Ferrite Phase Field Research Articles

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.

Engineering Hierarchical Microstructures via Advanced Thermo-Mechanical Processing of a Modern HSLA Steel

Advanced thermo-mechanical processing of mild steels in the ferrite phase field has recently achieved breakthrough in grain refinement into the submicron regime. However, these steels often suffer from grain boundary failure and low rates of work hardening. A potential approach to overcome these challenges is to process modern high-strength low-alloy steels with multi-scale hierarchical microstructures. Thus, the applicability of advanced thermo-mechanical processing for achieving such microstructures in a high-strength low-alloy steel was studied. The microstructural evolution during warm deformation of a martensitic/bainitic starting microstructure using a Gleeble 3500 thermo-mechanical simulator at 600 °C followed by a direct aging step was investigated. The strain rate of 10 s−1 led to strain localization and, therefore, the formation of a macroscopic shear band. High-resolution characterization techniques such as electron channeling contrast imaging, electron backscatter diffraction, and transmission electron microscopy were used to reveal the ultrafine grain sizes (~ 0.5 μm) in this shear band. The mechanism behind this refinement is continuous dynamic recrystallization, as the initial grains subdivided into smaller crystallites that are confined by a mix of subgrain and high-angle grain boundaries. Two populations of precipitates were formed. Larger precipitates (mean diameter ~ 150 nm) decorate grain boundaries, whereas smaller precipitates (~ 15 nm) nucleate on dislocations and subgrain boundaries.

Influence of thermo-mechanical treatment in ferritic phase field on microstructure and mechanical properties of reduced activation ferritic-martensitic steel

The present study focuses on the evaluation of microstructure and mechanical properties of reduced activation ferritic-martensitic (RAFM) steel (9Cr-1W-0.06Ta) subjected to thermo-mechanical treatment (TMT) in ferritic phase field. The results obtained were compared with the steel in conventional normalised plus tempered (N+T) condition. The microstructure of the steel in N+T and TMT conditions was assessed by optical and scanning electron microscopes. Hardness, tensile and creep studies were carried out and the results were correlated with the microstructural studies. While the TMT processed steel resulted in coarser prior austenite grains and exhibited ferritic microstructure with large distribution of fine M23C6 and MX precipitates, the N+T steel reveals tempered martensitic structure with finer prior austenitic grains with coarser M23C6 and MX precipitates. Although ferritic structure is present in TMT processed steel, it exhibits better tensile and creep rupture strengths than N+T steel due to the presence of increased dislocation density and finer distribution of precipitates.

Abnormal grain growth in Eurofer-97 steel in the ferrite phase field

Reduced-activation ferritic-martensitic (RAFM) Eurofer-97 steel is a candidate material for structural applications in future fusion reactors. Depending on the amount of prior cold rolling strain and annealing temperature, important solid-state softening reactions such as recovery, recrystallization, and grain growth occur. Eurofer-97 steel was cold rolled up to 70, 80 and 90% reductions in thickness and annealed in the ferrite phase field (below ≈ 800 °C). Changes in microstructure, micro-, and mesotexture were followed by orientation mappings provided by electron backscatter diffraction (EBSD). Eurofer-97 steel undergoes abnormal grain growth above 650 °C and this solid-state reaction seems to be closely related to the high mobility of a few special grain boundaries that overcome pinning effects caused by fine particles. This solid-state reaction promotes important changes in the microstructure and microtexture of this steel. Abnormal grain growth kinetics for each condition was determined by means of quantitative metallography.

Short Communication on “Coarsening of Y-rich oxide particles in 9%Cr-ODS Eurofer steel annealed at 1350 °C”

Oxide-dispersion strengthened (ODS) Eurofer steel is targeted for structural applications in future fusion nuclear reactors. Samples were cold rolled down to 80% reduction in thickness and annealed at 1350 °C up to 8 h. The microstructural characterization was performed using Vickers microhardness testing, electron backscatter diffraction, scanning and scanning transmission electron microscopies. Experimental results provide evidence of coarsening of the Y-rich oxide particles in ODS-Eurofer steel annealed at 1350 °C within delta ferrite phase field.

