Quenching Partitioning Tempering Research Articles

Multi-interface migration mechanism induced by carbide precipitation during the quenching-partitioning-tempering process in a high-carbon steel

Phase-field finite element (PFFE) modeling of the quenching-partitioning-tempering (Q-P-T) process is proposed, and the two-dimensional PFFE-QPT model considering carbide precipitation and the interface migration between martensite and austenite is used to investigate microstructural evolution and the elastic/plastic strain distribution at quenching, partitioning and tempering stages in a high-carbon steel, respectively. The simulation results of the high carbon Q-P-T steel indicate that the precipitation strengthening of carbides occurs not only because they can block the movement of dislocations, but also because they can produce high internal stress. Meanwhile, the volume fractions of different phases (including primary martensite, retained austenite, secondary martensite, and carbide) and the carbon content in retained austenite predicted by the PFFE-QPT model are slightly better than those predicted by the novel one-dimensional QPT-LE (local equilibrium) model and much closer to experimental values. The PFFE-QPT model is also used to successfully predict the volume fractions of different phases in low-carbon and medium-carbon Q-P-T steels. More importantly, the microstructural morphologies closely related to mechanical properties can be demonstrated by the PFFE-QPT model and are comparable with the experimental observation. Therefore, the PFFE-QPT model will be a more powerful tool for guiding the process and microstructure design of Q-P-T steels compared with the QPT-LE model.

The Fabrication of Ultrahigh-Strength Steel with a Nanolath Structure via Quenching-Partitioning-Tempering.

A novel low-alloy ultrahigh-strength steel featuring excellent mechanical properties and comprising a nanolath structure was fabricated in this work using a quenching-partitioning-tempering (Q-P-T) process. The Q-P-T process comprised direct quenching and an isothermal bainitic transformation for partitioning after thermo-mechanical control processing (online Q&P) and offline tempering (reheating and tempering). The ultrafine nanolath martensite/bainite mixed structure, combined with residual austenite in the form of a thin film between the nanolaths, was formed, thereby conferring excellent mechanical properties to the steel structures. After the Q-P-T process, the yield and tensile strengths of the steels reached 1450 MPa and 1726 MPa, respectively. Furthermore, the Brinell hardness and elongation rate were 543 HB and 11.5%, respectively, with an average impact energy of 20 J at room temperature.

Effects of Retained Austenite and Softened Martensite on High‐Cycle Fatigue Behavior of Spring Steel under Two Loading Modes

A novel quenching‐partitioning‐tempering (Q‐P‐T) process is applied to 60Si2Mn spring steel, resulting in multiphase microstructures consisting mainly of retained austenite (RA) and softened martensite (SFM). The influence of multiphase microstructures on the high‐cycle fatigue behavior of Q‐P‐T and traditional quenching and tempering (Q&T) spring steels are investigated under rotating–bending and tension–tension loading. The results indicate that the fatigue strength of Q‐P‐T spring steel with RA, SFM, and lower quenching stress increases by 23.9 and 14.4% under cyclic rotating–bending and tension–tension loadings, respectively, compared with that of Q&T spring steel. Under both loading modes, the fatigue crack of the Q‐P‐T spring steel is mainly initiated from the interface between the martensitic matrix and inclusions or carbides. In contrast, the fatigue crack of the Q&T spring steel is mainly initiated from the plastic area around the martensitic matrix rather than inclusions. The RA, SFM, and lower quenching stress of the Q‐P‐T spring steel are conducive to the formation of dimples and tear ridges in the crack propagation and ultimate failure areas, whereas the inadequate SFM and higher quenching stress of the Q&T spring steel are prone to the formation of quasi‐cleavage facets and secondary cracks.

