Martensite Lath Width Research Articles

Study on the microstructure evolution and effect on mechanical properties of P92 steel during long term service

P92 heat-resistant steel is widely utilized in ultra-supercritical units due to its exceptional properties. This study examines the microstructure evolution and mechanical properties change of P92 during service spanning 0 to 93,000 h, while also exploring the correlation between microstructure changes and property variations. The results indicate that the matrix of P92 steel retains its characteristic tempered martensite structure, and the large-angle and small-angle grain boundaries in the steel remain nearly unchanged throughout extended service periods. However, the deterioration of P92 steel properties can mainly be attributed to alterations in precipitate size and martensitic lath width. The size of M23C6 and Laves particles increased from their initial dimensions of 150 nm and 0 nm to 260 nm and 370 nm, respectively. Moreover, the width of the martensitic lath increased from 350 nm to 700 nm. The martensitic lath contributes over 60 % of the theoretical tensile strength, playing a pivotal role in the exceptional strength of P92 steel during service. The presence of coarse Laves particles at the interface leads to a rapid reduction in impact energy, decreasing from approximately 130 J to 50 J, negatively affecting the impact energy of P92 steel.

Effects of Zr content on microstructure and properties of 9Cr ferritic/martensitic steel

The effect of the Zr content range in 0.03–0.63 wt% on the microstructure, mechanical properties and high-temperature oxidation behaviors of 9Cr ferritic/martensitic (FM) steels was investigated. The microstructural evolution of steels was studied using Thermo-Calc software, SEM, EBSD and TEM technologies. A full martensite structure was obtained in steels with various Zr contents. The small-sized M23C6 decrease and the larger-sized Zr(C, N) increase with the increasing Zr content, which leads to the increase of martensitic lath width and martensitic block size. The proper Zr content can enhance the oxidation resistance of steel, but Zr addition will reduce the mechanical properties. The decrease in strength and hardness can be ascribed to the reduction of precipitation strengthening and gain size strengthening. The increase in Zr content promotes the formation of Zr oxides, which reduces the formation of MnCr2O4 oxides and Cr2O3 oxides, and finally improves the oxidation resistance of steels.

The Effect of Multi-Step Tempering and Partition Heat Treatment on 25Cr2Ni3MoV Steel's Cryogenic Strength Properties.

In this study, the refinement of two microstructures was controlled in medium carbon 25Cr2Ni3MoV steel via multi-step tempering and partition (MTP) to achieve high cryogenic strength-ductility combinations. Microstructure evolution, the distribution of stress concentration, and microcrack formation and propagation during cryogenic Charpy impact testing were investigated. Compared with their performance in the quenching and tempering states (QT), the MTP steels showed a significant improvement in yield strength (1300 MPa), total elongation (25%), and impact toughness (>25 J) at liquid nitrogen temperature (LNT). The strengthening contributions mainly originated from the high dislocation density and refinement cementite (size: 70 nm) in the martensite lath (width: 1.5 μm) introduced by refined reversed austenite and its latter decomposition. The instrumented Charpy impact results indicated that cracks nucleated in the primary austenite grain (PAG) boundary for two steels due to the strain concentration band preferring to appear near PAGs, while cracks in the QT and MTP samples propagated along the PAGs and high-angle grain boundary (HAGB), respectively. The crystallized plasticity finite element simulation revealed that the PAG boundary with cementite precipitates of large size (>200 nm) was less able to dissipate crack propagation energy than the HAGBs by continuously forming a high strain concentration area, thus leading to the low-impact toughness of the QT steel.

Effect of relaxation before quenching on microstructure and properties of the steel

The effect of different relaxation time before online quenching on mechanical strength and toughness of high-strength medium-thickness martensitic steel plate was analysed in this work. Increasing the relaxation time from 15 s to 135 s resulted in an increase in the impact energy at −20 °C from 33 J to 148 J. However, the yield strength was decreased from 1050 MPa to 1002 MPa. Dislocation density and martensitic lath width are the main factors causing the strength change. Moreover, the dislocation density caused by deformation drops first sharply and then gently during the relaxation, resulting in an inverse trend of martensitic lath width. Additionally, there appears to be a linear relation between impact energy at −20 °C and dislocation density−1/2.

