Step Quenching Research Articles

Effect of Bainite/Martensite/Retained Austenite Triplex Microstructure on the High Strain Rate Behavior of a One Step Quenching and Partitioning Steel

The mechanical properties and microstructure evolution of a one step quenching and partitioning steel containing bainite/martensite/retained austenite mixed microstructure was studied though static and dynamic tensile tests (strain rates ranging from10-3 s-1 to 5×102 s-1 ).Scanning electron microscopy (SEM) and electron-back-scattered diffraction (EBSD) were applied to describe the microstructure evolution near the fracture. XRD characterization shows the volume fraction of retained austenite decrease exponentially with the strain rates increased.EBSD phase maps reveal that the first type of retained austenite is less sensitive to strain rate than the second type.

Martensite morphology and volume fraction evaluation of dual-phase X70 steel using magnetic Barkhausen noise technique

• The magnetic Barkhausen noise method is highly sensitive to changes in microstructure morphologies and martensite volume fraction. • The IQ treatment formed fine and fibrous martensite morphology uniformly distributed in the ferrite matrix. • The DQ treatment revealed hard martensite distributed in a ductile ferrite matrix. • The SQ treatment showed a banded morphology with a non-uniform distribution of the phases. • There are direct correlation between magnetic hardness (Hc) and mechanical hardness (HV) for each heat treatment. This paper aims to evaluate the martensite morphologies and volume fractions of dual-phase (DP) X70 microstructure (ferrite + martensite) using the Magnetic Barkhausen Noise (MBN) non-destructive technique. In order to develop different morphologies in X70 steel, three different heat treatments (intermediate quenching (IQ), direct quenching (DQ), and step quenching (SQ)) are done for half-hour. The inter-critical annealing temperature (IAT) ranges applied to the samples are from 740 °C to 820 °C. Measurements of MBN were carried out using the MikroMach instrument. Hardness measurements and microstructure examinations were conducted using a durometer and optical microscopy. Experimental results show that IQ treatment formed fine and fibrous martensite morphology uniformly distributed in the ferrite matrix. DQ treatment revealed hard martensite distributed in a ductile ferrite matrix. SQ treatment showed a banded morphology with a non-uniform distribution of the phases. Martensite Volume Fraction (MVF) and hardness get larger with IAT increases. These treatments permit obtaining the highest hardness in DQ treatment, and the lowest hardness has been identified in SQ treatment. Indeed, the MBN method has been proved to be highly sensitive to changes in microstructure morphologies and MVF. The highest MBN signal amplitude (MBNSA) for the SQ treatment has been identified at the lowest magnetic excitation field; in contrast, the lowest MBNSA for DQ treatment was located at the highest magnetic excitation field. The maximum MBNSA decreases whenever IAT increases. Finally, the direct correlation between magnetic hardness (Hc) and mechanical hardness (HV) for each heat treatment is established in this paper.

Effect of Step Quenching Heat Treatments on the Kinetics of Ferrite Formation and Quenching & Partitioning Modeling for a Commercial C-Mn-Si Steel

Aiming to meet current demands from the automotive industry, a third generation of advanced high strength steel (AHSS) has been developed as an alternative to previous generations. Different alloys and innovative processes have been deeply studied as, for example, quenching and partitioning (Q&P). The published papers highlight that the best heat treatment parameters for a satisfactory Q&P execution are strongly dependent on the austenite conditioning and they can be optimized if thermodynamic and kinetics calculations are performed. In this context, this work evaluated the effect of step quenching (SQ) heat treatments on the kinetics of ferrite formation and Q&P modeling for a commercial C-Mn-Si steel, predicting the microstructural evolution and the final phase fractions as a function of the heat treatment parameters. The JMAK model was optimally fitted to the ferrite fraction, as well as to the microhardness data. The kinetics characterization and the thermodynamic modeling showed that the combination of SQ and Q&P can provide a high retained austenite fraction in a multiphase microstructure. As a conclusion, it is possible to state that a well-planned SQ heat treatment followed by an optimized Q&P cycle has the potential to generate an advanced steel with a final microstructure assisted by the TRIP effect.

