Partitioning Steel Research Articles

Characterization of physical metallurgy of quenching and partitioning steel in pulsed resistance spot welding: A simulation-aided study

This study mainly focuses on the microstructure evolution of QP980 steel during resistance spot welding and its influence on the mechanical performance of resistance spot welds which is a critical influencing factor on the quality of body-in-white at the service condition. It is observed that the thermomechanically engineered microstructure of QP980 steel changes to form metastable phases such as martensite in the fusion zone and the heat affected zone due to rapid cooling induced by the thermal cycle of the welding. A finite element modeling of the welding process was used to predict the weldment heat distribution, thermal history and microstructure evolution in different welding zones. The modeled thermal history of the weldments shows that the peak temperature in the four-pulse resistance spot welding is delayed because of pulsed welding conditions and holding times between the welding pulses. This heat management approach in pulsed welding prevents void formation. The modeled thermal history and rapid heating, and cooling conditions are discussed here to predict the microstructure evolution and transformation in the fusion and reheated zones. The modeled results were helpful in the prediction of the microstructure at different weld zones. Then the strategic links between the microstructure and mechanical performance of the welded alloy are discussed thoroughly. The microhardness profile of the weld is discussed from a microstructural point of view to disclose the physical metallurgy of the welds. Softening phenomena were not observed in the sub-critical heat affected zone.

Influence of Quenching and Partitioning Times on Austenite Stability and Tensile Properties of CMnAlSi Quenching and Partitioning Steel

Influence of Quenching and Partitioning Times on Austenite Stability and Tensile Properties of CMnAlSi Quenching and Partitioning Steel

Analysis of hydrogen-induced cracking mechanism in quenching and partitioning steels

Quenching and partitioning (Q&P) steels are extensively used in the automotive industry due to their exceptional strength and ductility. However, the impact of hydrogen embrittlement (HE) poses a huge challenge to the safe service of Q&P steels, highlighting the necessity to investigate the hydrogen-induced cracking mechanisms. This study investigates the hydrogen-induced failure mechanisms in Q&P steels through experiments and finite element simulations, focusing on microstructure, local stress heterogeneity, hydrogen distribution characteristics, and hydrogen-induced cracking. The results reveal that hydrogen cracking nucleation in Q&P steel is primarily due to the further accumulation of hydrogen in local high-stress martensitic/austenitic regions.

Effects of residual elements on the microstructure and mechanical properties of a Q&P steel

Producing steel requires large amounts of energy to convert iron ores into steel, which often comes from fossil fuels, leading to carbon emissions and other pollutants. Increasing scrap usage emerges as one of the most effective strategies for addressing these issues. However, typical residual elements (Cu, As, Sn, Sb, Bi, etc.) inherited from scrap could significantly influence the mechanical properties of steel. In this work, we investigate the effects of residual elements on the microstructure evolution and mechanical properties of a quenching and partitioning (Q&P) steel by comparing a commercial QP1180 steel (referred to as QP) to the one containing typical residual elements (Cu+As+Sn+Sb+Bi<0.3wt%) (referred to as QP-R). The results demonstrate that in comparison with the QP steel, the residual elements significantly refine the prior austenite grain (9.7μm vs. 14.6μm) due to their strong solute drag effect, leading to a higher volume fraction (13.0% vs. 11.8%), a smaller size (473 nm vs. 790 nm) and a higher average carbon content (1.26 wt% vs. 0.99 wt%) of retained austenite in the QP-R steel. As a result, the QP-R steel exhibits a sustained transformation-induced plasticity (TRIP) effect, leading to an enhanced strain hardening effect and a simultaneous improvement of strength and ductility. Grain boundary segregation of residual elements was not observed at prior austenite grain boundaries in the QP-R steel, primarily due to continuous interface migration during austenitization. This study demonstrates that the residual elements with concentrations comparable to that in scrap result in significant microstructural refinement, causing retained austenite with relatively higher stability and thus offering promising mechanical properties and potential applications.

