Martensite Content Research Articles

Simultaneous Improvement of Strength and Ductility of Dual-Phase Steel Processed by Multi-Step Cyclic Rolling and Intercritical Annealing

In the present study, a multi-step (MS) cyclic rolling and intercritcal annealing process was proposed and applied for dual-phase (DP) steel. The MS process performed three times with 27% deformations and intercritical annealing, while the single-step (SS) process performed an 81% rolling, along with intercritical annealing. A microstructure with an average grain size of 3 μm and a martensite content of ~40% was obtained after MS treatment, which is similar to results obtained from the SS treatment. However, the distribution exhibits significant differences between the two different routes. A more homogenous distribution of ferrite–martensite was achieved after the multi-step compared with the single-step treatment. The yield strength of MS is slightly smaller than that of SS, while the ultimate tensile strength is better, which results in a decrease in yield ratio. Furthermore, the ductility was greatly improved after MS, which is mainly attributed to the uniform chain-like distribution of martensite.

Improving mechanical properties and corrosion resistance of 0.5 wt.% C TRIP steel by adjusting retained austenite stability and microstructural constituents

Here the effect of retained austenite stability and martensite-austenite island/fresh martensite on mechanical properties, fracture behavior, and corrosion resistance of 0.5 wt.% C TRIP steel was studied. The volume fraction of retained austenite is distinctly high at ∼ 30% in 0.5 wt.% C TRIP steel compared to the conventional TRIP steels (10–20%) while the morphologies of retained austenite are mainly film-like, mixed film-like and small blocky (0.3–0.7 μm), and large blocky (0.7–1.5 μm), respectively, in the steels after bainitic holding at 360 °C, 400 °C and 450 °C. In addition, the volume fraction of bainite decreases from 53 vol.% to 44 vol.% and 32 vol.%, and that of martensite-austenite island/fresh martensite content increases from 2 vol.% to 9 vol.% and 21 vol.% with increasing temperatures. The two-stage mechanical stability of retained austenite with large volume fraction and mixed film-like and blocky morphologies ensures stable and continuous TRIP effect. As a result, after bainitic holding at 400 °C for 30 min, the superiorly comprehensive mechanical properties are obtained for the steel, showing ultrahigh tensile strength of 1220 MPa and retaining good ductility of 44%. Moreover, the effects of multiphase on strength and ductility were discussed and the presences of martensite-austenite island/fresh martensite and blocky retained austenite lead to premature crack initiation. Crack initiation and propagation mainly happen along the interface between deformation-induced martensite and the ferritic matrix and that between M-A island/FM and the ferritic matrix. The decreasing carbon content in RA and the occurrence as well as increase of M-A island/FM can deteriorate corrosion resistance of 0.5 wt.% C TRIP steels.

Effect of austenitizing temperature on microstructure and properties of a high-speed cobalt steel

Abstract The effect of different austenitizing temperatures on the type, morphology, distribution of carbides and martensite content in cobalt high-speed steel is characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and energy dispersive spectrometry. The results show that the eutectic MC carbides hardly dissolve during austenitizing process, and the lamellar M2C carbides decompose into MC and M6C carbides at 1100 °C. A large amount of M23C6 carbides uniformly distributed on the matrix are dissolved into austenite at 1100 °C. With the increase of austenitizing temperature, alloy element dissolves into matrix and the effect of solid solution strengthening of martensite enhances, which increases the hardness of cobalt high-speed steel. However, when the austenitizing temperature exceeds 1050 °C, the excess alloying elements in the matrix reduce the Ms point and increase the volume fraction of retained austenite, resulting in decrease of hardness of cobalt high-speed steel. The peak hardness with 66.4 HRC appears when the austenitizing temperature reaches 1050 °C.

