Showing papers on "Austenite published in 1998"

Estimation of bainite plate-thickness in low-alloy steels

Abstract: Published experimental data on the thickness of bainite plates in silicon-rich steels have been analysed in order to develop a quantitative model for the plate-thickness. It is found that the strength of the austenite, and the chemical free energy change accompanying transformation, are by far the most important factors influencing plate-thickness. The transformation temperature does not have any independent effect within the limits of the analysis. These trends are expected from a metallurgical point of view and are discussed in the context of the mechanism of transformation.

A computational model for the prediction of steel hardenability

Abstract: A computational model is presented in this article for the prediction of microstructural development during heat treating of steels and resultant room-temperature hardness. This model was applied in this study to predict the hardness distribution in end-quench bars (Jominy hardness) of heat treatable steels. It consists of a thermodynamics model for the computation of equilibria in multicomponent Fe-C-M systems, a finite element model to simulate the heat transfer induced by end quenching of Jominy bars, and a reaction kinetics model for austenite decomposition. The overall methodology used in this study was similar to the one in the original work of Kirkaldy. Significant efforts were made to reconstitute the reaction kinetics model for austenite decomposition in order to better correlate the phase transformation theory with empiricism and to allow correct phase transformation predictions under continuous cooling conditions. The present model also expanded the applicable chemical composition range. The predictions given by the present model were found to be in good agreement with experimental measurements and showed considerable improvement over the original model developed by Kirkaldy et al.

Effect of martensitic phase transformation and deformation twinning on mechanical properties of Fe–Mn–Si–AI steels

Abstract: Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe–(15–30) wt-%Mn alloys with additions of Al and Si have been investigated. Tensile tests were carrie...

Numerical analysis of GTA welding process with emphasis on post-solidification phase transformation effects on residual stresses

Abstract: The objective of this work was to analyze the residual stress state in spot welds made in an HY-100 steel disk by an autogenous gas tungsten arc (GTA) welding process An uncoupled thermal-mechanical finite element (FE) model was developed that took into account the effects of liquid-to-solid and solid-state phase transformations Effects of variations in mechanical properties due to solid-state phase transformations on residual stresses in the weld were studied Extensive experimental testing was carried out to determine the mechanical properties of HY-100 steel The residual stresses in the disk with the spot weld were measured by a neutron diffraction (ND) technique The FE results are in good agreement with the ND measurements The results show that the volumetric changes associated with the austenite to martensite phase transformation in HY-100 steel significantly affect residual stresses in the weld fusion zone and the heat affected zone

Enhancement of the mechanical properties of a low-carbon, low-silicon steel by formation of a multiphased microstructure containing retained austenite

Abstract: Dual-phase and transformation-induced plasticity (TRIP)-assisted multiphase steels are related families of high-strength formable steels exhibiting excellent mechanical characteristics. This study shows how a ferrite-bainite-martensite microstructure containing retained austenite can improve the mechanical properties of a cold-rolled low-carbon, low-silicon steel. Such a multiphased microstructure is obtained by a heat treatment involving intercritical annealing followed by a bainite transformation tempering. Depending on the heat-treatment parameters, the samples present a variety of microstructures. Due to the presence of retained austenite, some samples exhibit a TRIP effect not anticipated with such a low silicon content. A composite strengthening effect also results from the simultaneous presence of a ductile ferrite matrix with bainite and martensite as hard second phases. A true stress at maximum load of 800 MPa and a true uniform strain of 0.18 can be obtained by forming a ferrite-bainite-martensite microstructure containing up to 10 pct of retained austenite. These properties correspond to a favorable evolution of work hardening during plastic deformation.

Microstructural and mechanical behavior of a duplex stainless steel under hot working conditions

Abstract: In the hot deformation of the duplex stainless steels, the complexity of the microstructure evolution and mechanical response is increased as compared with those of single-phase ferritic or austenitic stainless steels. In the present work, plane strain compression and torsion deformation modes have been used to analyze the microstructural evolution and the mechanical behavior of a duplex stainless steel in as-cast and wrought conditions, as a function of spatial phase distribution, the nature of interface, and the relative mechanical properties of both phases. The law of mixtures has been used to explain the different flow curves obtained when changing the phase distribution and/or the deformation mode. On deforming as-cast microstructures, the deformation partitions vary heterogeneously between both phases and some austenite areas act as hard nondeforming particles. Cracks have been observed to occur at the interface of such regions, from relatively low strains, for which the initial Kurdjumov-Sachs orientation relationship between ferrite and austenite is still present.