A metastable phase diagram for the dynamic transformation of austenite at temperatures above the Ae3

Abstract A method is proposed for calculation of the pseudobinary phase diagram associated with the dynamic transformation of austenite to ferrite. Here the driving force is taken as the difference between the austenite flow stress at the moment of initiation and the yield stress of the fresh Widmanstätten ferrite that takes its place. The energy opposing the transformation consists of the Gibbs energy difference between austenite and ferrite at temperatures above the Ae3 and the work of accommodating the shear displacements and dilatation associated with the phase change. A metastable phase diagram is calculated for a 0.30 wt.% Mn-0.01 wt.% Si steel by balancing the driving force against the three obstacles. The results show that, under dynamic conditions, the ferrite phase field extends all the way from room temperature to that for the formation of delta ferrite.

Hot Ductility of the 17-4 PH Stainless Steels

The mechanisms of loss of hot ductility and the mechanical behavior of 17-4 PH alloys were investigated using hot tensile testing at temperatures between 700 and 1100 °C and strain rates of 10−4, 10−2, and 10−1 s−1. Scanning electron microscopy was used in conjunction with the results of the tensile tests to find the temperature region of loss of ductility and correlate it with cracking observed during processing by hot upsetting prior to ring rolling. It is reported that 17-4 PH alloys lose ductility in a temperature range around 900 °C near to the duplex austenite + ferrite phase field. Furthermore, it is found that niobium carbides precipitated at austenite/ferrite interfaces and grain boundaries have a pronounced effect on the mechanical behavior of the alloy during high-temperature deformation.

Long-term microstructural stability of oxide-dispersion strengthened Eurofer steel annealed at 800 °C

Oxide-dispersion strengthened ferritic martensitic steels such as ODS-Eurofer grade are good candidates for structural applications in future fusion power reactors. Long-term annealing treatments in vacuum were carried out in cold-rolled samples (80% reduction in thickness) from 1h up to 4320h (6months) at 800°C, i.e. the maximum temperature in the ferritic phase field, to follow its softening behavior. The microstructural stability of this steel was mapped using several characterization techniques including scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, Vickers microhardness testing, X-ray diffraction texture measurements, low-temperature electrical resistivity, and magnetic coercive field measurements. ODS-Eurofer steel displays good microstructural stability. Discontinuous recrystallization occurs at the early stages of annealing resulting in a low volume fraction of recrystallized grains. Extended recovery is the predominant softening mechanism at this temperature for longer times.

Abnormal Grain Growth in Ferritic-Martensitic Eurofer-97 Steel

Abstract. Ferritic-martensitic steels like Eurofer-97 are candidate structural materials for future fusion reactors. In the tempered state, this steel contains fine particles dispersed in the ferritic matrix. The aim of this work is to investigate abnormal grain growth in Eurofer-97 steel. The microstructural evolution was followed by isothermal annealing between 200 and 800°C (ferritic phase field) after cold rolling to 70, 80, and 90% reductions. Representative samples were characterized by scanning electron microscopy in the backscattered electron mode. Microtexture was evaluated by electron backscattered diffraction. We propose a mechanism based on the size advantage acquired by nuclei with misorientation angles above 45º relative to their nearest neighbors to explain abnormal grain growth. Abnormal grain growth textures have components belonging to the α- and γ-fibers with predominance of {111}, {111}, and {100}.

Effect of Austenite Recrystallization on Toughness of Pipeline Steels

The factors affecting pipeline fractures are reviewed briefly, with particular emphasis on the influence of the {100} texture component. The deformation texture components introduced by rolling in the austenite temperature range are introduced, together with the component changes associated with recrystallization. The effect of the γ-to-α phase transformation on the austenite deformation and recrystallization texture components is described. The changes to the texture brought about by rolling in the ferrite (or in the intercritical) phase field are also outlined. The controlled rolling parameters that promote minimization of the texture intensity of the deleterious {100} component are summarized.

