Austenite Phase Field Research Articles

Effect of the supersaturation with nitrogen on the structure refinement and magnetic properties of mechanically alloyed and heated Fe14Cr ferritic alloys

Nano-nitride reinforcement of Fe–Cr alloys are under intense investigations, e.g., along with oxide-strengthened, nitride-strengthened reduced activation steels with superior high temperature mechanical properties are developed as materials for nuclear energy applications. Fe14Cr alloy powders were produced by mechanical milling for up to 170 h under a nitrogen atmosphere followed by heating under an Ar5%H2 atmosphere to 600°C–1120 °C. The samples were investigated by means of X-ray diffraction, DTA/TG and magnetic measurements. CrN, detected after milling for 58 h, reached 39 wt % (5.3 wt.% total N) at 170 h milling. The ferrite supersaturated with nitrogen shows a strong decrease in grain size and saturation magnetization, an increase in lattice constant, microstrain and coercivity, Hc. Upon heating, the metastable ultrafine α-ferrite and CrN in the as-milled samples undergo transformations and recrystallization into ferrite grains free of supersaturation, whose microstructure follows a continuous relaxation. The supersaturation with nitrogen was retained upon heating in the relatively stable ferrite crystalized at the milling stage. This microstructure undergoes a discontinuous relaxation in the 800°C–1120 °C region: a pronounced decrease of grain size (down to ∼25 nm), an increase of lattice constant, microstrain and Hc. These improvements were associated with features of discontinuous precipitation reaction of coherent nitride precipitates (involving a new α”-phase with expanded lattice), in connection with the heating in the austenite phase field. Such microstructure refinement or (nitride) strengthening of Fe14Cr alloys can be achieved on powders milled for much less time as compared to previously reported (oxide dispersion) strengthening of Fe14Cr–W–Ti ferritic steels.

Dynamic Ferrite Formation and Evolution above the Ae3 Temperature during Plate Rolling Simulation of an API X80 Steel

Thermo-mechanically controlled rolling is a technique used to produce steel strips and plates. One of the steels widely used in the production of heavy plates for application in oil and gas pipelines is API X80. The hot rolling process of this family of steels consists of applying deformation passes at high temperatures, mainly above Ae3, inside the austenite phase field. It has been shown that during deformation, the phenomenon of dynamic transformation (DT) of austenite into ferrite leads to lower hot deformation resistance within the stable austenite region. In this investigation, hot torsion simulations of an industrial rolling process under continuous cooling conditions were used to monitor the formation of ferrite by DT. Stress–strain flow curves and equivalent mean flow stresses followed by sample characterization via optical and electron microscopy showed the inevitable formation of ferrite above the Ae3. The employed 10-pass deformation schedule was divided into 5 roughing and 5 finishing passes, thereby promoting an increased volume fraction of ferrite and decreased critical strain for the onset of DT and dynamic recrystallization (DRX). A microstructural analysis confirmed the formation of ferrite from the first roughing strain until the last finishing pass. The volume fraction of DT ferrite increased due to strain accumulation, an increased number of deformation passes and as the temperature approached the Ae3, leading to a characteristic torsion texture at the end of the simulation.

Thermomechanical and thermodynamic behavior of deformed austenite in four different steel grades

In this work, the deformation behavior and thermodynamic aspect of four different steel grades were analyzed. This group of alloys includes a plain C–Mn, and three Nb-microalloyed steels grades. Two of the microalloyed steels are pipeline grades: a modified X-70 grade, and a typical X-80 steel. These materials were previously identified to produce dynamically transformed ferrite (combined with recrystallized grains) during physical simulation of hot rolling in the single austenite phase field of their phase diagrams. For C–Mn and microalloyed steels, hot deformation at a stain of 0.4 and strain rate of 1 s−1 generates 15 and 25 vol% of ferrite, respectively. In addition, the X-70 and X-80 steels yield 19 and 8 vol% of ferrite, respectively, after deformation strains of 0.4 and 0.2, and strain rate of 1 s−1. All the deformation was applied at least 53 °C above their respective Ae3 temperatures. The concept of transformation softening was applied to explain the occurrence of dynamic transformation during hot torsion tests. Here the driving force to dynamic transformation (DT) was taken by measuring the difference between the critical stress to DT in the austenite phase, and the yield stress of the ferrite that takes its place. The obstacle energy consists of the free energy between the parent and product phases and the shear accommodation and lattice dilatation work associated with phase transformation of austenite to ferrite. It is shown here that, for all the selected materials, the calculated driving force is higher than the energy obstacles, which thermodynamically explains the occurrence of DT.

