Effect of heating rate on reaustenitisation of low carbon niobium microalloyed steel

Abstract

Austenite formation during a continuous heating in a low carbon niobium microalloyed steel with a pearlite and ferrite initial microstructure has been studied. Characteristic transformation temperatures, Ac ₁, Ac _θ and Ac ₃ and the evolution of austenite formation have been determined by combining dilatometry and metallography in a range of heating rates from 0˙05 to 10 K s^–1. It has been observed that nucleation and growth of austenite depends highly on the applied heating rate. At low heating rates (0˙05 K s^–1) nucleation of austenite takes place both at pearlite nodules and at ferrite grain boundaries, while for higher heating rates (≥0˙5 K s^–1), nucleation at grain boundaries is barely present compared to the nucleation at pearlite nodules. The heating rate also affects the austenite growth path and morphology and, thus, the distribution of martensite in the dual phase microstructure obtained at room temperature.     

Isothermal transformations in advanced high strength steels below martensite start temperature

Abstract

The transformation products in advanced high strength steels have been studied during the isothermal decomposition of austenite, subsequent to initial martensite formation. Rapid cooling to various temperatures below martensite start was carried out in a dilatometer with the intention to form controlled volume fractions of initial martensite and austenite, followed by isothermal holding. The transformation kinetics was monitored by means of dilatometry and microstructural characterisation by scanning electron microscopy, electron backscatter diffraction and X-ray diffraction. Hardness measurements of the resulting microstructures were analysed. The results revealed that the microstructures formed below M_S are mainly composed of different fractions of tempered martensite, isothermal bainite with carbide precipitation and retained austenite.     

Anomalous kinetics of lath martensite formation in stainless steel

The kinetics of lath martensite formation in Fe–17·3 wt-%Cr–7·1 wt-%Ni–1·1 wt-%Al–0·08 wt-%C stainless steel was investigated with magnetometry and microscopy. Lath martensite forms during cooling, heating and isothermally. For the first time, it is shown by magnetometry during extremely slow isochronal cooling that transformation rate maxima occur, which are interrupted by virtually transformation free temperature regions. Microscopy confirms martensite formation after athermal nucleation of clusters followed by their time dependent growth. The observations are interpreted in terms of time dependent autocatalytic lath martensite formation followed by mechanical stabilisation of austenite during the transformation process.     

Carbon partitioning during bainite transformation in low alloy steels

Abstract

Carbon partitioning in untransformed austenite during bainite transformation has been studied using high speed dilatometry. It was found that in specimens partially transformed to bainite, during subsequent quenching to ambient temperature two martensite start temperatures M _s can be registered. Because M _s depends directly on a carbon content in austenite, the obtained results may indicate that the carbon concentration trapped in films of austenite between parallel subunits of bainitic ferrite is much larger than in the blocks of austenite. It would indicate the necessity of a substantial modification of bainite and martensite regions on the time–temperature–transformation (continuous cooling) diagrams.     

Thermally induced martensite properties in Fe-29%Ni-2%Mn alloy

Thermally induced martensite properties in Fe-29%Ni-2%Mn alloy were investigated according to martensitic transformation kinetics, morphology, magnetism of both austenite and martensite phases and also in terms of martensitic transformation start temperatures (M_s) for different austenite grain sizes of alloy. Kinetics of the transformation was found to be athermal. Also only lenticular martensite morphology was observed during investigations. On the other hand, Mössbauer spectra revealed a paramagnetic character for austenite phases and a ferromagnetic character for thermally induced martensitic phases. Determined M_s temperatures were found to be at − 128 °C for large grained samples and − 135 °C for small grained samples.     

Application of Koistinen and Marburger’s athermal equation for volume fraction of martensite to diffusional transformations obtained on continuous cooling 0·13%C high strength low alloy steel

Abstract

Koistinen and Marburger’s (KM) equation for the variation of volume fraction of athermal martensite y with temperature Tbelow the M _s has been applied to continuous cooling diffusional data. The data consisted of dilatometer curves obtained on continuous cooling of a 0·13%C high strength low alloy steel. The KM equation takes the form, ln(1 - y) = α(M _s - T ). Plots of -ln (1 - y) against temperature for what are thought to be grain boundary ferrite, intragranular ferrite, proeutectoid ferrite, and pearlite give a series of straight lines of increasing slope α. Intersections of these lines give the temperature of transformation points in good agreement with those on the dilatometry curves. Values of α obtained for each transformation are compared with those previously obtained for martensite in plain carbon and alloy steels and ferrite in Fe–9%Ni.     

