Interface Migration between Martensite and Austenite during Quenching and Partitioning (Q&P) Process

An Fe-0.2C-1.5Si-1.67Mn steel was subjected to quenching and partitioning （Q＆P） process, and the interface migration between martensite and austenite at an elevated partitioning temperature was observed. The interface migration is excluded in constrained paraequilibrium （CPE） model. Based on ＂endpoint＂ predicted by CPE model the thermodynamic condition of interface migration is analyzed, that is, the difference in the chemical potential of iron in both ferrite （martenisite） and austenite produces the driving force of the iron atoms to migrate from one phase to the other phase. In addition, the interface migration can change the austenite fraction; as a result, the austenite fraction at partitioning temperature may be higher than that at quenching temperature through the interface migration, but this phenomenon cannot be explained by CPE model.

Effect of Direct Quenching and Partitioning Treatment on Mechanical Properties of a Hot Rolled Strip Steel

Three different online heat treatment processes were designed to study the effects on the mechanical properties of a 0.19C-1.6Si-1.6Mn（wt%） hot rolled strip steel.The microstructures were characterized by means of SEM,TEM,EPMA,and XRD.The mechanical properties were estimated by tensile tests.Results showed that a satisfying combination of strength and ductility could be obtained through the ferrite relaxation and direct quenching and partitioning process.Analysis was also focused on this process.The microstructure contained proeutectoid ferrite grains,martensite packets and blocky or interlath retained austenite,and also contained carbide-free bainite in the case of relatively high quench temperatures.The retained austenite fraction was increased through proeutectoid ferrite and partial bainite transformation,while the tensile strength was also consequently decreased.The most of retained austenite transformed to ferrite under deformation and the elongation was obviously improved.

Effect of quenching temperature on the microstructure of Si-containing steel during quenching and partitioning

This study aims to investigate the effect of the 1-step quenching and partitioning （Q＆P） process on the microstructure and the resulting Vicker＇ s hardness of 0.3C-1.5Si-1.5Mn steel by using in-situ dilatometry ,optical microscopy （ OM ）, scanning electron microscopy （ SEM ）, X-ray diffractometry （ XRD ）, and Vicker ＇ s hardness measurement. Systematic analyses indicate that the microstructure of the specimens quenched and partitioned at 150℃ ,200 ℃ ,250℃ ,and 300℃ mainly comprises lath martensite and retained austenite. The dilatometry curve of the specimen partitioned at 150℃ is presumably ascribed to the formation of isothermal martensite. In the early stages of partitioning at 200℃,the nearly unchanged dilatation curve is closely related to the synergistic effect of isothermal martensite formation and transitional epsilon carbide precipitation. In the later stages of partitioning at 200 ℃ ,the slight increase in the dilatation curve is due to the continuous isothermal martensite formation. With further increase in partitioning temperature to 250℃, the dilatation increases gradually up to 3600 s, which is related to carbon partitioning and lower bainite formation. Partitioning at a higher temperature of 300 ℃ causes a rapid increase in the dilatation curve during the initial stages, which subsequently levels off upon prolonging the partitioning time. This is mainly attributed to the rapid diffusion of carbon from athermal martensite to retained austenite and continuous formation of lower bainite.

Feasibility of 0.02% Nb-Based Microalloyed Steel for the Application of One-Step Quenching and Partitioning Heat Treatment

To attain an enhanced combination of mechanical properties for low alloyed steel, the current study has been made to fulfill that growing need in the industry. Its results are introduced within this paper. One step Quenching and Partitioning (Q&P) heat treatment has been applied on Niobium-based microalloyed steel alloy with 0.2 %C, in the form of 2 mm thickness sheets. The target of this study is to investigate the viability of applying that significantly recommended, results-wise, heat treatment on the highly well-suited alloy steel samples, to achieve the main target of enhanced properties. A single temperature of 275°C was used as quenching and Partitioning temperature. Four Partitioning periods (30, 200, 500, and 1000 Seconds) were used for soaking at the same temperature. The results were analyzed in the light of microstructural investigation and mechanical testing. All applied cycles did not enhance the strength but moderately improved the ductility and toughness, mainly caused by the slightly high soaking temperature used. Niobium impact of grain refining was apparent through all cycles. The cycle of 500 Seconds Partitioning time obtained optimum values at that particular temperature. The 1000 Seconds Cycle obtained the worst combination of properties. A set of recommendations are set. More research is required at this point, where a lower Partitioning temperature is advised. In the light of the applied combination of parameters, the Partitioning period at such temperature is advised to be between 500 and 1000 Seconds. A high probability that periods closer to 500 than 1000 Seconds will produce better results. More research is needed between those two values of Partitioning time to precisely determine the optimum time at that temperature on that specific alloy.