Tensile properties and bake hardening response of dual phase steels with varied martensite volume fraction

A study has been made of the tensile properties on martensite plus ferrite dual phase (DP) steels. Tensile samples machined out of industrially produced DP steel sheet are quenched from the austenite plus ferrite phase field, so as to give a series of microstructures with varying martensite volume fraction (MVF) from 0.17 to 1. The strength of the produced DP steels is found to linearly increase with increasing the MVF, whereas their ductility showed a nonlinear decrease.Tensile samples of the produced DP-steels were taken to study the bake hardening (BH) response. The samples were aged at 100, 170 or 220 °C after applying pre-straining of 0, 2% or 5%. The change of BH-response with the time is found to go through two stages, an initial small increase in yield strength, followed by a larger strength increase at longer baking times. Prolonging the aging time results in decreasing the BH-response achieved in the second-stage (over-aging). The BH-response in the first stage is improved by increased pre-straining. This trend implies that Cottrell atmosphere formation is highly influenced by pre-straining. However, during the second stage the effect is reversed, with the non-pre-strained condition recording higher BH-responses up to 180 MPa. It is found also that changing the aging temperature does not change the characteristics of the aging-phenomenon, but it affects its rate. Saturation of increasing the BH-response with increasing MVF in the second strengthening stage is observed at MVF-range of 0.22–0.39.

In-situ X-ray diffraction evidence of dynamic transformation of austenite to ferrite during hot compression test in the single austenite phase field

Dynamic transformation (DT) of austenite to ferrite can take place in the single austenite phase field during thermomechanical processing as shown in the literature. However, despite significant progress on this topic, limited real-time evidence of DT has been published. To address this concern, an in-situ X-ray diffraction measurement was carried out on a pipeline steel before, during and after hot compression. The appearance of ferrite peaks is evident during deformation and these peaks disappear after 9 s of isothermal holding. This work provides an in-situ evidence of both forward and backward dynamic transformation complemented with energy calculations to validate the observations.

On the effect of thermo-mechanical treatment on creep deformation and rupture behaviour of a reduced activation ferritic-martensitic steel

Creep deformation and rupture behaviour of a reduced activation ferritic-martensitic (RAFM) steel subjected to thermo-mechanical treatment (TMT) is studied and compared with those of conventional normalized and tempered (N + T) steel. In TMT processing, the steel is warm rolled and aged in austenite phase field at 973 K before the martensite transformation on cooling and is then tempered at 1038 K. The TMT processing renders the steel with higher dislocation density, refinement in lath structure and large quantity of finer M23C6 and MX precipitates than those in the N + T steel. Creep tests are carried out at 823 K over the stress range 180–300 MPa. TMT processing of the steel decreases its minimum creep rate (ε˙min) with corresponding increase in time to onset of tertiary stage of creep deformation, rupture life (tr) and creep rupture ductility (εf). The stress exponent value (n), obtained from minimum creep rate vs. stress plot, increases upon TMT processing, indicating high resistance to creep deformation than in the N + T steel. Resisting stress as estimated based on the Lagneborg and Bergman method is found to increase on TMT processing and is associated with high damage tolerance parameter, defined asλ=εf/(ε˙min.tr). Enhanced creep deformation and rupture strength of the TMT steel, compared to N + T steel, is attributed to the microstructural refinement. Post-creep microstructural investigations show higher microstructural stability of the steel on TMT processing and are in line with the observed high damage tolerance parameter (λ), longer time to onset of tertiary creep and rupture life.

Effect of Number of Roughing Passes on the Dynamic Transformation of Austenite during Simulated Plate Rolling

When austenite is deformed within the austenite phase field, it partially transforms dynamically into ferrite. Here, plate rolling simulations were carried out on an X70 steel using rough rolling passes of 0.4 strain each. The influence of the number of roughing passes on the grain size and volume fraction of induced ferrite was determined. Up to three roughing passes applied at 1100 °C followed by 5 finishing passes at 900 °C were employed. The sample microstructures were analysed by means of metallographic techniques. Both the critical strain to the onset of dynamic transformation as well as the grain size decreased with pass number during the roughing simulations. For the finishing passes, the mean flow stresses (MFS`s) applicable to each schedule decreased when a higher number of roughing passes was applied. The volume fraction of dynamically formed ferrite retained after simulated rolling increased with the roughing pass number. This is ascribed to the increased amount of ferrite retransformed into austenite and the finer grain sizes produced during roughing. The forward transformation is considered to occur displacively while the retransformation into austenite during holding takes place by a diffusional mechanism. This indicates that both dynamic transformation (DT) and dynamic recrystallization were taking place during straining.

