Ferrite Martensite Research Articles

Investigation on strength and plasticity enhancement mechanism of 1180 MPa-grade quenching and partitioning steel

A comparative study on microstructures and mechanical properties of a low carbon Fe–Mn–Si steel with and without quenching pretreatment is performed to investigate the strength and plasticity enhancement mechanism. Microstructural analysis and 3D APT observation show that the lamellar microstructure obtained by introducing pre-quenching at 990 °C for 20 min prior to quenching and partitioning (Q&P) process mainly consists of ferritic laths, Mn-enriched tempered martensite and retained austenite with high mechanical stability. Mn enrichment in tempered martensite helps maintain a relatively high tensile strength. The refined alternative laths consisting of ferrite and tempered martensite share the same crystallographic orientation, which should be easier to hinder the crack propagation and delay the fracture. Compared to the polygonal-structured sample treated without quenching pretreatment, both higher frequencies of Σ3 CSL boundaries and higher volume fraction of stable retained austenite allow the pre-quenched sample exhibit an excellent strength-plasticity balance. The plasticity of the pre-quenched sample increases by nearly 90% and the product of strength and elongation (PSE) reaches higher than 24 GPa·%.

Effect of Microstructure and Surficial Oxidation on the Wear Behavior of an UNS S41003 Stainless Steel

The UNS S41003 ferritic stainless steel is a low‐carbon alloy that has great corrosion and oxidation performances in wet and aqueous environments, as well as it has high mechanical strength and ductility when compared to the most ordinary low‐carbon steels. These great characteristics, allied to its relatively low manufacturing cost, have made it a potential option to replace structural steels in many applications. Although it is a current trend, there are still few published studies that relate this steel manufacturing process with microstructural evolution and mechanical behavior, mainly regarding its wear behavior, which is a substantial and sometimes limiting characteristic for its applications. In this context, this article presents a pioneering study about the use of biphasic microstructures (ferrite–martensite) and controlled surficial oxidation to enrich the wear behavior of a UNS S41003 steel type. This study concludes that, if well planned, both the increase of martensite fraction and the controlled growth of an adherent and compact oxide layer on the steel surface significantly improve its wear performance, decreasing its wear rate up to 93%.

Improvements in welding continuous cooling transformation diagram of high-strength low-alloy steel weld

The welding continuous cooling transformation (CCT) diagram concerning microstructure evolution in a fine zone of the weld joint provides essential data for microstructure control in practice. In this study, a welding CCT diagram was improved to be elaborate by using the expanded lever rule. Processes for obtaining pieces of phase information are explained in detail. Improvements are: (i) phase areas are clearly presented, (ii) phase volume percentages are given, and (iii) overlapped phase areas are revealed. To meet high-strength requirement, a heat input of 21 kJ cm−1 was referenced from the welding CCT diagram, for achieving acicular ferrite and lath-like martensite. Transformed phase volume fractions are estimated and high strength and good toughness are obtained due to the elaborate phase information.

A low-cost strategy to improve strength-ductility-toughness balance in a low-carbon steel

ABSTRACT The current research introduces a low-cost strategy to improve strength-ductility-toughness balance in low-carbon steel containing 0.08 wt-% carbon by simple heat treatment. As the intercritical annealing temperature increased, the fraction of martensite enhanced. By increasing the annealing temperature from 770 to 830°C, the hardness of dual-phase steel was increased from 183 HV to 212 HV, which was due to the increment of martensite fraction from 0.28 to 0.49. The strength of all dual-phase steel samples produced by intercritical annealing treatment was higher than that of the initial sample owing to the presence of hard martensite in the soft ferrite. The interesting point was that the ductility of the dual-phase steels was almost the same as the initial sample. This led to a remarkable increase in the toughness of dual-phase steels compared to the initial sample. All dual-phase steels revealed a perfect ductile fracture.