Role of prior austenite grain boundary and retained austenite in strain localization of medium-carbon high-strength steels

Exploring the micromechanical behavior of lath-martensitic quenching and tempering (Q&T), as well as quenching and partitioning (Q&P) steels presents a significant challenge due to their complex multi-phase constituents and hierarchical structures. This study conducts a comprehensive comparative analysis using medium-carbon Q&T and Q&P steels with identical chemical compositions. Atom probe tomography (APT) was used to investigate the distribution of chemical elements, while high-resolution digital image correlation (HR-DIC) provided insights into nanoscopic strain localization and partitioning. Both Q&T and Q&P steels exhibit significant strain localization at prior austenite grain boundaries (PAGBs) due to substantial carbon segregation. Strain localization in Q&T steel at PAGBs and packet boundaries strongly depends on preferred geometric orientations, with boundaries inclined 45° to the loading direction experiencing severe deformation. Conversely, in Q&P steel, PAGBs adjacent to large clusters of fine lath martensite, lacking a packet with a longitudinal direction parallel to boundaries and a feasible in-lath slip system, primarily undergo deformation. Carbon partitioning from martensite to retained austenite (RA), along with coexisting carbon and manganese segregation at phase interfaces, stabilizes the RA, resulting in a neighborhood-dependent deformation behavior of RA. The existence of RA not only absorbs carbon, maintaining low strength and high deformability, but also uniformly distributes and induces stable interfaces to hinder the formation of large packets with easily activated in-lath slip transmission.

Thermodynamic approach for designing processing routes of 4Mn quenching and partitioning steel

AbstractThe study addresses the design and optimization of chemical composition and processing routes of new quenching and partitioning medium-Mn alloy using theoretical and experimental approaches. The thermodynamic calculations using Thermo-Calc and JMatPro software were carried out to characterize the influence of Mn, Si and Al contents on cementite formation and precipitation processes. The evolution of individual phases as a function of temperature under thermodynamic equilibrium conditions was estimated. The investigations included the determination of continuous cooling transformation (CCT) and the time–temperature transformation (TTT) diagrams of a model 4Mn alloy. The calculated equilibrium diagrams were compared with the experimental diagrams determined using dilatometric tests. Microstructural observations were carried out to verify the results of dilatometric measurements. The results of thermodynamic calculations and experimental tests showed the moderate agreement. It is related to the inaccuracy of currently available models in the used software and/or non-equilibrium conditions of experimental tests.

Effects of resistance spot welding parameters on microstructure and mechanical properties of the dissimilar welded joint of quenching and partitioning steels

Abstract In this study, the effect of welding currents and welding times on the microstructure and mechanical properties of the dissimilar welded joints of Q&amp;P980 and Q&amp;P1180 steel was investigated. The macrostructure and microstructure of the dissimilar welded joints were characterized and the relationship between the welding parameters and the mechanical performance was analyzed using confocal laser scanning microscope, scanning electron microscope, and mechanical properties testers. Results show that with the increasing welding current and welding time, the nugget diameter, indentation rate, and maximum shear force of the dissimilar joint increase. The absorption energy of the dissimilar joint increases when the welding current rises, while it increases first and then decreases with elevating welding time. All the hardness distributions of the dissimilar Q&amp;P980/Q&amp;P1180 joints exhibit the highest hardness value in the fusion zone and a gradually decreasing hardness value in the heat-affected zone. Moreover, with increasing current and time, much higher hardness occurs at the FZ/HAZ boundary. The microstructure characterization illustrates the martensite fraction in the intercritical heat-affected zone of the Q&amp;P1180 side is higher than that of the Q&amp;P980 side after the welding process. With the increase of welding current and time, the lath martensite in the fusion zone gradually coarsens. The coarsening martensite and the nugget diameter are responsible for the change in the shear force and energy absorption of the dissimilar Q&amp;P980/Q&amp;P1180 joints.

