Fine Austenite Grain Research Articles

Solidification Behaviour and Microstructural Development of Iron-based Alloys under Conditions Pertinent to Strip Casting – 200 Series Stainless Steels

The castability and microstructures produced from strip casting simulations of three compositions in the 200 series stainless steels have been examined. The nucleation density was similar for all three compositions.The as-cast microstructure showed very fine austenite grains of 10–20 μm in width. Retained delta ferrite was observed in the inter-dendritic regions, and was likely to be stabilised by the segregation of Cr into these regions. An analysis of the crystallography expected of different solidification sequences is presented, but a strict adherence to the Kurdjumov-Sachs orientation relationship was not found in these samples.

Surface Nanocrystallization of 3Cr13 Stainless Steel Induced by High‐Current Pulsed Electron Beam Irradiation

The nanocrystalline surface was produced on 3Cr13 martensite stainless steel surface using high‐current pulsed electron beam (HCPEB) technique. The structures of the nanocrystallized surface were characterized by X‐ray diffraction and electron microscopy. Two nanostructures consisting of fine austenite grains (50–150 nm) and very fine carbides precipitates are formed in melted surface layer after multiple bombardments via dissolution of carbides and crater eruption. It is demonstrated that the dissolution of the carbides and the formation of the supersaturated Fe (C) solid solution play a determining role on the microstructure evolution. Additionally, the formation of fine austenite structure is closely related to the thermal stresses induced by the HCPEB irradiation. The effects of both high carbon content and high value of stresses increase the stability of the austenite, which leads to the complete suppression of martensitic transformation.

Development of Bimodal Ferrite-Grain Structures in Low-Carbon Steel Using Rapid Intercritical Annealing

Mixed ferrite grain structures, which have fine- and coarse-grain regions and showing “bimodal” grain size distributions, have been produced by rapid intercritical annealing of warm-rolled (or cold-rolled) samples. Microstructural changes have been analyzed using dilatometric studies, size prediction of transformed and recrystallized grains, and microtexture measurements. Fine austenite grains (<5 μm) developed during rapid annealing and transformed into fine-ferrite grains (2 to 4 μm) after cooling. Coarse-ferrite grains (28 to 42 μm) resulted from the recrystallization and growth of deformed ferrite. The effect of heating rate on microstructural morphologies during intercritical annealing has also been studied. A slow rate of heating (30 K/s) developed a uniform distribution of fine-ferrite grains and austenitic islands, while rapid heating (300 K/s) generated coarse blocks of austenite, elongated along the prior-pearlitic regions, in the ferrite matrix. As expected, bimodal ferrite grain structures or fine-scale dual-phase structures showed superior combination of tensile strength and ductility, compared to the ultrafine-grained steels.

Growth of austenite from as-quenched martensite during intercritical annealing in an Fe–0.1C–3Mn–1.5Si alloy

The growth of austenite from as-quenched martensite during intercritical annealing was studied in a quaternary Fe–0.1C–3Mn–1.5Si alloy. Fine austenite grains either grew from interlath-retained austenite films or were newly nucleated at lath and martensite packet boundaries. Both types grew to a size comparable to the width of the martensite lath. It was found both metallographically and by dilatometry that the austenite grew to an amount in excess of the volume fraction at final equilibrium. Simulation by DICTRA, which assumed local equilibrium at the α/γ boundary, confirmed that the development of austenite is composed of three stages: initial negligible-partitioning growth controlled by rapid carbon diffusion in ferrite, which is gradually replaced by carbon diffusion in austenite; intermediate slow growth, controlled by diffusion of Mn and/or Si in ferrite; and a final stage controlled by diffusion of substitutional elements in austenite for final equilibration, which may result in the shrinkage of austenite. The formation of austenite in excess of the equilibrium amount is considered to occur due to very slow substitutional diffusion in the growing austenite compared to the boundary migration.