Effect of tempering temperature on microstructure and mechanical properties of a low carbon bainitic steel treated by quenching-partitioning-tempering (QPT) process

The quenching-partitioning-tempering (QPT) treatment was carried out on a low carbon bainitic steel, and the effect of tempering temperature on the microstructure and mechanical properties of the QPT processed steel was investigated. The findings demonstrate that as the tempering temperature increases, the yield strength decreases gradually, while the tensile strength decreases first and then increases. However, the elongation exhibits a trend of increasing first and then decreasing with increasing the tempering temperature. When the tempering temperature is 340 °C, the best mechanical properties can be obtained with the yield strength of 1495 MPa, tensile strength of 1806 MPa, elongation of 17.7%, and product of strength and elongation up to 32.92 GPa%. Such excellent mechanical properties are related to the synergistic combination of tempered martensite, low carbon bainite, retained austenite and carbides.

Effects of Compositional Inhomogeneity on the Microstructures and Mechanical Properties of a Low Carbon Steel Processed by Quenching-Partitioning-Tempering Treatment

Quenching-partitioning-tempering (Q-P-T) heat treatment is a relatively novel approach to attain excellent ductility in high-strength steels. In the present work, the microstructural evolution and the mechanical properties of a low carbon microalloyed advanced steel were systematically investigated after the Q-P-T process. The microstructural evolution was explored by employing X-ray diffraction, transmission electron microscopy and scanning electron microscopy. The results indicate that the multiphase microstructures strongly depend on both the initial microstructure and the processing parameters of the quenching and partitioning process, especially the quenching temperature. Compositional inhomogeneity during the Q-P-T process results in multiphase microstructures, in which the mechanical properties of the quenching and partitioning steels may be strongly impacted by the distribution of heterogeneous austenite phase in the steel matrix.

Universality of quenching-partitioning-tempering local equilibrium model

• Microstructural amount and C γ in low-C and medium-C Q-P-T steels were quantitatively characterized. • The QPT-LE model is a novel tool for the design of Q&P/Q-P-T steels. • The universality of QPT-LE model was firstly verified. The design of metastable retained austenite (RA) and its stability have always played a vital role in developing advanced high strength steels (AHSSs). The quenching-partitioning-tempering local equilibrium (QPT-LE) thermo-kinetic model was established by us, with consideration of carbide precipitation and interface migration. As a tool for designing AHSSs, the above model can well predict the microstructural amount and the carbon content in RA ( C γ ) in a high-C quenching-partitioning-tempering (Q-P-T) steel. In this work, QPT-LE model predicates the microstructural amount and C γ in low-C and medium-C Q-P-T steels more accurately than constrained carbon equilibrium (CCE) model without consideration of carbide precipitation and interface migration, and more accurately than quenching and partitioning-local equilibrium (QP-LE) model without consideration of carbide precipitation, which verifies the universality of QPT-LE model. The advantages of QPT-LE model are also discussed.

Study on Thermal Process and Microstructure & Properties of High Strength Automotive Steel

This paper studies how the Nb content affected mechanical properties and microstructures of automotive steel processed by quenching and tempering (Q-T) and quenching-partitioning-tempering (Q-P-T) with the help of tensile testers, X-ray diffractometers, scanning electron microscopes, transmission electron microscopes and other equipment. It was demonstrated that when the Nb content remained the same, Agt (total elongation at maximum force), A (elongation after fracture) and R m·A (the product of strength with ductility) of the steel processed by Q-P-T were higher than those of the steel processed by Q-T; and when the steel was treated by the same thermal process, as the Nb content increased, its R p0.2 (yield strength) and R m (tensile strength) would grow, while A gt, A and R m·A would rise first and then decline. No matter if treated by Q-T or Q-P-T, the microstructures of the steel containing 0% and 0.03% of Nb separately were always lath martensite and film-like retained austenite located between laths. When processed by Q-P-T, the steel containing 0.03% Nb saw the largest strength-ductility product. This was because Nb could help refine the sizes of the original austenite grain and the lath martensite and precipitate nano-sized NbC; and the Q-P-T process was more effective in thickening the film-like retained austenite in steel and promoting the dispersed precipitation of the NbC.

