Martensite Packet Research Articles

Multilevel Model for Describing Martensitic Transformation: Formation of the Polyhedral Martensite Structure

Multilevel models of inelastic deformation that take into account microstructure evolution are promising for technology development for creating functional material structures with optimal performance characteristics. The paper discusses the mathematical formulation of a direct multilevel model to describe the inelastic deformation of a polycrystal representative volume (analogous to a macrosample), taking into account the formation and the martensitic structure evolution during the transformation process. The model considers three structural-scale levels. At the macro level, the boundary value problem is solved, the fields of stresses, strains and other model variables are determined. At mesolevel-I, a homogeneous original austenite grain is considered, in which a martensitic transition occurs due to external influences. For a detailed description of the material response at the grain level, an auxiliary scale level is introduced into consideration, i.e. mesolevel-II. At this level, the geometric features of the martensite packet formation are explicitly studied. An original method has been developed to describe martensite polyhedral structure, the construction of which is carried out when the new phase volume fraction in the austenite grain reaches a critical value. A packet description as union of polyhedra consisting of thin plates allows one to introduce the geometric characteristics of the structural elements into the model, in particular, plates and packet boundaries, linear dimensions, volumes etc., and supplement them with crystallographic orientations. The resulting geometric characteristics of martensite package with subsequent processing is transferred to an individual grain level. This makes it possible to take into account the mechanisms of deformation and hardening that occur during the interaction of phases. The results are presented of polyhedral martensite packet structure formations in AISI 304 steel in numerical experiments on uniaxial deformation at room temperature and strain rate 10–5s–1.

Microstructure evolution, crack initiation and early growth of high-strength martensitic steels subjected to fatigue loading

Fatigue loading induced microstructure evolution featured by Fine Granular Area (FGA) in high-strength martensitic steels have close ties to the fatigue crack initiation and early growth, which are worthy of being investigated. On this issue, we conducted Very High Cycle Fatigue (VHCF), High Cycle Fatigue (HCF) and Low Cycle Fatigue (LCF) tests on three different types of specimens. Combined with post-mortem/quasi in-situ Scanning Electron Microscope (SEM) and Electron Back-Scattered Diffraction (EBSD) observations, as well as further Transmission Kikuchi Diffraction (TKD) and Transmission Electron Microscope (TEM) analyses, we captured two different types of microstructure evolution behaviors during the fatigue loading. One is the “dynamic precipitation”, a kind of phase transformation from martensitic matrix (BCC) to Nb- and Mo-rich small carbides (MC, M7C3) accelerated by repeated slight plastic deformation, whose formation does not rely on the stress concentration effect and also has no ties to the plastic strain localization and fatigue crack initiation. Another is the “local grain refinement” around those stress singularities (crack tip, or interior inclusion) generating substantial amounts of fine sub-grains in VHCF, HCF and LCF regimes, which can then promote fatigue crack initiation and early growth along sub-grain boundaries, fine/coarse grain boundaries and martensitic packet boundaries.

Effect of the cooling behavior on phase transformation and mechanical property of RAFM steel

The phase transition behavior of RAFM steel during Hot Isostatic Pressing (HIP) diffusion bonding process would extensively impact the service performance of blanket component in the future fusion reactor. In this paper, the influence of cooling behavior on phase transformation and mechanical property of RAFM steel was studied using a combination of Electron Backscatter Diffraction (EBSD) and a thermal dilatometer. The results show that as the cooling rate decreases after austenitizing, the phase category transitions from a single-phase of martensite to the dual-phase of martensite and ferrite, along with an increase in the rate of ferrite, minor martensite packets and high-angle grain boundaries (HAGB). As the cooling rate decreases, the initial temperature for martensite transformation significantly increases, and the transformation driving force slowdown. Moreover, specimens processed with HIP cooling show lower strength, higher elongation, and a greater standard deviation of hardness compared to air cooling. These differences can be attributed to the formation of ferrite and the diffusion of carbon atoms within martensite. The research findings reveal the phase transition and microstructural evolution of martensite under varying cooling conditions, providing a reference for the controlling the cooling process after HIP bonding of blanket steel components in the future fusion reactor.

