Large Austenite Grains Research Articles

Evolution of ultrafine bainite microstructure during rapid heating process and the role of retained austenite

Rapid heat treatment is an important method to improve the properties of steel. Moreover, the study of the phase transformation process plays an important role in the formulation of rapid heat treatment. This paper investigates phase transformation behavior and microstructure evolution of ultrafine bainite at varying heating rates, and reveals the influence mechanism of retained austenite on austenitizing. Results showed that the decomposition of retained austenite was influenced by the diffusion of Si element. Therefore, the decomposition temperature of retained austenite is severely affected by the heating rate. Retained austenite completely transformed into cementite and ferrite when heating rate was below 5 °C/s. A small amount of retained austenite was kept when the heating rate was higher than 5 °C/s. The kept austenite effectively suppressed the hysteresis of Ac1 and resulted in a unique mechanism of austenite grain formation. When the heating rate range was 5–10 °C/s, some new austenite skipped the nucleation process and grew directly on the kept austenite, forming the large austenite grains. And other new austenite was obtained via nucleation and growth process. Thus, the grain size of bimodal distribution is formed. This leads to the phenomenon of austenite memory effect. When the heating rate was increased to 50 °C/s and higher, the austenite grain size was less than 14 μm. Therefore, rapid heating makes the phase transition lag. When the heating rate is high enough, part of the austenite is kept, which has an important influence on austenitizing process of the steel.

Strong and ductile medium-Mn steel achieved via controlled martensite tempering

The present work simplifies the previous room-temperature quenching and partitioning (RT Q&P) process down to a single furnace cooling process, delivering a medium Mn steel with an ultimate tensile strength (UTS) of 1.7 GPa and a uniform elongation of 9.5%. The martensite tempering during furnace cooling is featured with slightly reduced dislocation density and is free of cementite precipitation, which is beneficial for ultrahigh UTS. Although the carbon partitioning is limited, the banded large austenite grains have proper mechanical stability owing to pre-existing high carbon content and the strong martensite matrix, allowing the gradual transformation-induced plasticity (TRIP) effect to enhance the work hardening behaviour and thus the good uniform elongation.

Effect of Reheating Temperatures on Hot Ductility and Precipitation Behavior of Low‐Carbon Steels

The influence of reheating temperatures on hot ductility is investigated in two Ni‐, Mo‐, and Cu‐containing low‐carbon steels. The effects of solution treatment conditions at 1623, 1673 K, and remelting at 1803 K on the reduction in area (RA) are analyzed. The results show an RA trough in the range of 973–1173 K in all experimental conditions. At 973 K, which is close to the Ae3 temperature of both steel grades, ductility drop is caused by the formation of the thin ferrite film at the prior austenite grain boundary. Higher RA values are achieved after reheating to 1623 K, mainly due to smaller austenite grain size by the action of preformed precipitation throughout the matrix. In contrast, increased reheating temperatures result in relatively poor RA values because of the large austenite grain size due to the dissolution of preformed precipitate and reprecipitation at the grain boundaries during cooling. A comparison between two steel grades shows that better ductility at 973 K is achieved in low Ni‐containing steel due to the relatively thick ferrite film. The effects of alloy composition and preformed precipitate's existence on hot ductility are further discussed based on the matrix strengthening and reprecipitation behavior.

A feasible route to produce 1.1 GPa ferritic-based low-Mn lightweight steels with ductility of 47%

