Martensitic Structure Research Articles

Micromechanical analysis and finite element modelling of laser-welded 5-mm-thick dissimilar joints between 316L stainless steel and low-alloyed ultra-high-strength steel

As base metals (BMs), plates of 5-mm-thick low-alloyed ultra-high-strength carbon steel (LA-UHSS) with a tensile strength of 1.3 GPa and 5-mm-thick 316L austenitic stainless steel were laser-welded at two different energy inputs (EIs; 60 and 100 J/mm). The microstructural characteristics of the fusion zones (FZs) in the welded joints were examined using electron backscattering diffraction (EBSD) and transmission electron microscopy. The fine microstructural components, such as the prior austenite grain size (PAGS) and effective grain size of the fresh martensite promoted during welding, were analysed by processing the EBSD maps using MATLAB software. The micromechanical performance of the weldments was investigated using microindentation hardness (HIT) to display the mechanical responses of different zones. Uniaxial tensile testing was conducted to explore the joint strength and plasticity failure. The dominant phase structures promoted in the FZs at low and high EIs were similar, that is, martensite with a small fraction of austenite. The HIT values displayed a distinct variation in strength between different zones. The HIT values of 316L, LA-UHSS, and FZ were 1.95, 5.55, and 4.63 GPa, respectively. The PAGS increased from 45 to 70 μm with an increasing EI, and a finer martensitic grain structure with an average size of 2.62 μm was observed at high EIs. The mechanical tensile properties of the dissimilar joints at the studied EIs closely matched those of the BM 316L, demonstrating comparable yield and tensile strengths of 225 MPa and 650 MPa, respectively. This similarity can be attributed to the localized plastic tensile deformation occurring primarily within the relatively softer BM 316L, ultimately resulting in joint failure. The flow behaviour of the dissimilar joints under uniaxial tensile testing was analysed using finite element modelling to determine the stress and strain distributions. The plastic strain was mainly localised within the soft metal 316L owing to enhanced dislocation-mediated plasticity.

Development of a high-strength Fe–12Mn maraging steel via designing lath interfacial and intragranular nanostructures

A Heusler Fe2TiSi-phase strengthened 12Mn maraging steel Fe–12Mn–5Co–1Ti–1Mo–1Si was extensively studied using various experimental (EBSD, TEM and uniaxial tensile) techniques. The results indicate that the fine and dispersed Fe2TiSi particles, precipitating within the martensite, were the primary strengthening phase when aged at 450 °C regardless of whether the alloy is cold rolled or solid-solution treated. Furthermore, a soft reverted austenite nano-layer was observed at the lath interface, imparting significant toughness to the martensite matrix. By carefully optimizing the aging process parameters, the 12Mn alloy achieved a high tensile strength of ∼1700 MPa with a moderate elongation of ∼7% at break.When aged at 400 °C, unexpected precipitation of α-Mn particles within the martensite structure was observed, displacing the Fe2TiSi phase as the dominant strengthening phase. Simultaneously, elemental Mn segregation was found to occur at the lath interface, instead of the formation of reverted austenite. This Mn segregation layer was identified as the main factor contributing to brittle fracture behavior. These findings illustrate that synergistic improvement of strength and plasticity can be realized via an optimal combination of precipitation strengthening and interfacial layer toughening mechanisms.

Effect of low-temperature hot rolling on the microstructure and mechanical properties of air-cooling medium manganese martensitic wear-resistant steel