Influence of orientation-dependent lath martensite yielding on the hardening behavior of quenched martensitic steels

The onset of plasticity in quenched martensitic microstructures is characterized by a low initial yield stress, extreme initial hardening, and sudden saturation. The existing literature attributes these phenomena to residual stresses or microstructural heterogeneities. We introduce a novel perspective, suggesting that orientation-dependent yielding of lath martensite, induced by inter-lath sliding, significantly contributes to the observed behavior. To support this, we employ a numerical microstructural model, considering the yielding anisotropy of martensite packets due to sliding along their habit plane orientation. The combined response of early yielding in martensite packets with a favorable habit plane, along with those initially remaining elastic due to an unfavorable orientation, results in a macro-scale behavior with a low initial yield stress, followed by substantial initial hardening until the saturation stress level is approached. The simulations also qualitatively capture other observations reported for quenched martensitic steels, e.g. the effect of carbon content.

Determining the Hot Workability and Microstructural Evolution of an Fe-Cr-Mo-Mn Steel Using 3D Processing Maps.

The Laasraoui segmented and Arrhenius flow stress model, dynamic recrystallization (DRX) model, grain size prediction model, and hot processing map (HPM) of Fe-Cr-Mo-Mn steels were established through isothermal compression tests. The models and HPM were proven by experiment to be highly accurate. As the deformation temperature decreased or the strain rate increased, the flow stress increased and the grain size of the Fe-Cr-Mo-Mn steel decreased, while the volume fraction of DRX (Xdrx) decreased. The optimal range of the hot processing was determined to be 1050-1200 °C/0.369-1 s-1. Zigzag-like grain boundaries (GBs) and intergranular cracks were found in the unstable region, in which the disordered martensitic structure was observed. The orderly packet martensite was formed in the general processing region, and the mixed structure with incomplete DRX grains was composed of coarse and fine grains. The microstructure in the optimum processing region was composed of DRX grains and the multistage martensite. The validity of the Laasraoui segmented flow stress model, DRX model, grain size prediction model, and HPM was verified by upsetting tests.

Insight into austenite reversion and mechanical behavior of quenching and partitioning steel inherited hierarchical structure of martensite

In this paper, the start microstructure of full martensite and dual phases comprising ferrite and martensite were introduced into quenching and partitioning (Q&P) steel to tailor the microstructural characteristics and mechanical behaviors, which was achieved by means of a pre-quenching treatment with various temperatures. It is revealed that the conventional Q&P sample possessed bimodal microstructure with predominantly blocky feature, while gradually changed from combination of blocky and lath structures into individually lath feature in pre-quenched Q&P samples. The reversed austenite presented different variants, which predominantly reproduced the orientation of parent austenite and occasionally possessed twin relationship, low angle misorientation or extraneous with parent austenite. The twin-related austenite included annealing twin and reversed twin, whilst the latter showed twin relationship with both neighbored annealing twin and parent austenite. The variant type of reversed austenite significantly influenced the characteristics of its orientation relationship with neighbored pre-quenched martensite. The pre-quenching treatment was favorable to improve the carbon content of meta-stable austenite before Q&P treatment, while the final carbon content in retained austenite was mainly controlled by the synergistic carbon partitioning of martensite and bainite on basis of thermodynamic calculation, accompanied with the partitioning of Mn and Si elements. Moreover, the pre-quenched martensite deformed coordinately in a unit of martensite packet and was beneficial to retard necking to improve ductility. The optimal mechanical properties were obtained with pre-quenching temperature near Ac3, possessing yielding strength of 623 MPa, ultimate tensile strength of 861 MPa and product of strength and elongation exceeding 28 GPa%, attributing to the positive TRIP effect originating from lath-like retained austenite with high carbon content and multi-level hierarchical microstructure inherited from pre-quenched martensite.

Structure, Phase Composition and Mechanical Properties of a High-Strength Steel with Transition Carbide η-Fe<sub>2</sub>C

Abstract—The influence of quenching and tempering on the structure, phase composition and mechanical properties of high-strength Fe–0.34 C steel with 1.77 wt % Si is considered. The tempering at temperatures up to 500°C has virtually no effect on the structural characteristics of packet martensite formed during quenching. At tempering temperatures in the range of 200–400°C, the precipitation of transition η-carbide occurs, which leads to an increase in the yield strength to 1490 MPa and impact toughness to 35 J/cm2. The determined temperature of the brittle-ductile transition after tempering at 200°C is about –50°C. A decrease in the impact toughness and a decrease in the proportion of ductile fracture with a decrease in the test temperature is accompanied by a transition from transgranular to intergranular fracture. The precipitation of cementite particles along the boundaries of laths and blocks is observed after tempering at 500°C. This leads to a decrease in the yield strength, while the impact toughness of the steel remains unchanged.

