Kinetics and dilatometric behaviour of non-isothermal ferrite–austenite transformation

Abstract

A model that describes the ferrite–austenite transformation during continuous heating in Armco iron and three very low carbon, low manganese steels with a fully ferritic initial microstructure is presented. This model allows calculation of the volume fractions of austenite and ferrite during transformation as a function of temperature, and hence knowledge of the austenite formation kinetics under non-isothermal conditions in fully ferritic steels. Moreover, since dilatometric analysis is a technique very often used to study phase transformations in steels, a second model, which describes the dilatometric behaviour of the material and calculates the relative change in length that occurs during the ferrite–austenite transformation, has also been developed. Both kinetics and dilatometric models have been validated by comparison of theoretical and experimental dilatometric heating curves. Predicted and experimental results are in satisfactory agreement.     

Microstructure and mechanical properties of C–Si–Mn(–Nb) TRIP steels after simulated thermomechanical processing

Abstract

Continuous and discontinuous cooling tests were performed using a quench deformation dilatometer to develop a comprehensive understanding of the structural and kinetic aspects of the bainite transformation in low carbon TRIP (transformation induced plasticity) steels as a function of thermomechanical processing and composition. Deformation in the unrecrystallised austenite region refined the ferrite grain size and increased the ferrite and bainite transformation temperatures for cooling rates from 10 to 90 K s^-1. The influence of niobium on the transformation kinetics was also investigated. Niobium increases the ferrite start transformation temperature, refines the ferrite microstructure, and stimulates the formation of acicular ferrite. The effect of the bainite isothermal transformation temperature on the final microstructure of steels with and without a small addition of niobium was studied. Niobium promotes the formation of stable retained austenite, which influences the mechanical properties of TRIP steels. The optimum mechanical properties were obtained after isothermal holding at 400°C in the niobium steel containing the maximum volume fraction of retained austenite with acicular ferrite as the predominant second phase.     

Effect of austenitising temperature on tensile properties of Cu–Ni austempered ductile iron

Abstract

The effect of austenitising temperature on the microstructure, mechanical properties, and dimensional stability of a spheroidal graphite iron containing copper and nickel has been investigated. It was found that as the temperature increased the amount of carbon taken into solution by the austenite increased thus reducing the driving force of the original austenite to bainitic ferrite and high carbon austenite. As a consequence, the amount of retained austenite increased, but its stability decreased. This placed an upper limit on the austenitising temperature and on the amount of retained austenite permissible. All properties other than hardness showed maximum values after austenitising at 900°C. It was also found that increasing the solution treatment temperature increased the dimensional stability.

MST/1116     

Modelling thermal and microstructural evolution on runout table of hot strip mill

Abstract

A mathematical model has been developed to predict the thermal and phase transformation response of a 0·34 and a 0·05 wt-%C steel during cooling on the runout table of a hot strip mill. The model incorporates the cooling characteristics of laminar water bar sprays, the austenite–ferrite plus pearlite phase transformation kinetics as a function of the austenite grain size, and the heat of transformation. Overall heat transfer coefficients for the laminar water banks were determined from data obtained from in-plant trials carried out at the Stelco Lake Erie Works (LEW) hot strip mill. Isothermal and continuous cooling diametral dilatometer tests were performed on a Gleeble 1500 thermomechanical simulator at temperatures and cooling rates that simulate LEW hot strip mill conditions. The isothermal data were used to establish the phase transformation kinetics as afunction of austenite grain size and temperature. The continuous cooling results were used to obtain the relationship between cooling rate, transformation start temperature, and fraction of ferrite formed. The model was tested and validated by simulating the LEW cooling conditions while monitoring the phase transformation behaviour and by comparison of predicted and measured microstructural detail.

MST/1331     

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.     

Carbon composition and temperature dependence of relative change in length during isothermal decomposition of Fe–C austenite

Abstract

The carbon composition and temperature dependence of the relative change in length during isothermal decompos ition of Fe–C austenite has been modelled. Decomposition of austenite above and below the A₁ temperature has been considered, as well as a two-step procedure where a specimen decomposes into ferrite and carbon enriched austenite at a temperature above A₁ and is subsequently subjected to transformation below A₁ leading to ferrite and cementite. Analytical expressions have been given for the relative change in length as a function of carbon composition, isothermal decomposition temperature, and degree of transformation. Predicted changes in length have been compared with experimental results.     