Effect of molybdenum on phase transformation and microstructural evolution of strip cast steels containing niobium

Molybdenum (Mo) is known to have a complex effect on phase transformations and precipitation in steels manufactured by conventional casting. The present work aims to examine the effect of Mo on phase transformations in Nb-containing steels produced by strip casting. Advanced experimental techniques have been utilised to simulate the strip casting process, and the microstructural features of the rapid solidification are retained for further study. Two cooling conditions from the austenite phase field were examined, isothermal holding and continuous cooling. It was found that at high cooling rates, the addition of Mo delayed the nucleation of bainite and lowered the bainite start temperature, but did not alter the bainite growth rate. The addition of Mo was also found to result in a slower transformation rate of polygonal ferrite under both isothermal and continuous cooling conditions. Thermodynamic simulations indicated that Mo did not affect the growth velocity of the polygonal ferrite, and quantitative metallography showed the nucleation density was significantly reduced by Mo addition. For the slowest continuous cooling rate, the addition of Mo completely inhibited pearlite formation, with bainitic ferrite forming instead. This has been suggested to be the result of the suppression of pearlite nucleation, rather than inhibition of growth.

The effect of molybdenum on interphase precipitation and microstructures in microalloyed steels containing titanium and vanadium

Despite much research into steels strengthened through interphase precitation, there remains much that is not clear, such as the role of a range of elements, particularly Mo, in the interphase precipitation process. Four steels were manufactured with identical composition, but with variations in Ti, V, Mo and N content to investigate the effect of composition on interphase precipitation. Alloys were rapidly cooled from the single austenite phase field and isothermally transformed at 630 °C and 650 °C for 90min. The addition of Mo was found to significantly reduce the austenite to ferrite transformation kinetics, particularly for the V steel. Interphase precipitation was observed in all alloys at both transformation temperatures. For the Ti bearing steel, the two types of precipitate were observed throughout the sample, namely TiC (finer) and Ti2C (coarser), while for the V bearing steels, VC (finer) and V4C3 (coarser) were observed. Where Mo was present in the alloy, it was found dissolved in all carbide types. The (Ti,Mo)C and (V,Mo)C formed by classical planer interphase precipitation (PIP) while the (Ti,Mo)2C and (V,Mo)4C3, that had a much wider row spacing, formed through curved interphase precipitation (CIP). Each adopted one variant of the Baker-Nutting orientation relationship. The Ti-microalloyed steels exhibited the smallest precipitates of all the steels, which were approximately the same size irrespective of whether Mo was present in the alloy and irrespective of the transformation temperature. However, the addition of Mo to the V bearing steels resulted in a significant increase in precipitate volume fraction and a reduction in precipitate size. The mechanisms of interphase precipitation leading to the coincident production of two different precipitate types is considered and the role of Mo on the interphase precipitation process is discussed. The resultant effect on strength is considered.

Effect of Grain Size and Residual Strain on the Dynamic Transformation of Austenite under Plate Rolling Conditions

When austenite is deformed within the austenite phase field, that is, above the equilibrium transformation temperature Ae3, it partially transforms dynamically into ferrite. Here, rough rolling simulations are conducted at 1100 °C on an X70 steel to study the influence of grain size and residual strain on this type of transformation. Three simulated roughing passes are applied with pass strains of 0.4 at a strain rate of 1.0 s−1. The resulting stress–strain curves are recorded and analyzed. The volume fractions of ferrite and martensite (prior austenite) are determined on quenched samples by means of metallographic techniques. It is observed that dynamic transformation took place during the first roughing pass and that the volume fraction of transformed ferrite increases with the cumulative strain. Both the critical strain to the onset of dynamic transformation as well as the grain size decrease with pass number.