Analysis of phase transformation thermodynamics and kinetics and its relationship to structure-mechanical properties in a medium-Mn high strength steel

The difference in transformation mechanisms between the normal and reverse phase transformation in medium Mn steel was revealed using thermodynamic and kinetic analysis. The results indicate that the austenite formation during the reverse phase transformation is a fast reaction, significant amount of 46.9 vol% retained austenite is obtained after subjected partial reversion at 650 °C × 1 h. At the nucleation stage, the lath-like structure, dislocations, martensitic packet boundaries, and grain boundaries provide high-density nucleation sites for austenite formation. The critical temperature of austenite formation is reduced by the enrichment of Mn and C. However, no ferrite transformation occurred during the normal phase transformation (650 °C × 6 h isothermal treatment and subsequent furnace-cooling process). At the nucleation stage, the undersaturated Mn and C in the prior austenite grain interior provide insufficient concentration fluctuation, leading to a low nucleation rate. At the phase growth stage, the atomic mobility of Mn and C in the parent austenite (control the ferrite growth) is 1/69 and 1/282 than that in ferrite (control the austenite growth) at 650 °C. The differences in nucleation site density, driving force, and element diffusion rate lead to significant variations in phase transformation behaviors. The excellent mechanical property with the product of strength and elongation (PSE) exceeding 20 GPa % is achieved via the sustained transformation-induced plasticity (TRIP) effect by metastable retained austenite over a large strain range.

The effect of high-temperature ECAP on dynamic recrystallization behavior and material strength of 42CrMo steel

With the development of mechanical equipment towards long life and high reliability, the traditional heat treatment method is challenging to meet the high strength requirements of 42CrMo steel for mechanical equipment. High-temperature equal channel angular pressing (ECAP) of 42CrMo steel was carried out above austenite transformation temperature, and the microstructure and material strength of the specimen at different radial positions were characterized and tested. The dynamic recrystallization (DRX) kinetic model of 42CrMo steel was established. The distribution of strain, strain rate, temperature and DRX volume fraction in high-temperature ECAP was studied by coupling finite element software and DRX kinetic model, and the formation mechanism of microstructure and material strength difference was revealed. The results show that the different degrees of DRX caused by the uneven distribution of strain and strain rate on the specimen is the main reason for the gradient distribution of microstructure and material strength in the radial direction of the specimen. From the top to the bottom of the specimen, with the decrease of strain and strain rate, the degree of DRX increases continuously, the microstructure inhomogeneity decreases, the size of the martensite packets and blocks increases gradually, and the strength decreases gradually. The grain boundary strengthening caused by the decreased size of martensite packets and blocks is the main reason for the radial difference in material yield strength. Grain boundary strengthening and the coordinated deformation of geometrically necessary dislocations at the soft-hard grain interface of heterogeneous materials are the main mechanisms leading to the tensile strength gradient distribution. This study reveals the dynamic recrystallization behavior and material strength variation of high-temperature ECAP, which contributes to the qualification of the ECAP process and promotes the development of severe plastic deformation on the further strength enhancement of high-strength steel.

Fatigue crack propagation of the gradient surface-modified layer of high-strength steel

A gradient modified layer can be produced on the high-strength steel surface by carburizing heat treatment. In this study, microstructure characterization and fatigue crack propagation test were carried out on the gradient surface-modified layer of the 18CrNiMo7-6 high-strength steel. With increasing depth of the surface-modified layer, the yield strength and the kernel average misorientation value decrease, and the equivalent grain size of the slatted martensite structure and the number of the large-angle boundaries increase. The orientation difference angle of the point to point pre- and post-fatigue at the crack of the transition layer and the matrix layer increases from less than 2° to approximately 10° and about 4°, respectively. With the increase of the depth of the surface-modified layer, the degree of deformation inside the martensitic packet decreases, while the extent of macroscopic deformation of the material increases. Deformation occurs inside the martensitic packet at the main crack, and there is essentially no deformation inside the martensite packets away from the main crack. On both sides of the main crack, a large number of extrusions parallel to the martensitic lath appear. The large-angle boundaries of the original austenite grain boundary and martensite boundary hinder the propagation of the fatigue crack.