Effect of the Cooling Rate of Thermal Simulation on the Microstructure and Mechanical Properties of Low-Carbon Bainite Steel by Laser-Arc Hybrid Welding

A new kind of low-carbon bainite steel with excellent strength and toughness was developed, serving as the bogie of the next-generation high-speed train. However, the softening of the heat-affected zone (HAZ) in laser-arc hybrid welding (LAHW) needs to be overcome. In this study, the effect of the cooling rate of the LAHW process on the microstructure and mechanical properties in the HAZ was explored via thermal simulation. The results showed that with increased cooling rate, the grain size increased, the content of lath martensite decreased, and the lath bainite gradually changed to a granular shape in the thermal simulation specimen. With the decrease in the cooling rate, i.e., with the increase of t8/5, the strength–toughness matching of the material showed a downward trend. The thermal simulation specimen with a t8/5 of 6~8 s had higher strength and good toughness, which can be considered a potential welding parameter reference. The content of martensitic austenite (M-A) constituents was the main factor that determined the strength and toughness of the joint. During the tensile test, the axial force caused the material to tighten, and the transverse stress as obvious in the part of the M-A constituents that are prone to microcracks and many defects, resulting in cracks, paths, and multi-component layers in the center. As a result, the thermal cycle specimens had mixed fracture characteristics.

Innovative Online Measurement and Modelling Approach for Property-Controlled Flow Forming Processes

The production of complex multi-functional, high-strength parts is becoming increasingly important in the industry. Especially with small batch size, the incremental flow forming processes can be advantageous. The production of parts with complex geometry and locally graded material properties currently depicts a great challenge in the flow forming process. At this point, the usage of closed-loop control for the shape and properties could be a feasible new solution. The overall aim in this project is to establish an intelligent closed-loop control system for the wall thickness as well as the α’-martensite content of AISI 304L-workpieces in a flow forming process. To reach this goal, a novel sensor concept for online measurements of the wall thickness reduction and the martensite content during forming process is proposed. It includes the setup of a modified flow forming machine and the integration of the sensor system in the machine control. Additionally, a simulation model for the flow forming process is presented which describes the forming process with regard to the plastic workpiece deformation, the induced α’-martensite fraction, and the sensor behavior. This model was used for designing a closed-loop process control of the wall thickness reduction that was subsequently realized at the real plant including online measured feedback from the sensor system.

Salt-Fog Corrosion Behavior of GCr15 Steels Treated by Ultrasonic Strengthening Grinding Process

In this paper, the corrosion resistance of four GCr15 steel samples has been investigated. Three samples were initially surface-treated by ultrasonic shot peening (USP), a dry ultrasonic strengthening grinding process (DUSGP), and a wet ultrasonic strengthening grinding process (WUSGP), respectively, while another one was only polished. Then, all the processed samples were subjected to salt spray corrosion. Sample properties, such as capacitance arc, self-corrosion potential (Ecorr), self-corrosion current density (Icorr), corrosion morphology, and corrosion products were analyzed. Results show that the sample treated by WUSGP has the best corrosion resistance, which can be attributed to the high dislocation density, small grain size, needle-like and lath-like shape of martensite content formed in the treatment, which hinders Cl− from eroding the matrix. The obtained corrosion pits and cracks in Sample WUSGP show a width of approximately 1.4 µm and 2.1 µm, respectively, degrading 78% and 75% compared to polishing. The possible corrosion mechanism of the samples has been discussed. The findings denote that the treatment fused ceramic balls, strengthened liquid, and corundum in USP could be an efficient method to improve the corrosion resistance of some mechanical parts.

Numerical Study to Promote the Residual Stresses Development during ISF Process with Improvement in Two Point Incremental Die Forming

Metastable austenitic stainless steel (MASS) has been the material of choice for the fabrication of disc springs employing incremental sheet forming (ISF) processes due to its high creep, fatigue, and chemical resistance, as well as its good surface quality. Previous research has shown that the presence of martensite enhances the formation of beneficial compressive residual stresses. However, if the ISF is accelerated to improve efficiency, the rise in temperature during ISF operation suppresses the deformation that causes martensite transition (DIMT). In essence, the cooling channel shapes are developed with numerical assistance such that its impact on residual stress induction is low. Variation in ISF process parameters, such as tool diameter, tool step-down, and contact force, as well as variation in cooling channel size, are used to construct the computational analysis. To analyze the finally produced residual stresses in the disc spring, the non-linear isotropic/kinematic hardening combined with the TRIP formulation is simulated. According to the comparison, the channel size must be between 0.8 and 1.2 mm in radius to minimize residual stress fluctuation. Additionally, when moving across the die with cooling channels, the force-controlled ISF produces more consistent results. Based on the numerical findings, it is conceivable to greatly enhance the ISF process speed and dissipate process heat by cooling the sheet on sides, allowing residual stresses and martensite content to be adjusted in a stable manner. As a result, the ISF process may be greatly expedited, making it more appealing for industrial applications.