Tensile behaviour of duplex stainless steel at low temperatures

Abstract

The tensile behaviour of the ferrite and austenite phases of Fe–22Cr–5Ni (wt-%) duplex stainless steel containing a maximum of 17·2% austenite was investigated in the temperature range 65–298 K. The results indicate that mechanical twinning occurred in the testing temperature range, and that austenite impeded the growth of twinning. Mechanical twinning in ferrite was well decorated with a ‘dislocation shell’, and the density of dislocations at the coherent twin boundary and within a twin was much higher than in the matrix above the ductile–brittle transition temperature (DBTT). This supported the occurrence of slip localisation next to coherent twin boundaries. Dislocations in the material with no austenite tested below the DBTT were characterised by coplanar slip dislocation on the { 110} plane, and both coplanar slip on { 110} and cross-slip dislocations were observed above the DBTT. Dislocation in ferrite was negligibly affected by the presence of austenitic particles. Strain induced martensite transformation occurred in austenitic particles at or below 220 K, and the characteristics of the transformation were essentially similar to those in type 304 stainless steel. The DBTT of the material was lowered from ～140 to 110 K in the presence of austenite, independent of the volume fraction of austenite. This suggests that the decrease in the DBTT of the material was mainly due to austenite scavenging carbon and other interstitial elements from the ferritic matrix. The fracture of the material at low temperatures was primarily controlled by the fracture of twin boundaries in ferrite.     

Effect of austenite grain size on isothermal bainite transformation in low carbon microalloyed steel

Abstract

The effect of austenite grain size on isothermal bainite transformation in a low carbon microalloyed steel was studied by means of optical microscopy, SEM and TEM. Two widely varying austenite grain sizes, a fine average grain size (～20 μm) and a coarse average grain size (～260 μm), were obtained by different maximum heating temperatures. The results showed that the morphology of isothermal microstructure changes from bainite without carbide precipitation to bainitic ferrite with a decrease in holding temperature. Coarse austenite grain can retard the kinetics of bainite transformation and increase the incubation time of bainite transformation by reducing the number of nucleation site, but it does not influence the nose temperature of the C curve of bainite start transformation, which is ～534°C.     

Morphology and constitution of the phases in as-welded microstructure of austempered ductile iron

Abstract

It was found by optical and electron microscopic examination of the microstructure of as-weld austempered ductile iron that the weld matrix is composed of austenite and bainite, the volume fractions of which were determined. In addition, the carbon content of austenite was measured and therefore the average carbon content of the matrix was calculated. In the matrix of the weld metal two types of bainite, bainite ferrite and lower bainite, were found. According to the morphology and distribution of the bainite plates, the nucleation and growth modes of bainite was inferred.     

Transition from bainite to acicular ferrite in reheated Fe–Cr–C weld deposits

Abstract

Factors controlling the transition from acicular ferrite to bainite in Fe–Cr–C weld metals have been investigated. It appears that the presence of allotriomorphs of ferrite at austenite grain boundaries has the effect of suppressing the formation of bainitic sheaves. This in turn allows the acicular ferrite plates to develop on intragranular nucleation sites. A theoretical analysis indicates that bainitic transformation is prevented from developing at the allotriomorphic ferrite/austenite boundaries by the carbon concentration field present in the austenite at the allotriomorphic ferrite/austenite interface. This field does not homogenise within the residual austenite during the time scale of the experiments.

MST/1217     

Austenite films in bainitic microstructures

Abstract

Bainitic microstructures in which fine platelets of ferrite are intimately mixed with films of austenite are known to exhibit good combinations of strength and toughness. It isfound that the thickness of these austenite films can be estimated by assuming that the carbon diffusion field around an existing plate of ferrite prevents the close approach of another parallel plate. This is because the regions of austenite with the highest carbon concentration are unable to transform to bainite.

MST/3052     

Microstructure evolution during thermomechanical processing in 3Mn-0.1C medium-Mn steel

ABSTRACT

Medium-Mn steels are energetically investigated as a candidate of the third generation advanced high strength steels (AHSSs). However, their phase transformation and microstructaure evolution during various heat treatments and thermomechanical processing are still unclear. The present study first confirmed the kinetics of static phase transformation behaviour in a 3Mn-0.1C medium-Mn steel. Hot compression tests were also carried out to investigate the influence of high-temperature deformation of austenite on subsequent microstructure evolution. It was found that static ferrite transformation was quite slow in this steel, but ferrite transformation was greatly accelerated by the hot deformation in austenite and ferrite two-phase regions. Characteristic dual-phase microstructures composed of martensite and fine-grained ferrite were obtained, which exhibited superior mechanical properties.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

Austenite stability for quenching and partitioning treated steel revealed by colour tint-etching method

Abstract

The complex microstructures of quenching and partitioning treated (QP980) steel have been investigated using two-step colour tint-etching method and further verified by X-ray diffraction, electron backscattering diffraction, magnetisation measurements and Mössbauer spectroscopy. The colour tint-etching method can quantitatively discriminate the ferrite, martensite and retained austenite by obviously colour differences. It is found that retained austenite was observed inside both martensite and ferrite, and the fraction of retained austenite in martensite was statistically higher but more scattering than that in ferrite. Moreover, the retained austenite in martensite is a little bit more stable than that in ferrite by comparing the change of volume fraction retained austenite in both phases after tension.     