Influence of thermomechanical processing and tempering temperature on tensile flow and work hardening behaviour of India specific reduced activation ferritic martensitic steel

In the present work, an indetailed investigation has been carried out on the effect of thermomechanical treatment (TMT) and tempering temperature on microstructural features, tensile flow characteristics and work hardening behaviour of the India specific reduced activation ferritic martensitic (INRAFM) steel. The flow and work hardening characteristics of the studied steel are described by using the Voce constitutive equation and Kocks- Mecking (K-M) approach respectively in the temperature range of 300–873 K. The steel subjected to TMT and tempering for 90 min at 1033 K shows a refined martensitic lath width, finer size (Cr/W/Fe)23C6 and (V/Ta) (C/N) precipitates with an increment in number density. Furthermore, increase in tempering temperature coarsened the refined structural features. Voce equation closely fits the flow curves at 823 K and 873 K, but shows a slight deviation at temperatures below 773 K. The parameters derived from Voce equation are fairly in good correaltion with the experimental values. The strain hardening rate (θ) vs. true stress (σ) plots portrayed stage-I and stage-III work hardening stages and their corresponding phenomena were discussed. In addition, the effect of TMT and tempering treatment on the work hardening parameters obtained from K-M approach and inter-barrier spacing of the steel were also presented in this study. The work hardening characteristics of the steel signifcantly improved upon TMT due to refinement in microstructural features. The flow stress and work hardening parameters decreases with raise in test temperature due to the dominant effect of dynamic recovery.

Nondestructive Ultrasonic Evaluation of Ferritic CrW Steel upon Long-term Aging

The microstructural evolution of heat-resistant ferritic CrW steel during long-term aging was characterized by ultrasonic wave velocity measurement. Long-term aging was conducted at 700°C for 4000 h. The ultrasonic wave velocity increased monotonically upon long-term aging. The ultrasonic wave velocity increased rapidly during the initial 1000 h of aging and moderately thereafter. The MSUB23/SUBCSUB6/SUB carbide and Fe2W Laves phases underwent coarsening through the Ostwald ripening mechanism, causing a decrease in Cr and W solute atom concentration of the iron matrix during 4000-h aging. The dislocation density also decreased because of the heat treatment. The width of martensite lath increased owing to dislocation recovery at the lath boundaries. The ultrasonic wave velocity depends on the Young’s modulus of the material, which may influence microstructural features such as dislocation substructure, precipitates, solute atom concentration, and martensite lath boundaries. This study utilizes ultrasonic wave velocity to quantify the damage level and microstructural changes during the long-term aging of ferritic CrW steel.br/

Study on the improving effect of Nb-V microalloying on the hydrogen induced delayed fracture property of 22MnB5 press hardened steel

Press-hardened steel (PHS) is widely used in manufacturing automobile parts and is sensitive to hydrogen embrittlement (HE). A novel 0.04Nb-0.04 V (wt%) PHS (22MnB5NbV) was developed based on 22MnB5. The constant-strain bending test shows the duration of no-cracking for 22MnB5NbV in 0.5 mol/L HCl is > 300 h even though the pre-bending stress is 1500–1600 MPa, and that for 22MnB5 is 12 h. The prior austenite grain size and martensite lath width in 22MnB5NbV decrease from 13.4 to 8.2 μm and from 0.35 to 0.15 μm due to Nb-V microalloying, respectively, resulting in the hydrogen trapped in reversible hydrogen traps increases by 2.09 times according to thermal desorption spectrometry analysis. The leaching experiment presents that 88% Nb and 25% V exist in the form of precipitates rather than in solution in 22MnB5NbV. The nano-precipitates (< 10 nm) are mainly (Nb, Ti, V)C, in which Nb-bearing precipitates account for the largest proportion and supply the heterogeneous nucleation sites for V-bearing precipitates, this is confirmed by atom probe tomography (APT). APT confirms that the hydrogen is mainly trapped at the interface of Fe matrix/carbide. The dispersed nano-carbide and refined microstructure resulting from the Nb-V microalloying are the key factors improving the HE of 22MnB5NbV.