ІNCREASE IN WEAR RESISTANCE DUE TO MANGANESE ALLOYING OF ADI CAST IRON

The effect of manganese content on the wear resistance was studied on ADI materials , that are isothermally quenchеd at different temperatures. ADI samples with Mn content of 0.78% and 0.24% were analyzed. The final structure of the cast iron was created by austenitizing heating at 900 ° C for 30 minutes. and subsequent isothermal quenching in liquid tin at temperatures of 310, 330, 350, 380 ° C for samples with high manganese content and at 350, 370 ° C for samples with low content. It is shown that increasing the manganese content increases the amount of residual austenite under the same quenching conditions. This enhances the positive role of the TRIP effect on the strengthening of the subsurface layer and wear resistance. Wear resistance studies were performed on laboratory equipment that simulated the wear of agricultural machinery during plowing. Wear resistance tests were performed on samples of 10 x 10 mm2 in dry sand at a load of 5 kg at speed of 2.93 m/s which is approximately equal to the speed of machinery during agricultural work. The duration of the cycle corresponded to 5 km distance. After each cycle, the samples were weighed on analytical balances with an accuracy of 0.1 mg. It was found that under all conditions of quenching manganese alloying significantly increases wear resistance due to more intensive deformation-induced martensitic transformation in the near-surface layers. X-ray studies have shown that increasing the of manganese concentration doubles the content of deformation-induced martensite in the surface layers. The microhardness of the surface layer after shear deformation during wear increases by 1.5 times compared to the sample without wear. One third of the hardness impressions show abnormally high microhardness Hμ = 700 - 900. These values ​​correspond to the microhardness of martensite in cast irons of this class. The obtained results of laboratory experiments are a prerequisite for the wide practical use of TRIP - effect as an effective mechanism of deformation strengthening of the surface layers of agricultural machinery operating in the mode of wear.

Martensite transformation behavior and mechanical properties of two kinds of low‐nickel austenite stainless steel

AbstractIn this study, tensile tests were performed to study the effect of martensitic transformation on mechanical properties. The effect of the martensitic transformation on the growth of austenite grains was also investigated. The result indicated that, with higher martensite content, alloy 2 possessed finer austenite grains and higher tensile strength. To compare the martensitic transformation behavior of the two steels, the Ms and Md30 temperatures, austenite stacking fault energies, and driving force were calculated by empirical equations. Equilibrium phase diagrams were calculated using Thermo‐Calc software. The kinetics of martensite volume fraction versus true strain was fitted by the sigmodal model. It was found that martensite volume fraction and deformation behavior exhibited significant differences in the two low‐nickel austenitic stainless steel. Both stacking fault energy and the driving force of transforming austenite to martensite have a critical role in strain‐induced martensite deformation. Beyond that, the prediction of martensite content by the sigmoidal equation was consistent with the experimental data.

Influence of Mn content on rolling performance of lean duplex stainless steel

ABSTRACT Cold rolling test as well as room temperature compression was carried out to investigated the influence of Mn content on the cold deformation properties of lean duplex stainless steel. The cold deformation mechanism was also discussed by observation of cold-rolled samples with cracks and analysis of the work hardening curve. The results demonstrated that the morphology and size of grains were both influenced by the Mn content. The quenched martensite content varied with Mn content, indicating that Mn plays critical role in both the difficulty of stress-induced martensite transformation of austenite grains in LDSS and the transformation process from austenite to martensite during solution treatment. LDSS with a lower Mn content exhibits lower austenite stability. After quenching, plenty of the austenite was transformed into martensite which performs much higher hardness than ferrite. With the increase in hardness difference between phases, the plasticity of the alloys decreased caused by each phase bore more uneven deformation.