The divorced eutectoid transformation in steel

Abstract: Experiments are presented which show that the eutectoid transformation in steel can occur by two different modes for temperatures just slightly below A 1. In the normal mode, the transformation product is lamellar pearlite. The second mode occurs if the austenite contains cementite particles or nuclei with a spacing on the order of a few microns or less. In this case, the transformation product consists of spheroidal cementite particles in a ferrite matrix. This second mode is here called the divorced eutectoid transformation (DET), after recent work by Sherby and co-workers. A literature survey shows that the faster kinetics of the DET over lamellar pearlite in the presence of inhomogeneous austenite was established before 1940, but has received little attention. The inhomogeneities are generally small cementite particles. Experiments show that the DET does not occur by a shell of one phase (ferrite) forming around the other phase of the eutectoid (cementite), as is the case in divorced eutectic growth. Rather, a fairly planar austenite/ferrite front simply advances into the austenite, with no apparent effect on its shape being produced by the cementite particles. A first-order kinetic model is presented for the growth velocity as a function of undercooling below A 1 and is compared to the velocity vs undercooling for lamellar pearlite. The simple model indicates that the velocity of the divorced mode should be faster than the lamellar mode at low undercooling for cementite nuclei distributed in the austenite with spacings less than a few microns. This result is consistent with the experimental data.

Partitioning of chromium and molybdenum in super duplex stainless steels with respect to nitrogen and nickel content

Abstract: Experimental data of elemental partitioning of Cr, Mo, and Ni in a 5% Mn containing SAF 2507 super duplex stainless steel (SDSS) with varying nitrogen and nickel mass fractions are presented. Experimental results on phase equilibria and alloying element partitioning are compared to values calculated using Thermo-Calc software revealing good agreement. A trilinear model is proposed to describe the partitioning ratio of Cr, Mo and Ni with nitrogen and nickel mass percent in the austenite and the annealing temperature as variables. While nitrogen is found to reduce the partitioning of chromium and molybdenum, nickel enhances the partitioning ratio of these to alloying elements. Based on this semi-empirical model, a guideline for the future development of improved SDSS is formulated. With respect to corrosion resistance, a higher nitrogen level is not beneficial by itself but needs to be paralleled by increasing molybdenum and decreasing chromium contents.

Formability of stainless steel

Abstract: The forming behavior of austenitic stainless steels (types 201, 301, and 304) and ferritic stainless steels (types 437, 439, 444, and 468) was investigated. The tensile behavior and the forming-limit diagrams (FLDs) for these grades were determined. The ferritic alloys behave similarly to plain carbon steels and are relatively insensitive to small variations of strain rate and temperature. The formability of the austenitic alloys is influenced greatly by martensitic transformation during straining. The fraction of martensite transformed as a function of strain was found to be very sensitive to temperature, which, in turn, depends on the strain rate at typical testing rates (10−3 to 10−1/s). At low rates (when the specimen remains near room temperature), the formability of the austenitic alloys is markedly improved by transformation strengthening. The enhancement of formability is largest on the biaxial side of the FLD, because the fraction martensite transformed was found to depend on the absolute thickness strain, which is maximized in the balanced biaxial strain state.

Kinetics of γ → α phase transformation in Fe-Mn alloys containing low manganese

Abstract: The kinetics of the γ → α phase transformation in Fe-Mn alloys with low Mn contents was studied using differential scanning calorimetry. Analysis of the measured heat capacity yielded both the austenite fractions as a function of temperature as well as the enthalpy difference between ferrite and austenite which decreases with increases in both the temperature and the Mn concentration. The transformation was modelled using an interface controlled growth model. It was assumed that the interface velocity is proportional to the chemical potential difference of the Fe lattice. The interface mobility has a simple exponential temperature dependence. Taking into account the austenite grain size distribution, the activation energy and the pre-exponential factor for the interface mobility were estimated from the heat effects at 140 kJ mol−1 and 58 mm mol J−1 S−1 respectively.