Effect of Deformation on Microstructure and Mechanical Properties of Medium Carbon Steel During Heat Treatment Process

The current research of the Q-P and Q-P-T process has been focused on controlling the heating temperature and holding time, or adding alloy elements into the steel to induce precipitation strengthening and improve the strength and plasticity of the steel. In this article, based on a quenching-partitioning-tempering (Q-P-T) process combined with a hot deformation technology, a deforming-quenching-partitioning-tempering (D-Q-P-T) process was applied to medium carbon steel. The effect of the heat treatment parameters on the microstructure and mechanical properties of experimental steel under deformation was studied. Through use of a scanning electron microscope (SEM), transmission electron microscopy (TEM) and tensile tests, the optimal heat treatment conditions for realizing high strength and plasticity that meet the safety requirements were obtained. The mechanism for the D-Q-P-T process to improve the strength and plasticity of experimental steel was discussed. A multiphase composite structure of lath martensite and retained austenite was obtained. Compared with the Q-P-T process, use of the D-Q-P-T process can increase the strength of steel by 57.77 MPa and the elongation by 5%. This study proposes a method to improve the strength and plasticity of steel.

Effect of microstructure on hydrogen embrittlement susceptibility in quenching-partitioning-tempering steel

Hydrogen embrittlement (HE) restricts the application of high strength steel in sustainable energy productions. As one type of efficient hydrogen trap sites, the NbC carbide precipitation is a superior approach to mitigate the HE susceptibility. The microstructure, mechanical properties and HE susceptibility were investigated in different high strength steels treated by quenching and partitioning (Q&P), quenching-partitioning-tempering (Q-P-T) and intercritical annealing quenching and partitioning (IAQP) processes in this study. The results show that the NbC particles can significantly improve the HE resistance of high strength steel treated by Q-P-T process. The NbC carbide precipitation has four different morphologies with varied sizes, which can effectively trap a large amount of diffusible hydrogen atoms. The retained austenite phases with different morphologies and locations have different hydrogen trap ability, but ferrite phase does not show strong hydrogen trap ability. Meanwhile, the NbC carbide precipitation can enhance the yield strength of steel effectively through precipitation strengthening, but has little effect on its ductility.

Revealing carbide precipitation effects and their mechanisms during quenching-partitioning-tempering of a high carbon steel: Experiments and Modeling

Quenching-Partitioning-Tempering (Q-P-T), as a modified process of quenching and partitioning (Q&P), is a promising process to treat ultra-high strength steels with a good balance of strength and ductility. The essences of the Q-P-T process are to stabilize metastable austenite via carbon partitioning from martensite into austenite during partitioning and to strengthen the martensite matrix via nano-precipitation of micro-alloyed carbides during tempering. The competitive reactions, e.g. carbon segregation to dislocations, transition carbide precipitation and austenite decomposition, could occur during partitioning/tempering, which are expected to play a substantial role in the microstructures of the Q-P-T steels. In this contribution, the complex microstructure evolution during the Q-P-T processing of an Fe-0.67C-1.48Mn-1.53Si-0.038Nb steel was systematically characterized by various techniques. A concise QPT-LE (Local Equilibrium) thermo-kinetic model with dual interfaces (martensite/carbide and martensite/austenite) migration was established to predict the evolution of austenite fraction and its carbon content based on the consideration of competitive reactions mentioned above. In the QPT-LE model the effect of carbide precipitation is introduced, which is different from CCE (constrained carbon equilibrium) thermodynamic model and QP-LE thermo-kinetic model. Therefore, the QPT-LE model can be used to reveal the effects of carbide precipitation on the retained austenite fraction and its carbon content, while the prediction accuracy of carbide fraction can be further improved by considering the effect of carbon segregation to dislocation. In general, the QPT-LE model can better predict the experimental results compared with popular CCE model and QP-LE model.