The Effect of Surface Hardening on The HQ 705 Steel Camshaft Using Static Induction Hardening and Tempering Method

Induction hardening (IH) is a popular choice for automotive components such as camshafts for its ability to harden portions of a component selectively. The camshaft will contact the tappet, connected to the rocker arm, to open and close the valve whenever the engine is running. This contact between the camshaft and the tappet causes wear on the camshaft surface. IH of the camshaft is required to improve wear resistance and service life, as well as core elasticity to absorb high torsional stresses. It is known that studies about IH on camshafts are still very limited. This study aims to determine the effect of the induction hardening and tempering treatment on the mechanical properties of the camshaft made of HQ 705 steel. The induction hardening carried out in this study uses different parameter settings such as heating time and output current. The camshaft specimen is hardened by static induction and then quenched in oil. The specimens are tempered after induction hardening with different temperatures and holding times to adjust the hardness level and reduce brittleness. Hardness, macro photographs, micrograph, and wear tests were conducted to determine the mechanical properties of the camshaft specimen after the induction hardening and tempering process. This study indicates that induction hardening with an output current of 747 A for 15 seconds followed by tempering at 150 °C for 15 seconds on specimen 1 produced the best mechanical properties. On the surface of these specimens found more martensite content while there was no microstructural change on the inside. The surface hardness of these specimens is 44 HRC (Rockwell C Hardness), while the inside is 26 HRC. Meanwhile, specific wear decreased by 45.45%.

Microstructure of Ultrafine Acicular Bainite and Mechanical Properties of 3Cr2MnNiMo Mold Steel during Austempering

Herein, the effect of austempering treatment on martensite/bainite multiphase microstructure evolution and mechanical properties of 3Cr2MnNiMo mold steel are investigated, and the mechanical properties are compared with the conventional quenching and tempering (QT) treatment. It indicated that the morphology of martensite/bainite can be effectively tailored by changing the austempering temperature. With the increase of temperature from 280 to 360 °C, the microstructure changes from tempered martensite + bainite + blocky retained austenite (RA) to martensite + coarse bainite plate + blocky RA and irregular martensite/austenite (M/A) island. When the temperature reaches 420 °C, the martensite content reaches the maximum. The bainite morphology is surprisingly transformed from coarse bainite plate to ultrafine acicular shape owing to the reduction of the bainitic driving force and the sharp increase of carbon diffusion coefficient. Moreover, blocky RA and M/A islands disappear and are replaced by nanoscale filmy RA. The alternating distribution of variants has a refinement effect on the microstructure and the effective grain size reaches a minimum value. Compared with the conventional QT treatment, the tensile strength of austempering at 420 °C increased by 702 MPa, but the toughness and ductility levels are almost the same.

Effect of Variation of Martensite with a Constant Carbon Content on Mechanical Behavior and Sliding Wear of Dual Phase Steels

In a dual phase steel, an increase in the martensite volume fraction leads to a decrease in its carbon content, and hence, elucidating the effect of martensite content on the mechanical and tribological behaviors has not been possible. For the first time, the present work dealt with this subject by maintaining a constant amount of carbon in different volume fractions of martensite. Tribological properties was evaluated via a pin-on-disk sliding tribometer. The study of wear tracks and debris revealed oxidative wear as the dominant wear mechanism. Increasing normal load from 10 to 30 N resulted in a decrease in the specific wear rate, which was related to the high work-hardening capacity. This resulted in higher surface hardness, accommodating the formation of a more durable tribofilm on the surface, and a lower coefficient of friction. However, the increase in normal load from 30 to 60 N resulted in an increase in the specific wear rate and reduction of oxide coverage on the sliding surface due to high surface stresses. The worn debris were mainly identified as metallic in the running-in stage and Fe3O4 oxide in the steady-state stage.Graphical Abstract