In this paper, the low-temperature hot rolling was conducted on air-cooled medium manganese martensitic steel, utilizing a finishing rolling temperature (FRT) that was significantly lower than the conventional used temperature. The effect of FRT on the microstructure, mechanical properties and wear resistance was systematically investigated. The results revealed that the pancaked austenite grain formed after low-temperature hot rolling could still be completely transformed into martensite structure under air-cooled condition. The lower FRT significantly refined the martensite multi-level microstructures, increased the grain boundaries density and dislocations density, and promoted the nano-scaled V(C, N) precipitation. These resulted in enhanced refinement strengthening, dislocation strengthening and precipitation strengthening. Compared to the FRT-900 sample, the FRT-700 sample exhibited increased yield strength, tensile strength, and Brinell hardness, with values rising from 973 MPa, 1526 MPa, and 454 HB to 1174 MPa, 1701 MPa, and 482 HB, respectively. In addition, the refined microstructure effectively increased the dimple region in the fractography of impact samples, preventing the propagation of crack and thereby enhancing the impact absorbing energy. Consequently, the impact absorbed energy at −40 °C of experimental steel was increased from 28 J to 38 J, while also increasing strength. Moreover, the low-temperature hot rolling effectively increased the surface hardness of the experimental steel, inhibited abrasive embedding and improved impact wear resistance. The mass loss rates of low-temperature hot rolling samples were 0.052 mg/s and 0.063 mg/s under the impact energy of 1 J and 2.5 J, respectively.

Surface hardening and plastic stability of Ti–6Al–4V alloy by cryo-quenching

Ti–6Al–4V alloy was heat-treated at 850 °C followed by air cooling, water- and cryo-quenching. The quenched samples were further work-hardened and plastically stabilised under tension. Interestingly, the surface hardness was more improved by the cryo-quenching, which led to the enhanced formation of complex α′ martensite structure at the outermost surface than the water-quenched sample.

Research on thermal creep behavior of Chinese low-activation ferritic/martensitic steel

A series of thermal creep experiments are conducted on Chinese low-activation ferritic/martensitic steel (CLF-1 steel) at a temperature of 550, 575, and 600 °C with various normal stress (from 150 to 300 MPa), respectively. The results are analyzed by creep curves, microstructure, and creep properties. Finished samples display adequate fracture surface in short creep rupture tests (within 3000 h). The comparative creep curve shows no difference related to the thickness of CLF-1 steel plate (10 and 30 mm). Microstructure images before and after creep reflect a tempered lath martensite structure and carbides decorating both in prior austenitic grain boundaries and lath boundaries. Comparison is discussed between 30 mm plate and 10 mm plate. All current results reflect that all CLF-1 steel samples shows good creep behavior and proves that a series excellent progress of CLF-1 steel R&D work has been successfully achieved. Besides, collected data will contribute a lot to the design and R&D work of the Chinese Helium Cooled Ceramic Breeding Test Blanket Module (CNHCCB-TBM).

Microstructural investigation on a rail fracture failure associated with squat defects

Squats, observed in numerous railway networks worldwide, are a significant concern for rail track maintenance. This work presents a specific case study of a rail fracture failure associated with squat defects. A detailed microstructural analysis was conducted to investigate the causes of squat formation, subsequent crack propagation, transverse defects, and fracture. According to the findings, the initiation of squats can be attributed to the presence of White Etching Layers on the rail surface and rolling contact fatigue. After the initiation of Squats, the cracks extended downwards into the bulk rail at a shallow angle until reaching a sizeable martensitic island located in the subsurface of the rail along the path of crack propagation. The brittleness of the undesired martensitic structure triggered rapid crack development, facilitated transverse cracking, and ultimately resulted in rail fracture failure. Transmission electron microscopy examination confirmed that the abnormal defects are plate martensite with a twin substructure and high carbon content, which could be generated as a result of improper heat treatment during the manufacturing of rails.

Parameter Impacts of Martensitic Structure on Tensile Strength and Hardness of TIG Welded SS410 with characterized SEM Consequences

Parameter Impacts of Martensitic Structure on Tensile Strength and Hardness of TIG Welded SS410 with characterized SEM Consequences

Microstructures and Tensile Properties of Friction Stir Welded 0.2%C-2%Si-Cr Steels

The 0.2C-2Si (mass%) steels with the addition of 0–4 mass% Cr were prepared by hot rolling followed by subsequent annealing for normalization. The steels were subjected to friction stir welding (FSW) conducted above A3 temperature. For all the steels, sound FSWed joints were obtained. Microstructures and tensile properties using small tensile specimens were investigated for both base materials and stir zones. The base materials showed a relatively good balance of strength and ductility when the Cr content is over 3 mass% presumably owing to the relatively fine microstructures of ferrite and martensite. The tensile properties of stir zones were substantially enhanced by FSW, and the stir zone of the 0.2C-2Si-4Cr joint with fully martensitic structure exhibited the surprisingly high tensile strength of 1720 MPa compared with that of the conventional martensitic steel of 0.2 mass%C together with the excellent balance of ductility. This is assumed to be caused by the refinement of block size in the fresh lath martensite and/or the formation of ausformed martensite induced by the dynamically recrystallized fine austenite grains by FSW of the Cr added steels.