Effect of microstructural inheritance window on the mechanical properties of an intercritically annealed Q&P steel

Different initial microstructures significantly influence the final microstructures and mechanical properties of the intercritically annealed quenching and partitioning steels. Previous studies have primarily focused on the mechanism for the inheritance of different initial microstructures into the final microstructures, which affects the phase fraction and mechanical properties. However, these studies have overlooked the existence of an inheritance window in the intercritical annealing process. In this study, we investigated the inheritance window and explored the impact of varying initial microstructures on the reverse transformation of austenite, final phase fraction, and mechanical properties. Our findings reveal that the varying initial microstructure exhibits minimal influence on the final microstructure and mechanical properties for short or long annealing times. However, for the intercritical annealing treatment for 60 s, the initial microstructure of martensite with more nucleation sites accelerated the austenite reverse transformation fraction, enhanced the reverse-transformed austenite content, increased the primary martensite content, and improved the yield strength. Conversely, the coil-cooled sample, with initial microstructures consisting of ferrite and pearlite without dissoluble Mn-rich cementites, reduced the austenite reverse transformation rate, decreased the reverse-transformed austenite content, enhanced the ferrite and RA content, and improved ductility.

Deformation-induced martensitic transformation kinetics in TRIP-assisted steels and high-entropy alloys

In this study, the sigmoidal kinetics of deformation-induced martensitic transformation (DIMT) in the transformation-induced plasticity (TRIP)-assisted alloys, including metastable high-entropy alloys (HEAs), austenitic stainless steels, and advanced high-strength steels (AHSSs) such as low-alloyed TRIP steels, medium-Mn steels, and quenching and partitioning steels, were studied using various models. To tune mechanical stability, the influences of stacking fault energy (SFE), chemical composition, austenite grain size, deformation temperature, strain rate, and stress state were considered. The novel Fe47Co30Cr10Ni5V8-xSix (x = 3, 4, and 6 at.%) alloys were the main investigated HEAs in the present work, in which silicon addition effectively led to lowering SFE and faster DIMT kinetics by promoting the formation of body-centered cubic α΄-martensite. The Olson-Cohen, Guimaraes (based on Johnson-Mehl-Avrami-Kolmogorov analysis), Shin, and Ahmedabadi models were analyzed and critically discussed. Moreover, a Hill-based sigmoid model was formulated as fα' / fsat = 1 – p / (p + εq) with the parameters p and q characterizing the stability of austenite, and fsat representing the saturated volume fraction of α΄-martensite. It was demonstrated that χ = p × q can be considered the sole austenite stability parameter. It was concluded that the proposed Hill-based model, demonstrating the highest accuracy among the studied models, can effectively simulate the kinetics of α΄-martensite formation.

Dynamic deformation behavior of Fe-0.22C-1.65Si-1.98Mn quenching and partitioning steel over a wide range of strain rates and constitutive model establishment

During traffic accidents, automobiles often experience rapid deformation at high strain rates spanning from 10² to 10³ s⁻¹. The dynamic deformation behavior and the constitutive model of the materials are crucial for precisely analyzing and simulating vehicle collisions and damage. This study focuses on a high-strength quenching-partitioning steel (Q&P steel) compositionally tailored for automotive applications, specifically Fe-0.22C-1.65Si-1.98Mn. Tensile tests were conducted under quasi-static (10⁻⁴–10⁻¹ s⁻¹) and dynamic (1–10³ s⁻¹) conditions to obtain stress-strain data, based on which the deformation behavior and instability mechanism of the Q&P steel were revealed. A Johnson-Cook (J-C) constitutive model was established to predict the stress-strain behavior. This model was further improved by incorporating the nonlinear influence of strain rate on stress, resulting in a significant reduction of prediction error by 60.9 % in the high strain rate range. Based on the modified J-C model, the finite element simulations of the high-speed collision for U-shaped and tubular components were conducted. The simulations exhibit excellent agreement with experimental results, thus confirming the accuracy of the model in predicting high-speed deformation behavior of the Q&P steel. This validation enables precise simulations of dynamic collision characteristics and energy absorption.