Micromechanical Modeling of Strain‐Hardening Behavior in Dual‐Phase Steels

A mathematical model is developed to consider the impact of microstructural parameters, including the volume fraction and the average particle size of martensite, on the flow stress and strain‐hardening behavior of dual‐phase microstructure. In this regard, the micromechanical approach is applied for partitioning the stress and strain in ferrite and martensite. Martensite carbon content and geometrically necessary dislocations, generated from austenite‐to‐martensite transformation, and strain accommodation at the ferrite–martensite interface, are involved to modify the partitioned stress of martensite and ferrite, respectively. Having partitioned stress in each phase, the global stress is estimated as the function of steel chemical composition, ferrite grain size, martensite particle size, aspect ratio, and volume fraction. To evaluate the applicability of the proposed model, four dual‐phase steels containing 12, 25, 34, and 48% volume fractions of martensite are prepared from the intermediate quenching process, and then after the strain‐hardening stages are investigated. Comparing the experimental result and model output reveals that the presented model shows good predictive capabilities to identify strain‐hardening stages and estimate the inverse of the strain‐hardening exponent.

Physical Simulation and Numerical Simulation of Flash Butt Welding for Innovative Dual Phase Steel DP590: A Comparative Study.

In this study, the microstructure and performance of newly designed dual-phase steel (DP590) after joining by flash butt welding (FBW) for vehicle wheel rims was analysed and compared by two simulations, i.e., physical simulation and numerical simulation, due to the high acceptance of these two methodologies. Physical simulation is regarded as a thermal-mechanical solution conducted by the Gleeble 3500 simulator and which can distribute the heat-affected zone (HAZ) of the obtained weld joint into four typical HAZs. These are coarse-grained HAZ, fine-grained HAZ, inter-critical HAZ and sub-critical HAZ. A combination of ferrite and tempered martensite leads to the softening behaviour at the sub-critical HAZ of DP590, which is verified to be the weakest area, and influences the final performance due to ~9% reduction of hardness and tensile strength. The numerical simulation, relying on finite element method (FEM) analysis, can distinguish the temperature distribution, which helps us to understand the relationship between the temperature distribution and real microstructure/performance. Based on this study, the combination of physical and numerical simulations can be used to optimise the flash butt welding parameters (flash and butt processes) from the points of temperature distribution (varied areas), microstructure and performance, which are guidelines for the investigation of flash butt welding for innovative materials.

Austenite Nucleation and Growth as a Function of Starting Microstructure for a Fe–0.15C–5.56Mn–1.1Si–1.89Al Medium‐Mn Steel

The effects of starting microstructure and intercritical annealing temperature on the phase‐transformation kinetics and microstructural evolution of a prototype Fe–0.15C–5.56Mn–1.1Si–1.89Al medium‐Mn third‐generation advanced high‐strength steel are examined. The starting microstructures comprise 1) an as‐received cold‐rolled (CR) microstructure containing a significant fraction of ferrite and tempered martensite and 2) an austenitized and quenched martensite–ferrite (MF) microstructure. Based on the microstructural observations, two different scenarios for austenite formation during intercritical annealing are proposed. For the CR starting microstructure, austenite can nucleate at ferrite/cementite interfaces, and at ferrite grain boundaries. In the case of the MF starting microstructure, which contains thin films of interlath retained austenite (RA), austenite forms on the martensite lath boundaries or grows directly from the existing interlath RA. The studies are interpreted using the DICTRA module of Thermo‐Calc. At 665 °C, the austenite reversion kinetics of the MF samples are faster than the transformation kinetics in the CR structure. At higher temperatures where most of the cementite has dissolved, the austenite fraction in both starting microstructures is very similar.

A comparative analysis of the effect of laser surface treatment on the dry sliding wear behavior of ductile cast irons with different microstructures