Carburising behaviour of low carbon steel with nanocrystalline surface layer induced by ball milling

Ball milling, as a surface nanocrystallisation method, was employed to investigate the influence of severe plastic deformation on the carburisation treatment performed on low carbon steel. The results indicated an enhancement in the carburisation efficiency as a result of surface milling. This enhancement was attributed to the formation of a nanocrystalline layer in the surface of the treated samples. It was found that the main reasons for the accelerated kinetics of the carburisation process would be the considerable amounts of non-equilibrium defects and the finer austenite grains in the early and later stages of the treatment respectively, which facilitate the carbon diffusion.

The effect of single and double quenching and tempering heat treatments on the microstructure and mechanical properties of AISI 4140 steel

This investigation is concerned to evaluate the effect of double quenching and tempering (DQT) with conventional quenching and tempering (CQT) heat treatment processes on microstructure and mechanical behavior of a commercially developed hot rolled AISI 4140 type steel. Comparison of microstructure and mechanical properties of DQT and CQT heat treated specimens have been established in details. Optical and scanning electron microscopies have been used to follow impurity concentration and microstructural changes, and their relation to the associated mechanical properties. The results indicate that the improvement of mechanical properties particularly impact toughness of DQT heat treated specimens is much higher than that of the CQT condition, and this observation is rationalized in terms of finer austenite grain size developed in the DQT condition providing much finer martensitic packets within the grains and a lower level of impurity concentration of sulfur (S) and phosphorus (P) near the prior austenite grain boundaries as well.

Powder metallurgical nanostructured medium carbon bainitic steel: Kinetics, structure, and in situ thermal stability studies

It has been possible to produce incredibly fine plates of bainitic ferrite separated by a percolating network of retained austenite in a medium carbon steel produced by mechanical alloying followed by spark plasma sintering and isothermal heat treatment. This is because the sintering process limits the growth of the austenite grains to such an extent that the martensite-start temperature is suppressed in spite of the medium carbon concentration. Furthermore, the fine austenite grain size accelerates the bainite transformation, which can therefore be suppressed to low temperatures to obtain a nanostructure. Microscopy and in situ synchrotron X-ray diffraction were used to investigate the morphology and the thermal stability of the retained austenite during continuous heating. These latter experiments revealed a gradient of carbon concentration in the retained austenite and a reduced thermal stability in high carbon film-austenite. It was also possible to correlate the evolution of defect density and carbon depletion in both retained austenite and bainitic ferrite during tempering.

Strain‐Induced Development of Very Fine Ferrite‐Cementite Structures in Eutectoid Steels

AbstractThe development of duplex microstructures consisting of very fine ferrite grains and spheroidized cementite particles during concurrent hot deformation has been investigated by hot torsion in a eutectoid steel. It is shown that the hot deformation of the undercooled austenite at temperatures close to Ar1 induces and accelerates significantly the γ‐to‐pearlite transformation. Further deformation of the transforming pearlite does not lead only to the instantaneous spheroidization of the cementite lamellae but also refines the ferrite orientations units from 15 to 3 µm. Microstructures consisting of very fine ferrite grains (3 µm) and submicron cementite particles are produced at strain levels larger than ε = 1.5. The initial grains undergo progressive fragmentation by the generation of low angle boundaries (LABs) by dynamic recovery that progressively transform into high angle boundaries (HABs). The progressive increase of LABs misorientation is attributed to the presence of cementite particles and is referred to as continuous dynamic recrystallization (cDRX). Fine prior austenite grain size and moderate strain rates are found to promote the occurrence of cDRX. The resulting torsion textures consist of a strong 〈111〉 fiber dominated by the D {112} 〈111〉 component. During the subsequent annealing step, cementite particles coarsen preferentially at the triple junctions whereas the very fine ferrite grain are stable due to the strong pinning effect exerted by the cementite particles.