High- and medium-Mn (H/M-Mn) base lightweight steels are a class of ultrastrong structural materials with high ductility compared to their low-Mn counterparts with low strength and poor ductility. However, producing these H/M-Mn materials requires the advanced or high-tech manufacturing techniques, which can unavoidably provoke labor and cost concerns. Herein, we have developed a facile strategy that circumvents the strength–ductility trade-off in low-Mn ferritic lightweight steels, by employing low-temperature tempering-induced partitioning (LTP). This LTP treatment affords a typical Fe-2.8Mn-5.7Al-0.3C (wt.%) steel with a heterogeneous size-distribution of metastable austenite embedded in a ferrite matrix for partitioning more carbon into smaller austenite grains than into the larger austenite ones. This size-dependent partitioning results in slip plane spacing modification and lattice strain, which act through dislocation engineering. We ascribe the simultaneous improvement in strength and total elongation to both the size-dependent dislocation movement in austenite grains and the controlled deformation-induced martensitic transformation. The low-carbon-partitioned large austenite grains increase the strength and ductility as a consequence of the combined martensitic transformation and high dislocation density-induced hardening and by interface strengthening. Additionally, high-carbon-partitioned small austenite grains enhance the strength and ductility by planar dislocation glide (in the low strain regime) and by cross-slipping and delayed martensitic transformation (in the high strain regime). The concept of size-dependent dislocation engineering may provide different pathways for developing a wide range of heterogeneous-structured low-Mn lightweight steels, suggesting that LTP may be desirable for broad industrial applications at an economic cost.

The Effect of Direct Strip Casting on the Kinetics of Phase Transformation of a Dual Phase Steel

A dual phase steel has been produced directly from the liquid under conditions that simulate direct strip casting and thin slab casting. The kinetics of polygonal ferrite formation during the inter-critical anneal were quantified using the JMAK approach, and this revealed significantly retarded transformation kinetics in the strip cast samples compared to the commercial steel that was processed through the conventional hot rolling approach. The transformation rate in the strip cast samples were as much as three orders of magnitude slower compared to the commercial steel. It was found that the kinetics of the ferrite formation were retarded principally by the large prior austenite grain size in the strip cast samples, and this hypothesis was tested experimentally by both coarsening of the prior austenite grain size, and by refinement of the prior austenite grain size. However, even after grain size normalization, small differences in transformation kinetics between the direct strip cast and commercial steel specimens were observed. These differences were explained by investigation of MnS precipitation in the steels. It was found that the transformation rate is high when the solutes are in solid solution, and that the rate of transformation slows significantly when precipitation of nano-precipitates occurs.

Effect of boron segregation on bainite nucleation during isothermal transformation

The effects of boron segregation and austenite grain size on the bainitic isothermal transformation of a high-strength steel were studied independently. Dilatometry, microstructural observations and previous atom probe tomography analyses showed that bainitic transformation rate decreases with increasing boron excess at austenite grain boundaries. We also found that boron segregation causes an unexpected grain size effect: large austenite grains transform faster than small ones. A kinetic model assuming slow nucleation at austenite triple junctions and rapid growth of bainitic nuclei successfully described our experimental data. These results confirm that bainite nucleation is inhibited by segregated boron at grain boundaries. On this basis, austenitic grain size could be optimized to make the most of boron addition in advanced high-strength steels.

Interpretation of distribution of microstructural heterogeneity across the plate steel thickness

The relationships of the anisotropy behavior and other characteristics of structural heterogeneity over the thickness of the plate have been established and their microstructural interpretation has been presented using the example of low-alloy plate steel. The features of structure formation from the surface to the center of 100 mm thick plate metal have been considered with a detailed description of the structure evolution and the processes that caused these changes. It was found that a fine quasi-homogeneous structure consisting of lath bainite and granular bainite provides anisotropy Ka100 <1 and occupies a narrow layer from 5 to 17 mm from the plate surface. In the near-surface layer, due to the abnormal structure consisting of large austenite grains elongated along the rolling direction and decorated with allotriomorphic ferrite, the anisotropy is Ka100 > 1. In a quarter of the plate thickness, due to weak fragmentation of the structure, the anisotropy is Ka100 > 1. In the center of the plate, quasi-polygonal and polygonal ferrite absorbs all extended boundaries of the former austenite grains; therefore, Ka100 becomes again less than unity.