Homogeneous elasto-plastic deformation and improved strain compatibility between austenite and ferrite in a co-precipitation hardened medium Mn steel with enhanced hydrogen embrittlement resistance

A co-precipitation hardened medium-Mn steel with respective NiAl and Cu-rich particles in dual phases (austenite and ferrite) has been designed by using a quenching-partitioning-tempering (QPT) treatment. The influences of co-precipitates on the elasto-plastic transition and strain compatibility has been investigated based on the micro-mechanical behavior of constituent phases, the evolution of lattice strain and stress components. Compared to the Cu-Free steel, in which single NiAl precipitates form in ferrite, co-precipitation in the Cu-Added steel results in an improved strain compatibility between austenite (γ) and ferrite (α). The lattice strains of both γ and α phases show similar evolutions during the entire strain regime, and a homogeneous elasto-plastic deformation occurs together with a continuous yielding and weak transient of strain hardening rate. The improved strain compatibility leads to an alleviation of strain localization in the phase interfaces, so that the overall back stress increases slowly with the tensile strain increasing during the whole plastic deformation process, although the transformation-induced plasticity (TRIP) effect of the Cu-Added steel only occurs at low strains (<5%) due to the low mechanical stability of the retained austenite. In contrast, a non-continuous yielding of the constituent phases occurs in the Cu-Free steel leads to strong strain transfer across the α/γ interfaces due to a low deformation compatibility between austenite and ferrite. High internal stress in the phase interfaces results in easy damage-crack formation, but it can be accommodated by the TRIP effect which dominates in large strains, resulting in back stress decreasing and effective stress hardening during the subsequent deformation stage. However, the introduction of hydrogen can aggravate the dislocation accumulation and strain transfer between austenite and ferrite; thus, hydrogen-induced cracks (HICs) are prone to nucleate at the α/γ interfaces while the TRIP effect is not significant, leading to premature failure and significant elongation loss (85.4%) of the Cu-Free steel. Therefore, the improved strain compatibility of the Cu-Added steel contributes to a better resistance to hydrogen embrittlement, although both medium Mn steels possess similar tensile strength (1 GPa) and elongation (25%) values.

Investigation of hierarchical precipitation on bimodal-grained austenite and mechanical properties in quenching-partitioning-tempering steel

The influence of the hierarchical nanometer-sized precipitates (NPs) on the microstructure evolution and mechanical properties of ultra-low carbon medium manganese quenching-partitioning-tempering (QPT) steel was studied. A unique combination of hierarchical NPs including lenticular Cu/Ni precipitates (>5 nm) and spherical (<1.5 nm) NiAl/Cu co-precipitates was obtained during QPT treatment. The shear mechanisms of spherical NPs mainly contribute to a strength enhancement of approximately 500–700 MPa, whereas the lenticular Cu/Ni precipitates result in an enhanced driving force for an abnormal austenite growth owing to the localized element enrichment and the interfacial incoherency, thereby obtaining a bimodal-grained distribution of austenite which possesses a wide mechanical stability. Thus, a continuous transformation-induced plasticity (TRIP) effect over the entire strain regime occurs, contributing to a good strain hardening capacity and superior ductility enhancement (uniform elongation change from 8% to 16%). However, the introduction of the hierarchical NPs not only inevitably creates a strong concentration of stress within the weak grain boundaries, but also leads to a detrimental transformation product from the blocky austenite with a low mechanical stability, thus resulting in the formation of intergranular cracks and a low impact toughness (20 J).

Influence of Partial Replacement of Si by Al on Microstructure and Properties of Nanostructured Martensitic Steel.

Nanostructured martensitic steels comprising nanoscale-martensite and retained austenite were obtained by quenching–partitioning–tempering (Q–P–T) treatment. The influence of partial replacement of Si by Al on the microstructure and properties were studied. Results showed that with partial replacement of Si by Al, except nanoscale-martensite and retained austenite, a little ferrite was also clearly observed. By contrast, with partial replacement of Si by Al, although the ultimate tensile strength (1392 MPa against 1215 MPa) was slightly lower, the elongation after fracture (16.7% against 19.9%) and the toughness (equivalent to the area under the stress–strain curve) (43.1 × 106 against 37.1 × 106 J/m3) were relatively higher. This was because the retained austenite in the steel with partial replacement of Si by Al had higher carbon content, similar volume fraction of film-like morphology, lower volume fraction of blocky morphology and was surrounded by ferrite, which played significant role in the transformation-induced plasticity (TRIP) effect.

