Search

In this work, the effects of cold rolling (CR) and cold rolling–cryogenic treatment (CR–CT) on the microstructure and mechanical properties of Fe–32Ni alloy were studied via optical microscopy methods, OM, SEM, XRD, TEM, tensile strength and hardness tester, and tensile testing. The results reveal the grain refinement in the alloy after rolling deformation. When the deformation is higher than 85%, the polygonal austenite grains become layered, and a small amount of martensite forms. Because of the inhibitory effect of cold-rolling deformation before cryogenic treatment on martensitic transformation, the amount of martensite form phase after cryogenic treatment decreases with the increase of deformation. The hardness and strength of the sample, independent of whether the cryogenic treatment is performed, increase with the increase of deformation degree. Under the same deformation rate, the hardness of the CR–CT sample is higher than that of the CR sample, which is related to the hard martensite phase with high dislocation density obtained during cryogenic treatment. The strain hardening behavior of the sample is greatly affected by the deformation degree. With the increase of true strain, the work hardening exponent of CR and CR–CT samples undergoing severe plastic deformation is lower than that at small deformation degree and low dislocation density, which is attributed to the earlier entanglement of high dislocations in CR and CR–CT samples with large deformation degrees. [ABSTRACT FROM AUTHOR]

A low-carbon 18Mn3Si2CrMo steel was subjected to austempering at different temperatures below M s , and low-temperature bainite was introduced. The multiphase microstructures of martensite, bainite and retained austenite with different volume fractions were obtained, and the relationship among their phase contents and mechanical properties was studied. The 18Mn3Si2CrMo steel with a volume fraction of martensite ( V M ), bainite ( V B ) and retained austenite ( V γ ) of approximately 80%, 16% and 4%, respectively was obtained by austenitizing at 900 °C followed by austempering at 205 °C for 2 h, and then exhibited the optimal mechanical properties with a yield strength of 944 MPa, a tensile strength of 1512 MPa, a total elongation of 15.4% and an impact toughness of 119 J/cm 2 . When the austempering temperature was increased from 205 °C to 330 °C, the yield strength, tensile strength and impact toughness of the steel decreased. The poorest comprehensive mechanical properties were observed after 330 °C of austempering. The introduction of low-temperature bainite enhanced the ductility and toughness of the 18Mn3Si2CrMo steel. [ABSTRACT FROM AUTHOR]

A novel process, quenching to a low temperature far below M s followed by a short austempering treatment (Q-A-T), was carried out on a low-carbon martensite steel. Results show that a notably improved toughness without losing strength was obtained on the Q-A-T specimen as compared with the oil-quenched specimen. Microstructure analyses reveal that the Q-A-T specimen consisted of low-carbon martensite, high carbon–low temperature bainite and retained austenite. The fast quenching process effectively refined the martensite plate and the austempering process introduced the high strength phase–low temperature bainite, both of which are responsible for the improved mechanical properties. [ABSTRACT FROM AUTHOR]

The cooling process is important for the formation of duplex microstructure in Zr–Nb alloy. To further understand its effect on the microstructure, three cooling methods, furnace cooling, air cooling and water cooling, were carried out on the annealed specimens. The results show that the equiaxed microstructure can be obtained in furnace-cooled specimens with equiaxed α grains and residual β phase, and the duplex microstructure can be obtained via the air cooling and water cooling processes, in spite of the different transformation mechanisms and phase constitutions. The duplex microstructure for the air cooling process was formed via diffusion-type transformation and consisted of primary α phase, secondary α phase, ω phase and the residual β phase, while for the water cooling process it was formed via displacive-type transformation and consisted of primary α phase, α′ phase and ω phase. There were many dislocations within the α′ phase for the microstrain during the phase transformation. The strength increased and elongation decreased with increasing cooling rate. However, the air- and water-cooled specimens possessed a high value of product of strength and elongation, revealing a high security for the two duplex microstructures, in spite of their differences. [ABSTRACT FROM AUTHOR]