Exploring the effect of complex hierarchic microstructure of quenched and partitioned martensitic stainless steels on their high cycle fatigue behaviour

Recent studies have demonstrated the viability of quenching and partitioning (Q&P) treatment for processing martensitic stainless steels showing an improved balance of high strength and sufficient ductility. However, to date, the fatigue behaviour of these materials has not been explored. This study examines the effect of their complex hierarchic microstructure on high cycle fatigue performance. Three steels with different alloying element contents underwent Q&P processing, resulting in multiphase microstructures rich in retained austenite. High cycle fatigue tests and analysis of fatigue fracture surfaces were performed using SEM and EBSD techniques. The results indicate satisfactory high cycle fatigue performance in Q&P treated martensitic stainless steels, surpassing traditional counterparts. Fatigue cracks predominantly form and propagate along martensite packet and block boundaries, while prior austenite grain boundaries and MnS inclusions have minimal influence on fatigue crack formation and growth. Microplastic deformation at the fatigue crack tip enhances local KAM values and triggers localized transformation of retained austenite grains. It is hypothesized that the developed Q&P treated martensitic stainless steels exhibit improved resistance to low cycle fatigue.

Strength improvement over 2 GPa and austenite grain ultra-refinement in a low carbon martensite steel achieved by ultra-rapid heating and quenching

This study focuses on enhancing the strength of low-carbon martensitic steel through the refinement of the prior austenite grain size. After subjecting the steel to heavy cold deformation, ultra-rapid heating in a salt bath was employed. The steel was then quenched in water. The results showed that the ultra-rapid heating for 5 s achieved an impressive tensile strength exceeding 2 GPa, accompanied by a higher hardness of over 500 kgf/mm2. By introducing more nucleation sites through heavy cold rolling, it was possible to achieve an ultrafine-grain structure in the steel, with the prior austenite grain size ranging from 1∼3 μm. In some regions, even smaller grain sizes of around 1 μm or less were observed. The boundaries between martensite packets or blocks disappeared, replaced by two or three lath variations. The study also investigated the effect of high-temperature holding time on the austenite phase. It was found that increasing the holding time resulted in the growth of the austenite grains and a subsequent decrease in tensile strength. Considering carbide dissolution, the optimal temperature holding time was approximately 5 s at 900 °C. A shorter holding time allowed more carbides to remain, which weakened the strengthening effect of the carbon solid solution. The relationship between strength and grain size was analyzed by fitting experimental data. The Hall-Petch slopes were calculated to be about 862 MPa mm1/2 and 1022 MPa mm1/2 for the prior austenite grain size and the effective grain size, respectively. Moreover, excluding the influence of martensite packet or block boundaries, the strength contributions from the carbon solid solution and dislocation strengthening were determined to be approximately 583 MPa and 268 MPa, respectively.

Influence of Martensite/Bainite Dual Phase-Content on the Mechanical Properties of EA4T High-Speed Axle Steel.

In this work, we have investigated the effect of martensite/bainite dual phase content on the mechanical properties of EA4T high-speed axle steel. For evaluation and control of the strength, ductility, and toughness of steel, the microstructure of lath martensite (LM) and granular bainite (GB) was clarified through an optical microscope (OM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). Besides, the tensile fracture morphology was studied by scanning electron microscopy (SEM). For this purpose, this study conducted a quantitative analysis of the LM and GB fractions using the Pro Imaging software-2018 of OM. The remarkable effect of the LM/GB structure on mechanical properties is discussed. The results have shown that by increasing the volume fraction of the GB structure, the LM structure is refined and its microhardness and strength are improved. Meanwhile, the micro strength of LM follows the Hall-Petch relationship with the lath martensite packet size. Subsequently, the mechanical property prediction model of EA4T steel based on the LM/GB content was established by regression analysis of all experiment dates. When the LM fraction in the steel is about 40-70%, a superior combination of strength, ductility, and toughness can be obtained in EA4T steel.

Effect of Hot-Rolling on the Microstructure and Impact Toughness of an Advanced 9%Cr Steel

A 9%Cr martensitic steel with Ta and B additions was subjected to thermo-mechanical treatment (TMT) including rolling in the range of metastable austenite at 900–700 °C followed by water quenching and tempering at different temperatures. Applied TMT with tempering at T ≥ 700 °C substantially improved the impact toughness. The application of the TMT with subsequent tempering at 780 °C decreased the ductile–brittle transition temperature from 40 to 15 °C and increased the upper shelf energy from 300 to 380 J/cm2 as compared to the normalized and tempered (NT) condition. The microstructural observations with scanning and transmission electron microscopes showed the precipitation of fine Ta-rich MX carbonitride and M23C6 carbide during TMT and subsequent tempering. The analysis of the cleavage facets and the secondary cracks with electron back-scattered diffraction (EBSD) revealed that the brittle fracture occurred via cleavage cracking along {100} planes across the laths, while the high-angle boundaries of martensite blocks and packets were effective barriers to the crack propagation. The increased impact toughness of the tempered TMT steel sample was attributed to enhanced ductile fracture owing to the uniform dispersion of the precipitates and favorable {332}⟨113⟩ crystallographic texture.