In this study, the wear behavior of ferritic (GJS 400) and pearlitic (GJS 700) ductile cast iron (DCI) specimens after laser surface hardening were investigated comparatively. In general, multi-pass laser applications are inevitable in a laser surface treatment (LST) applied for surface hardening because the laser spot diameter is quite small compared to the applied surface area. This situation may cause a decrease in wear resistance due to residual stress formation because of the thermal gradient and softening effect due to the overlap ratio. In this study, a single-pass laser surface hardening (LSH) process with a laser pulse area of 20 × 5 mm2 compatible with the laser energy density (6.28 J/mm3) used in the literature, was applied to overcome these limitations. Thus, it is aimed to make it possible to evaluate the comparative analysis more reliably. After LSH processes, hardness values of DCI samples increased approximately 4.3 times compared to untreated ones due to the transformation hardening effect. In the microstructure of the laser-treated pearlitic DCI samples, martensite was present as the dominant phase in addition to a small amount of residual austenite. As for the ferritic DCI samples, the ledeburite and martensite phases were detected. Dry sliding wear tests were performed using different loads (5 N, 10 N) and sliding speeds (10 mm/s, 20 mm/s, 30 mm/s). As a result, the wear volume loss values of laser-treated ferritic and pearlitic DCI samples could be reduced by approximately 26.6% and 30.7%, respectively, compared to untreated ones. The COF values of the untreated samples increased with increasing load, while those of LSHed decreased. Severe plastic deformation and delamination were observed for untreated samples, while mild plastic deformation and micro-grooves were observed for samples with LSHed.

Microstructure development and mechanical properties of a C-Mn-Si-Al-Cr cold rolled steel subjected to quenching and partitioning treatment

The microstructural evolution and mechanical behavior of Fe-0.2C-2.7Mn-1.2Si-0.8Al-0.3Cr (wt.%) quenching and partitioning steel were studied. Partitioning was performed at 400, 440, and 480 °C for various durations from 30 to 300 s to obtain different fractions of retained austenite. Scanning electron microscopy, dilatometry, and X-ray diffraction analysis as well as uniaxial tensile testing were employed to characterize the microstructural evolutions and mechanical properties. The resulting microstructures were primarily composed of tempered lath martensite, bainitic ferrite, retained austenite, and blocky fresh martensite. The lower partitioning temperature of 400 °C was appropriate for efficient carbon partitioning and stabilization of retained austenite at room temperature. However, the retained austenite fraction decreased with increasing partitioning temperature, due to accelerated bainite formation and carbide precipitation at higher partitioning temperatures. The tensile results revealed that all specimens had tensile strength values of more than 1128 MPa and yield strength values of over 830 MPa. Additionally, prolonging the isothermal holding time had a negative effect on the yield strength due to bainite formation, while it enhanced the elongation. Fractography analysis showed that specimens treated at the partitioning temperature of 400 °C were fractured in a relatively ductile mode, but the fracture nature altered to an intergranular mode after partitioning at higher temperatures.

Enhanced strength-plasticity matching of lamellar 1 GPa-grade dual-phase steels via cyclic intercritical quenching

In this work, an innovative cyclic intercritical quenching process has been proposed for achieving excellent mechanical properties in terms of strength and plasticity, which results in a lamellar martensitic-ferritic dual-phase steel. The microstructural evolution under cyclic heat treatment was thoroughly investigated, and the mechanism that enables the lamellar structure to enhance its strength-plasticity matching was also explored. The experimental results revealed that the incomplete austenitization process inhibits the coarsening of the reversed austenite, and the cyclic intercritical quenching procedure finer and more evenly distributes the reversed austenite. The single intercritical quenching (C1) generated mostly massive martensite with traces of ferrite, whereas three-cyclic intercritical quenching (C3) yielded a fine and uniformly distributed lamellar structure. A comparison in terms of the mechanical properties between C3 and C1 steels indicates that uniform elongation, total elongation, and the product of strength and uniform elongation of the C3 steel increased by 85.4%, 47.3%, and 65.0%, respectively, while ultimate tensile strength was dropped by only 11.0%. In addition, the yield ratio of C3 steel is just 0.48. The slight decrease in strength is due to the fine lamellar microstructure. The high plasticity is attributed to the good deformation compatibility between lamellar ferrite and lamellar martensite with excellent plastic deformability. Therefore, cyclic mechanical heat treatment is very promising for the production of low-cost dual-phase steels.

