Ferrite-martensite Interface Research Articles

Microvoid Formation of Ferrite-martensite Dual-phase Steel via Tensile Deformation after Severe Plastic Shear-deformation

One of the objectives for the development of high-strength dual-phase (DP) steel is improving the stretch-flangeability. Large-strained sheared edges are deformed and frequently cracked during stretch-flange formation. Considering shearing as the first deformation, the stretch-flange deformation may be regarded as a secondary deformation. To improve the stretch-flangeability of the DP steels, many researchers have analyzed the microvoid formation. However, in these analyses, the shearing process was not considered. With this background, ex-situ mini-bending tests combined with scanning electron microscopy (SEM) monitoring of microvoid formation were conducted during the secondary deformation. Prior to the secondary deformation, several microvoids were observed on the sheared surface and fine subgrains formed in the ferrite. During secondary deformation, the preliminary microvoids present at the ferrite-martensite interface propagated into the ferrite phase. In contrast, this behavior was not observed for the reamed surface deformation, which was formed without preliminary deformation. Furthermore, microvoids were initiated on ferrite grains that were not present at the ferrite-martensite interface, and martensite islands were not cracked during secondary deformation. This result is noteworthy because martensite cracking was the main factor involved in microvoid initiation, in the absence of shearing. Electron backscattering diffraction analysis revealed that the work hardening of ferrite, prior to the secondary deformation, caused a deviation in the strain concentration sites from those found in the reamed surface deformation. Therefore, this study elucidated microvoid formation on preliminary deformed surfaces via shearing and provided insights for material development considering deformations on the sheared surfaces of materials.

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.

Analysis of Nano-hardness Distribution Near the Ferrite-martensite Interface in a Dual Phase Steel with Factorization of Its Scattering Behavior

Analysis of Nano-hardness Distribution Near the Ferrite-martensite Interface in a Dual Phase Steel with Factorization of Its Scattering Behavior

Effect of Tempering on Stretch-Flangeability of 980 MPa Grade Dual-Phase Steel

In this study, the effect of tempering on the stretch-flangeability is investigated in 980 MPa grade dual-phase steel consisting of ferrite and martensite phases. During tempering at 300 oC, the strength of ferrite increases due to the pinning of dislocations by carbon atoms released from martensite, while martensite is softened as a consequence of a reduction in its carbon super-saturation. This strength variation results in a considerable increase in yield strength of the steel, without loss of tensile strength. The hole expansion test shows that steel tempered for 20 min (T20 steel) exhibits a higher hole expansion ratio than that of steel without tempering (T0 steel). In T0 steel, severe plastic localization in ferrite causes easy pore formation at the ferrite-martensite interface and subsequent brittle crack propagation through the highly deformed ferrite area during hole expansion testing; this propagation is mainly attributed to the large difference in hardness between ferrite and martensite. When the difference in hardness is not so large (T20 steel), on the other hand, tempered martensite can be considerably deformed together with ferrite, thereby delaying pore formation and hindering crack propagation by crack blunting. Eventually, these different deformation and fracture behaviors contribute to the superior stretch-flangeability of T20 steel.

A new 1.2 GPa-strength plain low carbon steel with high ductility obtained by SRDR of martensite and intercritical annealing

In the current research, a new technique to produce ultra-high strength ferrite-martensite dual phase (DP) steels with high ductility was developed for a low-cost plain low carbon steel with 0.16 wt% C. A combination of austenitization, water quenching, single-roll drive rolling (SRDR), and intercritical annealing at 830 °C for three different holding times was used. The microstructure and mechanical properties of DP samples were investigated. It was found that the martensite islands were very uniformly distributed in both RD–TD and RD–ND planes of DP steels due to performing the SRDR process. The average sizes of the martensite islands for the holding times of 1 (DP1), 5 (DP5), and 15 (DP15) min were 7.1, 11.6, and 15.8 μm, respectively. Also, the volume fraction of martensite was about 0.29, 0.47, and 0.68 for the DP1, DP5, and DP15 steels, respectively. With increasing the annealing time, the hardness, strength, and ductility increased. The DP15 showed a superior hardness (354.1 H V) strength (1172.5 MPa) and high ductility (13.5%). The martensite phase in the DP15 steel exhibited remarkable plastic deformation because its hardness decreased by reducing the carbon content and therefore the strain hardening ability of the martensite phase increased. Achieving superior strength with high ductility in the DP15 was attributed to the uniform distribution of martensite caused by the SRDR process. As the holding time increased to 15 min, two stages of strain hardening transformed to one stage owing to increasing the ability of strain hardening of martensite with lower carbon content. All DP steels mainly consisted of dimples indicating the ductile fracture, however, some cleavage facets were observed on the fracture surfaces. In DP1 and DP5 steels voids formed mainly by decohesion at the ferrite-martensite interface. With increasing the holding time to 15 min, the severity of decohesion decreased due to increasing the critical strain required for the formation of voids through interface decohesion.