Effect of thermo-mechanical treatment on tensile properties of reduced activation ferritic-martensitic steel

Effect of thermo-mechanical processing on tensile behaviour of reduced activation ferritic martensitic (RAFM) steel has been assessed and compared with those of the normalized and tempered steel. The thermo-mechanical treatment (TMT) was carried out by performing hot rolling and ageing in the austenite phase field, air cooled followed by tempering. TMT of the steel increased its hardness compared to the normalized and tempered condition. Tensile strength of the steel increased on TMT over the investigated temperature range of 300–923 K and more importantly was accompanied with increase in ductility. The strength increase on TMT was accomplished with the refinement of microstructure of the steel having finer martensite lath size and intra-granular precipitates. The tensile flow behaviour of steel has been assessed in the frame work of Voce's constitutive equation. The strength related parameters of the constitutive equation i.e. initial stress and saturation stress increased on TMT, while the recovery related parameter (nv) decreased marginally. The enhancement of tensile strength and ductility of the steel on TMT is considered due to microstructural refinement and its enhanced stability as reflected in appreciable lower relaxation and creep rates and the delayed in onset of tertiary stage of creep deformation.

Effects of Post-Weld Heat Treatment on the Microstructure and Toughness of Flash Butt Welded High-Strength Low-Alloy Steel

Effect of post-weld heat treatment on the weld microstructure, texture, and its correlation to the toughness of flash butt welded joints were investigated. Upon flash butt welding, the α and γ-fiber in the parent material converted to Goss (110)[001], rotated Goss (110)[1\( \bar{1} \)0], and rotated cube (001)[1\( \bar{1} \)0], (001)[\( \overline{11} \)0] textures along the fracture plane. Formation of these detrimental texture components was a result of shear deformation and recrystallization of austenite at temperatures above Tnr resulting in a drop of toughness at the weld zone. Inter-critical and sub-critical annealing cycles proved to be less effective in reducing the Goss (110)[001], rotated Goss (110)[1\( \bar{1} \)0], and rotated cube (001)[1\( \bar{1} \)0], (001)[\( \overline{11} \)0] texture components, and therefore, toughness values remained unaffected. Post-weld heat treatment in the austenite phase field at 1000 °C for 5 seconds resulted in the formation of new grains with different orientations leading to a reduction in the texture intensities of both Goss and rotated Goss components and therefore improved weld zone toughness. Prolonged annealing time was found to be ineffective in improving the toughness due to grain growth.

Dynamic Transformation during Plate and Strip Rolling

Torsion simulations were carried out of both plate (long interpass times) and strip (short interpass times) rolling. Both isothermal and continuous cooling conditions were employed. The dynamic transformation of austenite to ferrite was observed under all conditions and at all temperatures within the austenite phase field. About 8 to 10 volume percent ferrite was formed in a given pass, leading to about 50 - 70 % ferrite at the end of selected simulations. During the interpass intervals, some retransformation to austenite took place, the amount of which increased with holding time and temperature and decreased with the addition of alloying elements. It is shown that the driving force for the transformation is the softening associated with the replacement of work-hardened austenite grains by the softer alpha phase. The implications with respect to rolling load (i.e. mean flow stress) are also discussed.

Formation of Widmanstätten ferrite at very high temperatures in the austenite phase field

Compression tests were carried out on a 0.06wt%C-0.3wt%Mn-0.01wt%Si steel at temperatures high in the austenite phase field. Eight deformation temperatures were selected in the range from 1000 to 1350 °C at 50 °C intervals. The quenched samples were examined using optical microscopy and EBSD techniques. It was observed that dynamic transformation took place and that the volume fraction of transformed ferrite first decreased with temperature (up to 1050 °C) and then increased as the delta ferrite temperature domain was approached. The EBSD results revealed the presence of Widmanstätten ferrite plates under all testing conditions, right up to 1350 °C.

Effect of variant transformations in fusion zones of gas metal arc welds

The macro- and microtextures of gas metal arc welds fabricated using both conventional ferritic and low transformation temperature (LTT) filler metals were examined. Both welds were largely composed of acicular ferrite. The weld textures were found to be distinct, which is observed to be due to variant selection during the austenite to ferrite transformation. In situ electron backscatter diffraction performed during thermal cycling of the LTT fusion zone into the austenite phase field confirmed that these transformations satisfy the expected crystallographic relationships. Implications for welding consumable design and finite element process modelling are drawn.