Impact of Cryogenic Treatment Process on the Performance of 51CrV4 Steel.

The working load on automotive components is continuously rising, and the mechanical performance requirements for component materials are rising along with the growth trend toward light weight and high dependability in automobiles. In this study, the response characteristics of 51CrV4 spring steel were taken to be its hardness, wear resistance, tensile strength, and impact toughness. Prior to tempering, cryogenic treatment was introduced. Through the Taguchi method and gray relational analysis, the ideal process parameters were discovered. The ideal process variables were the following: a cooling rate of 1 °C/min, a cryogenic temperature of -196 °C, a holding time of 24 h, and a cycle number of three. An analysis of variance revealed that the holding time had the greatest effect on the material properties, with an effect of 49.01%. The yield limit of 51CrV4 was increased by 14.95% and the tensile strength was increased by 15.39% with this group of processes, and the wear mass loss was reduced by 43.32%. The mechanical qualities had a thorough upgrade. Microscopic analysis revealed that cryogenic treatment resulted in refinement of the martensite structure and significant differences in orientation. Additionally, bainite precipitation occurred, exhibiting a fine needle-like distribution, which positively influenced impact toughness. Analysis of the impact fracture surface showed that cryogenic treatment led to an increase in dimple diameter and depth. Further analysis of the elements revealed that calcium (Ca) weakened the negative effect of sulfur (S) on 51CrV4 spring steel. The overall improvement in material properties provides guidance for practical production applications.

Investigation on post-weld heat treatment (PWHT) inputs for controlling average fusion zone hardness in fibre laser-welded modified 9Cr–1Mo steel

Post-weld heat treatment (PWHT) generally relieves stress and enhances weldments’ mechanical properties. Optimisation of PWHT parameters is very much needed to control the process parameters for achieving the desired property requirements. In this research, the PWHT parameters for laser-welded P91 steel were optimised using the response surface methodology-based Box–Behnken methodology. Tempering temperature, heating rate and holding time are considered the input parameters and using analysis of variance (ANOVA), their impact on average fusion zone hardness has been analysed. The results indicate that the hardness values are highly influenced by temperature followed by holding time. A parametric combination of 780°C tempering temperature, 6 hours holding time, and 200°C/h heating rate have evolved as the optimal solution. The autogenous nature of the welding process resulted in a heterogeneous microstructure with untempered lath martensite structure showing precipitate dissolution in the as-welded fusion zone, ultimately resulting in a high average fusion zone hardness of 502 HV0.3, whereas PWHT with the optimal solution resulted in an average fusion zone hardness value of 252 HV0.3. The decrease in hardness value followed by heat treatment is primarily due to the tempering process, which resulted in precipitation within and over the grain boundaries. The validation experiment result using the optimum combination of parameters has been observed to be aligned significantly with the expected values, indicating the adequacy of the developed model.

Phase Composition and Temperature Effect on the Dynamic Young’s Modulus, Shear Modulus, Internal Friction, and Dilatometric Changes in AISI 4130 Steel