The microstructure and the mechanical properties of a low-carbon Al/Si-alloyed carbide-free bainitic steel austempered at temperatures between 300 and 350 °C have been investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, x-ray diffraction analysis, and mechanical property tests. The results show that an excellent combination of tensile strength, ductility, and impact toughness is obtained at the austempering temperature of 320 °C-but neither at the lowest temperature of 300 °C nor at the highest temperature of 350 °C. These results are correlated with the features of the constituent phases of the microstructure, especially the amount and size of retained austenite, which are largely dependent on the austempering temperature. The observations are also due to the inconsistent effects of strain-induced martensitic transformation on strength, ductility, and toughness. The results of the present study suggest that an optimized isothermal temperature may also exist for any other bainitic steel, at which, if austempering treatment is carried out, an improved combination of tensile and impact properties can be obtained. [ABSTRACT FROM AUTHOR]

• A SFE-dependent strategy was carried out on a medium Mn TRIP steel. • High density dislocations and high order nanotwins were obtained. • Martensitic transformation within nanotwins induced improved mechanical properties. In this work, we developed a strategy based on SFE controlling to improve the mechanical properties of a medium Mn steel. Two successive cycles of deformation were conducted in the stacking fault energy ranges dominated by dislocation slip and twinning deformation, which developed steel with high density dislocations and nanotwins. Dislocation strengthening results in high yield strength. Meanwhile, martensitic transformation within nanotwins provides continuous hardening capacity and consequently high strength and acceptable ductility via alleviating shear strain accumulation and retarding crack initiation. [ABSTRACT FROM AUTHOR]

Abstract High tensile strength of ~ 1550 MPa, uniform elongation of ~ 30%, and impact energy of ~ 80 J were achieved in a high-C high-Si bainitic steel, by austempering around the nose-tip temperature of the time–temperature–transformation (TTT) diagram. This bainitic microstructure revealed multiple lengths of nanobainite plates and various sizes and shapes of retained austenite. Its bainite transformation during austempering and microstructure evolution during tensile deformation were discussed in depth. Results demonstrated that large blocks of retained austenite (with inhomogeneous carbon distribution) transformed partly and gradually to multiple martensite grains as strain increases, due to the strong effect of carbon on mechanical stability of retained austenite. Ductile long nanobainite plates (~ 0.13 at% C) suppressed the propagation of microcracks, because of stress relief effect ahead of the current microcracks. [ABSTRACT FROM AUTHOR]

In the present study, the microstructure evolution and the corresponding mechanical properties of V-containing low carbon ultra-fine bainitic steel during tempering were studied. The results show that during the initial stages of tempering, part of retained austenite in V-containing ultra-fine bainite steel transforms into bainite. The stability of the retained austenite improves when the tempering period is increased to 5 h. Simultaneously, nano-sized VC precipitates are precipitated in the microstructure. On the one hand, these phases can give full play to the transformation-induced plasticity (TRIP) effect by modulating the carbon content of the retained austenite. On the other hand, they can also reduce the lattice distortion in the matrix and combine it with the lower dislocation density, thereby improving the plasticity of the specimen. At the same time, the fine precipitated phase can also effectively improve the toughness of the cleavage crack region in the transition zone, preventing the micro-cracks from generating at the interface of the precipitated phase and then extending to the adjacent matrix to cause fracture, so that the uniform elongation, total elongation, impact toughness at room temperature and impact toughness at low temperature of the 5 h specimen are as high as 8.8 ± 0.2 %、16.2 ± 0.7 %、113 ± 4 J/cm2 and 76 ± 2 J/cm2, respectively. Furthermore, the primary contribution of the strengthening mechanism in the microstructure has shifted from fine grain strengthening and dislocation strengthening to precipitation strengthening, and part of the retained austenite undergoes a martensitic transformation during cooling, resulting in the highest yield strength of 1086 MPa for the 5 h specimen. The tensile strength is effectively maintained at 1402 MPa by increasing the tempering period, which has good tempering resistance, to achieve the comprehensive performance optimization of bainitic steel. [ABSTRACT FROM AUTHOR]