Additive manufacturing of a nanocrystalline lathy Ti6Al4V5Cu alloy with high strength and ductility combination

Among various types of microstructures in titanium alloys, martensitic microstructure has the highest strength, but with poor ductility, which restricts its applications in the engineering practice. In this study, we have efficiently refined the martensite lath width from 500-1000 nm to 50–200 nm through 5% copper alloying in the Ti6Al4V alloy through laser powder bed fusion. The strength of the material increased from 1090 MPa of Ti6Al4V to 1460 MPa of Ti6Al4V5Cu with the elongation of 11.2%. The nanocrystalline lathy Ti6Al4V5Cu alloy prepared in this study can provide reference for the material design of new ultra-high strength titanium alloys.

Fabrication of ultrafine-grained Ti-15Zr-xCu alloys through martensite decompositions under thermomechanical coupling conditions

• Ultrafine-grained microstructure was successfully achieved in Ti-15Zr- x Cu system by hot rolling. • Cu alloying increased dynamic recrystallization nucleation rate and formed Ti 2 Cu along grain boundaries, which inhibited grain growth. • The comprehensive mechanical properties of the ultrafine grained Ti-15Zr-5Cu alloy are better than that of the Roxolid Ti-Zr alloy currently used for dental implants. Grain refinement is a well-recognized method to simultaneously increase the strength and ductility of metallic materials. Fabrication of ultrafine-grained metals in bulk using a simple low-cost approach is a long-term goal for material scientists. In this work, based on the chemical composition of a biomedical Ti-15Zr alloy, a series of novel Ti-15Zr- x Cu ( x = 0, 3, 5, 7 wt.%) alloys were designed and fabricated. The alloys were quenched in the single β phase region to obtain a martensitic microstructure and deformed in the temperature range of 710–750 °C to obtain an ultrafine-grained microstructure through martensite decomposition under thermomechanical coupling conditions. Experimental results showed that Cu alloying could increase the dynamic recrystallization (DRX) nucleation rate due to its role in both refining martensitic lath width and increasing dislocation density. Cu alloying could also suppress grain growth due to the precipitated Ti 2 Cu particles exerting pinning forces on the grain boundaries. The optimal Cu content in the Ti-15Zr- x Cu alloy was determined to be 5 wt.%. After being subjected to a compression leading to a 70% height reduction at 730 °C and 1 s –1 , the grain size of the Ti-15Zr-5Cu alloy was only 180 ± 70 nm. The tensile strength of the as-prepared alloy reached 975 ± 10 MPa, which was 45% higher than that of the conventional Ti-15Zr alloy (673 ± 16 MPa). This increase in strength was achieved without any reduction in ductility. The comprehensive mechanical properties of the ultrafine-grained Ti-15Zr-5Cu alloy are better than that of the Roxolid Ti–Zr alloy currently used for dental implants.

Effects of different silicon content on irradiation defects and hardening in 9Cr ferritic/martensitic steel