Age hardening behaviour during reverse transformation from martensite to austenite in Fe–31·1wt-%Ni

Abstract

The effects of thermomechanical treatments on the reverse transformation behaviour from twinned plate martensite to austenite in Fe–31·1%Ni have been studied. The variation of both diffusion controlled and diffusionless reverse transformation with temperature and time was examined. Diffusional reversion was dominant at lower reheating temperatures and led to a fine martensite–austenite duplex microstructure with a grain size of 0·01–0·1 μm, which caused a remarkable hardening ?H_v of 170–230 HV during aging. Cold working of the martensite promoted diffusional reversion and enhanced age hardening. X-ray analysis indicates that the age hardening is caused mainly by elastic strain resulting from coherent precipitation of austenite in martensite.

MST/1414     

Thermally activated growth of lath martensite in Fe–Cr–Ni–Al stainless steel

The austenite to martensite transformation in a semi-austenitic stainless steel containing 17 wt-%Cr, 7 wt-%Ni and 1 wt-%Al was investigated with vibrating sample magnetometry and electron backscatter diffraction. Magnetometry demonstrated that, within experimental accuracy, martensite formation can be suppressed on fast cooling to 77 K as well as on subsequent fast heating to 373 K. Surprisingly, martensite formation was observed during moderate heating from 77 K, instead. Electron backscatter diffraction demonstrated that the morphology of martensite is lath type. The kinetics of the transformation is interpreted in terms of athermal nucleation of lath martensite followed by thermally activated growth. It is anticipated that substantial autocatalytic martensite formation occurs during thermally activated growth. The observation of a retardation of the transformation followed by a new acceleration during slow isochronal (i.e. at constant rate) cooling is interpreted in terms of the combined effect of the strain energy introduced in the system during martensite formation, which thermodynamically and/or mechanically stabilises austenite, and autocatalytic nucleation of martensite.     

Effect of cooling rate on grain size of ferrite in a carbon steel

Abstract

Ferrite grain refinement by accelerated cooling has been studied in a carbon steel. The size of ferrite grains d_α formed by continuous cooling transformation from polygonal austenite has been measured as a function of cooling rate and austenite grain size d_γ. In the cooling rate range studied (q= 0·05–5 K s^?1), d_α was found to be proportional to q^?0·26d_γ^0·46. The mechanism of grain refinement by accelerated cooling is discussed, and it is shown that this occurs in the transformation where the ratio of nucleation to growth rate increases with a decrease in temperature. The austenite grain size dependence of ferrite grain size is shown to become progressively large as the nucleation mode changes from homogeneous to grain surface to edge to corner. A theoretical estimation of ferrite grain size formed by continuous cooling transformation was attempted on the basis of nucleation and growth rates. In the alloy studied, ferrite grain size was theoretically estimated to be proportional to q^?0·17d_γ^0·33. This was in close agreement with the dependence obtained in the present experiment.

MST/466     

Mechanical stabilisation of bainite

Abstract

Compression experiments in which plastically deformed austenite is allowed to transform to bainite have revealed that bainite, like martensite, is susceptible to mechanical stabilisation. The overall transformation kinetics becomes slower and the maximum attainable fraction of bainite decreases in deformed austenite. This is because the motion of the transformation interface is hindered by the accumulated debris of dislocations in the austenite. The number density of nucleation sites is increased in deformed austenite, resulting in a more refined microstructure. Severe deformation eventually leads to a recovery in the maximum attainable fraction of bainite because of the corresponding increase in nucleation site density.

MST/3148     

Pressure induced martensite transformation in plain carbon steel

Abstract

In the present study, plain low carbon steel with 0·033 wt-% carbon content was subjected to severe pressure during continuous cooling from austenite region. The pressure increased gradually and then suddenly released by the breakdown of ram under pressure. As a result, a microstructure composed of 80% lath martensite and 20% ferrite was produced. Results showed that the martensite formation is not due to the effect of cooling rate but the effect of hydrostatic pressure on the austenite to ferrite transformation start temperature Ar₃.     

Phase transformations during the decomposition of austenite below Ms in a low-carbon steel

The purpose of this study is to investigate and understand the phase transformations during the decomposition of austenite, which occurs during isothermal treatments below the martensite start temperature (M_s) in a low-carbon steel. Isothermal holding treatments after rapid cooling to various temperatures (forming a controlled volume fraction of initial martensite) were carried out in a dilatometer. Results obtained by dilatometry, microstructural characterization and hardness were analyzed. This combination of results shows that the microstructures formed below the M_s temperature are mainly bainitic, mixed with tempered martensite. The kinetics of isothermal bainite formation was described by a nucleation-based transformation model. The complex competition and interactions between their transformation mechanisms during the isothermal holding at different temperature regimes are discussed.     