Elastic properties of materials and their changes with temperature are important for their applications in engineering. In the present study the influence of phase composition and temperature of AISI 4130 alloy on Young’s modulus (Ed), shear modulus (Gd), and damping (Q−1) was carried out by the impulse excitation technique (IET). The material characterization was performed using confocal microscopy, XRD, SEM, HV, and dilatometry. A stable structure, composed of ferrite (BCC) and pearlite (α-Fe + Fe3C), was obtained by annealing. Metastable structure of martensite (BCT) was obtained by quenching. The Ed, Gd, and Q−1 were measured by varying the temperature from RT to 900 °C. The values of Ed and Gd, at RT, were determined as 201.5 and 79.2 GPa (annealed) and 190.13 and 76.5 GPa (quenched), respectively. In the annealed steel, the values Ed and Gd decrease linearly on heating up to 650 °C, with thermal expansion. In the quenched steel, weak changes occurred in the dilatometric curve, Ed, Gd, and Q−1, in the range of 350–450 °C, which indicated decompositions of the martensitic phase. A sharp decrease in the moduli and high peak of Q−1 were observed for both samples around 650–900 °C, revealing low lattice elastic stability of the phases during transformations α(BCC) + Fe3Cγ(FCC).

Influence of the in-situ heat treatment during manufacturing on the microstructure and properties of DED-LB/M manufactured maraging tool steel

Due to high productivity, additive manufacturing (AM), and especially Directed Energy Deposition using laser and metallic powder (DED-LB/M) is attractive for manufacturing tools with integrated functionalities. This investigation was dedicated to DED-LB/M manufacturing of experimental maraging tool steel, characterization of the build microstructure with advanced electron microscopy and evaluation of hardness properties. High printability and low porosity of the final builds were observed, relative density was not lower than 99.5% for specimens manufactured with 600 W and 800 W, but microstructure and properties of the build had a gradient along the height. The characteristic hardness profile and microstructure, which were dependent on the manufacturing parameters, were observed. The top layers of manufactured maraging steel samples had a structure of martensite with precipitates presumably formed during solidification. The top layers were therefore softer to the depth of the austenitization isotherm. The higher hardness was measured in the inner regions which was a result of an in-situ heat treatment that the manufactured material was subjected to during layer-by-layer manufacturing. Thermal cycles during manufacturing resulted in precipitation hardening effect in the inner regions. Scanning and transmission electron microscopy confirmed the formation film-like and round particles in the as-build material, in top and inner regions. However, the quasicrystalline nano-sized R′-phase precipitates were observed only in the inner regions. The formation of the R′-phase precipitated during manufacturing as a result of the in-situ heat treatment was discussed as a reason for higher hardness (440 – 450 HV1) measured in the inner regions.

The improvement of the shape memory effect of Cu-13.5Al–4Ni high-temperature shape memory alloys through Cr-, Mo-, or V-alloying

This study presents the effect of small additions of Cr, Mo, or V on the microstructure, crystallographic features of martensitic transformation, and shape memory effect of Cu-13.5Al-4Ni master alloy. The results show that the studied alloys consist of dominant β'1 (18R) martensitic structure and fine bcc precipitates which are riched in the alloying elements but poor in the Cu content. The martensitic transformation start temperatures of the alloys are 414K, 385K, and 394K when X = 0.5Cr, 0.5Mo, or 0.5V, respectively. More importantly, a drop of grain size was achieved in the studied alloys, leading to the improvement of the shape memory effect in Cu-13.5Al-4Ni alloys. As the result, the studied alloys exhibit a steady increase of shape memory effect as the pre-strain changed from 6% to 10%, which the maximum shape memory effect reaching 6.2%, 5.8%, and 5.5% for X = 0.5Cr, 0.5Mo, and 0.5V, respectively.

Microstructure and Mechanical Property of MXene-Added Ti–6Al–4V Alloy Fabricated by Laser Powder Bed Fusion

Laser powder bed fusion (L-PBF) has been utilized to prepare a high-strength titanium alloy builds using 0.3 mass% MXene-decorated Ti–6Al–4V alloy particles produced by an electrostatic self-assembly. The MXene/Ti–6Al–4V powder mixture had similar spherical morphology and powder size, while showing higher laser absorptivity as compared with the Ti–6Al–4V powders. Under high-energy laser irradiation, MXene was fully decomposed and uniformly dissolved into the Ti matrix. Microstructure observations illustrated that the MXene/Ti–6Al–4V build was completely consisted of ultrafine carbon-saturated martensite structures. Consequently, the MXene/Ti–6Al–4V build exhibited a high tensile strength of ∼1.4 GPa, attributing to the refinement of martensitic structure and the solid solution strengthening of carbon. These findings of this study may broaden the potential application of MXene and pave up the way towards the fabrication of advanced titanium parts via L-PBF.