Annealing Behavior of RAFM ODS-Eurofer Steel

Oxide-dispersion-strengthened (ODS) ferritic-martensitic steels are candidates for applications in fusion power plants where microstructural long-term stability at temperatures of ˜650°C to 700°C are required. The microstructural stability of 80% cold-rolled reduced-activation ferritic-martensitic 9% Cr ODS-Eurofer steel was investigated within a wide range of temperatures (300°C to 1350°C). Fine oxide dispersion is very effective to prevent recrystallization in the ferritic phase field. The low recrystallized volume fraction (<0.1) found in samples annealed at 800°C is associated with the nuclei found at prior grain boundaries and around coarse M23C6 particles. The combination of retarding effects such as Zener drag and concurrent recovery decrease the local stored energy and impede further growth of the recrystallization nuclei. Above 900°C, martensitic transformation takes place with consequent coarsening. Significant changes in crystallographic texture are also reported.

Study of deformation-induced phase transformation in plain low carbon steel at low strain rate

Hot-deformation behavior of super-cooled austenite within the austenite–ferrite (γ–α) phase field was studied in low carbon steels (0.15 wt.%) at various strain rates. Flow softening was observed during the hot-deformation at temperatures lower than critical temperature. The critical temperature was observed to be significantly higher than T 0 temperature. This was due to large shift of γ–α phase-equilibrium to a higher temperature during hot-deformation. A thermodynamic model to account the effect of non-uniform deformation-induced stress on the γ–α phase-equilibrium during hot-deformation was proposed and its prediction was in good agreement with the experimentally measured critical temperature.

Annealing behavior of ferritic–martensitic 9%Cr–ODS–Eurofer steel

Oxide dispersion strengthened ferritic–martensitic steels are potential candidates for applications in future fusion power plants. High creep resistance, good oxidation resistance, reduced neutron activation and microstructural long-term stability at temperatures of about 650–700 °C are required in this context. In order to evaluate its thermal stability in the ferritic phase field, samples of the reduced activation ferritic–martensitic 9%Cr–ODS–Eurofer steel were cold rolled to 50% and 80% reductions and further annealed in vacuum from 300 to 800 °C for 1 h. The characterization in the annealed state was performed by scanning electron microscopy in the backscattered electron mode, high-resolution electron backscatter diffraction and transmission electron microscopy. Results show that the fine dispersion of Y-based particles (about 10 nm in size) is effective to prevent recrystallization. The low recrystallized volume fraction (<0.1) is associated to the nuclei found at prior grain boundaries and around large M 23C 6 particles. Static recovery was found to be the predominant softening mechanism of this steel in the investigated temperature range.

The Effect of Thermal Expansion on Spallation Behavior of Fe-Base Alumina-Forming Alloys

Long-term cyclic oxidation behavior was compared for commercial FeCrAl alloys and model Fe-Al and FeCrAl alloys, and their coefficients of thermal expansion (CTE) were measured. For single-phase disordered (ferritic) Fe(Al) alloys, the CTE increased only slightly with Al content and was similar to that of FeCrAl alloys. More significant CTE increases were observed at ≥20%Al, as intermetallic phases, Fe3Al and FeAl, formed. As expected, the intermetallic compositions showed increased oxide spallation rates during cyclic oxidation at 1100° and 1200°C. However, after extensive spallation and loss of Al from the substrate, the compositions of Fe3Al and FeAl specimens entered the ferritic phase field, and the amount of scale spallation decreased. Among commercial oxide dispersion strengthened (ODS) FeCrAl alloys, a composition containing Mo (ODM751) exhibited the lowest thermal expansion and showed the slowest degradation rate in long-term testing at 1100°C. The concept of minimizing CTE as a route for alloy development was investigated.