Tempering Behavior of Ductile 1700 MPa Mn-Si-Cr-C Steel Treated by Quenching and Partitioning Process Incorporating Bainite Formation

We obtained a good combination of strength and ductility in a 0.4C-2.0Mn-1.7Si-0.4Cr（wt%） steel,namely,;.7 GPa of ultimate tensile strength and;6% of elongation,by conducting a Q-P-T（quenching-partitioning- tempering） process incorporating the formation of carbide-free bainite. The tempering behavior of this steel was discussed by using experimental finding(scanning electron microscopy,X-ray diffraction（XRD）,transmission electron microscopy and dilatometer) and CCE（constrained carbon equilibrium） modeling. The XRD results combined with CCE calculation prove that carbon partitioning from martensite to austenite occurs during tempering. Consequently,the thermodynamic stability of retained austenite is enhanced. This idea can be utilized to design novel Q-P-T processes in future.

Effect of Heat Treatment on Microstructure and Mechanical Properties of Quenching and Partitioning Steel

In order to investigate the effect of microstructural characterization on the mechanical properties and retained austenite stability, a different type of quenching and partitioning steel（I-Q＆P） through intercritical annealing before the quenching and partitioning process was designed, which can realize lamellar intercritical microstructure compared to the conventional quenching and partitioning（Q＆P） process. The morphology of ferrite and martensite/retained austenite is lamellar in the I-Q＆P steel while it is equiaxed after being heat-treated by conventional Q＆P process. The I-Q＆P steel is proved to have better formability and mechanical properties than conventional Q＆P steel, which is due to the highervolume fraction of retained austenite in the I-Q＆P steel and confirmed by electron backscattering diffraction patterns and X-ray diffraction. Furthermore, the stability of retained austenite in I-Q＆P steel is also higher than that in conventional Q＆P steel, which is investigated by tensile tests and differential scanning calorimetry.

Effect of bainite microstructure during two-step quenching and partitioning process on strength and toughness properties of a 0.3%C bainitic steel

The effect of bainite transformation and microstructure on the mechanical properties in 0.3%C bainitic steel was investigated via the heat treatment process of quenching at higher initial temperature and partitioning below martensite-start temperature. The results show that bainite transformation takes place with the partitioning time increasing during partitioning below martensite-start temperature. The microstructure of samples treated by this two-step quenching and partitioning process consists of lath bainite, lath martensite and retained austenite. This kind of multiphase microstructure exhibits better strength of 1420 MPa, ductility of 21.8 % and the product of strength and elongation of 30.8 GPa%. Furthermore, the excellent impact toughness of 103 J is exhibited by partitioning at 280 ℃ for 3 h. In addition, the coalescence of bainite platelets was found in the sample treated by partitioning for 8 h, leading to the deterioration of toughness.

Quasi-Situ Characterization of Deformation in Low-Carbon Steel with Equiaxed and Lamellar Microstructure Treated by the Quenching and Partitioning Process

The relationship between microstructure morphology and mechanical properties of the low-carbon steel(Fe-0.20 C-2.59 Mn-2.13 Si)treated by diff erent intercritical annealed quenching and partitioning(Q&P)processes was investigated through interrupted tensile tests plus quasi-situ electron backscatter diff raction measurements.Results show that size and distribution of retained austenite(RA)directly aff ect the sequence of deformation induced martensitic transformation.As strain increases,the equiaxed RA grains wrapped by ferrite transform fi rst,followed by the equiaxed and fi lm-like RA grains adjacent to martensite.Compared with traditional intercritical annealed Q&P steel with equiaxed structure,the steel with quenching pretreatment contains uniform lamellar structure and the relatively fi lm-like type of RA,leading to the higher yield strength,tensile strength,and elongation,as well as the steady increase in dislocation density upon straining.