Carbides precipitation and kinetics of H13 steel subjected to deep cryogenic treatment

The influence of deep cryogenic treatment (DCT) on carbides precipitation and isothermal softening behaviour as a function of both temperature and time during tempering of H13 steel was investigated. The results show that the retained austenite transformed into martensite leading to an increase in hardness. The carbon content of martensite decreased after DCT and tempering according to the calculation of the X-ray diffraction spectrum. This indicates that more fine carbides precipitated from the martensitic matrix after DCT. The morphology of microstructure and distribution of carbides have verified the properties improvement mainly depending on the retained austenite transformation and more fine carbides precipitated after tempering. In addition, the carbides precipitating and coarsening are the main causes of thermal softening.

Influence of Deep Cryogenic Treatment on Microstructural Evolution and Transformation Kinetics Simulation by Finite Element Method of Low‐Carbon High‐Alloy Martensitic‐Bearing Steel

Nowadays, deep cryogenic treatment (DCT) is taken as a promising technique for improving the performance of steel. This article is focused on the influence of different heat treatment processes involving DCT on the microstructural evolution of a high Cr–Co–Mo high‐temperature‐bearing steel. Moreover, a multiphysical field coupling numerical model is built to investigate the martensitic transformation kinetics during DCT. The results show that DCT significantly improves the hardness and promotes the transformation of retained austenite to martensite at low temperatures. DCT not only refines the retained austenite as thin‐film morphology but also increases the precipitation of carbides and induces the carbides more homogenous distribution. This indicates that DCT promoted the segregation of carbon atoms and reduced the carbon content of martensite. The finite element model simulation shows that the retained austenite transformation is still incomplete, and the simulation results agree well with the experimental data by X‐Ray diffraction and transmission electron microscope analysis. This implies that the finite element simulation can better reflect and verify the experimental results.

Experimental Study on the High Temperature Impact Torsional Behavior of Ti-1023 Alloy.

Based on the modified Hopkinson torsion bar, a high-temperature dynamic shear test method was proposed for the Ti-1023 alloy, and the microstructural evolution of the tested material at different temperatures was studied. By using the modified high-temperature Hopkinson torsion bar, high-temperature testing within 1000 °C can be achieved. As the specimen-heating rate was fast, and the temperature gradient of the experimental environment was small, valid experimental data can be ensured during the experiment. The experimental results show that stress-induced martensite can significantly enhance the strength of the Ti-1023 alloy. Dynamically recrystallized grains similar to those in adiabatic shear bands appear in the microstructure of the Ti-1023 alloy after severe plastic deformation. Therefore, it is possible to regulate the content of stress-induced martensite in the microstructure to improve the mechanical properties of other alloys that are similar to β titanium alloys.

Novel Corrosion‐Resistant Tool Steels with Superior Wear Properties

This work presents the novel high‐strength cast steels Fe81Cr15Mo1V2C1 and Fe80Cr15Mo1V3C1 (wt%) with high resistance against abrasion and acid corrosion. By means of a tailored alloy design and a special casting technique, implying high solidification rates, multiphase microstructures composed of martensite, austenite and a complex arranged carbidic network are formed. The increase of the vanadium concentration by 1 wt% results in an enhanced content of martensite and a decreased fraction of metastable retained austenite as detected by X‐ray diffraction and magnetization studies. This affects significantly the deformation, strengthening and wear behavior of the presented cast steels. Nevertheless, both steels achieve very high compressive (>4500 MPa) and tensile strengths (>1100 MPa) already in the as‐cast state. Furthermore, a distinctly higher abrasive wear resistance than the conventional X90CrMoV18 cold‐work steel was determined for the novel alloys. In conclusion, the results of the microstructural, mechanical, tribological and corrosive characterizations illustrate the impact of alloy design and special solidification conditions on the microstructure and resulting properties for the newly developed cast steels. In addition, the high potential of Fe81Cr15Mo1V2C1 and Fe80Cr15Mo1V3C1 as materials for high‐performance tools, requiring high strength besides high resistance against abrasive wear and acid corrosion, is demonstrated.