Influence of Intercritical Heating on the Microstructure and Mechanical Properties of Al‐Rich Quenching–Partitioning–Tempering Steel

The microstructure and mechanical properties with traditional and intercritical heating and quenching–partitioning–tempering (referred as Q–P–T and I&amp;Q–P–T, respectively) treatment for the Al‐rich steel are revealed through optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD), electron probe microanalysis (EPMA), and tensile testing. Results indicate that the multiphase microstructures of the Q–P–T and I&amp;Q–P–T‐treated steel consist of martensite (M), ferrite (F), and retained austenite (RA). Meanwhile, the I&amp;Q–P–T‐treated steel has much more ferrite and relatively less RA than those of the Q–P–T‐treated one. During tensile testing, more RA with optimized stability transforms to martensite in the I&amp;Q–P–T‐treated specimen, which leads to a more pronounced transformation‐induced plasticity (TRIP) effect. Thus, with more ferrite and a remarkable TRIP effect, the elongation and product of strength–elongation of the I&amp;Q–P–T steel are higher than those of the Q–P–T‐treated one.

Introduction of nanoprecipitation and transformation-induced plasticity in ultra-low carbon medium manganese quenching-partitioning-tempering steels

ABSTRACTCombined nanoprecipitation hardening and transformation-induced plasticity (TRIP) effects have been introduced in ultra-low carbon medium manganese steels for enhanced strength, plasticity and toughness. The mechanical properties and designed microstructure evolution were investigated after an innovative quenching-partitioning-tempering (QPT) treatment with different initial conditions, cold rolling (CR) and hot rolling (HR). The transmission electron microscopy (TEM) combined with 3D atom probe tomography (APT) observed plenty of block austenite (30%) with dispersed precipitation in CR-QPT steels, while less amounts of film austenite (18%) with higher density of nanoprecipitates in HR-QPT steels. The controlled multiphase microstructure evolution is based on the Mn diffusion and segregation process. The unloading-reloading tests reveals the respective roles of precipitation hardening and TRIP in the overall mechanical properties according to the Baushinger effect (BE): the nanoprecipitation results in a higher back stress strengthening, while the deformation-induced martensite transformation in a wide strain regime degrades the large stress concentration in grain boundaries, leading to a back stress softening but effective stress hardening in the later deformation stage.

Effect of Al replacing Si on mechanical properties of high carbon Q–P–T martensitic steels

ABSTRACTFe–0.67C–1.55Mn–1.99Al–0.60Cr–0.038Nb (Fe–2Al) and Fe–0.63C–1.52Mn–1.49Si–0.62Cr–0.036Nb (wt.%) (Fe–1.5Si) were designed and both subjected to novel quenching–partitioning–tempering (Q–P–T) process, and the effect of 2 wt.% Al replacing 1.5 wt.% Si on mechanical properties was studied. The Fe–2Al Q–P–T martensitic steel exhibited a tensile strength of 1640 MPa and elongation of 21.1% accompanied with product of strength and elongation (PSE) of 43.65 GPa %, which is superior to the third generation advanced high strength of 30 GPa %. While the tensile strength of Fe–1.5Si Q–P–T martensitic steel is 1.950 GPa, and the total elongation only is 12.4%, whose PSE is less than 30 GPa %. High ductility of Fe–2Al Q–P–T martensitic steel origins from that the dislocation absorption by retained austenite effect and high mechanical stability of retained austenite due to the addition of Al, which avoids the formation of more brittle strain –induced twin-type martensite from retained austenite transformation. The lower strength of Fe–2Al Q–P–T steel than Fe–1.5Si Q–P–T steel is attributed to lower elastic modulus of former steel after Q–P–T process.