Abstract: The serrated flow behavior of a Fe–Mn–C twin-induced plasticity steel was investigated at room temperature. Tensile tests were performed at various strain rates under extensometer-measured strain control, as compared to conventional crosshead displacement-controlled tests. Different types of serrations were observed with type C ones identified for the first time. [Copyright &y& Elsevier]

In this paper, martensite-to-austenite reversion during intercritical annealing was applied to a conventional low-alloy transformation-induced plasticity steel to form reverted austenite, which was followed by austempering to form bainite, and hence the resulting microstructures and mechanical properties were investigated. The obtained microstructure is composed of alternating lamellae of intercritical ferrite and reverted austenite on microscale, with the latter consisting of bainitic ferrite laths and retained austenite films on nanoscale. This unique microstructure exhibits excellent strength–ductility combinations, which surpass those of conventional polygonal blocky-structured counterparts, providing a strategy for developing high-performance structural materials at low production cost. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The effect of magnetic field on bainite microstructure and mechanical properties of 70Si3MnCr steel was studied. Microstructure observation shows that introducing magnetic field during the austempering process, the microstructure was significantly refined and the volume fraction of the bainitic ferrite was increased. The high nucleation rate for bainitic ferrite caused by the high magnetostatic energy should be responsible for this positive effect. As compared with samples without magnetic field treatment, the magnetic samples also show a simultaneously increased strength and ductility. The ultra-fine bainitic ferrite with higher dislocation density, in magnetic samples increased the tensile strength, and carbon-riched filmy retained austenite and small blocky retained austenite in magnetic samples have a tremendous effect on ductility. High magnetic strength in magnetic samples increases the strength of the bainitic ferrite, which acts as a stress shielding source to prevent the retained austenitic phase from higher stress and increase the plasticity. Moreover, high magnetic strength can alter the spins of the free radicals between the dislocations and obstacles from S state to T state which had weak bonding with dislocations. Hence, depinning of dislocations is facilitated which results in increased plasticity. Therefore, due to the magnetoplasicty, high magnetic properties, and ultra-fine microstructure, magnetic samples show a more excellent strength and ductility as compared with the normal samples. [ABSTRACT FROM AUTHOR]

A novel heat-treatment process, below- Ms austempering, was performed on two low-C high-Si/Al steels, and accordingly the resultant microstructures and mechanical properties were investigated. The results showed that bainitic transformation occurs at isothermal temperatures below Ms and unexpectedly, the transformation process is accelerated significantly by the prior formation of martensite. Furthermore, as compared with the conventional above- Ms austempering process, the below- Ms austempering refines effectively the bainitic plates and reduces the size and volume fraction of blocky martensite/austenite phases, giving rise to much enhanced impact toughness and improved tensile properties. [ABSTRACT FROM AUTHOR]

We show more persistently-enhanced strain hardening with improved tensile strength and maintained high plasticity in Fe–18Mn–1.0C than in Fe–22Mn–0.6C steel. This is primarily attributed to the gradual release of more densely-spaced thinner twins and twin bundles favored by the increased dynamic interactions between carbon and dislocations in the carbon-increased steel. [ABSTRACT FROM AUTHOR]