The nanocluster formation and vacancy behavior of step-quenched Al–Mg–Si alloy and its effect on transition to β" phase via advanced methods

The effect of step-quenching (SQ) on two/multi-step aging and the bake-hardening response in Al-0.9Mg-1.0Si (in wt.%) alloy was investigated in terms of nanocluster formation and vacancy behavior using advanced methods, including atom probe tomography (APT) and positron annihilation lifetime spectroscopy (PALS) analysis. The SQ treatment was found to be highly effective at suppressing the negative effects of natural aging during two/multi-step aging, compared with pre-aging (PA) treatment. The characteristic behaviors of nanoclusters formed by PA and SQ treatments were completely different. SQ is likely to preferentially form Si–V complexes, which are different than Cluster (1), and Si-rich clusters, as detected by the distribution of cluster compositions using APT analysis. Si–V complexes pre-formed during SQ treatment result in the formation of Mg–Si clusters with higher atomic density, inducing favorable transformation to the pre-β" phase because there are fewer vacancies inside the Mg–Si cluster. Also, the Si–V complexes reduce the fraction of Si-rich clusters commonly formed during natural aging (NA). The step-quenched Al–Mg–Si alloy with great bake-hardening response has promising potential for automotive outer panel applications.

Effect of Martensite Morphologies on Corrosion in 5% H2SO4 Solution of Borided X70 Dual Phase Steel

In the present investigation, some electrochemical properties of dual phase X70 steels with different martensite morphologies which have undergone boriding were studied. To obtain a variety of martensite morphologies, Direct Quenching (DQ), Intermediate Quenching (IQ) and Step Quenching (SQ) heat treatments were applied at an intercritical annealing temperature (IAT) of 760℃. The treatment (DQ) allowed the formation of fine martensite evenly distributed in the ferrite matrix. (IQ) treatment showed the formation of martensite along the ferrite / ferrite grain boundaries. In contrast, treatment (SQ) induced the formation of a banded morphology of martensite and ferrite. The realization of borides on X70 (DP) steel was carried out in a powder mixture containing 5% of B4C as source of boron, 5% of NaBF4 as activator and 90% of SiC as diluent at 950℃ for a period of time from 4 h. The corrosion behavior of X70 (DP) steel has been explored by the Tafel extrapolation method in a 5 wt. % H2SO4 solution. The corrosion resistance of steel which has undergone boriding (BDP) is higher than that of steel which has not undergone it (DP).

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.

Effect of various quenching treatments on microstructure and mechanical behavior of a laser additively manufactured 12CrNi2 alloy steel

• The prior austenite grain size of martensite in the martensite-ferrite dual-phase microstructures was adjusted. • The nanotwins were observed as a main substructure of the refined martensite. • The refined martensite in the dual-phase steel led to a higher total elongation with respect to the coarse one. • The IHAZ retained in the steel after DQ treatment was conducive to enhance the post-uniform logarithmic strain. A ferrite-austenite 12CrNi2 alloy steel additively manufactured by laser melting deposition (LMD) has been treated by various quenching methods, including direct quenching (DQ), inter-critical quenching (IQ) and step quenching (SQ), respectively, to form the martensite-ferrite dual-phase steel with a similar major volume fraction (62 vol.%) but different grain sizes and lath substructures of martensite. The results demonstrate that SQ, DQ and IQ help to refine the grains of martensite in order, correspondingly decreasing the yield strength, ultimate tensile strength but increasing in the uniform logarithmic strain of the alloy steel. The effect of not only the interface structures and substructures of martensite but also the quenching-route-dependent structures of ferrite on mechanical properties was interpreted in detail. Curiously, the directly quenched steel displays a higher post-uniform logarithmic strain as compared to the inter-critically quenched steel. The abnormal result occurs due to that DQ cannot completely homogenize the microstructural difference between the laser-scanned layers and the interlayer heat-affected zones of the as-deposited alloy steel.

Effects of the Phase Interface on Spallation Damage Nucleation and Evolution in Dual‐Phase Steel

The dual‐phase steel sample obtained by step quenching (SQ) of AISI 1020 steel is shock loaded using one‐stage light‐gas gun experiments. The free surface particle velocity is measured by Doppler pin system (DPS) during the loading experiment. The effect of phase interface on spallation damage nucleation and evolution in dual‐phase steel is investigated with optical microscopy (OM), nanoindentation, and electron backscatter diffraction (EBSD) techniques. The evolution mechanism of spallation damage in dual‐phase steel is systematically analyzed. The results show voids nucleated within martensite rather than at the martensite/ferrite interface under dynamic loading. The nucleation of voids in martensite was not random, the martensite block boundaries with a great Taylor Factor (TF) mismatch and a large angle to the impact direction were the preferred nucleation positions of the voids. Voids nucleate, grow and further coalesce into microcracks. Most of the microcracks propagate across the phase interface and the propagation direction hardly deflects. Due to the existence of many martensite blocks with high‐angle grain interface boundaries inside the martensite, the resistance of microcracks propagation in martensite is greater than that in ferrite. Furthermore, the size of microcracks is related to the angle between the direction of martensite blocks and shock direction.