Effect of Tempering Process on Microstructure and Properties of Resistance Spot-Welded Joints of δ-TRIP Steel

In this paper, a medium-frequency inverter spot welder was used for resistance spot-welding experiments on 980 MPa grade cold-rolled δ-TRIP(Transformation-induced Plasticity) steel. The effects of the tempering process on the morphology, microstructure, element distribution, and properties of spot-welded joints were studied by Scanning Electron Microscope (SEM), Transmission Electron Microscopy (TEM), and Electron-Probe MicroAnalysis (EPMA). The microstructure of the nugget zone obtained by single-pulse process was δ ferrite, lath martensite, and twin martensite. After adding tempering under the single-pulse process, the microstructure was δ ferrite and lath martensite. However, the morphology of the microstructure was still dendritic, which remained unchanged. The tensile shear failure of spot-welded joints under the two processes was an interface failure, and the fractures were cleavage fractures. After adding tempering, the interface fracture surface presents two kinds of fracture characteristics: the outer cracks’ growth direction was consistent with the columnar crystal growth direction, and the inner crystal cracks occurred in the nugget core and finally grew along the columnar grain boundary. Due to the significant hardness difference between δ ferrite (283 HV) and martensite (533 HV), the low-strength δ ferrite phase was the main channel of crack propagation. After adding tempering, the hardness distribution of the spot-welded joints was more uniform and the tensile shear force increased (7.4 kN→8.5 kN).

Interfacial gradient M7C3 carbides precipitation behavior and strengthening mechanisms of stainless steel/carbon steel clad plates

The mechanical incompatibilities between ferrite and martensite at the cladding interface of stainless steel (SS)/carbon steel (CS) clad plates can result in stress concentration and interface cracks. In this paper, 304 SS/Q235 CS clad plates were prepared by liquid-solid (L-S) composite casting, and the mechanical incompatibilities between ferrite and martensite reduced by constructing gradient carbides in the interfacial ferrite. The precipitation behavior of the gradient carbides was analyzed, and the strengthening mechanism was revealed. The ultimate strength of the SS/CS clad plates was 370 MPa when stretched perpendicular to the cladding interface, and the SS/CS clad plates fractured in the CS that was far from the cladding interface. The interfacial microstructures from the CS to the SS were ferrite, M7C3 carbides, and lath martensite, and the carbides content gradually decreased toward the CS. During the L-S composite process, a micro-melting region with decreased volume fraction of interdendritic liquid to the CS formed at the cladding interface. The C, Cr, and Ni mixed and diffused in the interdendritic liquid, and the gradient distribution of elements resulted in the precipitation of coherent M7C3 carbides whose volume fraction decreased toward the CS. The coherent strengthening and precipitation strengthening of carbides strengthened the interfacial ferrite and resulted in a gradient strengthening, which reduced the mechanical incompatibilities between ferrite and martensite at the cladding interface. Additionally, the gradient carbides changed the stress state at the cladding interface and improved the interfacial bonding strength.

Yielding behavior of triplex medium Mn steel alternated with cooling strategies altering martensite/ferrite interfacial feature

• Solute segregation at various interfaces characterized atomically in triplex medium Mn steel. • Two cooling strategies produced different martensite/ferrite interfaces for distinct yielding behavior. • Density and mobile ability of dislocations near martensite/ferrite interfaces determine yielding behavior. In this paper, we report the influence of cooling processes on the yielding behavior of a medium Mn steel (MMS) with triplex microstructure, i.e. austenite (γ), ferrite (α) and as-quenched martensite (α'). After the intercritical annealing (IA) at both 725 ℃ and 750 ℃, the steel was subjected to the two cooling processes, i.e. air cooling (AC) and water quenching (WQ). It exhibits the discontinuous yielding after the AC following the IA at 750 ℃ while the continuous yielding after the WQ. Compared with WQ process, both the dilatometry and the microstructural examinations show that the AC process leads to lower M s temperature, larger retained austenite (RA) fraction and less martensite, the latter is always companied with geometry necessary dislocations (GNDs) generated near the α/α' interfaces. Considering the complexity of nanosized tri-phases in this steel, the presence of martensite with key features in the resultant specimens was systematically examined by atom probe tomography (APT) on the samples prepared by the specific target lift-out method. The APT results directly revealed the C/Mn co-segregation at the α'/α interfaces in the AC samples but not in WQ samples. The numerical simulation results further suggest that the segregation of C and Mn at the α'/α interfaces may be due to different mechanisms. We conclude that the yielding of triplex MMS is determined by both the quantity of GNDs generated near the α/α' interfaces, which increases with martensite fraction, and the extent of their immobilization resulting from the interfacial segregation of solute atoms when the presence of martensite is sufficient. WQ tends to suppress the discontinuous yielding of MMS since the rapid cooling may promote more martensite formed with the increasing quantity of GNDs and prevent the interfacial segregation of both C and Mn.