レーザピーニングが浸炭鋼の表面特性および曲げ疲労特性に及ぼす影響

To investigate the effects of laser peening (LP) on the surface and bending fatigue properties of carburized steel (JIS SCM420), surface roughness measurements, residual stress measurements, retained austenite measurements, and four-point bending fatigue tests were conducted. After LP, the compressive residual stress was introduced on the surface, with the effect extending to a depth of more than 1 mm. Due to the high compressive stress, the bending fatigue strength of LP specimen was 1.46 times higher than that of the as-heat-treated specimen even though the surface roughness was increased by LP. Notably, the amount of retained austenite was 10.9％ at the top surface of LP specimen, and high-density twins were observed in the lath martensite structure. This can be attributed to high strain rate deformation by LP.

Corrosion behaviour of microstructurally modified elastic rail clip steel under crevice and aggressive salt fog exposure

In the present work, existing Indian elastic rail clip steel (tempered martensite) is microstructurally modified to bainite using austempering route. Crevice corrosion of elastic rail clip (ERC) is of major concern as metal dissolution can cause the loosening of the rail clip assembly (such as rail clipinsert and rail clipliner) and lack of performance. Furthermore, the surface of rail clip exposed to the atmosphere could also be affected by the corrosive environment. The present work concentrates on the findings of crevice corrosion and wetdry salt fog exposure of the austempered ERC steels in 3.5 wt% NaCl solution for 30 days and compared with the asreceived (existing) ERC steel which consists of tempered martensitic structure. The severity of corrosion has been investigated by studying the surface dissolution and the rust formed on the exposed surfaces of the ERC steels after the crevice and salt fog tests. Scanning electron microscopy (SEM) and optical profilometry have been used for the analysis of surface dissolution behaviour. Moreover, the rust formed on the exposed surfaces characterised using SEM shows the presence of various rust phases, such as αFeOOH, βFeOOH, γFeOOH, αFe2O3, etc. Furthermore, Raman spectroscopy has been used for the confirmation and quantification of these rust phases. The higher protective ability index (PAI) calculated from the quantitative assessment of different rust measured from Raman spectroscopy shows the better corrosion resistance of the austempered ERC stee

Constant plane shift model: Structure analysis of martensitic phases in Ni50Mn27Ga22Fe1 beyond non-modulated building blocks

Martensites of Ni-Mn-Ga-based alloys consist of hierarchical twinning domains spanning from micro- to nanoscale. This affects the diffraction pattern and thus can decrease the accuracy of the determination of the crystal structure. We propose a method to obtain different martensitic phases in Ni-Mn-Ga-Fe alloy with simplified variant microstructures and domain sizes of more than 2 micrometers. The use of simplified variant microstructures allows the influence of nanometer-scale domains on diffraction line position to be circumvented and enabls the comparison of the lattice parameters of non-modulated (NM), five-layered modulated (10M), and seven-layered (14M) phases in the same temperature range due to the large hysteresis of the intermartensitic transformations. It is found that the short crystallographic axes in NM, 14M, and 10M martensites at the same temperature have different lengths. As a result, equilibrium NM structure building blocks cannot be used to build the crystal structures of 14M and 10M martensites. Instead, we introduce a constant plane shift model with identical shift values of the nearest planes (110) along [1¯10] or [11¯0] as a replacement for the tetragonal building blocks model. The work demonstrates that plane shift values differ dramatically between martensites, which agrees with ab initio calculations. The application of the constant plane shift and hard sphere models in modulated lattices for atomic-level twinning considerations is discussed.