Understanding plasticity in multiphase quenching & partitioning steels: Insights from crystal plasticity with stress state-dependent martensitic transformation

This study develops a novel crystal plasticity (CP) model incorporating deformation-induced martensitic transformation (DIMT) and transformation-induced plasticity (TRIP) effect to predict the complex interplay between microstructural evolution and mechanical behavior in a third-generation advanced high-strength steel QP980. This model introduces phenomenological theory of martensite crystallography (PTMC) based TRIP theory and DIMT kinetics grounded on nucleation-controlled phenomenon. Notably, the DIMT model is improved by utilizing a geometric approach for calculating shear band intersections. A virtual multiphase representative volume element (RVE) based on the Voronoi tessellation is generated for the QP980 steel, which comprises ferrite, martensite, and retained austenite (RA). The study highlights how phase transformation affects mechanical properties, notably the strengthening from transformed martensite and the mechanical alterations in RA due to the TRIP effect. The DIMT kinetics dependent on stress states such as uniaxial tension (UT), uniaxial compression (UC), plane strain tension (PST), and equi-biaxial tension (EBT) are analyzed using the developed model. The role of microstructural surroundings on martensitic transformation is also examined. Furthermore, analysis under biaxial loading angles using the model reveals an asymmetric yield surface, with more pronounced changes in yield stress in the tensile region due to accelerated transformation behaviors, as opposed to the more gradual transformations in the compressive region. This study provides valuable insights into the deformation mechanisms of the third-generation advanced high-strength steels including relationship between plastic anisotropy, transformation kinetics, and microstructural evolution.

Mitigating hydrogen embrittlement sensitivity in quenching and partitioning steel by optimizing the microstructure of segregation bands

Segregation bands (SBs) in quenching and partitioning (Q&P) steel plates will increase hydrogen embrittlement (HE) sensitivity, posing a major challenge in industrial production. This study addresses the issue by optimizing the microstructure of SBs through an additional double rapid tempering (DRT) heat treatment. The treatment effectively optimized the microstructure of the SBs, transforming combined martensitic islands into a mixed composition comprising isolated martensite islands, ferrite, and reverted austenite. This microstructure optimization of SBs altered the hydrogen crack nucleation characteristics from the thickness center of the steel plate to the corners of the specimen. Such a simple process significantly alleviates the adverse effects of SBs on HE sensitivity, providing a valuable reference for industrial applications.

Effect of partitioning treatment on the strengthening and plasticising mechanism of one-step quenching and partitioning steels

The effects of partitioning temperature and time on the strengthening and plasticising mechanism of a one-step quenching and partitioning (Q&P) steel were investigated. The results show that as the partitioning temperature decreased, the lath martensite became finer with higher dislocation density, while the dislocation density and RA amount decreased with increasing partitioning time. Lower quenching temperature and shorter partitioning time are conducive to achieve the high ultimate tensile strength of 1377 MPa and the better total elongation of 12.3% for low-carbon one-step Q&P steels. Meanwhile, the dislocation strengthening caused by martensite and the solid solution strengthening are main strengthening mechanisms of one-step Q&P steels. In addition, a new martensite transformation kinetic model considering the influence of silicon element was established, and simultaneously, the modified CCE model predicted RA fractions more accurately for high-silicon Q&P steels, especially at lower quenching temperature.

High-performance room temperature quenching and partitioning (RT Q&P) steel fabricated through laser powder bed fusion

The present investigation focuses on analyzing the microstructure and tensile properties of room temperature quenching and partitioning (RT Q&P) steel manufactured using laser powder bed fusion (LPBF). The as-built specimen reveals a dual-phase microstructure composed of retained austenite (RA) grains within the martensite matrix, with the absence of the conventional Mn banded microstructure. However, the as-built sample demonstrates poor tensile properties, exhibiting only a 2.5% total elongation, largely due to significant residual stress. To mitigate this issue, the as-built sample undergoes tempering at low temperatures (300, 350, and 400 °C) for 10 min. This treatment results in a reduction of the average compressive residual stresses from −214.4 to −132.1 MPa, and a transformation of the residual stresses in RA from a compressive state of −52.4 MPa to a tensile state of 136.4 MPa after tempering at 350 °C. Furthermore, the volume fraction of RA increases by approximately 10%, and there is a slight increase in the C content after tempering. The tempered RT Q&P steel demonstrates an ultra-high tensile strength ranging from 1.3 to 1.5 GPa, coupled with a substantial total elongation of 10–15%. This exceptional combination of strength and ductility is attributed to the transformation-induced plasticity (TRIP) effect and the reduction of residual stresses. Notably, this study marks the first confirmation that strong and ductile RT Q&P steel can be produced through the LPBF process.