Relationship between the Microstructure and Impact Toughness of the Coarse‐Grained Heat‐Affected Zone for Offshore Engineering Steels with Different Mg Contents

The relationship between the microstructure and impact toughness of coarse‐grained heat affected zone for offshore engineering steels with 0.0002 and 0.0042 wt% Mg, termed as 2 and 42Mg, respectively, is clarified. With increasing the Mg content from 0.0002 to 0.0042 wt%, the average impact energy at −40 °C decreases from 201 to 59 J. The prior austenite grain size (PAGS) increases from ≈92 to ≈163 μm, the average thickness of lath bainites increases from ≈234 to ≈361 nm. The addition of Mg makes the particle increase in number and refine in size, but the amount of the effective pinning particles at the temperature close to 1350 °C in 42Mg is smaller than that in 2Mg, which leads to the larger austenite grain growth rate and larger PAGS in 42Mg. The larger PAGS with the higher bainite transformation temperature and smaller PAG boundary density in 42Mg makes the bainite coarser. So, in 2Mg, the effective bainitic grain size with misorientation above 45° is smaller and the density of high‐angle grain boundaries (HAGBs) is larger, resulting in its higher toughness. Besides, the coalesced bainite containing cementite colonies in 42Mg makes the bimodal bainitic lath thicknesses and heterogeneous microhardness distribution, reducing the toughness.

Revealing the intrinsic nanohardness of retained austenite grain in a medium Mn steel with heterogeneous structure

The present work investigates the intrinsic nanohardness of retained austenite in medium Mn steel with heterogeneous structure. By increasing the austenite stability using Mn alloying, the martensitic transformation in both small and some large austenite grains is effectively suppressed during nanoindentation measurement. The average intrinsic nanohardness of small and large austenite grains are found to be 4.4 GPa and 5.5 GPa, respectively. The higher nanohardness of large austenite grains as compared to the small austenite grains is due to the higher dislocation density. Despite of grain boundary strengthening to the ferrite phase, the nanohardness of ferrite is still lower than that of small austenite grains. The intrinsic nanohardness of austenite in the present work could be used to generate valuable input parameters (i.e. yield strength, strain hardening) for the modeling work on high strength steels with retained austenite.

Improving the yield strength of an antibacterial 304Cu austenitic stainless steel by the reversion treatment

As an implant material, Cu-bearing austenitic stainless steels can possess the antibacterial property, but their mechanical strength is low. In order to improve the yield strength of a 304Cu (17%Cr–7%Ni–3%Cu) alloy through substantial grain refinement, a research investigation has been taken up to conduct the reversion annealing treatment comprising a heavy (71%) cold rolling reduction followed by annealing at various temperatures (650–950 °C) and durations (1–5400 s). The microstructure evolution was examined by electron backscatter diffraction and further characterized by magnetic measurements, and mechanical properties were determined by tensile and hardness testing. The precipitation of Cu was confirmed by transmission electron microscopy. It was found that the reversion of deformation-induced martensite to austenite took place by the shear mechanism, followed by subgrain formation and continuous recrystallization resulting in quite non-uniform grain size distribution. The finest reversed grains were around 0.6 μm in size, but also much larger austenite grains and a small fraction of unreversed martensite existed in the final structure despite annealing at least up to 800 °C. Coherent Cu particles were observed after aging for 1.5 h at 700 and 650 °C, while the yield strength could be improved to 507 and 791 MPa, respectively, i.e. by ~2–3 times that of the annealed steel. The ductility of the steel remains still high, the fracture elongation being 36%.