Effect of fast cooling process on microstructure and toughness of heat affected zone in high strength pipeline steel X120

A process for improving the heat affected zone (HAZ) toughness by fast cooling during welding was developed. The holding time at high temperature above Ac3 and the cooling time from 800 to 500°C were decreased when the welding thermal cycle was experienced a fast cooling process. The microstructures in the HAZ with varied thermal cycles were examined by optical and scanning electron microscopes. The shorten holding time led to thinner HAZ width and finer austenite grains in the fusion line and coarse grained HAZ, while the decreased cooling time from 800 to 500°C resulted in finer bainitic ferrite in the HAZ. Martensite–austenite constituents in the fusion line were reduced in amount and size and predominantly changed into film-like shape. Experimental results indicated that the fast cooling process played an important role in improving the HAZ toughness of pipeline steel X120.

Effect of Cooling Rate and Nb Composition on Non-Isothermal Austenite Grain Growth Kinetics in Nb-HSLA Steel during Hot Rolling

Abstract. The strength of a final product of steel is affected by its final austenite grain size.Almost applied models for grain growth based on Beck and Sellars equation , which has the isothermal condition assumption; whilst most of the materials processing take place under non-isothermal condition. The purpose of this research is to find the effect of Nb and cooling rate to the austenite grain growth model of HSLA-Nb steel to predict the Austenite grain size after hot rolling process in non-isothermal condition .The two composition of 0.019 and 0.056% Nb of HSLA-Nb was hot-rolled about 0.3-0.4 deformation at a temperature of 900-11000 C, followed by cooling rate of 7-11 0 C/s, in a time period of 25-40 second, and quenched by using water jetspray. Austenite final grain size was measured . The work shows that the higher Nb content of steel with higher cooling rate will reduce the final austenite grain size . It was also found that the new non-isothermal austenite grain growth after hot-rolled illustrated as a function of cooling rate,which obtained by modifying the previous established model by a cooling rate parameter 1/Crm, and it was more precision compared to previous grain growth equation.

鍛造型板厚大圧下によるオーステナイト組織の微細化機構

To obtain fine and uniform microstructures, appropriate forging conditions were examined using high-speed large-reduction forging. Swing-type forging equipment was employed in hot-forging experiments using low-carbon steel (0.14%C-0.64%Mn). Also, a microstructure analytical system was developed by combining thermomechanical FE analysis with microstructural analysis to discuss the metallurgical phenomena occurring during forging. It was observed that coarse austenite grains can be refined to 20-30 μm uniformly using the proposed forging technology in the case of 59% reduction in thickness at 1273 K. In this case, dynamic recrystallization is a dominant factor for obtaining fine and uniform austenite grains. In the case of 43% reduction in thickness, the recrystallization is inhomogeneous, resulting in an inhomogeneous microstructure. Thus, about 60% reduction in thickness is necessary to obtain fine and uniform microstructures using the proposed technology when applied to the roughing of hot strip mills.

Microstructure and Crystallographic Features of Martensite Transformed from Ultrafine-Grained Austenite in Fe&amp;ndash;24Ni&amp;ndash;0.3C Alloy

We studied microstructure and crystallographic features, especially orientation relationship, of martensite transformed from austenite with mean grain sizes ranging from 35μm to 750 nm in an Fe­24Ni­0.3C alloy. The austenite structures with ultrafine or fine grains were fabricated through intense straining by accumulative roll-bonding process and subsequent annealing. The morphology of martensite transformed from the austenite with various grain sizes was all lenticular type. On the other hand, the martensite plate size decreased with the decrease in the austenite grain size. The orientation relationship of martensite transformed from coarse-grained austenite with mean grain size (d) of 35μm changed depending on the location within martensite plate: i.e., it was Greninger­Troiano relationship at the center part of each martensite plate, while it changed to Kurdjumov­Sachs relationship near the interphase boundary. In contrast, the whole area of each martensite plate, transformed from fine-grained austenite (d = 2.5 μm) and ultrafine-grained austenite (d = 750 nm), satisfied Greninger­Troiano relationship. Furthermore, high density of dislocations and low angle boundaries within the ultrafine-grained austenite resulted in a large scatter of observed orientation relationship. [doi:10.2320/matertrans.MD201121]

Influence of peritectic phase transformation on hot ductility of high aluminium TRIP steels containing Nb