Strain‐Induced Precipitation Behavior of a Nb–Ti–V Steel in the Austenite Phase Field

AbstractThe precipitation behavior of a Nb (0.041 wt%), V (0.11 wt%), and Ti (0.018 wt%) micro‐alloyed steel influenced by a deformation process in the austenite region has been investigated by transmission electron microscopy and atom probe. After solution annealing at 1250°C, nearly the entire Nb and V amount is in solid solution. Ti does not go into in solid solution and stays stable as TiN precipitates. During the cooling process to deformation temperature, these TiN precipitates act as nucleation sites for Nb and V. Precipitates with a TiN rich core and a V‐rich shell are formed. Nb is homogenously distributed over the precipitate. After deformation, strain induced precipitates occur, which consist mainly of Nb, V, and only a little Ti. Smaller strain induced precipitates have a higher N‐content than C, with growth the C‐content increases.

Plastic deformation of low carbon steels

The plastic deformation of two steels was studied by means of axisymmetric compression tests carried out in a computer driven servohydraulic machine. The tests were conducted within the temperature range of 700–1100°C for steels that have similar carbon contents, but differed in other alloying elements. Little difference in strength between the steels was found when deformation took place in the austenite phase field, but the strength of the material varied as soon as ferrite was present. The critical temperatures were evaluated by means of thermal analyses, which were conducted by inserting a thermocouple within samples of the two steels; these temperatures were found to be close to those predicted by empirical equations.

Comparative study of the prediction of microstructure and mechanical properties for a hot-stamped B-pillar reinforcing part

Automobile manufacturers have been increasingly adopting hot-stamped parts for use in newly designed vehicles to improve crash worthiness and fuel efficiency. However, the hot-stamped parts require extreme mechanical properties with ultimate tensile strengths as high as 1500 MPa (∼450 Vickers hardness) while still maintaining adequate formability during the stamping operation. The ultra high strength of hot-stamped components is attributed to the martensitic phase transformation that occurs after the part has been formed at temperatures corresponding to the austenite phase field where formability is enhanced. In the present study, a computer-aided design method incorporating Kirkaldy and Venugopalan type phase transformation models has been implemented following a thermo-mechanical coupled finite element analysis to predict the mechanical properties of hot-stamped parts made with a boron-modified steel. Three empirical models which are typically used for hot stamping analysis are employed and the prediction capability of the models is compared using continuous cooling dilatometry and forming experiments of a modified B-pillar part.

Demonstration of elemental partitioning during austenite formation in low-carbon aluminium alloyed steel

Austenite formation is an important and much studied process in steel research, being intensively investigated in low and medium carbon steels.. In ferritic-pearlitic steels this process can be split into two: nucleation of austenite in pearlitic colonies with rapid growth of austenite consuming pearlite followed by the slower growth of austenite consuming ferrite [1-6]. Studies of the effect of substitutional elements such as manganese and chromium on austenite formation have shown that they slow down the transformation when partitioning of the elements occurs during austenitization [2, 3]. There has been little research concerning the effect of aluminium as a substitutional element in steel in amounts higher than 0.1 wt. % despite a general awareness that Al is a ferrite stabilizer and tends to expand the ferrite + austenite phase field of the equilibrium phase diagram. This could be due to technological concerns over the production of high Al-content steels. Commercial interest has emerged recently with studies on Al additions, of the order of 0.5–2 wt.%, to low carbon high strength strip steels to produce a multi-phase

Effect of silicon on the spheroidization of cementite in hypereutectoid high carbon chromium bearing steels

The effect of silicon on the spheroidization of cementite in hypereutectoid high carbon chromium bearing steels has been investigated on the basis of microstructural analysis and thermodynamic calculations. The results showed that an increase of silicon content in high carbon chromium bearing steels retards the spheoridization of cementite. The thermodynamic calculations revealed that the shrinkage of the austenite phase field in bearing steels with increasing silicon content gave rise to an increase of volume fraction of cementite at an annealing temperature, possibly resulting in incomplete spheroidization. Furthermore, due to the low solubility of silicon in cementite, an increase of silicon content can raise the activity or chemical potential of carbon atoms in austenite at the austenite/cementite interfaces. Consequently, the difference in chemical potential of carbon atoms at the interfaces would be reduced with increasing silicon content, causing a decrease of the driving force for their diffusion from cementite to austenite.