Analysis of preliminary local hardening close to the ferrite–martensite interface in dual-phase steel by a combination of finite element simulation and nanoindentation test

This study investigated the local preliminary hardening of ferrite near ferrite–martensite interfaces in dual-phase (DP) steel. Geometrically necessary dislocations (GNDs), generated in the case of interfacial misfit between different phases, may cause preliminary hardening around such interfaces. Firstly, finite-element simulations of nanoindentations were conducted using an ideal cylindrical DP model without preliminary hardening around the ferrite–martensite interface. The simulations indicated gradually decreasing nano-hardness with increasing distance from the ferrite–martensite interface. Second, nano-hardness measurements of ferrite near the ferrite–martensite interface were performed for laboratory-made DP steel containing 10 vol.% martensite. The measurements were consistent with the simulation without preliminary hardening around the interface. That is, interfacial preliminary hardening of ferrite was not indicated by our measurements. The GND density, which has been reported to cause preliminary hardening around the interface, supported this result. The density on ferrite was increased around the interface, but the area was very small compared with the nanoindentation size. We conclude that another hardening mechanism, e.g., GND accumulation under a high strain gradient around the interface, acts as a microstructural local strengthening factor in DP steel deformation.

An RVE-Based Study of the Effect of Martensite Banding on Damage Evolution in Dual Phase Steels.

The intent of this work is to numerically investigate the effect of second phase morphology on damage evolution characteristics of dual-phase (DP) steels. A strain gradient enhanced crystal plasticity framework is used in order to capture the deformation heterogeneity caused by lattice orientations and microstructural size effects. The investigation is focused on two different martensite distributions (banded and random) that are relevant for industrial applications. The effects of martensite morphology are compared by artificially generated 2D plane strain microstructures with initial void content. The Representative volume elements (RVEs) are subjected to tensile deformation imposed by periodic boundary conditions. Evolution of voids are analyzed individually as well as a whole and characterized with respect to average axial strain. It is found that during stretching voids exhibit varying evolution characteristics due to generation of inhomogeneous strain fields within the structure. The behavior of individual voids shows that the stress-state surrounding the void is different from the imposed far field macroscopic stress-state. The voids at the ferrite martensite interface and in ferrite grains of the randomly distributed martensite grow more than in the banded structure. On the other hand, voids formed by martensite cracking growth shows an opposite trend.

Analysis of Nano-hardness Distribution Near the Ferrite-martensite Interface in a Dual Phase Steel with Factorization of Its Scattering Behavior

Analysis of Nano-hardness Distribution Near the Ferrite-martensite Interface in a Dual Phase Steel with Factorization of Its Scattering Behavior

Enhancing plasticity by increasing tempered martensite in ultra-strong ferrite-martensite dual-phase steel

The present study focuses on the influence of tempered martensite (TM) on the mechanical properties and fracture mechanisms of an ultra-strong dual-phase (DP) steel. The steel was subjected to intercritical annealing combined with an aging process. The high fraction of martensite (above ∼70 vol%) results in a high strength level above 1300 MPa, and the presence of TM ensures a good ductility up to ∼10% total elongation. The microstructures of the tested steels were analyzed by the scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Results showed that a higher fraction of TM significantly improves ductility, mainly due to its beneficial effects on damage tolerance. More specific, the damage behavior alters from martensite cracking to ferrite-martensite interface decohesion, with increasing TM fractions. This results in higher post uniform elongation (PUE) values. With the increase of intercritical annealing temperature, the yield strength (YS) increases, but both the ultimate tensile strength (UTS) and strain hardening rate decrease. The strain hardening rate was discussed based on the influence of carbide precipitates and decreasing geometrically necessary dislocation (GNDs). Besides, we found that the aging process could significantly increase the volume fraction of TM and improve the plasticity.

