Transformation Of Retained Austenite Research Articles

Role of the martensitic microstructure in the stabilized residual stresses under cyclic loading and in the fatigue behavior of two steels

The influence of martensitic microstructure on the evolution of residual stresses during cyclic loading has been investigated using X-ray diffraction, both in martensite and in retained austenite. Different microstructures containing retained austenite fractions of 25% and 40% were obtained by vacuum carburizing applied to the 14NiCr11 steel. Additionally, other microstructures, such as quenched or tempered martensite of the 42CrMo4 steel were also studied.The results indicate that, for the same hardness and similar stabilized residual stresses in retained austenite, the greater the longitudinal residual stresses in quenched martensite, the greater the improvement in fatigue strength.From initial biaxial isotropic residual stresses, cyclic loading close to the endurance limit created anisotropic residual stress relaxation in quenched martensite and retained austenite. After cyclic loading, the longitudinal residual stresses were higher than the transverse residual stresses, regardless of the type of loading (bending in the case of 14NiCr11 carburized steel or combined bending and torsion in the case of the 42CrMo4 steel). The high level of longitudinal residual stresses after cyclic loading was correlated with the high retained austenite transformation to martensite in the case of 14NiCr11 carburized steel. The anisotropy of the stabilized residual stresses (after a few 10th of cycles) is due to the metastability of quenched martensite under cyclic loading.

Impact abrasive wear behaviour of the multiphase microstructure in a medium carbon quenched and partitioned bainitic steel

To enhance the impact abrasive wear resistance of steel for mining and metallurgy industries, a 0.3C steel with a multiphase microstructure of bainitic, martensitic and retained austenite was produced by a Q&P process involving bainitic transformation, and its wear performance under impact abrasive conditions was evaluated in contrast with fully martensitic and fully bainitic steels. The results revealed that the multiphase microstructure demonstrates outstanding combination of strength (1612 MPa), toughness (105 J), hardness (48.7 HRC) and elongation (19.2%), along with superior impact abrasive wear resistance. After 90 min of impact abrasive wear test, the multiphase microstructure shows lower wear loss than that for the fully martensitic and fully bainitic steels, which can be attributed to its high surface microhardness and increased toughness. The formation of nanograins and an increase in dislocation density as well as the retained austenite transformation leads to the higher surface microhardness. In addition, some larger craters can be observed on the wear surface of fully martensitic and fully bainitic steels, and the crack propagation depth is larger under the wear surface. In contrast, the wear surface of the multiphase microstructure presents some delamination damage, and the crack propagation depth is shallow because of the thicker deformed layer and presence of film-like austenite retained in the deformed microstructure. These results suggested that the multiphase microstructure can be used as a potential candidate structure for enhanced resistance to impact abrasive wear.

Carbides precipitation and kinetics of H13 steel subjected to deep cryogenic treatment

The influence of deep cryogenic treatment (DCT) on carbides precipitation and isothermal softening behaviour as a function of both temperature and time during tempering of H13 steel was investigated. The results show that the retained austenite transformed into martensite leading to an increase in hardness. The carbon content of martensite decreased after DCT and tempering according to the calculation of the X-ray diffraction spectrum. This indicates that more fine carbides precipitated from the martensitic matrix after DCT. The morphology of microstructure and distribution of carbides have verified the properties improvement mainly depending on the retained austenite transformation and more fine carbides precipitated after tempering. In addition, the carbides precipitating and coarsening are the main causes of thermal softening.

Influence of Deep Cryogenic Treatment on Microstructural Evolution and Transformation Kinetics Simulation by Finite Element Method of Low‐Carbon High‐Alloy Martensitic‐Bearing Steel

Nowadays, deep cryogenic treatment (DCT) is taken as a promising technique for improving the performance of steel. This article is focused on the influence of different heat treatment processes involving DCT on the microstructural evolution of a high Cr–Co–Mo high‐temperature‐bearing steel. Moreover, a multiphysical field coupling numerical model is built to investigate the martensitic transformation kinetics during DCT. The results show that DCT significantly improves the hardness and promotes the transformation of retained austenite to martensite at low temperatures. DCT not only refines the retained austenite as thin‐film morphology but also increases the precipitation of carbides and induces the carbides more homogenous distribution. This indicates that DCT promoted the segregation of carbon atoms and reduced the carbon content of martensite. The finite element model simulation shows that the retained austenite transformation is still incomplete, and the simulation results agree well with the experimental data by X‐Ray diffraction and transmission electron microscope analysis. This implies that the finite element simulation can better reflect and verify the experimental results.