In order to explore the effect of silicon (Si) content on the irradiation behavior of ferritic/martensitic (F/M) steel, 9Cr F/M steel with Si of 0, 0.4%, 0.7%, and 1.0% in weight percent were prepared. All specimens were irradiated with 2.4 MeV iron ions at 550 °C to the fluence of 6.41 × 1016 ions cm−2, corresponding to a peak damage dose of 70 dpa. Transmission electron microscopy (TEM) was used to characterize the microstructure of the alloys before and after irradiation, and nanoindentation tests were used to assess the irradiation hardening. The addition of Si inhibited the growth of both M23C6 and MX precipitates and promoted the nucleation of MX precipitates, and the nucleation of the M23C6 precipitates was inhibited when the added Si was less than 0.7 wt.%, indicating that the precipitates can be regulated by Si content. The martensite lath width and the packet size of 0.4 wt.% Si content alloys are the largest and then decrease with the increase of Si content. As the Si content increases, the average size of dislocation loops decreases while the number density increases. Alloys without Si have the smallest void number density, followed by alloys with 0.7 wt.%, 0.4 wt.%, and 1.0 wt.% Si content. The hardness of the unirradiated alloy increased with the increase of Si content. After irradiation, the most severe radiation hardening occurred in the 0.4 wt.% Si alloy and the lowest in the 0.7 wt.% Si alloy.

Effects of V–Nb microalloying on the microstructure and properties of spring steel under different quenching-tempering times

The effects of V–Nb microalloying on the microstructure and mechanical properties of ultra-high-strength spring steel at various austenitizing and tempering times were investigated. To achieve this, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron backscatter diffraction, and tensile tests were performed. The equations for austenite grain coarsening at 900 °C and (V,Nb)C carbide growth at 400 °C were obtained. The results indicated that the tensile properties of the investigated steel significantly improved after V–Nb microalloying with the same quenching-tempering treatment. With increasing austenitization or tempering time, the tensile properties of the experimental steels dramatically declined owing to the coarsening of the grain size or of the martensite lath width and carbide size. Because V–Nb microalloying increased the densities of the grain boundaries and carbides, the full width at half maximum 211α, microstrain, and dislocation density decreased slowly with prolonged tempering time. The strength increment among the investigated steels mainly resulted from the grain refinement strengthening, dislocation strengthening, and most importantly, precipitation strengthening of (V,Nb)C carbide.

Experimental And Numerical Modelling of Cyclic Softening and Damage Behaviors for a Turbine Rotor Material at Elevated Temperature

In order to better understand the physical process of deformation and cyclic softening a 12% Cr martensitic stainless steel FV566 has been cyclically tested at high temperature in strain control. Increase in temperature was found to increase the cyclic life, softening rate and viscous stress magnitude. An increase in the dwell time led to the acceleration of the material degradation. The microstructure changes and dominating deformation mechanisms were investigated by means of scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. The results have revealed a gradual sub-grain coarsening, transformation of lath structure into fine equiaxed sub-grains, and misorientation angle development in blocks and packets until material failure. Further, a unified viscoplastic constitutive model coupled with a physically-based damage variable, is proposed to capture the cyclic mechanical behavior and microstructural evolution of the material at elevated temperature. The mechanical strength can be reduced by the decrease in the dislocation density, the coarsening of the martensitic lath and the loss of the martensitic structure under cyclic loading. The proposed physically-based damage variable is driven by the evolutions of dislocation density and martensitic lath width. The good comparisons with test results mean that the proposed model can reasonably model the cyclic elastic-viscoplastic constitutive behavior of the material at high temperature.

The size effect of martensite laths and precipitates on high strength wear-resistant steels

Low alloy high strength wear resistant steels are with high toughness, low cost and good abrasion resistance. It can effectively resist the propagation of wear cracks and prolong the service life of machine components. This paper focuses on the internal relationship between macroscopic physical properties and microscopic martensite lath and precipitate size throughout thickness of wear resistant steel. Four kinds of 40mm thickness wear resistant steels with different alloy chemical composition were produced and investigated. Results show the strength and hardness performance of ARIV are obviously higher than other three steels. ARI have a relatively large strength difference through thickness. The impact toughness of ARIV is relatively uniform, which is greater than that of the ARIII at middle layer and lower than that of the ARIII at 1/4 layer. The width of martensite lath of ARIV is relatively small, mainly 100 ∼ 300 nm, while that of ARII and ARIII is mainly 200 ∼ 400 nm. ARIV steel has shorter martensite lath band and more precipitates below 50 nm. It indicates that the size of martensite laths and precipitates of wear-resistant steels are important factors to determine its performance throughout thickness.

