Intercritical Region Research Articles

Reduced spheroidization in rail flash-butt welding by selective alloying

Thermal cycle (welding simulations) at the inter-critical region were performed on an Si and V micro-alloyed (not heat-treated) rail steel. The beginning of austenitization temperature (Ac1) in the micro-alloyed rail steel occurred significantly above that observed in an alternative heat-treated rail steel. Very little pearlite alteration was observed until austenitization at a temperatures above 750°C, where there was a reduction in hardness and onset of spheroidization. The maximum spheroidization together with the lowest observed hardness occurred at 760°C. It was also observed that there is a linear correlation between hardness and volume fraction of spheroidization of cementite. By the dilatometric variation, contraction due to austenitization was observed only from 760°C onwards in the 60 s isothermal period at the austenitization temperature. These results may lead to a new approach to weldability and reduction of the softened region concerning rail welding.

Effect of prior austenite grain size on austenite reversion, bainite transformation and mechanical properties of Fe-2Mn-0.2C steel

To elucidate the response of austenite reversion and subsequent bainite transformation behavior to the prior austenite grain (PAG) size, the transformation kinetics and amount of reverted austenite in the intercritical region, the subsequent bainite transformation kinetics, and the tensile properties were investigated by dilatometers, scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and the mechanical tests. The results show that, when annealed at 730 °C, fine-grained specimens exhibit faster reverted austenite transformation rate than coarse-grained specimens due to the presence of more PAG boundaries. As the intercritical temperature rises to 760 °C, more intragranular globular austenite forms in the coarse-grained specimen, which accelerates the reverted austenite transformation rate. Compared to fine-grained specimens, coarse-grained specimens exhibit faster bainite transformation and higher amount of bainitic ferrite. The microstructure after austempering consists of intercritical ferrite, bainitic ferrite, RA and fresh martensite. With increasing the austempering time, more bainitic ferrite is formed and the amount of RA and fresh martensite decreases. With increasing the austempering time, the increase in the amount of bainitic ferrite leads to an increase in the yield strength, while the decrease in the amount of RA leads to a decrease in the elongation. Moreover, compared with coarse-grained specimens, the finer microstructure in fine-grained specimens improves yield and tensile strengths, while higher amount of RA contributes to larger uniform and total elongations.

Double Soaking of Medium Manganese Steels

Double soaking (DS) has been proposed as an alternate processing route for medium manganese steels. DS consists of soaking in the intercritical annealing region to stimulate manganese enrichment of austenite by depletion of ferrite followed by secondary soaking at a higher temperature and cooling to room temperature to obtain a martensite/austenite microstructure. DS is different from more traditional medium manganese heat treating which usually involves a single soaking step in the intercritical region to generate a ferrite/austenite microstructure. DS has been shown effective at generating attractive tensile properties notably tensile strength levels in excess of typically observed levels in medium manganese steels. A review of properties and microstructural evolution obtained by DS of medium manganese steels is presented here.

Effects of above- or below-AC3 austenitization on bainite transformation behavior, microstructure and mechanical properties of carbide-free bainitic steel

The isothermal bainite transformation kinetics, microstructure, crystallography and mechanical properties of a carbide-free bainitic steel, subjected to pre-quenching treatment, followed by austenitization at various temperatures above or below AC3 and then by above-MS austempering, have been investigated via dilatometric measurements, microstructural characterization and mechanical tests. The results show that, as the austenitization temperature decreases from 980 °C to 860 °C (above AC3), bainite transformation first decelerates and then accelerates; this is primarily a result of the competition between the availability of parent austenite grain (PAG) boundaries as nucleation sites for bainite transformation and the shear resistance of bainite growth, both of which increase with decreasing the PAG size but play a reverse role in bainite transformation rate. Further decreasing the austenitization temperature down to 820 °C and 800 °C (below AC3) even more notably accelerates bainite transformation and shortens the transformation completion time, since the intercritical ferrite (or tempered martensite), which is formed in the intercritical region, favor the nucleation of initial bainitic laths. The resulting microstructure after austempering is composed of bainite, retained austenite and intercritical ferrite and/or fresh martensite, the feature of which depends on the austenitizing temperature below or above AC3. Below-AC3 austenitization largely refines the microstructure and decreases the amount of large, brittle fresh martensite/austenite blocks, mainly attributed to the synergetic role of the refinement of austenite grains and pre-existence of intercritical ferrite prior to austempering. In addition, the intercritical ferrite and its adjacent bainitic laths have nearly the same crystallographic orientation and belong to the same block. As compared with above-AC3 austenitization, below-AC3 austenitization shows large increases in elongation, especially for specimens subjected to shorter time austempering, despite some decreases in yield and tensile strengths.