Two-step austempering process has been demonstrated to be an effective method to accelerate the bainitic transformation at low temperature, but the rule of first-step controlling is still unclear. This paper comprehensively studies the effects of the first-step transformation on bainitic transformation, microstructural evolution and the related mechanical properties of ultra-fine bainitic steel obtained by two-step processes. Results show that the two-step austempering process notably accelerates the bainitic transformation, with the maximum reduction in total transformation time of 93.1% when the first-step bainitic fraction is controlled to be 22%. The thickness and length of bainitic ferrite plates and the size of blocky retained austenite of the two-step austempering samples are smaller than those of the conventional first-step austempering sample. Although the hardness and strength of the two-step austempering samples decrease slightly as compared with traditional one-step sample, they also meet the requirements of the product, such as bearing, especially the sample with 22% and 50% first-step fractions, which achieve higher toughness and yield strength, respectively. This study provides a theoretical support on process design of ultra-fine bainitic steel, based on short-period heat treatment process, optimized microstructure and mechanical properties. [ABSTRACT FROM AUTHOR]

Abstract: The monotonic tensile properties and deformation mechanisms of Fe–Mn–C twinning-induced plasticity (TWIP) steels have been extensively studied; however, the low-cycle fatigue (LCF) properties of this series of advanced steels have not been well understood. The present paper addresses the cyclic deformation behavior and the deformed microstructure of an as-annealed TWIP steel. Fully reversed push–pull LCF tests were performed at room temperature under total strain amplitude control with a strain rate of 0.006s−1 and strain amplitudes ranging from 0.002 to 0.01. The results show initial rapid cyclic hardening within the initial 10% of the fatigue life at all strain amplitudes, and demonstrate an obviously enhanced cyclic yield strength. Different types of cyclic stress responses were revealed, which are featured by initial cyclic hardening followed by cyclic saturation, or followed by cyclic softening and saturation, or followed by cyclic softening without saturation till the final fracture, depending on the strain amplitude applied. The microstructure prior to and after fatiguing were examined by means of optical and transmission electron microscopy. The typical optical microstructure of fatigued samples is characterized by increases in slip band density with increasing strain amplitude or number of cycles at a given strain amplitude applied. The substructures of the deformed samples are featured by the formation of stacking faults and vein/labyrinth dislocation structures, while fine twins and cell or wall dislocation structures, besides those generated at lower strain amplitudes, are formed at high strain amplitudes. [Copyright &y& Elsevier]

Nanobainite shows high tempering stability. However, studies of the multi-tempering behavior of nanobainite hot-working die steel are rare. In the present work, the microstructure and mechanical properties of Si–H13 nanobainite steel, subjected to multi-tempering processes, have been investigated via microstructural characterization and mechanical tests. The results show that the filmy retained austenite decomposed into ferrite and granular V 2 C during the first tempering process. The size of V 2 C increased slightly after the second tempering. VC precipitated from bainitic ferrite of the third tempered sample. Because of hindering effects that decomposition products of filmy retained austenite have on the combination of bainitic ferrite laths, the increment of thickness of bainitic ferrite lath is only 29% (26 nm) after multi-tempering processes. The yield strength of multi-tempered samples is obviously higher (by more than 50%) than that of austempered samples. With an increase in number of tempering, the dominant strengthening contribution changes from grain boundary strengthening and dislocation strengthening to precipitation strengthening. Moreover, the precipitation of carbides results in secondary hardening during the tempering process. Although the thickness of bainitic ferrite lath became coarse, the hardness of the multi-tempered sample was approximately the same as that of austempered sample. Hence, the Si–H13 nanobainite steel exhibits good tempering stability. • The Si–H13 nanobainite steel is subjected to multi-tempering processes and exhibits good tempering stability. • The precipitation of carbides results in secondary hardening and precipitation strengthening during the tempering process. • The decomposition products of filmy retained austenite can hinder the combination of bainitic ferrite laths. • The contribution of precipitation strengthening increases greatly after the multi-tempering processes. [ABSTRACT FROM AUTHOR]