Increasing Al from 0·05 to 1% in Nb containing transformation induced plasticity steel resulted in deepening and considerable widening of the hot ductility trough. Further increase in the Al level to 1·5% produced a trough similar to the low Al steel but having better ductility in the temperature range of 650–800°C. This improved ductility could be ascribed to its finer austenite grain size. Nb(CN) was able to precipitate readily in these steels and was important in influencing the hot ductility of the 0·05 and 1·5%Al steel in the temperature range of 750–1000°C, with ductility improving as the particle size increased with test temperature. No AlN was found in 0·05%Al containing steel, and there was no significant dendritic precipitation of AlN in 1·5%Al containing steel, although precipitation of AlN in plate form was readily observed. In 1%Al steel, copious dendritic precipitation of AlN was present at the γ grain boundaries, leading to rock candy fracture. The poor ductility shown in 1%Al containing steel is due to a combination of this dendritic precipitation, which took place only in a steel of peritectic carbon composition, and its coarse grain size. Both low and 1·5%Al containing steels had compositions outside the peritectic range. It is strongly advised that for this type of steel, the composition should be designed to fall outside the peritectic carbon range.

Enhanced mechanical properties of type AISI301LN austenitic stainless steel through advanced thermo mechanical process

The effect of annealing temperature and time on the grain size of reversed austenite and mechanical properties was studied based on AISI301LN austenitic stainless steel in this paper. Cold rolling at 0 °C was employed to obtain the strain-induced martensite, followed by annealing treatment at the temperature range of 650–900 °C for 1–20 min. The relationship between the volume fraction of strain-induced martensite and cold reduction was analyzed by Ferritescope MP30 and X-ray diffraction. The grain growth of reversed austenite was observed by FEG-SEM and the mechanical properties were determined by tensile tests and Vickers hardness tests. Austenite grain sizes under 3 μm can be obtained after annealing in the range of 700–900 °C for 1–20 min. The finest austenite grain size was produced after annealing at 700 °C for 20 min, which had a good combination of yield strength (≈830 MPa), tensile strength (≈953 MPa) and elongation (≈36%).

Advanced technology in creep life prediction and damage evaluation for creep strength enhanced ferritic steels

Creep damage mechanisms of recently developed boiler and steam-turbine steels are not well clarified, however, creep life prediction technology for such creep strength enhanced ferritic steels, grades 91, 92 122, etc. with tempered martensitic structure including welds has been strongly demanded by power plant operators and equipment manufacturers. In this paper, the technologies related to the creep life prediction and damage evaluation for creep strength enhanced ferritic steels being studied from the various aspects in Japan will be surveyed and presented. The creep life prediction of this type of steel must be grounded on the findings on microstructural degradation and creep softening in martensitic structure composed of very fine martensite lath, block, packet and prior austenite grains, and precipitation and dislocation structures. Physical properties response to the creep degradation and measurement and detection of localized creep damage/strength would be useful tools to develop diagnostic techniques for life prediction of creep strength enhanced ferritic steels. Other important techniques to support the creep life prediction based on creep-strain/rupture data are creep modelling and data analysis which have been successfully investigated to date.

Non-Isothermal Austenite Grain Growth Kinetics in the HAZ of a Microalloyed X-80 Linepipe Steel

Non-isothermal austenite grain growth kinetics under the influence of several combinations of Nb, Ti and Mo containing complex precipitates has been studied in a microalloyed linepipe steel. The goal of these studies is the development of a grain growth model to predict the austenite grain size in the weld heat affected zone (HAZ). A detailed electron microscopic investigations of the as-received steel proved the presence of Ti-rich, Nb-rich and Mo-rich precipitates. Inter and intragranular precipitates of ~5-150 nm have been observed. The steel has been subjected to austenitizing heat treatments to selected peak temperatures of 950, 1150 and 1350°C at various heating rates of 10, 100 and 1000°C/s. Thermal cycles have been found to have a strong effect on the final austenite grain size. The increase in heating rate from 100 to 1000°C/s has a negligible difference in the austenite grain size irrespective of the austenitizing temperature. However, the increase in grain size has been noticed at 10°C/s heating rate for all the austenitizing temperatures. The austenite grain growth kinetics have been explained taking into account the austenite growth in the presence of precipitates.