Microstructure and mechanical behaviour of Nb- containing high Al ferritic low density steels

Two high Al ferritic low density steels, Fe-11Al-5Mn-1Nb-1C (Alloy 1) and Fe-11Al-10Mn-1Nb-1C (Alloy 2) were prepared by vacuum induction melting. Their recrystallization behaviour is investigated. Hot rolling of the steels was carried out at 1100 °C. The temperature was chosen to ensure that the alloys were hot rolled in the intercritical region. Hot rolling led to a ferritic microstructure which was only partially recrystallized even after 92% rolling reduction. Annealing of hot rolled alloys at 1000 °C for 1 h results in a significant Static Recrystallization of the ferrite matrix. This suggests that dynamic recrystallization is difficult in these high Al ferritic steels. A further reduction of 33% was given to the hot rolled and annealed alloys by cold rolling. Annealing of cold rolled alloys at 1000 °C for 1 h leads to almost complete recrystallization. The hot rolled alloys exhibit better Yield and Ultimate Tensile Strength whereas the annealed alloys show higher total elongation. The best combination of properties is obtained in cold rolled and annealed samples. Alloy 2 exhibit superior tensile properties in comparison to Alloy 1. NbC precipitates pin the grain boundaries and may delay recrystallization. This necessitates further cold working and annealing to achieve complete recrystallization. NbC precipitates result in a significant refinement in grain size in annealed alloys. Nb addition also permits cold rolling without edge cracking. Thus, Nb may be an important alloying addition for high Al ferritic low density steels.

Effect of Forming and Heat Treatment Parameters on the Mechanical Properties of Medium Manganese Steel with 5% Mn.

Medium manganese steels fall into the category of modern third-generation high-strength steels. Thanks to their alloying, they use a number of strengthening mechanisms, such as the TRIP and TWIP effects, to achieve their mechanical properties. The excellent combination of strength and ductility also makes them suitable for safety components in car shells, such as side reinforcements. Medium manganese steel with 0.2% C, 5% Mn, and 3% Al was used for the experimental program. Sheets with a thickness of 1.8 mm without surface treatment were formed in a press hardening tool. Side reinforcements require various mechanical properties in different parts. The change in mechanical properties was tested on the produced profiles. The changes in the tested regions were produced by local heating to an intercritical region. These results were compared with classically annealed specimens in a furnace. In the case of tool hardening, strength limits were over 1450 MPa with a ductility of about 15%.

Influence of Austenite Stability on the Machinability of Different Grades of Austempered Ductile Iron (ADI) During Dry Turning Machining

This work evaluated the machinability characteristics of three different austempered ductile irons using three machinability indicators: cutting force, tool wear, and surface roughness. Additionally, this paper addresses the interplay among the production conditions of ADIs, microstructural development, the TRIP effect arising from transformations of the retained austenite into martensite, and the machinability behavior of ADIs. Various characterization studies including hardness measurement, X-ray diffraction analysis, light and electron microscope were used. Machinability results indicated that the increase in cutting force is always associated with a corresponding increase in the martensite content when the cutting depth was increased from 0.5 to 2.0 mm. Such correlation is more evident with ADI grade austempered at high austempering temperature, 375 °C (ADI-375). It was noticed that although ADI-375 has only 0.46% martensite in the structure before machining, this ratio increases with the highest depth of cut of 2.0 mm to reach about ~ 23% martensite with a corresponding increase in the cutting force. Ferrite content and morphology have a great impact on the overall surface finish of each grade as it leads to deteriorating the surface characteristics of the investigated alloys. This could explain the highest roughness values across all processing experiments obtained in the ADI grade austenitized in the intercritical region (IADI), which mainly contains the highest ferrite content. Tool wear and chip characteristics were also studied in this work.