The previous research on tempered nanostructured bainite has been mainly focused on the carbide precipitation at high temperatures with few details on nanostructured bainite tempered at lower temperatures. In this research, the microstructure evolution and the corresponding mechanical properties of the nanostructured bainite tempered at a temperature ranged from 180 °C to 400 °C were studied via X-ray diffraction, transmission electron microscope and high-resolution thermal dilatometer. Results show that retained austenite occurred bainitic transformation when the tempering temperature ranged from 210 °C to 320 °C. The amount of newly formed bainite increased at first and then decreased slightly with increasing tempering temperature. When the tempering temperature reached 380 °C and higher, the cementite precipitated from retained austenite and reduced the stability of over-stable retained austenite. As a result, it can be concluded that stress-induced martensitic transformation would occur easily in retained austenite during deformation process, leading to the optimum increase of retained austenite contribution to toughness. coefficient of carbon increased sharply, the dispersed cementite particles precipitated between the nanostructured bainitic plates, inhibiting coarsening of bainitic plates when the tempering temperature reached 380 °C. Finally, nanostructured bainitic plates only coarsened slightly. Therefore this high-carbon high-silicon nanostructured bainitic plate possess a high dimensional stability. [ABSTRACT FROM AUTHOR]

Abstract: The tensile and impact properties of two low-carbon bainitic transformation-induced plasticity (TRIP) steels, one being Al-free and the other being Al-containing, have been investigated. The two steels are featured by the existence of different sizes and shapes of retained austenite showing different mechanical stabilities. Compared with the Al-containing steel, the Al-free steel shows significantly improved tensile strength, uniform strain and total elongation, but exhibits much decreased impact toughness. These results demonstrate conflicting effects of strain-induced martensitic transformation of retained austenite on tensile ductility and impact toughness of TRIP steels. It is indicated that a wide distribution of size and shape of retained austenite is favorable for sustained high strain hardening rate and increased uniform tensile strain; however, thin films of retained austenite, which is mechanically more stable, tends to enhance impact toughness. [Copyright &y& Elsevier]

Vacuum hot-rolled bonding (VHRB) titanium (Ti)-steel clad composites have been increasingly serving in both severe corrosion and heavy load conditions, and their overall properties severely depend on the microstructure of interfacial reaction layer. Notably, introducing an appropriate interdiffusion barrier to minimize the brittle Fe–Ti intermetallics has still been a popular scheme for optimizing their interfacial properties. In this work, with two low-alloyed steels of 0.06 and 0.16 wt% carbon (C) contents employed, the pure Ti -steel composite samples were successfully prepared via vacuum hot-compressed bonding (VHCB) in Gleeble-3500 system under the identical temperature/reduction/rate condition of 850 °C/70 %/0.01s−1. The interfacial reaction, microstructure and tensile property in two Ti-steel composites were investigated to estimate the C content effect via various characterizations, and the mechanisms were clarified. Results indicated that the diffusion of Ti, Fe and C mainly occurred in the interfacial reaction layers of two composites, and accordingly the reaction products of TiFe, TiFe 2 and TiC formed, together with the C-depleted α-Fe zone. The nanoscale TiC particles were mainly located at the steel side, and increased with the increasing C content. Correspondingly, the brittle TiFe layer and TiFe 2 layer decreased due to the increased TiC layer acting as a diffusion barrier. This led to an evident decrease of percent cleavage fracture area (CA , %) from 77% to 52% and a significant increase of interfacial tensile strength from 182 to 344 MPa. [ABSTRACT FROM AUTHOR]