Ultra-high strength ausaged steels with excellent cryogenic temperature toughness

Abstract: An ultra-high strength, weldable, low alloy steel with excellent cryogenic temperature toughness in the base plate and in the heat affected zone (HAZ) when welded, having a tensile strength greater than about 830 MPa (120 ksi) and a microstructure comprising (i) predominantly fine-grained lower bainite, fine-grained lath martensite, fine granular bainite (FGB), or mixtures thereof, and (ii) up to about 10 vol % retained austenite, is prepared by heating a steel slab comprising iron and specified weight percentages of some or all of the additives carbon, manganese, nickel, nitrogen, copper, chromium, molybdenum, silicon, niobium, vanadium, titanium, aluminum, and boron; reducing the slab to form plate in one or more passes in a temperature range in which austenite recrystallizes; finish rolling the plate in one or more passes in a temperature range below the austenite recrystallization temperature and above the Ar3 transformation temperature; quenching the finish rolled plate to a suitable Quench Stop Temperature (QST); stopping the quenching; and either, for a period of time, holding the plate substantially isothermally at the QST or slow-cooling the plate before air cooling, or simply air cooling the plate to ambient temperature.

Solidification microstructure and M2C carbide decomposition in a spray-formed high-speed steel

Abstract: The solidified carbide morphology, the decomposition behavior of the M2C carbide, and the carbide distribution after forging of an Fe-1.28C-6.4W-5.0Mo-3.1V-4.1Cr-7.9Co (wt pct) high-speed steel prepared by spray forming have been investigated. The spray-formed microstructure has been characterized as a discontinuous network of plate-shaped M2C carbides and a uniform distribution of fine, spherical MC carbides. The metastable M2C carbides formed during solidification have been fully decomposed into MC and M6C carbides after sufficient annealing at high temperatures. Initially, the M6C carbides nucleate at M2C/austenite interfaces and proceed to grow. In the second stage, the MC carbides form either inside the M6C carbides or at the interfaces between M6C carbides. With this increasing degree of decomposition of the M2C carbide, the carbides become more uniformly distributed through hot forging, which produces a significant increase in ultimate bend strength. The decomposition treatment of M2C carbide has been found to be most important for obtaining a fine homogeneous carbide distribution after hot forging.

Factors affecting chromium carbide precipitate dissolution during alloy oxidation

Abstract: Ferrous alloys containing significant volumefractions of chromium carbides were formulated so as tocontain an overall chromium level of 15% (by weight) buta nominal metal matrix chromium concentration of only 11%. Their oxidation at 850°C inpure oxygen led to either protectiveCr2O3 scale formation accompaniedby subsurface carbide dissolution or rapid growth ofiron-rich oxide scales associated with rapid alloy surface recession, which engulfedthe carbides before they could dissolve. Carbide sizewas important in austenitic alloys: an as-castFe-15Cr-0.5C alloy contained relatively coarse carbides and failed to form aCr2O3 scale, whereas the samealloy when hot-forged to produce very fine carbidesoxidized protectively. In ferritic alloys, however, evencoarse carbides dissolved sufficiently rapidly to provide the chromium flux necessary to formand maintain the growth of a Cr2O3scale, a result attributed to the high diffusivity ofthe ferrite phase. Small additions of silicon to theas-cast Fe-15Cr-0.5C alloy rendered it ferritic and led toprotective Cr2O3 growth. However,when the silicon-containing alloy was made austenitic(by the addition of nickel), it still formed aprotective Cr2O3 scale, showing that the principal function of silicon was inmodifying the scale-alloy interface.