Microstructures resulting from the interaction between ferrite recrystallization and austenite formation in dual-phase steels

The present work investigates the interactions between ferrite recrystallization and austenite formation in dual-phase steels by experiments performed at high heating rate (100 °C/s). It was shown that both ferrite recrystallization and austenite formation are strongly coupled and interdependent. The kinetics of ferrite recrystallization is strongly affected by the formation of austenite and can be even inhibited in some cases. The microstructure is more heterogeneous and anisotropic when both the austenite formation and the ferrite recrystallization overlap. It was highlighted that the degree of anisotropy depends on the volume fraction of austenite at a given temperature. Furthermore, an unusual behavior for austenite growth was highlighted. It is characterized by a much higher volume fraction than those obtained under OrthoEquilibrium and ParaEquilibrium. The results, especially those at 715 °C close to the eutectoid plateau, at which the driving force for austenite growth is classically low, suggest a diffusionless transformation for austenite.     

Overview Inclusion assisted microstructure control in C–Mn and low alloy steel welds

Abstract

Inclusion assisted microstructure control has been a key technology to improve the toughness of C–Mn and low alloy steel welds over the last two to three decades. The microstructure of weld metals and heat affected zones (HAZs) is known to be refined by different inclusions, which may act as nucleation sites for intragranular acicular ferrite and/or to pin austenite grains thereby preventing grain growth. In the present paper, the nature of acicular ferrite and the kinetics of intragranular ferrite transformations in both weld metals and the HAZ of steels are rationalised along with nucleation mechanisms. Acicular ferrite development is considered in terms of competitive nucleation and growth reactions at austenite grain boundary and intragranular inclusion nucleation sites. It is shown that compared to weld metals, it is difficult to shift the balance of ferrite nucleation from the austenite grain boundaries to the intragranular regions in the HAZ of particle dispersed steels because inclusion densities are lower and the surface area available for ferrite nucleation at the austenite grain boundaries tends to be greater than that of intragranular inclusions. The most consistent explanation of high nucleation potency in weld metals is provided by lattice matching between ferrite and the inclusion surface to reduce the interfacial energy opposing nucleation. In contrast, an increase in the thermodynamic driving force for nucleation through manganese depletion of the austenite matrix local to the inclusion tends to be the dominant nucleation mechanism in HAZs. It is demonstrated that these means of nucleation are not mutually exclusive but depend on the nature of the nucleating phase and the prevailing transformation conditions. Issues for further improvement of weldment toughness are discussed. It is argued that greater numbers of fine particles of a type that preferentially nucleate acicular ferrite are required in particle dispersed steels to oppose the austenite grain boundary ferrite transformation and promote high volume fractions of acicular ferrite and thereby toughness.     

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.     

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.     

High temperature decomposition of austempered microstructures in spheroidal graphite cast iron

Abstract

Spheroidal graphite (SG) cast iron is often plasma nitrided for corrosion resistance, and plasma nitriding has been proposed as a surface engineering treatment to improve wear resistance. However, the microstructure of austempered SG iron comprises constituents that may be unstable at nitriding temperatures. Therefore, the thermal stability of austempered SG cast iron has been studied at high temperature. Differential scanning calorimetry shows that microstructures obtained by austempering at low (300°C) and intermediate (380°C) temperatures, and which contained retained austenite, underwent a large exothermic transition during heating to typical nitriding temperatures. The transition began at approximately 470°C and peaked at 510–520°C, and was due to the decomposition of retained austenite to ferrite and cementite. A microstructure obtained by austempering at a higher temperature (440°C), and which consisted entirely offirst and second stage bainite, was stable up to nitriding temperatures. After tempering for 2 h at 570°C all austempered microstructures consisted offerrite and cementite, but cementite was most finely distributed in the material that had been austempered at 300°C, and coarsest in that austempered at 440°C. It is concluded that if SG cast iron is to be nitrided conventionally at temperatures >500°C, then prior austempering to obtain controlled microstructures is of limited value.

MST/3106     

Microstructure and microtexture of ultrafine ferrite formed in 0.1%C steel using new strip rolling process

Abstract

A new hot strip rolling process is discussed which is capable of producing ultrafine, equiaxed ferrite grains (i.e. less than 2 µm)in the surface region of steel strip. Both microstructural and texture analysis of low carbon steel strip that has been rolled using this method are used to show that the ferrite forms by strain induced transformation. Analysis by electron backscatter diffraction (EBSD) indicates that a strong ferrite microtexture exists within the individual austenite grains in which the ferrite nucleates. The results from bulk X-ray texture analysis confirm that the ferrite forms as a result of transformation from austenite that has undergone heavy shearing during rolling, with nucleation occurring on the austenite substructure. In the centre region of the strip, a bainitic microstructure forms after rolling during air cooling. In the transition region between the surface and the centre of the strip, ferrite is shown to nucleate to form closely spaced parallel ‘rafts’ of ferrite grains traversing individual austenite grains. Again, EBSD is used to show that the ferrite located within these rafts is strongly textured, which, in combination with microstructural evidence, suggests that this ferrite nucleates along intragranular shear bands that form in the austenite in this region of the strip during rolling.     

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