Development of Low-Alloyed Low-Carbon Multiphase Steels under Conditions Similar to Those Used in Continuous Annealing and Galvanizing Lines

In the present work, a Cr+Mo+Si low-alloyed low-carbon steel was fabricated at laboratory scale and processed to produce multiphase advanced high-strength steels (AHSS), under thermal cycles similar to those used in a continuous annealing and galvanizing process. Cold-rolled steel samples with a microstructure constituted of pearlite, bainite, and martensite in a matrix ferrite, were subjected to an intercritical annealing (817.5 °C, 15 s) and further isothermal bainitic treatment (IBT) to investigate the effects of time (30 s, 60 s, and 120 s) and temperature (425 °C, 450 °C, and 475 °C) on the resulting microstructure and mechanical properties. Results of an in situ phase transformation analysis show that annealing in the two-phase region leads to a microstructure of ferrite + austenite; the latter transforms, on cooling to IBT, to pro-eutectoid ferrite and bainite, and the austenite-to-bainite transformation advanced during IBT holding. On final cooling to room temperature, austenite transforms to martensite, but a small amount is also retained in the microstructure. Samples with the lowest temperature and largest IBT time resulted in the highest ultimate tensile strength/ductility ratio (1230.6 MPa-16.0%), which allows to classify the steel within the third generation of AHSS. The results were related to the presence of retained austenite with appropriate stability against mechanically induced martensitic transformation.

Strengthening of Low Carbon Steel by Nano-sized Vanadium Carbide in Ferrite and Tempered Martensite

Strengthening of Low Carbon Steel by Nano-sized Vanadium Carbide in Ferrite and Tempered Martensite

Effect of ferrite grain shape in friction element welds on cross tension strength

High-strength steels (HSSs) have been increasingly used in car bodies in order to simultaneously achieve the weight reduction and high collision safety of vehicles. In resistance spot welding, which is widely used for joining car bodies, low cross tension strength (CTS) of joints using HSSs is a problem. In our previous study, we focused on friction element welding (FEW) of HSSs to improve CTS and reported that the poor local ductility of the area quenched from two-phase temperature region (inter-critically annealed and quenched area) due to the presence of soft ferrite (α) and hard martensite (M) causes a decrease in CTS of FEW. The local ductility of steels with dual phase structure is known to be affected by the microstructural morphology. Therefore, in this study, we prepared joints of FEW with both the equiaxial α (C30) and the acicular α (C30L) in dual phase structure by controlling parent structures prior to the inter-critical annealing and quenching during FEW. C30L exhibited the higher CTS than C30. C30L fractured along the thickness direction of the lower sheet from the inter-critically annealed and quenched area near the edge of the joint to the softened area of heat affected zone, while C30 fractured along the inter-critically annealed and quenched area; however, the dimples were observed on the fracture surfaces irrespective of materials. Taking into account the fact that C30 and C30L showed no difference in the hardness distribution of the joints and the hardness difference between α and M hardness, it was suggested that morphology of α plays an important role in determining CTS. It is inferred that the local strain concentration in the inter-critically annealed and quenched area is relaxed by the presence of fine acicular α, which leads to the improvement of the local ductility, i.e. CTS.

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.