Effect of quenching-partitioning treatment on the microstructure, mechanical and abrasive properties of high carbon steel

The present work employed the X-ray diffraction, scanning electron microscopy, electron backscattered diffraction, and electron probe microanalysis techniques to identify the microstructural evolution and mechanical and abrasive behavior of high carbon steel during quenching-partitioning treatment with an aim to enhance the toughness and wear resistance of high carbon steel.Results showed that, with the increase in partitioning temperature from 250 to 400℃, the amount of retained austenite(RA) decreased resulting from the carbide precipitation effect after longer partitioning times.Moreover, the stability of RA generally increased because of the enhanced degree of carbon enrichment in RA.Given the factors affecting the toughness of high carbon steel, the stability of RA associated with size, carbon content, and morphology plays a significant role in determining the toughness of high carbon steel.The analysis of the wear resistance of samples with different mechanical properties shows that hardness is the primary factor affecting the wear resistance of high carbon steel, and the toughness is the secondary one.

Microstructure and Tensile Properties of a Nb–Mo Microalloyed 6.5Mn Alloy Processed by Intercritical Annealing and Quenching and Partitioning

The transformation behavior, microstructural evolution and mechanical properties were compared in a coldrolled Nb–Mo microalloyed 6.5Mn alloy after intercritical annealing（IA） and quenching and partitioning（Q ＆ P）,respectively. The thermodynamic calculation and theoretical analysis were used to determine the optimal heat treatment parameters. The Q ＆ P samples exhibited relatively higher strength with relatively low ductility, mainly due to the hard martensite matrix, which resulted in continuous yielding behavior upon loading, whereas the IA samples showed the significantly improved ductility, which benefited from the more sufficient transformation-induced plasticity（TRIP） effects and the softer ultrafine ferrite matrix. The dependence of yield point elongation（YPE） of IA samples on grain size demonstrated that the YPE value was in the reverse proportional relationship to the average grain size, which agreed well with theoretical analysis.

The Effect of Nb on Mechanical Properties of Quenching and Partitioning Steels

In order to study the effect of alloy elements on mechanical properties of quenching and partitioning steels,the Q and P heat treatments on different chemical composition steels were carried on in lab.The tensile test results indicated the strength of Nb+Ti-bearing steel was not increasing as expected,but lower than that of the Nb+Ti-free steel,and the elongation was raised to 26% from 9%.The Nb+Ti-bearing steel microstructures after tensile test were detected by TEM and found a certain amount of twins in the deformed microstructure while the deformed microstructure mainly was lath martensite in Nb+Ti-free steel,which means the addition of Nb and Ti elements could cause the twinning induced plasticity by inhibiting the phase transformation from austenite to martensite.Based on above analysis,adding trace Nb element could greatly increase the stacking fault energy of the retained austenite,which is beneficial to the formation of twins,and the formation of twins would lower the strength slightly and raise the elongation drastically.

Correlation of isothermal bainite transformation and austenite stability in quenching and partitioning steels

The possible decomposition of metastable austenite during the partitioning process in the high end quenching and partitioning （Q＆P） steels is somewhat neglected by most researchers. The effects of primary martensite and alloying elements including manganese, cobalt and aluminum on the isothermal decomposition of austenite during typical Q＆P process were studied by dilatometry. The transformation kinetics was studied systematically and resulting microstruc tures were discussed in details. The results suggested that the primary martensite decreased the incubation period of isothermal decomposition by accelerating the nucleation process owing to dislocations especially on phase and grain boundaries. This effect can be eliminated by a flash heating which recovered dislocations. Co addition significantly promoted the bainite transformation during partitioning while A1 and Mn suppressed the isothermal bainite transformation. The bainite transformation played an important role in carbon distribution during partitioning, and hence the amount and stability of austenite upon final quenching. The bainite transformation during partitioning is an important factor in optimizing the microstructure in Q＆P steels.