Effect of Austenite Grain Size on the Fatigue Crack Growth and Arrest Behavior of Carburized Steel

The effect of austenite grain size on the fatigue crack growth and arrest behavior of carburized steel was studied. On the fatigue test of same stress amplitude, the fatigue crack initiation of fine austenite grain steels is delayed in comparison with coarse austenite grain steels, but the difference of crack initiation life between fine grain steels and coarse grain steels is very slight against 107 cycles. The improvement of crack arrest property is dominant for increasing fatigue limit by refinement of austenite grain size. The critical stress intensity factor for crack arrest under fatigue tests at the stress amplitude of the each fatigue limit of carburized steels is calculated by considering the effect of compressive residual stress. As a result, it becomes clear that the finer the austenite grain size, the larger becomes the critical stress intensity factor for crack arrest, and crack arrest property is improved remarkably by the refinement of austenite grain size.

Effect of direct and reheated quenching on microstructure and mechanical properties of CLAM steels

Direct and reheated quenching on microstructural mechanical properties of China Low Activation Martensitic (CLAM) steel was investigated. Three direct quenching and tempering (DQ&T) CLAM steels were rolled at the same finish rolling temperature with different quenching rate, and their microstructures and mechanical properties were compared with those of a reheated quenching and tempering (RQ&T) CLAM steel. DQ&T process was proven to strengthen the tensile property but lower the toughness compared to RQ&T process due to the finer austenite grain size formed under RQ&T process. Tensile property first increased with quenching rate and then decreased. The reason for this was discussed.

Nonisothermal Austenite Grain Growth Kinetics in a Microalloyed X80 Linepipe Steel

Nonisothermal austenite grain growth kinetics under the influence of several combinations of Nb, Ti, and Mo containing complex precipitates has been studied in a microalloyed linepipe steel. The goal of this study is the development of a grain growth model to predict the austenite grain size in the weld heat affected zone (HAZ). Electron microscopy investigations of the as-received steel proved the presence of Ti-rich, Nb-rich, and Mo-rich precipitates. The steel has then been subjected to austenitizing heat treatments to selected peak temperatures at various heating rates that are typical for thermal cycles in the HAZ. Thermal cycles have a strong effect on the final austenite grain size. Using a mean field approach, a model is proposed for the dissolution of Nb-rich precipitates. This model has been coupled to a Zener-type austenite grain growth model in the presence of pinning particles. This coupling leads to accurate prediction of the austenite grain size along the nonisothermal heating path simulating selected thermal profiles of the HAZ.

Electron microscopy and microanalysis of steel weld joints after long time exposures at high temperatures

The structural changes of three trial weld joints of creep resistant modified 9Cr-1Mo steels and low alloyed chromium steel after post-weld heat treatment and long-term creep tests were investigated. Smooth cross-weld specimens ruptured in different zones of the weld joints as a result of different structural changes taking place during creep exposures. The microstructure of the weld joint is heterogeneous and consequently microstructural development can be different in the weld metal, the heat affected zone, and the base material. Precipitation reactions, nucleation and growth of some particles and dissolution of others, affect the strengthening of the matrix, recovery at high temperatures, and the resulting creep resistance. Therefore, a detailed study of secondary phase's development in individual zones of weld joints can elucidate mechanism of cracks propagation in specific regions and the causes of creep failure. Type I and II fractures in the weld metal and Type IV fractures in the fine prior austenite grain heat affected zones occurred after creep tests at temperatures ranging from 525 to 625 °C and under stresses from 40 to 240 MPa. An extended metallographic study of the weld joints was carried out using scanning and transmission electron microscopy, energy-dispersive and wave-dispersive X-ray microanalysis. Carbon extraction replicas and thin foils were prepared from individual weld joint regions and quantitative evaluation of dislocation substructure and particles of secondary phases has been performed.