EFFECT OF POST-WELD HEAT TREATMENT ON MICROSTRUCTURE, MECHANICAL PROPERTIES, RESIDUAL STRESSES AND ELECTROCHEMICAL BEHAVIOR OF AISI 4140 TIG WELDMENTS

Welded structures, specifically their fusion and heat-affected zones, are majorly prone to embrittlement and enhanced corrosion due to the induced residual stresses resulting from the complexity of the heating and cooling cycles during welding. In this work, TIG welding of AISI 4140 alloy steel (chromium–molybdenum steel) was done using the filler wire ER80S-B2 (AWS A5.28) followed by post-weld heat treatment. A comparison of base metal, as-welded, and post-weld heat-treated samples is made based on residual stresses, corrosion resistance, and mechanical properties. Due to the presence of stresses and the formation of unstable martensitic structure, the as-welded samples depicted the highest corrosion rate (8.982 mpy) as compared to the post-weld heat-treated sample (5.707 mpy) which is closer to that of base metal (5.627 mpy). Post-weld heat treatment relieves the residual stresses which results in the enhancement of corrosion resistance. The tensile strength for the base metal, as-weld and PWHT samples come out to be 739, 763, and 744 MPa, respectively. Ductility, on the other hand, is restored by post-weld heat treatment which was compromised in the as-welded samples.

Enhancing the Mechanical Properties of Cu-Al-Ni Shape Memory Alloys Locally Reinforced by Alumina through the Powder Bed Fusion Process.

A classical problem with Cu-based shape memory alloys (SMAs) is brittle fracture at triple junctions. This alloy possesses a martensite structure at room temperature and usually comprises elongated variants. Previous studies have shown that introducing reinforcement into the matrix can refine grains and break martensite variants. Grain refinement diminishes brittle fracture at triple junctions, whereas breaking the martensite variants can negatively affect the shape memory effect (SME), owing to martensite stabilization. Furthermore, the additive element may coarsen the grains under certain circumstances if the material has a lower thermal conductivity than the matrix, even when a small amount is distributed in the composite. Powder bed fusion is a favorable approach that allows the creation of intricate structures. In this study, Cu-Al-Ni SMA samples were locally reinforced with alumina (Al2O3), which has excellent biocompatibility and inherent hardness. The reinforcement layer was composed of 0.3 and 0.9 wt% Al2O3 mixed with a Cu-Al-Ni matrix, deposited around the neutral plane within the built parts. Two different thicknesses of the deposited layers were investigated, revealing that the failure mode during compression was strongly influenced by the thickness and reinforcement content. The optimized failure mode led to an increase in fracture strain, and therefore, a better SME of the sample, which was locally reinforced by 0.3 wt% alumina under a thicker reinforcement layer.

Evidence for austenite to non-modulated martensite transformation crystallography and variant organization in Ni-Mn-Ga-Co ferromagnetic shape memory alloys

Knowledge of transformation crystallography and variant organization of product phase was a prerequisite for tuning microstructure and enhancing functionalities in materials with displacive structure-transformation. However, the transformation orientation relationship (OR) between parent austenite and non-modulated (NM) martensite in traditional NiMnGa ferromagnetic shape memory alloy was not yet well-determined via direct experimental results, thus leading to ambiguity in understanding the self-accommodated configuration. In this work, by generating a microstructure with coexisting austenite and NM martensite through minor Co substitution and proper heat treatment, the transformation OR was unambiguously determined to be the N-W relation with {111}A//{101}NM and <2¯11>A//<101¯>NM on the basis of accurate EBSD orientation measurements. Analysis on deformation gradient matrix constructed from the N-W OR revealed that, the internal nano-twined structure allowed a maximum profit for eliminating the overall latticed deformation caused by transformation, overweighing the “sandwich” micro-variant pair structure. Such strain accommodation mechanisms consequently contributed to the resultant self-accommodated hierarchically twinned structure of martensite. Moreover, it was found that the prior austenite grain boundaries (PAGBs) provided dominated nucleation sites for NM martensite variants, where the preferential variant selection at PAGBs mainly depended on the inclination angle between the PAGB and matching close-packed direction (<2¯11>A//<101¯>NM) of variants. The present study provided comprehensive information on transformation crystallography and displacive characters of austenite to NM martensite transformation, which was useful for the efforts in property optimization and theoretical simulation.