by a process of rapid intragranular nucleation during, or immediately after, deformation This process of inducing intragranular nucleation of ferrite by deformation is referred to as strain-induced transformation