Grain Structure of Fe-0.3mass%C-9mass%Ni Steel Processed through .ALPHA..RAR..GAMMA..RAR..ALPHA.' Transformation Caused by Spontaneous Reverse Transformation

α→γ→α' (ferrite→austenite→martensite) transformation caused by spontaneous reverse transformation (SRT) in Fe-0.3C-9Ni mass% steel has been demonstrated using laboratory rolling mill, and grain structure of the plate has been investigated. SRT is a new process to obtain an ultra refined austenite, and resulting ultra refined transformation products, which occurs due to adiabatic heating caused by deformation at temperatures in ferrite phase field. SRT occurred all through the plate thickness by intensive rolling with 90% reduction and final microstructure was ultra-refined martensite. Mean lineal intercepts of the martensite grains were 0.90 μm at mid thickness and 0.83 μm at quarter thickness. Ultra-refined grain structure attributes to enhancement of austenite nucleation due to the intensive rolling before α→γ→α' transformation. The martensite texture after the α→γ→α' transformation caused by SRT composed of RD//‹110›α and ND//‹111›α fibers. However, these were not sharp, thus crystallographic grain orientation was quite randomised in spite of the intensive rolling. Two consecutive transformations weakened the initial ferrite rolling texture remarkably. Randomization of the grain orientation was more prominent at the quarter-thickness than at the mid-thickness due to a shear strain.

Effect of Equalisation Time on the Austenite Grain Size of Simulated Thin Slab Direct Charged (TSDC) Vanadium Microalloyed Steels

The thin slab direct charging process (TSDC) as applied to microalloyed steels has had a considerable development due to its relatively low production costs and ability to produce steels with good mechanical and toughness properties, comparable to those manufactured by the conventional cold charging process (CCC). This research is concerned with the effect of equalisation time on the microstructures. Three equalisation times (53 min, 318 min, and 1 333 min) at 1 150°C were investigated for a vanadium microalloyed steel, which contained a low carbon content (0.073 wt%) and a high nitrogen content (0.021 wt%). The experimental results show that MnS precipitates provided the main pinning forces during casting due to the high solubility temperature, but MnS particles did not provide a sufficient pinning force to prevent the austenite grains from growing. The austenite grain growth appears more likely to be abnormal in this stage. AlN precipitates were only identified at the longest equalisation time due to a low precipitation rate. The AlN pinning effect was evident at 1 333 min equalisation time, and was able to prevent further austenite grain growth by providing an additional pinning force. The study shows that vanadium does not have any effect on austenite grain growth at 1 150°C because of its low solubility temperature in austenite. When the steel was water-quenched, vanadium remained supersaturated in the solution but tempering in the ferrite phase field (650°C) can promote VN or VCN precipitation often using AlN as the nuclei.

Influence of martensite composition and content on the properties of dual phase steels

A study has been made of the mechanical properties of dual phase (martensite plus ferrite) structures produced when Fe-Mn-C alloys are quenched from the austenite plus ferrite phase field, so as to give a series of alloys with constant ferrite and martensite compositions but varying percent martensites. It is found that the strength of a dual phase structure is dependent on the ferrite grain size and the volume fraction of martensite, and is independent of the composition and strength of the martensite. In agreement with previous work the ductility of these steels is superior to that for standard HSLA steels at the same tensile strength. As shown in a previous paper the strength and ductility as a function of percent martensite are in agreement with Mileiko’s theory of composites of two ductile phases. This theory and the results indicate that the superior ductility of dual phase steels is largely a consequence of the high strength (fine grained), highly ductile (low interstitial content) ferrite matrix.

The effects of experimental rolling on microstructure and mechanical properties of a Ni-Cu-Cb steel

The effects of hot rolling and warm working on mechanical properties and microstructure of Ni-Cu-Cb steel were investigated. In the hot rolled condition optimum strength and toughness resulted from rolling in the upper portion of the austenite plus ferrite phase field. This treatment produced a fine-grained structure consisting of equiaxed and acicular ferrite. Unusually high strength-toughness combinations were realized by warm working the hot rolled steels at 1200°F.