Effects of quenching and tempering heat treatment on microstructure, mechanical properties, and fatigue crack growth behavior of 51CrV4 spring steel

Herein, the microstructure, mechanical properties, and fatigue crack growth (FCG) behavior of 51CrV4 spring steels under different quenching and tempering heat treatment were studied. Results show that tempering temperature would not affect the prior austenite grain (PAG) size, while quenching temperature has a great influence on it; the volume fraction of carbide precipitates decreased with increasing quenching temperature. The width of martensite lath has little relationship with the prior austenite grain size. The mechanical properties of the investigated steels decreased with an increasing tempering temperature. Tempering temperature had a greater influence on the fatigue crack growth behavior compared with quenching temperature. The FCG of all the investigated steels were well interpreted by Paris model. When the quenching temperature was 840 °C, the stable expanding stage of the fatigue fracture was representative fatigue striations, fatigue steps and secondary cracks; multiple failure mechanisms coexisted in the rapid expanding stage; the fracture morphology of unstable failure stage was mainly dimples. When the quenching temperature increased to 880 °C and 920 °C, quasi-cleavage was observed in all the stages compared with the samples quenched at 840 °C. Secondary cracks, fatigue steps and crack closure could retard the FCG rate by consuming the driving force near the tip of fatigue crack.

The influence of post-weld tempering temperatures on microstructure and strength in the stir zone of friction stir welded reduced activation ferritic/martensitic steel

Abstract Reduced activation ferritic/martensitic (RAFM) steels are among the most competitive candidates of structural materials for nuclear fusion reactors, due to their superior comprehensive properties. Friction stir welding (FSW) was investigated in joining RAFM steel, considering its potential advantages in obtaining an optimal microstructure and mechanical properties of welded joint. To evaluate the feasibility of FSW in joining RAFM steel, an in-depth understanding of the microstructure-property relationships for friction stir welded joints of RAFM steel is necessary. In this research, the quantitative relationships between microstructural evolution and tensile properties in the stir zone (SZ) of friction stir welded RAFM steel after post-weld tempering treatment (PWTT) were systematically studied. Three different post-weld tempering temperatures namely 720 °C, 760 °C, and 800 °C were adopted. Then the uniaxial tensile properties were tested at room temperature and 550 °C, respectively. Electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and the Thermo-Calc Calphad software were adopted to systematically investigate the microstructural evolution. Martensite lath width, precipitate number density, equilibrium solid solubility of alloying elements in the matrix, and geometrically necessary dislocation (GND) density were analyzed quantitatively. With the results obtained, we assessed the contribution of each strengthening mechanism to the 0.2% offset yield strength. According to the effective inter-barrier spacing theory, a microstructure-sensitive yield strength model was obtained to well predict the change in yield strength at different conditions. Finally, the results calculated by equivalent strengthening effect indicated that the crucial microstructure determining the yield strength of the SZ for RAFM steel after PWTT is the high density of dislocation substructures.

Microstructure evolution of low alloy wear resistant steels during heat treatment procedure

Microstructure evolution of low alloy wear resistant steels during heat treatment procedure was studied in this paper. The results showed that During furnace cooling in homogenizing, Chromium/iron, Niobium, Vanadium and other hardly soluble carbides formed. But Chromium/iron carbides could resolve into austenite during quenching procedure, while the other carbides barely changed. Carbon addition grew the carbides into shuttle shapes and inflated the austenite grains. But Ni addition broadened the martensite lath width without dilating the austenite grains. And it hardly influenced the carbides formation. Vanadium addition seemed that the martensite lathes were cut into several discontinues sections. With the temperature rising, the boundaries got blurred, which might correlated with the decomposing of retained austenite.