The excellent properties of transformation-induced plasticity (TRIP) steels with a lamellar structure are generally claimed to mechanically stabilize retained austenite (RA) grains, enabling the TRIP effect to persist up to higher strains. Nevertheless, the present work finds that nearly no RA grains are transformed into martensite at the strain range of 0.10–0.14 in lamellar-structured specimens. Furthermore, the strain-hardening exponent remains nearly unchanged. To explain the underlying mechanisms of this strain-hardening behavior, the block/packet/prior austenite grain boundaries of the hierarchically lamellar-structured TRIP steel were reconstructed. Moreover, the evolutions of full-field strain distribution and back stress hardening with straining were examined through the in situ micro digital image correlation technique and cyclic loading–unloading–reloading tensile tests. Results show that the block structure controls the microstructural deformation behavior of lamellar-structured specimens. Within a block, ferrite and bainite layers could deform synergistically. However, next to block boundaries, ferrite layers are susceptible to a large strain gradient due to the different deformation tendencies of adjacent blocks. Compared to conventional polygonal-structured specimens, the more refined ferrite grains of lamellar-structure specimens reduce the strength difference between ferrite and bainite layers. The strains are more uniformly partitioned among various constituents, thus delaying the occurrence of tensile necking. Back stress hardening dominates the strain-hardening in conventional polygonal-structured specimens, facilitating the increase of flow stress but decreasing the toughness. In lamellar-structured specimens, combined moderate back stress and dislocation hardening should be responsible for the persistent strain-hardening exponent after the strains exceed 0.10. [ABSTRACT FROM AUTHOR]

Obtaining a fine bainitic ferrite for high carbon steel at low temperature requires a long period. Thus, creating strategies to accelerate bainite transformation is important. This investigation aims to explore the introduction of vanadium carbide at the bay region on bainitic transformation kinetics and mechanical properties by comparing isothermal bainitic transformation treatment directly and in combination with bay treatment prior to bainite transformation. Results show that introduced fine VC precipitation during bay treatment notably accelerates the initial bainite transformation with the incubation period being reduced by 91%. Decrease in activation energy for bainite nucleation at vanadium carbide/austenite interfaces and generation of preferred nucleation sites lead to a favorable acceleration of bainitic formation kinetics. However, indirect refinement of grain size for intragranular bainitic ferrite nucleation and consumption of available vanadium carbide/austenite interfaces result in overall rate of the subsequent bainite transformation. For the sample with prior bay treatment, the lath directions of bainitic ferrite are more complex and the size of block RA is smaller than that of the conventional sample. Moreover, the introduction of vanadium carbide at the bay region can simultaneously enhance the tensile strength and impact toughness of the steel. [ABSTRACT FROM AUTHOR]

The isothermal transformation kinetics, microstructure and mechanical properties of a low-carbon bainitic steel, subjected to below- Ms /above- Ms austempering with or without prior ausforming, have been investigated via dilatometric measurements, microstructural characterization and mechanical tests. The results show that for all austempered samples, prior ausforming largely refines the bainitic laths and enhances the mechanical stability of supercooled austenite, the latter leaving more austenite untransformed at the finish of isothermal treatment. Nevertheless, divergent consequences on the final microstructure arise: for the above- Ms austempered samples, prior ausforming increases the amount of large, unwanted brittle martensite/austenite blocks and decreases the volume fraction of retained austenite at room temperature; however, for the below- Ms austempered samples, prior ausforming decreases the size and amount of martensite/austenite blocks and increases the volume fraction of retained austenite. Accordingly, the below- Ms austempered samples with prior ausforming exhibit a product of strength and ductility of ∼ 43 GPa% and impact toughness of ∼180 J/cm2, in sharp contrast with those of the above- Ms austempered samples with prior ausforming, ∼33 GPa% and ∼80 J/cm2, respectively. The present results clearly demonstrate a favorable effect of the combined process of ausforming and below- Ms austempering against the adverse combining effect of ausforming and above- Ms austempering. Unlabelled Image • The coupling effects of ausforming and below- Ms /above- Ms austempering in a low-C Si-rich bainitic steel were investigated. • A low-carbon superfine bainitic microstructure with a PSE of 43 GPa% and impact toughness of 180 J/cm2 has been obtained. • Ausforming followed by below- Ms austempering largely refines bainitic laths and decreases the amount of large M/A blocks. • Ausforming followed by below- Ms austempering significantly increases the amount of retained austenite. • Ausforming followed by above- Ms austempering refines bainitic laths but increases the amount of unwanted large M/A blocks. [ABSTRACT FROM AUTHOR]