Effect of Heat Treatment on Microstructure and Mechanical Properties of an Fe-6.5Mn-0.08C Medium-Manganese Steel

Effect of heat treatment on microstructure and mechanical properties of an Fe-6.5Mn-0.08C medium-manganese steel is investigated in this study. Three kinds of medium-manganese steel specimens are fabricated by varying heat treatments of intermediate quenching (IQ), step quenching (SQ), and intercritical annealing (IA). Hardness and tensile tests are performed to examine the correlation of microstructure and mechanical properties for the Fe-6.5Mn-0.08C medium-manganese steel specimens. The IQ and SQ specimens have microstructures of martensite matrix with ferrite, whereas IA specimen exhibits microstructure of acicular ferrite matrix with martensite. The tensile test results show that the SQ specimen with martensite matrix has the highest yield strength and the lowest elongation. On the other hand, the SQ specimen has the highest hardness due to the relatively lower reduction of carbon content in martensite during intercritical annealing. According to the fractography of tensile tested specimens, the SQ specimen exhibits a dimple and quasi-cleavage fracture appearance while the IQ and IA specimens have fully ductile fracture appearance with fine-sized dimples caused by microvoid coalescence at ferrite and martensite interface.

Effect of Step Quenching on Multi-Step Aging Behavior in Al-Mg-Si Alloys.

Two types of nanoclusters were detected using DSC thermal analysis operating at -50 °C with liquid nitrogen. The formation of Cluster (1) was substantially suppressed by the formation of Cluster (2) during step quenching at 100 °C for 0.06 ks. The hardness increase was retarded for a period during two-step aging at 170 °C in the case of natural aging for 604.8 ks. The hardness decrease was determined to be due to the dissolution of nanoclusters in the early stage of two-step aging at 170 °C. On the other hand, hardness was directly increased in the step-quenched samples during multi-step aging at 170 °C. It is noted that Cluster (2) directly transforms into the strengthening phase. The bake hardening response is fairly enhanced by the formation of Cluster (2), which is formed by the step quenching process. It was confirmed that the step-quenched samples had a higher number of precipitates than the naturally-aged ones based on TEM images. The structure of the precipitate was identified to be the β″ phase by analyzing HRTEM image.

Unraveling the Effect of Martensite Volume Fraction on the Mechanical and Corrosion Properties of Low‐Carbon Dual‐Phase Steel

The effect of martensite volume fraction on the mechanical and corrosion properties of low‐carbon dual‐phase steel is studied based on both step quenching (SQ) and intercritical annealing (IA) routes. For SQ samples, hardness and ultimate tensile strength decrease with increasing holding time at the intercritical temperature and reach a plateau, which is related to the decrease in the amount of martensite during annealing. Conversely, for the IA samples, hardness increases during holding at the intercritical temperature due to austenitization and reaches the same plateau. At a same martensite volume fraction, the work‐hardening behavior of SQ samples is better than IA samples, which is related to both the finer grain size and smaller martensite islands in the former. At low martensite fractions, the corrosion properties are comparable with the as‐received ferritic–pearlitic sample. It is revealed that by decreasing the volume fraction of martensite in SQ samples, the corrosion current density (icorr) decreases almost linearly, and at martensite content of zero, it reaches the icorr of the fully ferritic microstructure. The latter is found to be lower than the icorr of the as‐received sample. Therefore, the real effect of martensite on corrosion properties is unraveled for the first time.

Processing Route Effects on the Mechanical and Corrosion Properties of Dual Phase Steel

The mechanical and corrosion behaviors of low carbon DP steel were studied based on different processing routes: (1) intercritical annealing (IA), (2) step quenching (SQ) via austenitization and quick transferring of the sample to the second furnace, and (3) Slow SQ via furnace cooling to the desired temperature. The properties were found to be highly dependent on the volume fraction of martensite (VM) and the density of ferrite/martensite interfaces. However, at the same martensite content, the mechanical properties of Slow SQ sheet were inferior than those of SQ and IA sheets, which were related to the relatively poor work-hardening behavior due to the severe partitioning of Mn between ferrite and martensite phases. The latter was also responsible for an increase in the corrosion current density (icorr) via galvanic corrosion. These results were analyzed based on the polarization curves and Nyquist plots obtained from the electrochemical impedance spectroscopy test. This study revealed that the SQ route can result in both better mechanical performance and higher corrosion resistance.