Relationship Between Microstructure, Delamination Behavior, and Toughness of Steel Plates Rolled in the Intercritical Region Under Variable Deformation

Herein, the effects of microstructure on delamination behavior and toughness are discussed in detail. Steel plates have been fabricated by intercritical rolling (IR) under various strains (0.5, 0.8, and 1.1). The results show that increasing applied strain enhances the degree of laminated microstructure, and three kinds of impact fracture occur in sequence. 1) No delamination occurs in the IR0.5 steel plate with approximately equiaxed microstructure. 2) Delamination takes place in the IR0.8 steel plate with obvious laminated microstructure. But a crack parallel to the impact load direction forms beneath the delaminated crack. 3) Delamination is enhanced in the IR1.1 steel plate with enhanced laminated microstructure, and no crack forms below the delaminated crack. The delaminated crack mainly propagates along the laminated interfaces, so the laminated microstructure is the key reason for forming delamination. The more apparent laminated microstructure causes more remarkable laminated tearing at room temperature, large‐scale severe plastic deformation in the microstructure below the delaminated crack at low temperatures, and longer zigzag cracks at lower temperatures. Therefore, to improve toughness, the microstructure needs enough laminated structure to inhibit the formation of cracks parallel to the impact load direction below the delaminated crack.

VANADIUM ALLOYING IN S355 STRUCTURAL STEEL: EFFECT ON RESIDUAL AUSTENITE FORMATION IN WELDED JOINTS HEAT AFFECTED ZONE

The inter-critical heat affected zone (ICHAZ) appears to be one of the most brittle sections in the welding of high-strength micro-alloy steels (HSLA). Following multiple heating cycles in the temperature range between Ac1 and Ac3, the ICHAZ undergoes a strong loss of toughness and fatigue resistance, mainly caused by the formation of martensite-austenite constituent (MA). The presence of micro-alloying elements in HSLA steels induces variations in the formation of some microstructural constituents, more or less beneficial, which allow to improve the mechanical performance of a welded joint. The behavior in the inter-critical region of a S355 grade steel with 0.1wt% V addition is reported in this paper. Five double-pass welding thermal cycles were simulated using a dilatometer, with the maximum temperature of the secondary peak in the inter-critical area, in the range between 720 ° C and 790 ° C. The residual austenite dependence on inter-critical temperature is analyzed and related to the hardness behavior.

Yield point phenomenon in low-alloyed TRIP-aided steel: role of retained austenite

Multiphase TRIP-aided steels are generally characterised by a continuous yielding behaviour. The processing route involving a deformation step in the intercritical (ferrite + austenite) region showed an undesirable discontinuous yielding. This is attributed to the formation of fresh dislocations in the soft ferrite matrix that trap the diffusing carbon and form Cottrell atmospheres. The discontinuous yielding disappears when fresh mobile dislocations are introduced through transformation of the retained austenite to martensite.

Effect of intercritical annealing on the mechanical properties of dual-phase steel

ABSTRACT A dual-phase steel chemistry has been processed with varying intercritical annealing temperatures and the properties investigated. It was found that increasing the temperature in the intercritical region led to an increase in the volume fraction (52%–65%) and grain size (1.8–3.3 µm) of martensite whilst the volume fraction and grain size of ferrite decreased. The highest tensile strengths were found at low intercritical temperatures, achieving 935 MPa whilst the highest elongation values of 21% were achieved at the highest intercritical temperatures.

Hot Ductility Analysis and Flow Stress Prediction of (C-Mn-S-Al-Nb-V-Ti) Micro-alloyed Steel

The objective of this work is to study the hot ductility of (C-Mn-S-Al-Nb-V-Ti) micro-alloy steel of industrial production whose initial structural state is rolling stock. To simulate the thermomechanical treatments imposed we have deformed by pulling our samples after subjecting them to a solution treatment at 1200 °C and a precipitation treatment cycle before deformation. Hot deformations were carried out at temperatures from 700 °C and 1150 °C and deformation rates ranging between 10-2 s-1 and 5.10-4 s-1. The results show a decrease in hot ductility. Minimum values of hot ductility are determined at 800°C, and another decrease in hot ductility was observed at 900°C. Ferrite precipitation is observed at austenitic grain boundaries in the intercritical temperature range, causing intergranular embrittlement. Precipitation makes the hot ductility curve wider and deeper around 900°C. Hot ductility losses can explain by the presence of precipitates in the austenitic region and the presence of the two-phase structure in the intercritical region. Finally, based on experimental stress-deformation data, artificial neuron network (ANN) methods were used to predict flow stress of (C-Mn-S-Al-Nb-V-Ti) micro-alloy steel. This model is more effective in predicting flow stress and results can also be used in the mathematical simulation of hot metal formation processes.