High Strain Rate and Stress-State-Dependent Martensite Transformation in AISI 304 at Low Temperatures

Deformation-induced martensitic transformation as the basis of a hardening process is dependent, among others, on the stress state. In applications such as cryogenic cutting, where a hardened martensitic subsurface can be produced in metastable austenitic steels, different stress states exist. Furthermore, cutting typically occurs at high strain rates greater than 103s−1. In order to gain a deeper insight into the behavior of a metastable austenitic steel (AISI 304) upon cryogenic cutting, the influence of high strain rates under different loading conditions was analyzed. It was observed that higher strain rates lead to a decrease in the α′-martensite content if exposed to tensile loads due to generated adiabatic heat. Furthermore, a lath-like α′-martensite was induced. Under shear stress, no suppression of α′-martensite formation by higher strain rates was found. A lath α′-martensite was formed, too. In the specimens that were subjected exclusively to compressive loading, almost no α′-martensite was present. The martensitic surface generated by cutting experiments showed deformation lines in which α′-martensite was formed in a wave-like shape. As for the shear specimens, more α′-martensite was formed with increasing strain rate, i.e., force. Additionally, magnetic etching proved to be an effective method to verify the transformation of ferromagnetic α′-martensite.

Effect of Minor Carburizing on Microstructure and Mechanical Properties of M50 Bearing Steel

Herein, by using scanning electron microscopy, transmission electron microscope, X‐ray diffraction, residual stress analyzer, and mechanical‐property tests, the effect of minor carburizing on the microstructure and mechanical properties of M50 steel is studied. It is shown in the results that after minor carburizing, the austenite content in the surface layer of the quenched M50 steel increases, while the martensite content and the hardness decrease. After tempering, the retained austenite in the surface layer is transformed into martensite, and a large amount of uniformly distributed carbides are precipitated. The hardness of the surface layer increases to exceed that of the core region in the steel. After the minor carburizing, there is a carbon concentration gradient between the surface layer and the core region of the M50 steel, which influences the phase transformation behavior and the residual stress state in the surface layer during the tempering process. Minor carburizing increases the rotational bending fatigue limit of the steel by 16.7% and improves the tensile strength, yield strength, and impact absorption energy of the steel.

FEATURES OF MICROSTRUCTURE FORMATION IN THE WELDED PIPE JOINTS DURING HIGH-FREQUENCY WELDING AND SUBSEQUENT LOCAL HEAT TREATMENT

The transience of heating and cooling processes during high-frequency welding (HFW) of pipes leads to the formation in the heat-affected zone (HAZ) of a coarse-grained strained bainitic and/or martensitic structure with high hardness. Local or volumetric heat treatment is used to correct the microstructure of the HAZ. The paper presents the results of a study of the structure and properties of welded pipe joints produced at the Vyksa Steel Works. It has been established that the microstructure of the welded joint of pipes made of low-carbon steels (not more than 0.07% C) is mainly bainite of lath morphology, and for steel with a high content of carbon (~0.3%) and manganese ‒ martensite. To estimate the temperature field in terms of the angle and thickness of the pipe wall during local heat treatment a finite element model was created. The paper shows that the use of the model made it possible to optimize the heating parameters during the local heat treatment of pipes of various sizes. The experiments on local heat treatment of pipes with a diameter of 325 mm, made from rolled products 8 mm thick of steel 09Г2С, showed a significant refinement of the initial microstructure of the HAZ, an increase in uniformity and the disappearanc of structural heat-affected zones. In medium carbon steels (~0.3% C) with a high manganese content, the thermal cycle of HFC welding leads to formation of martensite. The local heat treatment in the form of normalization does not allow one to exclude the formation of martensite in the segregation zones due to the high cooling rate. The local heat treatment of welded joints in pipes made of steel 30Г (Russian Standard) in the high tempering mode (650–700 °C), with preliminary cooling of the welded joint to a temperature below the completion of the martensitic transformation (in segregation zones ~50 °C), makes it possible to release the segregation martensite and form the microstructure of sorbitol there.