An automated morphological classification of ferrite-martensite dual-phase microstructures

In this study, a simple classification scheme based on image processing is proposed to categorize optical microstructures of dual-phase steels with varying morphology (i.e., shape, size and distribution) of ferrite-martensite phases. Dual-phase microstructures with varying morphologies have been developed by employing different heat treatment schedules; namely, intercritical annealing (IA), intermediate quenching (IQ) and step quenching (SQ) using a low carbon micro-alloyed steel. The images of optical microstructures of the developed steels have been first segmented using Otsu thresholding technique and then, the ratio of the black (martensite) and white (ferrite) run-lengths for the phases have been calculated. The mean and median values of these ratios have been considered further, and it has been demonstrated that the mean values are more appropriate than the median values for the classification purpose. The mean and standard deviation of the former parameter for IA, IQ and SQ microstructures are 0.679 ± 0.040, 1.540 ± 0.090 and 0.989 ± 0.119, respectively. The simple classification technique is found to be advantageous from different aspects.

Effect of holding time at an intercritical temperature on the microstructure and tensile properties of a ferrite-martensite dual phase steel

To analyze the influence of the initial microstructure in the production of a dual phase (DP) steel on the mechanical properties, a commercial DP steel was subjected to two heat treatments (HT): step quenching (SQ) and intermediate quenching (IQ). SQ samples were austenitized at 930 °C for 20 min, then annealed at 830 °C for 5, 15, 30, 60 and 120 min and finally water quenched. Shorter annealing times show allotriomorphic ferrite and martensite while at longer time polygonal ferrite is formed. Tensile tests showed that yield and tensile strength are higher than those of the commercial DP steel and besides, the uniform elongation is smaller. IQ samples were water quenched after austenitization and then annealed at 830 °C for the same holding times and water quenched again. The microstructure consists of martensite dispersed in a ferrite matrix. The remarkable results are that these samples had similar tensile properties to that of the as-received DP steel. Tensile properties were analyzed as a function of the characteristic microstructure and the assessment of the alloying elements partition during the different HTs. Evaluation of the Kolmogorov-Johnson-Mehl-Avrami theory indicates that the transformation in SQ and in IQ samples corresponds to a diffusional process.

Comparing Microstructures and Tensile Properties of Intercritically Annealed and Quenched-Tempered 1.7Ni–1.5Cu–0.5Mo–0.2C Powder Metallurgy Steels

The aim of this study was to compare the influence of intercritical quenching (IQ), step quenching (SQ) and quenching plus tempering (QT) heat treatments on the microstructure and tensile properties of 1.7Ni–1.5Cu–0.5Mo–0.2C pre-alloyed powder metallurgy (P/M) steels. In the microstructures of the IQ and SQ specimens partial martensite having Ni-rich phases formed up in the soft ferritic matrix. It was observed that unlike Mo, a Cu alloying element dissolved homogeneously in the specimens. The martensite volume fraction (MVF) in the SQ specimens was higher than that in the IQ specimens. It was found that macrohardness, yield and tensile strengths increased, whereas microhardness of ferrite and elongation decreased with increasing MVF. However, with this increase, microhardness values of martensite phases decreased in the IQ specimen, while they increased in SQ specimens. It was observed that the yield, tensile, and elongation values of the QT specimens were lower than those of all intercritically annealed specimens having the same hardness values.

Influence of Heat Treatment Schedule on the Tensile Properties and Wear Behavior of Dual Phase Steels

The aim of present study is to investigate the influence of heat treatment schedule on the microstructure, tensile properties, and wear behavior of dual phase (DP) steels. In this regard, a cold-rolled low carbon steel (0.18% C, 1.25% Mn) is subjected to intercritical annealing (IA), step quenching (SQ), and intermediate quenching (IQ) heat treatment cycles to produce DP steels with different microstructures. The results show that the tensile strength, strength-elongation balance, and wear resistance of DP steels change in the order of SQ < IA < IQ. Variation of work hardening rate (θ = dσ/dϵ) with stress for the IA treated DP steel shows a one stage behavior corresponding to stage III, where θ linearly decreases with increasing flow stress. In contrast, for IQ and SQ treated specimens, a two stage behavior corresponding to an initial rapid decrease of θ with stress (transition stage) followed by stage III hardening are observed. The dominant wear mechanism for DP steels produced by IA and IQ treatments is a combination of abrasion and oxidation, while for the SQ treated specimen, a combination of oxidation and delamination is observed.