Antibacterial copper-bearing titanium alloy prepared by laser powder bed fusion for superior mechanical performance

• A boundary engineering strategy is proposed to improve the strength and ductility of materials simultaneously. • The addition of Cu successfully verified the feasibility of the boundary engineering strategy, and improved the comprehensive mechanical properties of LPBF martensitic titanium alloy. • The role of copper addition in microstructure development are clarified. • The strengthening mechanism of LPBF Ti, Ti-5Cu and Ti-6Al-4V is explained by calculation and fitting. Copper element was added in pure titanium to produce a new biomedical titanium-copper alloy by laser powder bed fusion (LPBF). Addition of copper can eliminate the mismatch of high strength but poor ductility problem caused by lath α' martensite, which is the usual microstructure of near α titanium alloy fabricated by LPBF. Instead of by the usual trade-off relationship between strength and ductility, which is a long-standing challenge for martensitic titanium alloys, in this study, we proposed a boundary engineering strategy and aim to synergistically enhance the strength and ductility of martensitic titanium alloy fabricated by LPBF. It is hypothesized that whilst both low-angle and high-angle grain boundaries are beneficial to the strength, high-angle grain boundary can simultaneously improve the ductility of materials. To test this strategy, a Ti-5Cu (wt.%) alloy is selected to compare against pure titanium and Ti-6Al-4V at the same laser processing conditions. EBSD, TEM and XRD analysis show that the as-fabricated LPBF Ti-5Cu alloy is comprised of partially tempered martensite with extraordinarily high number density of both high-angle and low-angle grain boundaries as well as low dislocation density. Such microstructure enables a high tensile strength of 940–1020 MPa, which is at a similar level as LPBF Ti-6Al-4V, and an excellent elongation of 13%–16%, twice as much as that of LPBF Ti-6Al-4V. The mechanism of microstructure refinement in LPBF Ti-5Cu at different levels from prior-β grains, martensitic packets, blocks to laths is also discussed.

Effect of Hot Deformation Parameters on Heat-Treated Microstructures and Mechanical Properties of 300M Steel.

The high strength of 300M steel originates from the heat treatment process after forging, but how hot deformation affects the heat-treated microstructure and mechanical properties is unclear. In this study, compression tests under different hot deformation parameters and post-deformation heat treatment experiments were carried out, and the martensite transformation process was investigated using in situ observation. The results show that the grain size of the specimen deformed at low temperature and high strain rate is smaller, and annealing twins will be formed. Both austenite grain boundaries and twin boundaries hinder the growth of martensite blocks, reducing the size of martensite units after heat treatment and thus resulting in higher yield strength. Besides, the mathematical models were established to describe the relationship between hot deformation parameters and grain size after deformation, martensite packet size and martensite block width, respectively, after heat treatment. The relationship between yield strength and hot deformation parameters was also analyzed. According to the results and models, the hot deformation parameters would be optimized more reasonably to improve the final mechanical properties of 300M steel forgings.

Carbon segregation and cementite precipitation at grain boundaries in quenched and tempered lath martensite

Tempering is widely applied to make carbon atoms beneficially rearrange in high strength steel microstructures after quenching; though the nano-scale interaction of carbon atoms with crystallographic defects is hard to experimentally observe. To improve, we investigate the redistribution of carbon atoms along martensite grain boundaries in a quenched and tempered low carbon steel. We observe the tempering-induced microstructural evolution by in-situ heating in a transmission electron microscope (TEM) and by compositional analysis through atom probe tomography (APT). Probe volumes for APT originate from a single martensite packet but in different tempering conditions, which is achieved via a sequential lift-out with in-between tempering treatments. The complementary use of TEM and APT provides crystallographic as well as chemical information on carbon segregation and subsequent carbide precipitation at martensite grain boundaries. The results show that the amount of carbon segregation to martensite grain boundaries is influenced by the boundary type, e.g. low-angle lath or high-angle block boundaries. Also, the growth behavior of cementite precipitates from grain boundary nucleation sites into neighboring martensite grains differs at low- and high-angle grain boundaries. This is due to the crystallographic constraints arising from the semi-coherent orientation relationship between cementite and adjacent martensite. We also show that slower quenching stabilizes thin retained austenite films between martensite grains because of enhanced carbon segregation during cooling. Finally, we demonstrate the effect of carbon redistribution along martensite grain boundaries on the mechanical properties. Here, we compare micro-scale Vickers hardness results from boundary-containing probe volumes to nanoindentation results from pure bulk martensite (boundary-free) probe volumes.