Effect of Nb and Cu on the microstructure and hardness of quenching–partitioning–tempering dual-phase steels

ABSTRACTThe effect of Nb and Cu on the microstructure and hardness of quenching–partitioning–tempering (Q–P–T) dual-phase steels was investigated. Nanostructured martensite and retained austenite were obtained when the steels were treated with a Q–P–T process. The hardness, volume fraction of retained austenite and final carbon content in austenite of the Nb + Cu-containing steel were higher than the Nb-containing steel. This was attributed to the promotion of Nb precipitation by the addition of Cu.

Dynamic tensile behavior of novel quenching-partitioning-tempering martensitic steel

ABSTRACTA Fe–0.20C–1.49Mn1.52Si–0.58Cr–0.05Nb low-carbon steel was treated by a novel quenching-partitioning-tempering (Q-P-T) process and the traditional quenching and tempering (Q&T) process for comparison, respectively. X-ray diffraction results indicate that there is 10.8% volume fraction of retained austenite (VRA) in the Q-P-T martensitic steel, while no diffraction peak of retained austenite was detected in Q&T martensitic steel. The Q-P-T steel and the Q&T steel were subjected to a dynamic tensile test at a strain rate of 500 s−1 and a quasi-static tensile test at a strain rate of 5.6 × 10−4 s−1, respectively. The results indicate that the high strain rate in the dynamic tension raises the strength of the Q-P-T and Q&T steels compared with their quasi-static tensions owing to strain rate hardening effect, however, the elongation of Q-P-T steel decreases slightly, while the elongation of Q&T steel evidently increases. This difference is attributed that the existence of the dislocation absorption by the retained austenite (DARA) effect in quasi-static tension is suppressed under dynamic tensile loading so that the enhancement influence of DARA effect on ductility cannot be effectively compensated by the adiabatic softening of the martensitic matrix, which leads to a slight decrease in the elongation of the Q-P-T steel in dynamic tension compared with that in the quasi-static tension. While Q&T steel has no DARA effect in quasi-static tension due to no retained austenite, and thus the adiabatic softening of the martensitic matrix increases the ductility of Q&T steel in dynamic tension.

Formability of quenching-partitioning-tempering martensitic steel

ABSTRACTA Fe-0.20C-1.49Mn-1.52Si-0.58Cr-0.05Nb (wt%) steel was treated by a novel quenching-partitioning-tempering (Q-P-T) process and traditional quenching and tempering (Q&T) for comparison, respectively. The researches of the mechanical properties and forming limit diagrams reveal that Q-P-T martensitic steel has more retained austenite (10.8%) than the Q&T martensitic steel (less than 3%), which makes Q-P-T martensitc steel possess higher strain hardening exponent and true uniform elongation than the Q&T martensitic steel. The high value of hardening exponent (n) and true uniform elongation (εu) stem from the low initial dislocation density in martensitic matrix in Q-P-T martensitic steel before deformation and the high strain hardening rate and dislocation absorption by retained austenite (DARA) effect of retained austenite during deformation. Moreover, the n, or the εu, is a most important parameter in all the apparent parameters affecting the formability of metal sheets, and this conclusion is of practical importance in the comparison of several steels’ formability.

Effect of heat treatment parameters on the microstructure of quenching–partitioning–tempering steel

ABSTRACTThe effect of quenching temperature and tempering time on the microstructure, content of retained austenite and carbon content in retained austenite of a copper and niobium alloyed medium carbon steel were investigated by using optical, electron microscopy and X-ray diffraction. Results showed the microstructure of the quenching–partitioning–tempering steel consisted of lath martensite, retained austenite and carbides. With the increase of quenching temperature, the martensite laths became coarsened and the content of retained austenite increased. With the increase of tempering time, the content of retained austenite initially increased, reached to the maximum at 600 s and then gradually decreased.