Effect of microstructure on wear and rolling contact fatigue behaviors of bainitic/martensitic rail steels

Two heat-treatment routes of bainite austempering, namely above-Ms austempering and below-Ms austempering, were applied on a low carbon rail steel. The obtained multiphase microstructures containing bainitic ferrite, martensite and retained austenite were quantitatively characterized by dilatometry, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. We investigated the influences of multiphase microstructure on the wear and rolling contact fatigue (RCF) behaviors in terms of weight loss, worn surface, microstructural evolution, and crack initiation. Results showed that the below-Ms austempered samples exhibit a less wear weight loss and a controllable RCF damage, compared to above-Ms austempered counterparts. The improved wear and RCF resistances of below-Ms austempered samples are attributed to the refined multiphase, increased mechanical stability of film-like retained austenite, as well as good deformation compatibility between bainitic ferrite and ductile martensite. Meanwhile, the correlations between microstructural evolution and crack initiation sites were established to discuss the competition between surface and subsurface crack initiation upon rolling contact.

Influence of Different Heat Treatment Temperatures on the Microstructure and Corrosion Behaviour of Dual-Phase EN8 Steel in 0.5 M Sulphuric Acid Solution

Dual-phase steels provide an excellent combination of strength and ductility, as well as improved energy absorption and anti-corrosion protection properties. This research aims at evaluating the microstructure and corrosion behaviour of EN8 steel under different heat treatment temperatures in 0.5 M sulphuric acid solution (H2SO4) using the EIS, potentiodynamic polarization, and gravimetric method (weight-loss method). Austenitizing is performed at 973 K, 1023 K, 1063 K, and 1173 K for 2 h followed by quenching in water to form a ferrite–martensite (F–M) dual-phase structure. From the results, it is seen that the corrosion rate increased with different heat treatment conditions depending on the change in the phase when immersed in 0.5 M H2SO4 at the temperature of 303 K, 313 K, 323 K, and 333 K. This work investigates the energy of activation, enthalpy, and entropy of activation. For dual-phase steel containing ferrite and martensite, the corrosion behaviour depends on the amount of martensite and ferrite. As the austenitization temperature increases from 1023 to 1173 K, the amount of martensite increases. This is reflected in the increase of micro galvanic corrosion cells in the region between the ferrite and martensite phases, which acts as active corrosion centres. The normalized specimen showed greater corrosion resistance compared to the water-quenched specimen at 1173 K. This is due to the presence of lower carbon content for normalized dual-phase steel containing ferrite–pearlite phase than the ferrite–martensite phase present in specimen austenitized at 1173 K. Surface characterization and XRD confirmed the corrosion behaviour of the specimens under investigation.

Effect of microstructure homogeneity on the impact fracture mechanism of X100 pipeline steel laser–MAG hybrid welds with an alternating magnetic field

Laser–metal active gas (MAG) hybrid welding has broad application prospects in pipeline steel welding because of the advantages of high penetration and efficiency. Because very little of the wire and shielding gas will reach the bottom of the molten pool, inhomogeneous microstructures will appear in the thickness direction of laser-MAG hybrid welds. In this study, we introduced an alternating magnetic field (AMF) into laser-MAG hybrid welding to study the effect of microstructure homogeneity on the impact fracture mechanism of X100 pipeline steel laser-MAG hybrid welds. The results illustrated that the arc and laser zones of the welds without an alternating magnetic field (WAMF) were composed mainly of acicular ferrite (AF) and lath martensite (LM), respectively. The distribution of inclusions in the thickness direction of the weld became homogeneous, and a homogeneous microstructure composed of AF and LM was obtained in the AMF weld. Among the WAMF and AMF welds, the laser zone of the WAMF weld had the lowest impact toughness due to the low density of the effective grain boundaries and high kernel average misorientation value. The difference in the impact toughness between the laser and arc zones of the WAMF weld led to a difference in the fracture mode. In addition, the impact fracture mechanism of the AF and LM mixed microstructures was investigated, which showed that AF provided a basis for crack deflection and void formation because of its highly effective grain boundary density. The large-angle deflection of the main crack, secondary cracks in the prior propagation direction, and the combined action of plastic deformation and voids improved the impact energy. The results of this study provide guidance for the study of fracture processes in heterogeneous microstructures.