Influence of intercritical temperature on microstructure of cold rolled DP980

In this study, Gleeble 3800 thermal simulator was used to simulate the continuous annealing process of industrial production of dual phase steel with the tensile strength over 980 MPa. In the experiment, four different annealing temperatures of 700 °C, 720 °C, 740 °C and 760 °C were set in the intercritical region while keeping other process parameters unchanged. The experimental results showed that the martensite content increases with the rise of intercritical annealing temperature. However, this growth is not persistent, the obvious change of martensite content can be seen only when the temperature exceeds 20 °C in current study. In addition, with the increase of temperature, the elongated ferrite grains tend to become more equiaxed.

The Significance of Mn Prepartitioning on Bainitic Transformation and Mechanical Properties of a Low‐Carbon Bainitic Steel

The significance of manganese prepartitioning treatment (Mn‐PrPT) in the intercritical region on bainitic transformation behavior, microstructure evolution, and mechanical properties of a low‐carbon bainitic steel is elucidated, herein. The results indicate that the Mn‐depleted and Mn‐enriched regions existed in the austenite after austenitizing as a result of Mn‐PrPT, which are evaluated by thermodynamic calculation, and validated by dilatometric and wavelength dispersive X‐ray spectrometry (WDS) measurement. The introduction of Mn‐PrPT before austenitizing has little effect on the prior austenite grain size, whereas it affects martensite start (M S) temperature remarkably, which increased by 30 °C. Moreover, the dilatation rate with Mn‐PrPT shows a two‐peak feature during the bainitic transformation, which is believed to be attributed to the varied bainitic transformation domain for Mn‐depleted and Mn‐enriched regions. In addition, the Mn‐PrPT contributes to an increase in both fraction of retained austenite and its carbon concentration, and consequently, the elongation is significantly improved without compromising the tensile strength, leading to an enhancement of the product of strength and elongation. However, it has a slightly negative effect on impact toughness due to an increment in the fraction of blocky retained austenite with low mechanical stability.

Effect of Intercritical Annealing on the Microstructure and Mechanical Properties of 0.1C-13Cr-3Ni Martensitic Stainless Steel

Standard heat treatment of martensitic stainless steel consists of quenching and tempering. However, this results in high strength and hardness, while Charpy impact toughness shows lower values and a large deviation in its values. Therefore, a modified heat treatment of 0.1C-13Cr-3Ni martensitic stainless steel (PK993/1CH13N3) with intercritical annealing between Ac1 and Ac3 was introduced before tempering to study its effect on the microstructure and mechanical properties (yield strength, tensile strength, hardness and Charpy impact toughness). The temperatures of intercritical annealing were 740, 760, 780 and 800 °C. ThermoCalc was used for thermodynamic calculations. Microstructure characterization was performed on an optical and scanning electron microscope, while XRD was used for the determination of retained austenite. Results show that intercritical annealing improves impact toughness and lowers deviation of its values. This can be attributed to the dissolution of the thin carbide film along prior austenite grain boundaries and prevention of its re-occurrence during tempering. On the other hand, lower carbon concentration in martensite that was quenching from the intercritical region resulted in lower strength and hardness. Intercritical annealing refines the martensitic microstructure creating a lamellar morphology.

Enhancing the Robustness and Efficiency in the Production of Medium Mn Steels by Al Addition

The narrow process window during intercritical annealing and discontinuous yielding have limited the commercialization of medium Mn steels. In this study, a double-annealing process based on the commercial continuous annealing line is proposed. The cold-rolled medium Mn steels were first fully austenitized and quenched during the first annealing, followed by intercritical annealing for reverted austenite transformation. The microstructure of duplex lath-shaped austenite and ferrite is produced and steel exhibits a desirable continuous yielding during tensile deformation. Al is added into the medium Mn steel to enlarge the process window and to improve the partitioning efficiency of Mn. The produced steel is more robust with temperature fluctuation during the industrial process due to the enlarged intercritical region. Mn partitioning is more efficient owing to the elevated annealing temperature, which results in the improvement of ductility in the Al-added steel with increased austenite stability.