The behaviour of AISI 4340 steel coatings on low carbon steel substrate produced by friction surfacing

In the present work, AISI 4340 alloy steel coatings were deposited on ASTM A36 low carbon steel substrates by friction stir surfacing with varying processing parameters. Temperature evolution during deposition was monitored by using an infrared camera. The coatings obtained were subsequently analysed in terms of their microstructure, mechanical properties and residual stresses. Prior austenite grain boundary analysis revealed that with increasing rotation and translational speeds, larger strain rates and a finer austenite grain size were obtained, which influenced the coating microstructure after cooling. The coatings with smaller austenite grain sizes exhibited average hardness values of 510 ± 60 HV0.025 and compressive residual stresses (between approximately −75 and 200 MPa). On the other hand, coatings produced with lower rotation and translational speeds developed relatively larger austenite grain sizes and exhibited mostly martensitic microstructures with average hardness values of 610 ± 60 HV0.025 and evolved tensile residual stresses (between approximately 10 and 90 MPa). Finally, it was possible to notice that the harder predominantly martensitic coatings with tensile residual stresses exhibited less adhesion with the substrate in comparison with the lower hardness coatings with compressive residual stresses.

Microstructural evolution of 2.25Cr-1Mo-0.25V submerged-arc weld metal

Heat-resistant V-modified 2.25Cr-1Mo-0.25V-weld metal is commonly used in petrochemical industry for heavy wall pressure vessels in high-temperature hydrogen service. In order to improve the reactor efficiency, the weldments have to endure even higher temperatures and pressures. Acicular ferrite (AF) is often regarded as the optimum microstructure due to its good combination of strength and toughness. As few literature about the evolution of microstructure and the final microstructure constituents of 2.25Cr-1Mo-0.25V weld metal is available, the current paper intends to provide comprehensive information by means of microscopy, crystallographic examination via electron backscatter diffraction and in situ observation of the austenite to ferrite phase transformation via high-temperature laser scanning confocal microscopy (HT-LSCM). The investigated weld metal exhibits a high density of complex aluminium-silicon-manganese oxides with a spherical shape and large prior austenite grains, which in combination is beneficial for intragranular nucleation of AF. Nonetheless, the examination of the transformed final microstructure was not sufficient to make an unambiguous statement about the presence of AF within the 2.25Cr-1Mo-0.25V weld metal. Via in-situ HT-LSCM of the phase transformation, intragranular nucleation of AF at non-metallic inclusions within the austenite grains was detected, which confirms that even though the microstructure of 2.25Cr-1Mo-0.25V weld metal is mainly bainitic, small amounts of AF are present.

Observations on the Relationship between Crystal Orientation and the Level of Auto-Tempering in an As-Quenched Martensitic Steel

Auto-tempering is a feature of the technologically important as-quenched low-carbon martensitic steels. The focus of this paper is on the morphology of the martensite and the orientation of the last forming untempered regions in relation to the earlier formed auto-tempered martensite in both small and large austenite grains. A low-carbon martensitic steel plate was austenitized for 24 h and quenched to room temperature. The resulting microstructure was characterized using electron microscopy and electron back scattered diffraction (EBSD) imaging. It was found that all the untempered regions in the martensitic microstructure were oriented with the plane normals {100} close to the thickness, or normal, direction of the plates. Variant analysis revealed that the untempered regions and the auto-tempered regions are part of the same packet.

High-strength medium Mn quenching and partitioning steel with low yield ratio

ABSTRACTA new microstructural design is proposed to develop a strong and ductile quenching and partitioning (Q&P) steel with low yield ratio. This steel has a heterogeneous dual phase microstructure which is developed by varying austenite thermal stability through Mn segregation. The heterogeneous microstructure contains large austenite grains which contribute to the low yield strength. The ultra-high tensile strength and good ductility are ascribed to the enhanced strain hardening behaviour resulted from the continuous transformation-induced plasticity (TRIP) effect. The present microstructural design enables a conventional medium Mn steel with high tensile strength, good ductility and low yield ratio, which promises easy forming and potential applications in automotive industries.This paper is part of a Thematic Issue on Medium Manganese Steels.