Anisotropic fracture behavior of the 3rd generation advanced high-strength – Quenching and Partitioning steels: Experiments and simulation

Advanced high-strength steels (AHSS) have revolutionized the automotive industry by reducing weight without compromising crashworthiness. The new third-generation steels, such as quenching and partitioning (QP) steels, offer exceptional strength and ductility. However, despite the extensive strength-ductility studies, there is a wide knowledge gap in the literature on the fracture behavior of QP steels under a large range of stress states and loading conditions for material forming operations. This study aims to systematically investigate the fracture behavior of QP1000 sheet metal through a combination of experimental and numerical approaches. In addition to the classic fracture dependency on stress states, we particularly focus on the anisotropic behavior in terms of both plasticity and fracture. Mechanical tests with digital image correlation are performed along three loading directions covering stress states from simple shear to plane-strain tension. The evolving non-associated Hill48 (enHill48) model is employed to describe anisotropic plasticity, while the fracture behavior is represented by a partially coupled anisotropic fracture model and a fully anisotropic fracture model. It is concluded that the investigated QP steel shows moderate anisotropic plasticity behavior yet strong fracture anisotropy, which intensifies with the increase of stress triaxiality. The partially coupled anisotropic fracture model, which has shown success for materials with minor anisotropic plasticity, fails to describe the anisotropic fracture and a fully anisotropic model provides significantly improved predictive capability.

Failure of dissimilar QP980/DP600 advanced high strength steels resistance spot welds

This study investigates the microstructure, failure characteristics, and mechanical properties of dissimilar resistance spot welds between QP980 quenching and partitioning steel and DP600 dual phase steel. The dissimilar spot welds exhibited a heterogeneous microstructure, with a fusion zone predominantly composed of martensite, significantly overmatching both base materials. A fully martensitic structure with grain size gradient was observed in the upper-critical heat affected zone (UCHAZ) on both sides of the dissimilar joint. In the sub-critical heat affected zone (SCHAZ), no significant phase transformation occurred on both sides. However, at high welding currents, a slight tempering of pre-existing martensite was detected on the QP980 side resulting in a minor hardness drop in this zone. There was a critical welding current at which the failure mode was changed from interfacial mode to pullout mode. The dissimilar QP/DP joint showed a lower tendency to fail via interfacial failure mode compared to similar QP/QP and DP/DP joints, attributed to a higher fusion zone hardness overmatching factor and greater nugget rotation. Pullout failure of dissimilar QP/DP combination was assessed in detail through interrupted tensile shear test. It was found that failure at stronger QP980 side occurred by ductile cracking mechanism initiated from the notch tip, while failure at softer DP600 side occurred by through-thickness localized necking. The former was found to be the prior failure mechanism controlling the load bearing capacity of the dissimilar joint. The mechanical properties of dissimilar QP/DP joints were discussed in terms of peak load and energy absorption, and then were compared to those of similar DP/DP and QP/QP joints in the light of weld microstructure and failure mechanisms.

Influence of Q&amp;P-Parameters and Al-Content on the Microstructural Evolution of Lean-Medium-Mn-Steels

Abstract This report investigates the impact of different heat treatment parameters and varying Al-contents on the microstructure of Quenching &amp; Partitioning (Q&amp;P) steels. Therefore, three lean-medium-Mn-steels with Al-contents between 0.3 and 0.9 wt-% underwent heat treatments according to Quenching &amp; Partitioning regimes. For comparison, the steels were subjected to a transformation induced plasticity (TRIP) aided-bainitic-ferrite (TBF) process. In both cases, the samples were fully austenitized at 900 °C for 120 s, using dilatometry. For the Quenching &amp; Partitioning process, the quenching temperature (TQ) ranged from 210 °C to 330 °C, while the transformation induced plasticity (TRIP) aided-bainitic-ferrite samples were cooled to 360 °C. Afterwards, the specimen were re-heated to the partitioning temperature (TP) of 400 °C and isothermally held for partitioning times (tp) of 40, 120 and 200 s. Subsequently, these steels were analyzed with regard to their phase fractions and hardness. The results indicated that in the Quenching &amp; Partitioning process, the microstructure was primarily influenced by the partitioning temperature (TQ), while partitioning times (tp) played a minor role due to the time-independent martensitic transformation during quenching. In general, rising quenching temperature (TQ) led to an increase in retained austenite (RA) fraction. In the transformation induced plasticity (TRIP) aided-bainitic-ferrite (TBF) process, a substantial influence of partitioning times (tp) was found, which can be explained by the kinetics of the isothermal bainitic transformation. Regardless of the heat treatment concept, an increasing Al-content contributed to elevated retained austenite contents.