Influence of friction-induced retained austenite transformation to martensite on the wear properties of a carburized layer of 23CrNi3MoA steel

This paper investigates the effect of the friction-induced retained austenite (RA) transformation to martensite on the surface wear performance of steel. 23CrNi3MoA steel was subjected to different carburizing processes to explore the effects of the RA volume fraction, carbon content and size on the stability of the metal. The results show that RA size is the key to controlling the mechanical stability of austenite in friction experiments. When the RA content is 20%–30%, the RA is small and has a uniform distribution and high mechanical stability. A small amount of friction-induced martensite is present in nanosheet form, and the particle size of the wear-stripped martensite is small, thereby reducing the amount of wear. When the content of RA is 30%–50%, the size of the RA is coarse, its distribution is uneven, the mechanical stability of the steel is low, the size of the exfoliated particles is coarse, and the wear performance of the steel is reduced. Through microstructural characterization of the worn surface, subsurface and matrix, it was found that cracks formed at the interface of the transformation induced martensite and the RA of the matrix during wear and penetrated the entire transformation induced martensite along the longitudinal direction of the interface. The bulk layer even extends into the deep matrix, thus eventually forming large exfoliations. The wear mechanism of RA during dry sliding friction is clarified, and a basis for the reasonable control of RA during production is provided.

The ratchetting and retained austenite transformation of medium‐manganese transformation‐induced plasticity steel during asymmetrical cyclic stressing

AbstractThe uniaxial ratchetting behavior and cyclic softening/hardening characteristics of Fe‐0.4C‐7Mn‐3.2Al steel are revealed by cyclic loading experiments. Electron backscattered diffraction (EBSD) and X‐ray diffraction (XRD) analyses have been carried out for assessing retained austenite (RA) transformation during cyclic deformation. Fe‐0.4C‐7Mn‐3.2Al steel exhibits a gradual transition from cyclic stabilization to slight cyclic hardening with increasing strain amplitude. RA transformation is one of the reasons for cyclic hardening. The ratchetting of Fe‐0.4C‐7Mn‐3.2Al steel shows two different characteristics under asymmetrical cyclic stressing. When the stress amplitude is below a certain value, the ratchetting is similar to that of typical cyclic stabilized materials in three stages of deceleration, stabilization and acceleration. However, when the stress amplitude reaches a certain level, the ratchetting evolution accelerates from the beginning to the fracture. The results show that the interaction between transformation‐induced plasticity and ratchetting strain can lead to accelerated accumulation of plastic strain.

Effects of Retained Austenite Transformation on the Tensile Deformation Response of 300M Steel

Effects of Retained Austenite Transformation on the Tensile Deformation Response of 300M Steel

Effect of carburizing process on high cycle fatigue behavior of 18CrNiMo7-6 steel

Carburizing heat treatment is widely used in the fabrication of gears to improve their fatigue performance. The effects of carburizing process on the material properties surface hardness, residual stresses, retained austenite of case-hardened 18CrNiMo7-6 and the consequential fatigue strength have not been studied in detail. In this research, the high-cycle fatigue performance of uncarburized and carburized 18CrNiMo7-6 gear steel were studied. In addition, the evolution processes of hardness, microstructure and residual stress of carburized specimens during the fatigue test were observed quasi-in situ. The initiation of fatigue cracks was also analyzed by SEM and the transformation of retained austenite during the fatigue test was observed by TEM. The results showed that the fatigue strength of carburized specimens has an increase-decrease trend with the increase of effective case depth. Besides, the fatigue failure mode of steel changes from surface failure to internal failure after being carburized, during which retained austenite plays an important part. In addition, if a large amount of retained austenite exists in the specimen with internal failure, the high-density twin martensite region formed by the retained austenite transformation can act as the crack initiation site, which is harmful to fatigue performance.

Microstructure and microhardness evolution of thermal simulated HAZ of Q&P980 steel

The microstructure evolution and microhardness of Q&P980 steel heat-affected zone (HAZ) was investigated systematically using weld thermal simulation technique. The phase transformation temperatures were determined by the dilatometry. Ac1 and Ac3 were observed to be quite constant, 750 and 935 °C, at the heating rates higher than 150 °C/s. The different regions of HAZ were obtained varying the peak heating temperature in the simulation experiments. With the peak temperature increased from 300 to 1350 °C, the microstructure evolved from martensite/ferrite/retained austenite to tempered martensite/carbides/ferrite, then finally turned to single martensite phase with fine lath or coarse lath morphology. The volume percentage of retained austenite reduced dramatically from 13% to 2% due to retained austenite transformation at high temperature. The lowest microhardness of 268 HV was obtained in sub-critical HAZ and the highest microhardness of 485 HV was obtained in fine grained HAZ. The welding continuous cooling transformation (CCT) diagram of Q&P980 steel corresponding coarse grained HAZ was constructed by dilatometric methods. There was ferrite, bainite and martensite transformation regions when the cooling rates ranged from 0.1 to 100 °C/s. The microhardness kept at a high certain level of 450–460 HV at high cooling rates (≥20 °C/s) because of the formation of martensite. By the CCT diagram determined, the effect of cooling rate on microstructure and microhardness can be deduced and utilized for optimizing the welding parameters.

Study on fatigue wear competition mechanism and microstructure evolution on the surface of a bainitic steel rail

The wear and fatigue competition mechanism of a bainitic rail steel was studied through rolling contact fatigue test. Results showed that the damage process could be divided into initial (before 2.0 × 105 cycles), middle (around 2.0 × 105 cycles) and late (after 2.0 × 105 cycles) stages, with the dominant damage mechanisms of wear, wear and fatigue, and fatigue in each respectively. With increasing rolling cycles, the retained austenite transformation played an important role in achieving dynamic equilibrium between wear and fatigue. Moreover, the untransformed retained austenite could release crack tip stress and effectively control the excessive propagation of fatigue cracks.