On the microstructural-textural characterization and deformation analysis of a nano/ultrafine grained Fe-20Cr-8Mn-0.3N duplex alloy with superior mechanical properties

The microstructure of a Fe-20Cr-8Mn-0.3N duplex stainless steel was tailored by combination of cold deformation and subsequent martensite reversion annealing. It was shown that by reversion treatment at proper temperature and time nano/ultrafine grained (NG/UFG) with superior mechanical properties (1GPa yield strength and 40% elongation to fracture) can be achieved. Supplementary dilatometry analysis and thermodynamic studies proved that the shear reversion mechanism of ά-martensite to austenite was responsible for the formation of NG/UFG microstructure. Detailed transmission Kikuchi diffraction (TKD) characterization revealed that austenite grain size distribution was broad and narrowed by increasing the reversion annealing time. The observed inhomogeneity in grain size distribution was ascribed to the large variation in morphology and deformation level of martensite. Additionally, it was proved that certain stable original orientations of austenite grains i.e. 〈110〉//ND are highly resistant to martensitic transformation, causing stability of originally large austenite grains. The austenite texture after reversion annealing in the present duplex alloy was found to be similar to that of single phase austenitic steels. It was shown that Brass and Goss textures, which were formed during cold rolling, persist after reversion annealing irrespective of austenite metastability degree. Transmission electron microscopy (TEM) revealed that superior mechanical strength observed in the NG/UFG material, stems from both grain refinement and chromium nitride precipitates which form during revision annealing. The extraordinary plasticity of the UFG microstructure was recognized to be due to the complex deformation mechanisms of NG/UFG austenite, specifically nano size twin/twin-like bands, as well as deformation accommodation by ferrite phase.

Influence of Austenite Grain Size on Mechanical Properties after Quench and Partitioning Treatment of a 42SiCr Steel

This paper examines how the initial austenite grain size in quench and partitioning (Q-P) processes influences the final mechanical properties of Q-P steels. Differences in austenite grain size distribution may result, for example, from uneven heating rates of semi-finished products prior to a forging process. In order to quantify this influence, a carefully defined heat treatment of a cylindrical specimen made of the Q-P-capable 42SiCr steel was performed in a dilatometer. Different austenite grain sizes were adjusted by a pre-treatment before the actual Q-P process. The resulting mechanical properties were determined using the upsetting test and the corresponding microstructures were analyzed by scanning electron microscopy (SEM). These investigations show that a larger austenite grain size prior to Q-P processing leads to a slightly lower strength as well as to a coarser martensitic microstructure in the Q-P-treated material.

Development of actions on optimization of chemical composition of carburized steel grade 16MnCrS5 to eliminate the causes of the growth of large austenite grains

The problem of friction, wear of machine parts and cutting tools, the need to increase the life of machines makes the task of creating new high-performance, energy-saving technologies of hardening one of the main in engineering. In mechanical engineering, the problem of improving the physical, mechanical and operational properties in thin surface layers (~10 microns) of such parts as shafts, gears, measuring tools, drills, cylinders of internal combustion engines, etc. is important. Currently, these tasks are often solved by applying reinforcing coatings. Cementation is economical. The technology of cemented steels smelting, which is currently used by metallurgical plants, does not ensure stable production of fine-grained structure in products. At the same time, the capacity of machines and units are growing, and to obtain parts that can withstand higher loads while maintaining their overall dimensions, new steel grades are needed. The complexity of the configuration of gears with a thin tooth and a massive sleeve, and the need for minimal warping make heat treatment as difficult and responsible as a complex tool, such as shaped cutters. Therefore, for the manufacture of gears you want to assign steel with small hereditary grain size. In this article we will focus on the development and production of cemented steel in the conditions of OJSC «BSW – Management Company of Holding «BMC», in particular, the steel brand 16MnCrS5 commissioned by the European manufacturer of gearboxes and motors. To determine the grain size of the metal must be subjected to special types of processing for the manifestation of certain characteristics. These results allow us to draw conclusions about the need to adjust the chemical composition of steel grade 16MnCrS5 by introducing a system of modifying elements, which will reduce the tendency of steel to overheating, therefore, reduce the size of austenitic grains. On the basis of the revealed regularities on the influence of carbide-forming elements, a further system of steel modification is determined, which includes a complex of elements V, Nb, Ti.