Stability of Retained Austenite Through a Combined Intercritical Annealing and Quenching and Partitioning(IAQP) Treatment

Intercritical annealing（IA） at various temperatures followed by quenching and partitioning（IAQP） treatments was conducted on a cold-rolled Fe-0.2C-1.42Si-l.87Mn（wt%） sheet steel.Optimized microstructure and enhanced mechanical properties were achieved through appropriate adjustment of IA temperatures.The steel which was annealed at1,033 K for 600 s,then quenched to 573 K and partitioned at 693 K for 20 min,designated as 1033 QP steel,exhibits maximum 16.3 vol% retained austenite（RA） with good mechanical properties（ultimate tensile strength 886 MPa and total elongation 27%）.It was found that the thermal and mechanical stabilities of RA are mainly influenced by the combined effect of its average carbon content and amount of adjacent martensite.Furthermore,the transformation-induced plasticity effect increased the peak n-values observed at the second stage of the work hardening of IAQP steels.

Tensile behavior and deformation mechanism of quenching and partitioning treated steels at different deforming temperatures

The effects of deforming temperatures on the tensile behaviors of quenching and partitioning treated steels were investigated. It was found that the ultimate tensile strength of the steel decreased with the increasing temperature from 25 to 100 ℃, reached the maximum value at 300 ℃, and then declined by a significant extent when the temperature further reached 400 ℃. The total elongations at 100, 200 and 300 ℃are at about the same level. The steel achieved optimal mechanical properties at 300 ℃due to the proper transformation behavior of retained austenite since the stability of retained austenite is largely dependent on the deforming temperature. When tested at 100 and 200 ℃, the retained aus tenite was reluctant to transform, while at the other temperatures, about 10 vol. % of retained aus- tenite transformed during the tensile tests. The relationship between the stability of retained austenite and the work hardening behavior of quenching and partitioning treated steels at different deforming temperatures was also studied and discussed in detail. In order to obtain excellent mechanical properties, the stability of retained austenite should be carefully controlled so that the effect of transforma tion-induced plasticity could take place continuously during plastic deformation.

Control of Austenite Characteristics and Ferrite Formation Mechanism by Multiple-Cyclic Annealing in Quenching and Partitioning Steel

Quenching and partitioning(Q&P)treatment is a novel method to produce advanced high strength steel with excellent mechanical properties.In this study,combination of multiple-cyclic annealing and Q&P process was compared with traditional cold-rolled Q&P steel to investigate the microstructural characteristics and austenite retention.The results showed that retained austenite in traditional Q&P sample was principally located in the exterior of austenite transformation products,while those in multiple-cyclic annealing samples were mainly distributed inside the transformation products.With the increase in cyclic annealing number,both of austenite fraction and austenite carbon content increased,attributing to higher initial austenite carbon content and larger number of austenite/neighbored phase interface to act as carbon partitioning channel.In traditional Q&P sample,the deformed ferrite was recrystallized by sub-grain coalescence,while the austenite was newly nucleated and grew up during annealing process.As a comparison,the ferrite in multiple-cycle annealing samples was formed by means of three routes:tempered martensite that completely recovered with retention of interior martensite variant,epitaxial ferrite that formed on basis of tempered martensite,ferrite that newly nucleated and grew up during the final annealing process.Both of lath martensite and twin martensite were formed as initial martensite and then tempered during partitioning process to precipitateεcarbide with C enrichment,Mn enrichment and homogeneous Si distribution.Compared with the traditional cold-rolled Q&P steel,the Q&P specimens after multiple-cyclic annealing show smaller strength and much larger elongation,ascribing to the coarser microstructure and more efficient transformation induced plasticity(TRIP)effect deriving from retained austenite with high carbon content and larger volume fraction.The application of double annealing treatment can optimize the mechanical properties of Q&P steel to show a striking product of strength and elongation as about 29 GPa%,which efficiently exploit the potential of mechanical performance in low carbon steel.