Nanoindentation plasticity and loading rate sensitivity of laser additive manufactured functionally graded 316L and 410L stainless steels

The present work is the first to show the effect of processing cooling rate on phase structure and the resulting nano-scale deformation mechanisms in functionally graded stainless steels produced by directed energy deposition. To this end, fabrication by laser additive manufacturing involved implementing a closed-loop feedback control approach to monitor and control the peak temperature of the molten pool and the corresponding cooling rate. Single-track wall structures were produced from 316-L austenitic and 410-L martensitic stainless steels deposited using open-loop conditions with varying processing parameters (laser power and beam scanning speed) to produce graded microstructures. These were then compared to uniformly deposited layers where a constant cooling rate was maintained to generate homogenous microstructures using these two alloys, containing well-aligned columnar austenitic grains and random martensitic packets. Microstructural aspects (grains and crystallographic texture) across different layers (bottom, middle, and top) of the consolidated stainless steel walls (austenitic and martensitic) were compared under various processing conditions (without and with closed-loop or feedback control) and characterized using electron backscattering diffraction analysis. The nano-scale plastic deformation of these austenitic and martensitic stainless steel structures were assessed using nanoindentation testing over a range loading rates from 1 to 50 mN/s. The indentation response using varying strain rates was investigated based on the plastic flow indenting transition behavior at a high loading rate in order to correlate their dependence on microstructural features (predominantly for the martensitic phase). According to the rate sensitivity analyses, the gradient in grain structure (size and morphology) in the case of additively manufactured austenitic microstructure was not sensitive to loading rate, while the martensitic grains had a high local loading rate sensitivity.

The Coupling Effect of Hierarchical Structures and MN‐Type Precipitates on the Creep Resistance for the High N Heat‐Resistant Martensitic Steel

Microstructure of the high nitrogen heat-resistant martensitic-steel (HNHM) crept using 140-170 MPa loading stresses at 923K, is investigated in correlation with creep lifetime and creep rupture strength. Upon creep deformation, both recrystallized grains and the pre-existed hierarchical structures grow via SIBM induced the degradation of strengthening. For pre-existed hierarchical structures in HNHM, martensitic packets and blocks are more inclined to form at higher stresses (170 MPa), causing the degradation of strength, while laths retain easily at lower stresses (140 MPa-155 MPa), retarding the creep damage. Additionally, under all stresses and upon 923K, the highly stabilized MN precipitates not only pin the hierarchical structural boundaries effectively, but also act as the core for the nucleation of Laves, realizing strengthening. The findings elucidate the coupling contribution of hierarchical structures and MN-type precipitates on the creep strengthening for HNHMs, challenging the common viewpoint that the coupling coarsening of lath and precipitates dominates the degradation of strengthening, and thus, deepens the understanding of the creep resistance of the HNHM and provides the first insights into obtaining reliable predicted creep life of the novel HNHMs. This article is protected by copyright. All rights reserved.

Ex situ analysis of high-strength quenched and micro-alloyed steel during austenitising bending process: numerical simulation and experimental investigation

This paper compares the microstructure and mechanical evolution in a high-strength quenched and micro-alloyed steel during the austenitising bending process. Simulation results indicated a new finding that the stress neutral layer (SNL) tends to move to the tension zone during straining. The hardness gradient detected from the centre to compression/tension zones was resulted from comprehensive factors: First of all, the location of SNL revealed a prominent impact on strength. Second, the dislocation accumulation would be responsible for the hardness gradient on the surfaces. In addition, the overall strength decrease during straining was mainly ascribed to integrated effects of dynamic recovery (DRV) and dynamic recrystallisation (DRX). Apart from that, overall smaller martensite packet size and coarser prior austenite grains resulted in the increased hardness value at a lower bending degree. Also, the high consistency between experimental and simulation results is instructive for the practical forming process of railway spring fasteners.