Anisotropic Pinning-Effect of Inclusions in Mg-Based Low-Carbon Steel.

In this study, the effect of austenite grain size on acicular ferrite (AF) nucleation in low-carbon steel containing 13 ppm Mg is determined. The average austenite grain size was calculated using OM Leica software. Results show that the predicted and experimental values of austenite grain size are extremely close, with a deviation of less than 20 µm. AF formation is difficult to induce by either excessively small and large austenite grain sizes; that is, an optimal austenite grain size is required to promote AF nucleation probability. The austenite grain size of 164 µm revealed the highest capacity to induce AF formation. The effects of the maximum distance of carbon diffusion and austenite grain size on the microstructure of Mg-containing low carbon steel are also discussed. Next, the pinning ability of different inclusion types in low-carbon steel containing 22 Mg is determined. The in situ observation shows that not every inclusion could inhibit austenite grain migration; the inclusion type influences pinning ability. The grain mobility of each inclusion was calculated using in situ micrographs of confocal scanning laser microscopy (CSLM) for micro-analysis. Results show that the austenite grain boundary can strongly be pinned by Mg-based inclusions. MnS inclusions are the least effective in pinning austenite grain boundary migration.

The significant impact of Ti content on microstructure–toughness relationship in the simulated coarse-grained heated-affected zone of high-strength low-alloy steels

ABSTRACTThis study aims to investigate the influence of Ti addition on microstructure and toughness in the simulated coarse-grained heated-affected zone (CGHAZ) of high-strength low-alloy steels. The steels with low and high Ti content respectively were subjected to 100 kJ/cm heat input welding thermal cycle. The results indicated that the second-phase particles were mainly oxide covered with MnS and fine (Ti,Nb)N precipitate in low-Ti steel, which were modified to the oxide surrounded by TiN and coarse (Ti,Nb)N precipitate in high-Ti steel. Compared with low-Ti steels, the coarser precipitates induced larger austenite grain in CGHAZ of high-Ti steel. Moreover, the wrapping of TiN decreases the ability of inclusion to promote the nucleation of acicular ferrite, resulting in lower fraction of acicular ferrite in CGHAZ of high-Ti steel. Content of martensite-austenite constituent increased in CGHAZ of high-Ti steel. They were all responsible for the degeneration in toughness in CGHAZ of high-Ti steel.

Evolution of the Macrostructure of Gray Cast Iron from Eutectic to Hypereutectic Composition

This work studies the evolution of the macrostructure of gray cast iron as the carbon equivalent (CE) increases from near-eutectic to highly hypereutectic values. The macrostructure of spherical samples of flake graphite irons of CE ranging between 4.24% and 5.23% was revealed by using direct austempering after solidification technique (DAAS). The results were analysed by applying the concept of coupled zone for gray cast irons. The coupled zone is a region in the stable Fe-C-Si diagram in which solidification proceeds as a cooperative growth of both austenite and flake graphite. The coupled zone for gray cast iron is claimed to be asymmetrical as the eutectic reaction is extended towards the prolongation of the austenite liquidus line. All the samples investigated in the present study show the presence of relatively large austenite grains, demonstrating that the number of active nucleation sites of austenite per unit volume is small. The observation of the microstructure shows the presence of several units of coupled growth inside each austenite grain, what proves that many eutectic colonies have grown from the same austenite nucleus, even for highly hypereutectic compositions. Colour etching was employed to reveal the “hidden” dendrites present in the microstructure. The results suggest that proeutectic graphite is not a good nucleating agent for austenite. These results contribute to the understanding of the solidification of hypereutectic gray irons and verify the earlier understanding of the solidification of eutectic gray irons and its extension to hypereutectic compositions.