Effect of Heat-Treatment Schedule on the Microstructure and Mechanical Properties of Cold-Rolled Dual-Phase Steels

Low-carbon (0.08 wt% C) steel has been subjected to three different heat treatments to obtain dual-phase steels with different microstructures. An understanding of structure–property was established through tensile tests, in conjunction with scanning electron microscope and transmission electron microscope. The results show that the steel after intermediate quenching (IQ) consisting of fine and fibrous martensite exhibited the intermediate strength, highest elongation and the best comprehensive performance of mechanical properties, whereas the steel subjected to intercritical annealing (IA) produced a network martensite along ferrite grain boundaries, having the lowest strength and intermediate elongation. Besides, step quenching (SQ) resulted in a coarse and blocky ferrite–martensite microstructure showing the worst mechanical properties of the three different heat-treatment conditions. The strain-hardening behavior was studied through the modified Crussard–Jaoul model, indicating two stages of strain-hardening behavior for all three samples. The highest magnitude of strain-hardening ability was obtained by IQ annealing routes. The analysis of the fractured surface revealed that ferrite/martensite interfaces are the most susceptible for microvoid nucleation. However, martensite microcracks were also observed in SQ sample, and the microvoids are nucleated within the ferrite grain in IA sample as well. The variations in strength, elongation, strain-hardening behavior and fracture mechanism of the steel with different heat-treatment schedules were further discussed in relation to the microstructural features.

Dilatometer Analysis of the One Step Quenching and Partitioning Process in a 1.5%Si Steel

A one step quenching and partitioning process was applied to a 0.2%C-2.0%Mn-0.5%Cr-1.5%Si steel by quenching austenitised samples to several different temperatures below the experimentallydetermined martensite start temperature of 397 °C and isothermally partitioning them beforequenching to room temperature using a quenching deformation dilatometer. These treatmentsyielded predominantly martensitic microstructures containing 5.6 vol.% to 7.5 vol.% retained austenite,as measured by x-ray diffraction. In each treatment, strong dilation was recorded during isothermalpartitioning, with little indication of phase transformation during subsequent cooling to room temperature.This behaviour lends weight to the idea that an isothermal phase transformation occurred duringpartitioning, and that the final microstructure is a mixture of athermally and isothermally formed constituents.These results also suggest that the final microstructure of this steel is mostly formed beforeand during partitioning.

Effect of Step Quenching and Zr Addition on the Microstructure and Mechanical Properties of Al-Mg-Si Alloy

The effects of substituting a step quenching process for the traditional water quenching and of adding zirconium into an Al-Mg-Si alloy, were investigated using differential scanning calorimetry (DSC) and transmission electron microscope (TEM), together with a tensile and hardness tester. The results indicated that step quenching and zirconium addition affected the microstructure and mechanical properties significantly. Step quenching at 100 ℃ for the Al-Mg-Si alloy was the optimal choice because it promoted the precipitation of the β′′ phase, afforded the maximum age-hardening response and strength increment, and restrained the negative effect of natural aging. Zirconium addition improved the strength while retaining good plasticity for the precipitation of Al3Zr. Zr partially promoted the precipitation kinetics of the β′′ phase when step quenching was conducted at 50 or 100 ℃. †(Received January 15, 2013)

Microstructure and Charpy impact properties of ferrite–martensite dual phase API X70 linepipe steel

The microstructural changes which occur during intercritical annealing treatment at 780°C of API X70 High Strength Low Alloy (HSLA) Steel have been investigated. Three treatments were developed to produce dual phase microstructure: Intermediate Quenching Treatment/Air (IQT/A), Intermediate Quenching Treatment/Water (IQT/W) and Step Quenching Treatment (SQT). The IQT/Water treatment resulted in the formation of fine and fibrous martensite morphology uniformly distributed in the ferrite matrix. However, the IQT/Air treatment showed a spherical network of martensite along the ferrite/ferrite grain boundaries. The SQT treatment yielded blocky and banded martensite and ferrite morphology. The experimental results show that the API X70 HSLA steel with finely dispersed microstructures (IQT/Water treatment) has higher Charpy impact properties and lower ductile–brittle transition temperature DBTT than HSLA steel with banded microstructures. A carefully conducted comparison of fracture surfaces of the representative specimens obtained after IQT and SQT treatments has been studied.