Analysis of Orientation Relationships,Carbon Partitioning and Strengthening Mechanism of a Novel Ultrahigh Strength Steel

The orientation relationships,carbon partitioning and strengthening mechanism of a novel ultrahigh strength steel were analyzed in depth during the complex process of heat treatment.The experimental results reveal that the(011)α//()γ,[100]α//[011]γ orientation relationships can be drawn between martensite and retained austenite.The position and angle of martensite and retained austenite are shown more clearly from the stereographic projections.Moreover,the calculated results show that the carbon content near the austenite interface is the highest in the shorter carbon allocation time.With the further increase of time,its carbon content gradually decreases.Furthermore,a model of the relationship between yield strength and strengthening mechanism was established.It was proved that the main strengthening components contributing to the yield strength include Orowan strengthening,grain-size strengthening and dislocation hardening.The main strengthening mechanism of steel in this experiment is dislocation strengthening.

Analysis of Microstructure and Mechanical Properties of Ultrafine Grained Low Carbon Steel

A novel design scheme of hot stamping, quenching and partitioning process was conducted in a quenchable boron steel to obtain the nanometric duplex microstructure comprising ultrafine retained austenite and martensite. It is shown that the materials possess excellent mechanical properties and the ductility can be further improved without compromising the strength. The newly treated steel shows excellent mechanical properties and the total elongation of the steel increases from 6.6% to 14.8% compared with that of hot stamped and quenched steel. Therefore, this kind of steel has become another group of advanced high-strength steels. The microstructure which is mainly responsible for such excellent mechanical properties was investigated.

A novel design to enhance the stability of local austenite and the volume fraction of retained austenite in a low-carbon Si Mn Q-P steel

Pre-quenching prior to intercritical annealing quenching and partitioning(Q-P)process was proposed to enhance the volume fraction of retained austenite and the mechanical properties of a low-carbon Si Mn steel.The intercritical austenite exhibited a lath morphology due to the martensitic microstructure maintained prior to intercritical annealing.Consequently,the alloy element enrichment of intercritical austenite,in which the alloy element was aggregated at the austenitic boundaries and further diffused inside,improved the stability of intercritical austenite and decreased the M_(s) of it.As a result,the fraction of retained austenite in steel was increased,which improved the mechanical properties of the experimental Q-P steel.

Prediction and evaluation of optimum quenching temperature and microstructure in a 1300 MPa ultra-high-strength Q&P steel

The quenching and partitioning steel is the representative of the third generation of advanced high-strength steel.The effect of quenching temperature on the microstructure and mechanical property of ferrite-containing quenching and partitioning steel was studied by intercritical annealing quenching and partitioning processes.When preparing a test steel with a tensile strength of 1300 MPa and total elongation of 19%,it is found that the actual optimum quenching temperature was lower than that calculated according to the constrained carbon equilibrium.The results indicate that the martensite start temperature of the austenite was overestimated when considering the diffusion of carbon only.Austenite grain size which is affected by low temperature and the existence of ferrite during intercritical annealing influenced the optimum quenching temperature.A scheme considering the diffusion of various alloying elements and austenite grain size was proposed and verified.Using this scheme,the optimum quenching temperature of intercritically annealed quenching and partitioning steel with complex microstructures was well predicted.

Partitioning of nitrogen atoms and its effect on retained austenite content in an ultra-low-carbon Cr-Mn-N stainless steel

The partitioning of nitrogen atoms and its effect on the retained austenite content(RAC)during quenching and partitioning(Q&P)process were investigated by dilatometry,X-ray diffraction,and field emission transmission electron microscopy with energy-dispersive spectrometer mapping in a 00Cr13Mn8N steel.Nitrogen partitioning by diffusion of N atoms from martensite to austenite occurred at 400℃after quenching.N atoms are enriched in austenite after partitioning,and the stability of these N-rich austenite is improved and retained at room temperature during subsequent cooling.The different quenching temperatures(QTs)result in different phase fractions after partitioning.With the increase in QT,RAC first increases and then decreases,and the maximum RAC is 28.5 vol.%after quenching at 80℃.A mathematical model was developed to rapidly and accurately characterize the phase fraction in Q&P process based on the relative length change of the samples partitioned after quenching at different QTs.