Influence of Vanadium on Microstrain Partitioning and Evolution of Microstructural Damage in DP1300 Steel

We have investigated the effect of vanadium addition on a DP1300 ferrite-martensite steel, particularly in terms of microstructure, deformation, and damage behavior at the microscopic level. DP steels with and without vanadium addition were given slightly altered heat treatments to produce similar martensite content and ultimate tensile strength in both steels. The addition of 0.14wt% vanadium to the reference alloy yields a dispersion of nano-scale vanadium carbonitrides throughout the microstructure along with a pronounced grain refinement. Microscopic digital image correlation (μDIC) was employed to analyze the local strain partitioning between ferrite and martensite at the grain level. It was found that the addition of V increases the mechanical compatibility between ferrite and martensite. The consequent enhanced mechanical homogeneity and reduced strain gradients at the ferrite-martensite interface suppresses damage, thus enhancing ductility of the V-added alloy. Interrupted tensile tests coupled with X-ray computed tomography confirm that microstructural damage growth is slower in the V-added steel during post-uniform elongation prior to fracture.

ひずみ付与したフェライト−マルテンサイト二相鋼における水素捕捉サイトの同定

There are risks of hydrogen embrittlement in ultra-high strength steels. We need to clarify the mechanism of hydrogen embrittlement and to develop steels which have superior hydrogen embrittlement resistance. Hydrogen is trapped at various trapping sites in steels. The influence of hydrogen on hydrogen embrittlement depends on the hydrogen trapping sites. Therefore, it is important to identify the kinds of hydrogen trapping sites in steels. The purpose of this study is to identify the hydrogen trapping sites in ultra-high strength steel sheets. 1180 MPa grade dual-phase steel was used. Various strain were applied to the samples by rolling, which was followed by cathodic electrolytic hydrogen charging. Hydrogen desorption rate was measured from –50 ºC using a Thermal Desorption Analysis device (TDA), which enables evaluation of each hydrogen trapping site. The TDA results were analyzed with using Gaussian function to identify each hydrogen trapping site. Four types of hydrogen trapping sites were identified in the DP steel. Hydrogen desorption peak appeared at 35°C was assigned to dislocations, peak appeared at 54°C was assigned to carbide in the martensite structure of the DP steel, Peak at 75°C was assigned to various boundaries in martensite and ferrite-martensite interface in the DP steel, and Peak at 110°C was assigned to a vacancy cluster.

Effect of ferrite-martensite interface morphology on bake hardening response of DP590 steel

The effect of martensite spatial distribution and its interface morphology on the bake hardening characteristics of a dual phase steel was investigated. In one case, typical industrial continuous annealing line parameters were employed to anneal a 67% cold rolled steel to obtain a dual phase microstructure. In the other case, a modified annealing process with changed initial heating rates and peak annealing temperature was employed. The processed specimens were further tensile pre-strained within 1–5% strain range followed by a bake hardening treatment at 170°C for 20min. It was observed that industrial continuous annealing line processed specimen showed a peak of about 70MPa in bake-hardening index at 2% pre-strain level. At higher pre-strain values a gradual drop in bake-hardening index was observed. On the contrary, modified annealing process showed near uniform bake-hardening response at all pre-strain levels and a decrease could be noted only above 4% pre-strain. The evolving microstructure at each stage of annealing process and after bake-hardening treatment was studied using field emission scanning electron microscope. The microstructure analysis distinctly revealed differences in martensite spatial distribution and interface morphologies between each annealing processes employed. The modified process showed predominant formation of martensite within the ferrite grains with serrated lath martensite interfaces. This nature of the martensite was considered responsible for the observed improvement in the bake-hardening response. Furthermore, along with improved bake-hardening response negligible loss in tensile ductility was also noted. This behaviour was correlated with delayed micro-crack initiation at martensite interface due to serrated nature.

Fatigue performance of dual‐phase steels for automotive wheel application

AbstractFatigue performance of ferrite–martensite (FM) and ferrite–bainite (FB) dual‐phase (DP) steels used in automotive wheels has been compared in terms of (i) high‐cycle fatigue performance and failure mechanisms and (b) low‐cycle fatigue performance (Δεt/2 = 0.002 to 0.01) and associated deformation mechanisms. FBDP steel exhibits moderately better high‐cycle fatigue performance, owing to delay in microcrack initiation. In FBDP steel, microcracks initiate predominantly along ferrite grain boundaries, while that at FB interface is significantly delayed in comparison with FMDP steel, where few microcracks appear at FM interface even below the endurance limit. During low‐cycle fatigue, however, FMDP steel performs considerably better than FBDP steel till Δεt/2 ≤ 0.005 attributed to initial cyclic hardening, followed by cyclically stable behaviour exhibited by FMDP steel. In sharp contrast, at all Δεt/2 > 0.002, FBDP steel undergoes continuous cyclic softening. The latter may cause undesirable deformation of wheels in service.