Effect of Cooling Rate and Austenite Deformation on Hardness and Microstructure of 960MPa High Strength Steel

Abstract In order to develop 960MPa grade high strength steel, the effects of cooling rate and austenite deformation on the hardness and the microstructure of high strength steel has been studied by Scanning Electron microscope (SEM), Transmission Electron Microscope (TEM), Gleeble-3500 thermal simulation testing machine and T2500 Vickers hardness tester. The results show that only when the cooling rate was higher than 10∘C/s and the final cooling temperature was lower than 250∘C, the microstructure mainly consists of martensite, and the strength of high strength steel could be above 960MPa; and austenite deformation could effectively refine the width of martensite lath, thus improving the strength and toughness.

Effect of Heat Treatments on Microstructural Evolution and Tensile Properties of 15Cr12MoVWN Ferritic/Martensitic Steel

In this study, the phase transformation temperature of 15Cr12MoVWN ferritic/martensitic steel was determined by differential scanning calorimetry to provide a theoretical basis for the design of a heat treatment process. An orthogonal design experiment was performed to investigate the relationship between microstructure and heat treatment parameters, i.e., normalizing temperature, cooling method and tempering temperature by evaluating the room-temperature and elevated-temperature tensile properties, and the optimum heat treatment parameters were determined. It is shown that the optimized heat treatment process was composed of normalizing at 1050 °C followed by air cooling to room temperature and tempering at 700 °C. Under the optimum heat treatment condition, the room-temperature tensile properties were 1014 MPa (UTS), 810.5 MPa (YS) and 18.8% (elongation), while the values are 577.5 MPa (UTS), 469 MPa (YS) and 39.8% (elongation) tested at 550 °C. The microstructural examination shows that the strengthening contributions from microstructural factors were the martensitic lath width, dislocations, M23C6, MX and grain boundaries of prior austenite grain (PAG) in a descending order. The main factors influencing the tensile strength of 15Cr12MoVWN steel were the martensitic lath width and dislocations.

Physically-based modeling of cyclic softening and damage behaviors for a martensitic turbine rotor material at elevated temperature

A unified viscoplastic constitutive model coupled with a physically-based damage variable, is proposed to capture the cyclic mechanical behavior and microstructural evolution of the material at elevated temperature. The mechanical strength can be reduced by the decrease in the dislocation density, the coarsening of the martensitic lath and the loss of the martensitic structure under cyclic loading. The proposed physically-based damage variable is driven by the evolutions of dislocation density and martensitic lath width. The good comparisons with test results mean that the proposed model can reasonably model the cyclic elastic-viscoplastic constitutive behavior of the material at high temperature.

Effects of Yttrium and Heat Treatment on the Microstructure and Mechanical Properties of CLAM Steel

The effects of normalizing and tempering temperatures on the microstructures and mechanical properties of China Low Activation Martensitic (CLAM) steel with and without yttrium were studied. Based on the optimized traditional heat treatment, two special intermediate heat treatments were studied to improve the mechanical properties of the steels. The AC3 temperature of the CLAM steel was increased by adding yttrium. The optimized traditional heat treatments were 1000 °C × 30 min + 755 °C × 90 min and 1050 °C × 30 min + 755 °C × 90 min for the C1 and C2 alloys, respectively. The intermediate heat treatment results indicated that the precipitation behavior of carbides was effectively controlled. The size of the M23C6 carbides was refined during the two intermediate heat treatments due to the priority precipitation of MX particles. However, coarsening of grains and martensite laths occurred during heat treatment with furnace cooling. The twice-quenched tempering samples had a smaller grain size and martensite lath width than the other samples. The ductile–brittle transition temperature was − 69 and − 103 °C for the C1 and C2 alloys with twice quenching, and the yield strengths were 745 and 760 MPa, respectively. An excellent balance of strength and impact toughness was obtained with the twice-quenching and once tempering heat treatment process.