Texture in hot rolled austenite and resulting transformation products

Abstract: By making use of a commercially processed TRIP (transformation induced plasticity) steel it was possible to directly measure the texture of hot rolled low alloyed austenite from the traces of retained austenite (9%) remaining at room temperature. This texture was used as the basis for analysing preferred orientation of the transformation products, ferrite and bainite, in the same steel. The EBSP (electron back scattering patterns) method was employed to determine textures of the different phases. The austenite condition was shown to be partially recrystallised, having a β -fibre texture in the deformed structure and a spread from cube to Goss in the recrystallised regions The polygonal ferrite and bainite textures have many similarities but also some differences, being significantly stronger in the bainite, especially the 100〈011〉 component. A model is presented which explains quite well the transformation texture based on the orientation relationship between austenite and ferrite. Where appropriate, variant selectivity is incorporated by reference to accepted theories of phase transformations in steels. Both nucleation and growth are considered to play roles in defining transformation textures, depending on the physical processes which are involved.

Recrystallization–Transformation Combined Reactions during Annealing of a Cold Rolled Ferritic–Austenitic Duplex Stainless Steel

Abstract: Solid state reactions taking place during annealing of 20% cold rolled ferrite-austenite duplex stainless steel (DIN W-Nr: 14462) have been studied by means of several complementary techniques: optical, scanning and transmission electron microscopy, X-ray diffraction analysis, microhardness and ultramicrohardness measurements and magnetic phase detection (ferritoscope) It has been found that after cold rolling austenite exhibited more strain hardening and a higher driving force for recrystallization than ferrite Extensive recovery took place in ferrite during annealing, while the deformation substructure of austenite remained nearly unrecovered until beginning of recrystallization The recrystallization kinetics in ferrite was faster than in austenite Recrystallization in austenite occurred in a more discontinuous manner than in ferrite The eutectoid transformation of ferrite to sigma phase plus austenite slowed down recrystallization kinetics of both ferrite and austenite phases A scheme is presented for sigma phase formation during recrystallization annealing

Microstructural evolution during the austenite-to-ferrite transformation from deformed austenite

Abstract: It is well established that the ferrite grain size of low-carbon steel can be refined by hot rolling of the austenite at temperatures below the nonrecrystallization temperature (Tnr). The strain retained in the austenite increases the number of ferrite nuclei present in the initial stages of transformation. In this work, a C-Mn-Nb steel has been heavily deformed by torsion at temperatures below the determined Tnr for this steel. After deformation, specimens are cooled at a constant cooling rate of 1 °C/s, and interrupted quenching at different temperatures is used to observe different stages of transformation. The transformation kinetics and the evolution of the ferrite grain size have been analyzed. It has been shown that the stored energy due to the accumulated deformation is able to influence the nucleation for low undercoolings by acting on the driving force for transformation; this influence becomes negligible as the temperature decreases. At the early stages of transformation, it has been observed that the preferential nucleation sites of ferrite are the austenite grain boundaries. At the later stages, when impingement becomes important, ferrite coarsening accompanies the transformation and a significant reduction in the number of the ferrite grains per unit volume is observed. As a result, a wide range of ferrite grain sizes is present in the final microstructure, which can influence the mechanical properties of the steel.

Ultra-high strength, weldable steels with excellent ultra-low temperature toughness

Abstract: A steel plate having a tensile strength of at least about 930 MPa (135Ksi), a toughness as measured by Charpy V-notch impact test at -40 °C (-40 °F) of at least about 120 joules (88 ft-lb), and a microstructure comprising at least about 90 volume percent of a mixture of fine-grained lower bainite and fine-grained lath martensite, wherein at least about 2/3 of said mixture consists of fine-grained lower bainite transformed from unrecrystallized austenite having an average grain size of less than about 10 microns and comprising iron and specified weight percentages of the additives: carbon, silicon, manganese, copper, nickel, niobium, titanium, aluminum, calcium, Rare Earth Metals, and magnesium, is prepared by heating a steel slab to a suitable temperature; reducing the slab to form plate in one or more hot rolling passes (10) in a first temperature range in which austenite recrystallizes; further reducing said plate in one or more hot rolling passes (10) in a second temperature range in which austenite does not recrystallize, quenching (12) said plate to a suitable Quench Stop Temperature (16); and stopping said quenching and allowing said plate to air cool (18) to ambient temperature.