Quantification of ferrite-martensite interface in dual phase steels: A first-principles study

The ferrite-martensite interfacial energy and equilibrium interfacial length as a function of martensite carbon content are assessed using first-principles atomistic simulations. The weight percent of carbon in the martensite phase was implicitly varied from 0.6 to 1.8 wt percent by modifying the lattice constant of body-centered tetragonal (BCT) martensite according to Kurdjumov and Kaminsky’s empirical expressions. With increasing carbon content, a decrease is found in both the interfacial energy and in the equilibrium distance between ferrite and martensite interfaces. Moreover, the Morse interatomic potentials between the atoms in the ferrite-martensite interface for four different martensite carbon contents are calculated, and the parameters of the Morse potential are correlated linearly with the martensite carbon content. In addition, the dissociation local strains during uniaxial loading in a direction normal to the interfacial plane are calculated from the interatomic potentials. The local strain at the interface needed for ferrite-martensite interface separation increases with increase in martensite carbon content. The fitted expressions can be used to predict the ferrite-martensite interfacial energy, equilibrium interfacial distance, dissociation local strain at the interface, and the Morse parameters as functions of martensite carbon content within the range of 0.6–1.8 wt percent. Furthermore, the introduced implicit method can potentially be used to study the mechanical properties of other materials with dopant impurities such as n-type and p-type semiconductors.

Micromechanical analysis of martensite distribution on strain localization in dual phase steels by scanning electron microscopy and crystal plasticity simulation

The morphology and distribution of the dispersed martensite islands in the ferrite matrix plays a key role in the formation of shear bands in dual phase steels. In this study, we investigate the relationship between the martensite dispersion and the strain localization regions due to the formation of shear bands in fine-grained DP 780 steel, employing experimental observations as well as numerical simulations. SEM studies of the deformed microstructure showed that voids nucleated at ferrite-martensite interface within larger ferrite grains and regions with low local martensite fraction. The experimental results were precisely analyzed by finite element simulations based on the theory of crystal plasticity. A parametric study was then performed to obtain a deeper insight in to the effect of martensite dispersion on the strain localization of the neighboring ferrite. Crystal plasticity simulation results revealed that in a more regular structure compared to a random structure, a greater region of the ferrite phase contributes to accommodate plasticity. In addition, these regions limit the formation of main shear bands by creating barriers against stress concentration regions, results in lower growth and interaction of stress concentration regions with each others.

Metallurgical Analysis of a Failed 17-4 PH Stainless Steel Pyro Bolt Used in Launch Vehicle Separation Systems

Abstract Precipitation hardenable stainless steels are widely used in aerospace applications because of their excellent combination of strength and corrosion resistance. In view of this advantage, Fe-17Cr-4Ni (17-4 PH) steel is used in pyro bolts in space ordnance systems for stage separation in a satellite launch vehicle. These systems typically consist of a pyro charge to sever a given material resulting in stage separation, and these materials are exposed to high strain rates. In one such application, although perfect severance took place, secondary cracks were noticed longitudinally, which are not desirable. This steel, in view of its composition, has delta ferrite in a martensitic matrix in the hardened and tempered condition. In wrought products worked unidirectionally, delta ferrite gets oriented along the direction of thermomechanical processing. The effect of delta ferrite orientation on the impact properties of 17-4 PH stainless steel was studied in various heat-treatment conditions with a view to understand the effect of microstructure on impact properties. It was found that the transverse impact properties are severely affected by the presence of highly oriented delta ferrite stringers for any given heat-treatment condition. This has been attributed to the weak delta ferrite–martensite interface, which facilitates easy crack propagation. Finally, it was suggested that the anisotropy in impact properties need to be carefully considered in the design of components for applications that involve high strain rates.