Continuous cooling transformation behaviour of Si–Mn and Al–Mn transformation induced plasticity steels

Abstract

The continuous cooling transformation (CCT) behaviour of two transformation induced plasticity (TRIP) steels was investigated using quench dilatometry. One was an established steel grade with a composition (wt-%) of Fe–0·2C–2Si–1·5Mn while the other steel was a novel composition where 2 wt-% Al replaced the silicon in the former grade. Characteristics of the α→γ transformation during reheating and the subsequent decomposition of austenite during continuous cooling were studied by dilatometry, and CCT diagrams were constructed for both steels. The effects of accelerated cooling and steel composition on γ transformation start temperature Ar ₃, phase transformation kinetics, and microhardness were investigated. The results showed that the Al–Mn steel had a much wider α→γ transformation range during reheating, compared with the Si–Mn steel. Furthermore, the Al–Mn steel exhibited no significant change in the rate of expansion during α→γ transformation. On the other hand, during continuous cooling, the Al–Mn steel exhibited higher Ar ₃, faster transformation kinetics, a higher volume fraction of polygonal ferrite in the microstructure, and lower hardness, compared with the Si–Mn steel. The addition of aluminium was found to have a significant effect on the products of phase transformation, kinetics, and form of the CCT diagram. For both steels, an increase in cooling rate lowered the Ar ₃ temperature, decreased the time of transformation, and increased the hardness.     

Acoustic emission monitoring of bainitic and martensitic transformation in medium carbon steel during continuous cooling

Abstract

This paper concerns acoustic emission (AE) measurements during continuous cooling of steel C45 using a Gleeble 1500 thermomechanical simulator. After austenising at a certain temperature, the studied specimen was cooled down and the root mean square (RMS) value of the continuous AE signal was measured. During cooling two distinct peaks in the RMS data were observed at temperatures of 200-300°C and 500-600°C, which have been attributed to martensite and bainite formation respectively. The observed bainite peak strongly indicated that the mechanism of bainite growth is displacive. The AE monitoring of bainite and martensite formation was supported by dilatation measurements, which were performed simultaneously. The effect of the austenite grain size on the evolution of the bainitic and martensitic transformation was studied by varying the austenising temperature T _a. It was found that upon lowering T _a, i.e. with decreasing austenite grain size, the bainite peak increases while the martensite peak decreases.     

Initial nucleation kinetics of martensite transformation

The initial rate of martensite transformation in Fe–Ni and Fe–Ni–Mn is described by the product of the probability of a nucleation site existing in an austenite grain times the probability of its propagation. The former depends on driving force, the latter on defect mobility. The onset of both athermal and isothermal martensite could be modeled in a consistent way, which suggests that both modes have common fundamentals.     

Microstructural evolution during thermomechanical processing of microalloyed medium carbon steels

Abstract

A laboratory study was carried out to determine the characteristics of austenite grain growth and recrystallisation, strain induced precipitation, and continuous cooling transformation kinetics for two microalloyed medium carbon steels (1541 + Ti,V and 1541 + Nb). Austenite grain refinement is achieved by a combination of undissolved carbonitride precipitates at the reheat temperature, deformation recrystallisation at temperatures above T _NR and strain induced carbonitride precipitation. Deformation below T _NR promotes transformation to grain boundary ferrite (GBF), intragranular ferrite (IGF), and pearlite (P) at the expense of bainite (B) in both steels. This is attributed to increased density of nucleation sites for ferrite and pearlite at austenite grain boundaries, twin boundaries, and deformation bands. The results suggest that thermomechanical forging schedules could be designed to produce refined F + P microstructure, and hence, to realise improved strength, toughness, and machinability in the forging.     

Martensitic transformation and work hardening of metastable austenite induced by abrasion in austenitic Fe-C-Cr-Mn-B Alloy - a TEM study

Abstract

The martensite transformation and work hardening of metastable austenite induced by abrasive wear in an austenitic Fe-C-Cr-Mn-B alloy were studied by TEM. The results show that an α' martensitic transformation occurred to form an elongated and equiaxial cellular dislocation substructure and the untransformed austenite matrix produced an equiaxial cellular dislocation substructure on the abraded surface. Electron diffraction patterns of the abraded material are composed of diffraction rings with series of broken arcs resulting from a fine grain structure and the deformation texture. The work hardening zone of austenite at the subsurface reveals heavy slip bands and deformation faults, at which the dislocations pile up. Examples of ? martensite induced by abrasive wear can be detected. The α' martensite transformation and metastable austenite work hardening should bring about an increase in surface hardness and wear resistance. Additionally, the cellular dislocation substructure of α' and γ have a significant effect on increasing the hardness of the wear surface. Observation by TEM indicates that the α' martensite transformation happens more easily in the austenite matrix near the carbide (Fe, Cr)₇C₃ as a result of the depletion of carbon and chromium.     