Microstructural evolution and mechanical properties of 9Mn steel during warm/cold rolling and subsequent intercritical annealing

The microstructural evolution of 9Mn steel during warm rolling in the intercritical region and cold rolling, both followed by subsequent intercritical annealing was studied. A significantly different microstructure was obtained by warm rolling and cold rolling, the former contained austenite and deformed ferrite/martensite and the latter consisted of deformed ferrite and transformed martensite, which significantly impacted the microstructure after intercritical annealing. The existence of austenite in warm-rolled sample reduced the elemental content in the deformed matrix, weakening the kinetics of austenite formation during subsequent annealing. In addition, the occurrence of dynamic recovery during warm rolling led to a low storage energy of recrystallization, resulting in a partial recrystallization of ferrite during IA. However, a slightly larger volume fraction as well as more heterogeneous austenite morphology with a wide austenite size distribution was obtained by combining both warm rolling and annealing. These austenite grains could transform to martensite in a more sustainable way, increasing the TRIP-related strain. Consequently, the warm-rolled + annealed steel showed a better combination of ultimate tensile strength (1380 MPa) and total elongation (29.11%).

Effect of Thermo-mechanical Processing on Structure and Properties of Dual-Phase Matrix ADI with Different Si-Contents

The improvement in the combination between strength and ductility of austempered ductile iron with dual matrix structure was investigated in two ductile irons having different silicon contents, namely 2.6 wt% and 4.0 wt%. The structure was produced in a thermo-mechanical simulator, equipped with a dilatometry system. The effect of silicon content on the transformation kinetics and mechanical properties was studied. For both ductile irons, the influence of introducing ferrite into the matrix on the structure development and mechanical properties was investigated and compared to those of completely ausferritic matrix. Increasing the Si-content widened the intercritical region and shifted it to higher temperature range. The former effect renders the intercritical annealing process more controllable. The introduction of the ferrite phase accelerated the ausferrite transformation kinetics and improved both the ductility and the formability index (ductility × ultimate strength), while both yield and ultimate strength declined.

Effect of Niobium Addition on Microstructure and Mechanical Properties of Fe–7Al–0.35C Low-Density Steel

The present work reports the effect of niobium addition on a Fe–7wt.%Al–0.35wt.%C-based low-density steel. A hot rolling temperature of 1100 °C was selected to carry out rolling in the intercritical (ferrite + austenite) region. The phases formed on adding Nb were predicted by thermodynamic calculations using ThermoCalc. All the alloys exhibited a significant (20% or more) tensile elongation. Niobium addition also resulted in a significant increase in strength and hardness of the hot-rolled steel. About 80% increase in the yield strength is observed as the Nb content increases from 0.2 to 1.0 wt.%. Nb carbides present at rolling temperature resist grain growth and lead to improved mechanical properties. This study demonstrates the advantages of adding Nb to Fe–Al–C-based low-density steels. Earlier attempts to add Nb to low-density steels resulted in very low ductility. This may be related to their susceptibility to hydrogen embrittlement. Melting of high aluminum low-density steels in a controlled atmosphere may lead to a considerable improvement in mechanical properties. Further, it is also important to maintain a high C/Nb ratio to avoid the formation of Laves phases.

Creep and fracture behavior of long-annealed weld HAZ in CB2 steel

CB2 steel for thick castings has been designed and investigated in the frame of COST-522 and C0ST-536 R&D actions of EU to work at supercritical conditions of the steam main lines. The steel exhibits loss of impact strength and hardness after long annealing and also of strength and life when creep tested after the long annealing. The most serious decrease of properties appears in weld’s heat-affected zones during the creep tests. Differences in microstructures of base metal (BM) and weld’s heat-affected zone (HAZ) have been studied in initial state and after long-term annealing/aging for 10 kh and 30 kh at 625 °C, followed by creep testing. The initial as-cast microstructure of CB2 is inhomogeneous, and the welding thermal cycle causes additional separation of phases in the intercritical region. TEM observations revealed appearance in HAZ of large ferrite grains between compact colonies of spheroidal carbides, associated with the decrease of creep strength and life in the HAZ.