EFFECT OF STRAIN RATE ON DYNAMIC DEFORMATION BEHAVIOR OF DP780 STEEL

Tensile properties and deformation behavior of DP780 steel were studied using servo-hydraulic high-speed material testing machine,SEM and TEM.The effects of strain rate and the mechanism were investigated.The results showed that the strength and ductility of DP780 steel remained almost unchanged as the strain rate increased at strain rates lower than 10°s~(-1). When the strain rate was over 10~1 s~(_1),the strength and the strain-hardening coefficient increased remarkably.Ductility of DP780 steel increased significantly at the strain rates ranging from 3×10~1 to 5×10~2 s~(_1).The deformation resistance increased with increasing the strain rate due to the stronger short range resistance induced by the acceleration of dislocation motion in the ferrite matrix. Increasing strain rate up to 3×10~1 s~(_1) resulted in a considerable increase of the amount of mobile dislocation,which was the main reason for the increasing uniform elongation and fracture elongation of DP780 steel at the strain rate ranging from 3×10~1 to 5×10~2 s~(_1).Interface of ferrite-martensite in DP780 steel was the main location for pile-up of dislocation,crack initiation and propagation. The ability of inhomogeneous plastic deformation of DP780 steel increased due to the decreasing plastic strain energy difference between the ferrite matrix and ferrite-martensite interface and the consequent delaying initiation and propagation of microvoids at ferrite-martensite interface induced by the increasing work hardening degree of ferrite matrix with the increasing strain rate.

Monitoring of the deformation and fracture process of dual phase steels employing acoustic emission techniques

Abstract In this paper, continuing our previous works, a new approach for detection of fracture micro mechanisms of ferrite–martensite dual-phase steels (DPSs) with various microstructures was investigated. For this purpose, dual phase steels with different volume fractions of martensite (VM) were produced by various heat treatment methods on a low carbon steel (0.1% C), and acoustic emission (AE) monitoring was then used during tensile testing of these DPSs. The AE signals from a tensile test using DPS in the range of 12–73% VM and various morphologies, like equiaxed or fibrous martensite phase, were captured. Principally, to understand the AE response and behavior of the martensite or ferrite phase separately, some samples of martensite and heat treated ferrite were tested. After the tests, by utilizing a new function named “sentry function”, we tried to relate the AE signals to various failure mechanisms of these steels. In confirmation of our earlier works, the results show that AE monitoring and sentry function are efficient tools to detect failure micromechanisms, consisting of ferrite–martensite interface decohesion and/or martensite phase fracture, identifying the correlation of failure mechanisms to microstructure in DPS. The results were verified with scanning electron microscopic observations and they indicate that AE monitoring is an efficient tool to detect micromechanisms identifying failure in DPSs.

Evaluation of Geometrically Necessary Dislocations Density (GNDD) near Phase Boundaries in Dual Phase Steels by Means of EBSD

The accumulation of dislocations around hard particles such as martensite in Dual Phase steel has a prominent influence on the mechanical properties of multiphase steels. The origin of these so-called Geometrically Necessary Dislocations (GNDs) is either due to the transformation strain, or to strain gradients that arise during deformation. The generation of deformation-GNDs is explained by Ashby’s theory [1] regarding deformation of a plastic mass that contains dispersed undeformable particles. It is argued that the GNDs pile up locally against the ferrite-martensite interface. This work reports the calculated density of GNDs from high resolution Electron BackScatter Diffraction (EBSD) measurements. By measuring the lattice orientation within the grains, the lattice curvature can be quan-tified, which can be directly related to the presence of GNDs. The density of the GNDs can then be estimated either directly through kernel average misorientations, or through the calculation of the dislo-cation tensor. From this first approximation of the GND density a GNDD map has been obtained by two recently developed approaches. This map shows an enhanced dislocation density around the mart-ensite particles due to volume change during transformation. The kernel choice and step size depend-ency of the results are also investigated.

Influence of ferrite-martensite microstructural morphology on tensile properties of dual-phase steel

The influence of ferrite-martensite microstructural morphology, volume fraction of martensite, epitaxial ferrite on the tensile behaviour of dual-phase steels, was studied. It was observed that increasing the martensite content and its aspect ratio raised tensile strength and ductility. Epitaxial ferrite in rolled material strongly reduced the strength and improved the ductility, suggesting that substructure strengthening of material, as well as increased stress transfer to the hard phase, contribute to the strength of thermo-mechanically processed material. Mettallographic analysis of deformed samples revealed that void nucleation occurs predominantly along the ferrite-martensite interface. The void density in the necked region increased towards the fracture surface in all samples and was higher for samples which exhibited localized necking.