Effects of solution treatment temperature on grain growth and mechanical properties of high strength 18%Ni cobalt free maraging steel

Abstract

The effects of solution treatment (ST) temperature (1073–1473 K) on the prior austenite grain size, microstructure, and mechanical properties of a 2000 MPa grade 18%Ni Co free maraging steel have been investigated. The results show that prior austenite grain size normally increases with increase of ST temperature. Strength and ductility in the solution treated condition are independent of both ST temperature and prior austenite grain size due to constant martensite lath spacing and dislocation tangles. In the solution treated + aged condition, the relationship between yield strength and prior austenite grain size follows the Hall- Petch equation, and ductility improves until the ST temperature used is >1373 K. Accordingly, the fracture mode transforms from intergranular to transgranular at a critical prior austenite grain size of ~ 150 μ m, because of severe segregation of Ni₃(Mo,Ti) and reverted austenite at prior austenite grain boundaries and martensite lath boundaries. The variation of Charpy V notch impact energy with increase of ST temperature in both the solution treated and solution treated + aged conditions is similar to that of the tensile ductility. The fracture toughness K_IC, however, increases with increase of ST temperature. No thermal embrittlement resulted from the Ti(C,N,S) inclusion segregation at prior austenite grain boundaries and martensite lath boundaries in the high temperature solution treatment.     

A JMAK-like approach for isochronal austenite–ferrite transformation kinetics in Fe–0·055 wt-%N alloy

Abstract

The kinetics of the isochronal austenite γ to ferrite α transformation of Fe–0·055 wt-%N alloy was investigated for cooling rates in the range of 5–15 K min^? by high resolution dilatometry. In accordance with thermodynamic characteristics of γ→α transformation investigated in this study and previous kinetic theory, a Johnson–Mehl–Avrami–Kologoromov (JMAK)-like approach for the kinetics of isochronal phase transformations was developed that incorporates three overlapping processes: site saturation nucleation, alternate growth modes (transition from interface- to diffusion-controlled growth) as well as impingement for random distribution nuclei. The JMAK-like approach has been employed to fit the experimental results, and the fitting results show that the γ→α transformation of Fe–0·055 wt-%N alloy does have two stages: a first, short interface-controlled growth stage and a second, long diffusion-controlled growth stage. In addition, soft impingement effect has been recognised to become serious in the later part of the second stage.     

On the austenite stability of cryogenic Ni steels: microstructural effects: a review

Austenite stability is essentially important in improving the cryogenic toughness of cryogenic Ni steels and guiding the development of Ni-saving cryogenic steels. The austenite stability in the cryogenic Ni steels is influenced by many microstructure features, making it a complicated issue which is lack of a systematic discussion. In this article, the microstructural effects on the thermal and mechanical stability of austenite in the cryogenic Ni steels are reviewed and discussed. The thermal stability of austenite (TSA) will be enhanced by the enrichment of austenite-stabilizing elements in the austenite which decreases the martensite-start (M_s) temperature. The grain refinement enhances the TSA by synergistically increasing the nonchemical driving force for the martensite transformation and the concentrations of austenite-stabilizing elements in the austenite. The excessive increase in the volume fraction of austenite weakens the TSA by decreasing the concentrations of austenite-stabilizing elements in the austenite. The film austenite is usually thermally more stable than the block austenite owing to its higher concentrations of austenite-stabilizing elements. The mechanical stability of austenite (MSA) is also influenced by the concentrations of austenite-stabilizing elements which affect the M_s temperature. The reports on the effect of grain size of austenite on the MSA are inconsistent. Both negligible and important effects of the grain size of austenite on the MSA are analyzed. The grain orientation of austenite affects the MSA via changing the Schmid factor and the additional driving force for the martensite transformation. The orientation which yields a larger value of Schmid factor would exhibit a lower MSA. The MSA is affected by the matrix or the neighboring phase due to the stress and strain partitioning among austenite and other constituent phases. The dislocation multiplication could weaken the MSA by assisting the nucleation and growth of martensite embryo and enhance the MSA by hindering the motion of embryo/austenite interfaces when dislocation density is sufficiently large. Austenite with a combination of a high TSA and a moderate or high MSA is considered to be effective strategies to enhance cryogenic toughness of the cryogenic Ni steels.