Multi-scale characterization of fatigue crack tip deformation and propagation in QP steel

A novel measurement method for multi-scale characterization of fatigue crack tip deformation and propagation behaviors in high strength and plasticity, multi-phase, fine-grain Quenching and Partitioning (QP) steel was proposed. The fatigue crack propagation (FCP) test system with the macro/micro images acquisition devices was built to measure the FCP rate and crack tip deformation simultaneously. First, at the mesoscale, the evolution characteristics of the crack tip deformation fields, plasticity induced crack closure (PICC) behaviors and plastic zone during the FCP process were analyzed using the sub-image stitching and matching digital image correlation (DIC) method. Then, the FCP models were established with special emphasis on the model considering the PICC effect based on the obtained crack opening load at the mesoscale. Moreover, the corresponding microscale surface crack and fracture morphologies, microstructure phase transformation and deformations were observed and analyzed by electron backscatter diffraction (EBSD) and scanning electron microscope (SEM) techniques.

Optimizing austenite and carbide content in medium carbon silicon-rich steel: A stepped partitioning strategy and analysis of strengthening and ductility enhancement mechanisms

To achieve the objective of enhancing the yield strength (YS) of quenching and partitioning (Q&P) steel without compromising tensile strength and ductility, a stepped partitioning (SP) strategy was proposed for Q&P treatment on medium carbon silicon-rich steel. The optimized partitioning strategy involves a two-stage partitioning, the first stage is conducted at temperatures above the martensite start (Ms) temperature, followed by the second stage carried out at a lower temperature for an extended period, involving isothermal partitioning. Experimental results demonstrate that the SP strategy yields a significant quantity of small carbides, with an average size of 59.02 nm, along with 15.71 vol% of retained austenite (RA) and a carbon concentration of 1.01 wt% in RA. Furthermore, the SP-treated specimen exhibits a remarkable balance of strength and ductility, showcasing YS, ultimate tensile strength (UTS), and total elongation (TE) values reaching 1732 MPa, 2263 MPa, and 14.9%, respectively. The microstructure evolution during tensile deformation demonstrates that RA in short-duration conventional partitioning specimens underwent premature martensitic transformation during tensile deformation due to early deformation, whereas RA in long time conventional partitioning specimens was overstabilized due to highest C concentration, which resulted in poor enhancement of strengthening and ductility by TRIP effect. In contrast, the SP strategy achieved a reasonable stability of RA, resulting in a simultaneous increase of tensile strength and ductility. Simultaneously, an analysis of precipitation strengthening indicates that the abundance of fine carbides in martensite contributes to an increase in YS due to precipitation strengthening.

A comparative study on the wear behavior of dual phase (DP) steel and quenching and partitioning (QP) steel

Present study aims to elucidate the wear resistance/morphology/mechanism at wear distances from 3 to 1000 m based on the microstructure and tensile properties in DP steel and QP steel with similar initial hardness by means of sliding wear test and electron microscopes. QP steel shows better wear resistance at higher wear distance over 150 m opposite to lower wear distance, compared to DP steel, due to continuous increase in hardened layer during wear test in QP steel. This is because of higher fraction of adhesive wearing (e.g., galling) without significant wear loss originated from higher capacity of strain−hardening caused by effective impacts of grain refinement of ferrite, transformation-induced plasticity, and multiphase hardening in QP steel.