Evaluation of Retained Austenite Transformation Behavior of Carburized SCM420H Steel during Fatigue Process

Evaluation of Retained Austenite Transformation Behavior of Carburized SCM420H Steel during Fatigue Process

Effect of tension-compression testing strategy on kinematic model calibration and springback simulation of advanced high strength steels

Various studies show that application of kinematic hardening models significantly improves the accuracy of springback simulation in sheet metal forming. The springback simulation using kinematic hardening models, e.g., the Yoshida-Uemori (YU) model, is highly dependent on the testing strategy and the calibration of model parameters. In this study, the effect of tension-compression (T-C) testing strategy on calibration of YU model parameters and springback simulation of advanced high strength steels was investigated. A single-element model with T-C conditions was built in LS-OPT to determine YU model parameters on the basis of T-C tests. U-bending tests of two 980 MPa grade steels were performed to evaluate the springback prediction. Results indicate that T-C strategies, including tension-compression sequence and prestrain level show little effect on the calibrated YU model parameters as well as springback simulation of a dual phase steel DP980. However, for a TRIP-assisted high strength steel QP980, T-C strategy significantly affects both the YU model parameters and springback simulation, which is related to the complex loading-path-dependent retained austenite transformation in the QP980. A compromise strategy of T-C testing is proposed to calibrate the YU model parameters of the steel QP980, which are capable to accurately simulate its springback in U-bending.

Tribological behaviour of ductile and austempered grey cast iron under dry environment

In the search of appropriate materials for producing machine tools, this study investigated the wear performance of Austempered grey cast iron (AGCI) and ductile iron (DI) done on a single way tribosystem. An alloy containing 1.2 kg of Fe-Si-Mg inoculated by 0.2%Ca-Si was introduced to the melt at 350 °C austempering temperatures after austenitizing at 900 °C for 180 min. However, the highest Coefficient of friction and resistance of materials to wear are independent of 2 N and 5 N applied loads, while 7 N, 8 N, and 10 N were strongly dependent on the load applied. The outcomes revealed that the Coefficient of friction generally rises as the applied load increases in AGCI and DI. Highest Coefficient of friction 0.8 was observed at 10 N in DI while 0.85 and 0.8 was observed at 10 N and 8 N for AGCI, due to strain-induced retained austenite transformation of the martensite and work hardening.

Local deformation and transformation behavior of retained austenite in 18CrNiMo7-6 after high-carbon carburizing treatment

The objective of this investigation is to investigate the retained austenite deformation and transformation behavior during nanoindentation of 18CrNiMo7-6 steel after a high-carbon case-hardening process. We conducted a high-carbon carburizing treatment that led to approximately 1.2 mass% carbon at the surface. Further heat treatment consisted of austenitization, quenching in oil, and low-temperature annealing. To transform large amounts of retained austenite into martensite and to stabilize the existent retained austenite, some specimens were cryogenically treated in liquid nitrogen. The resulting microstructures were analyzed as a function of carbon content in the gradient by means of scanning electron microscopy with energy dispersive X-ray spectroscopy and electron backscatter diffraction analysis. Nanoindentation was used to study locally the mechanical behavior and stability of retained austenite at the carburized surface. To better analyze the deformation behavior during indentation, atomic force microscopy was applied to measure the topography of individual indentation imprints and the deformation pattern around the indents. The results show that nanoindentation is suitable for mechanically induced phase transformation of retained austenite into martensite, which leads to pronounced pop-ins in the load-displacement curves at specific critical loads. The occurrence of pop-ins, and, thus the phase transformation is highly dependent on the local microstructure of the carburized specimens. The main influencing factors are the local amount of retained austenite in the deformed region and, predominantly, the crystallographic orientation of retained austenite. (001)-Oriented retained austenite is highly metastable, whereas (111) orientations show mostly stable austenitic deformation behavior with only isolated mechanically induced retained austenite transformation during indentation. Cryogenic treatment transformed larger amounts of retained austenite into martensite. However, even after cryogenic treatment, (111)-oriented retained austenite could undergo mechanically induced transformation into martensite. Furthermore, we used the progression of hardness as a function of indentation depth and topography images to analyze the extent of retained austenite transformation and the complex compound deformation/transformation behavior of preexisting martensite and retained austenite.

New crystallography insights of retained austenite transformation in an intercritical annealed quenching and partitioning steel

We studied the transformation of retained austenite in an intercritical annealing quenching and partitioning steel in aspect of mechanically induced martensite variant selection. The martensitic transformation process was characterized through the interrupted tensile tests plus ex-situ electron backscattering diffraction measurements. We have observed two kinds of retained austenite, the larger equiaxed austenite and the finer lamellar austenite. Results show that the variant selection becomes stronger as the retained austenite grain size decreasing. Equiaxed retained austenite tends to transform into a certain variant with close orientation to adjacent ferrite. In this case, phase transformation in specific grains could occur earlier under stress, regardless of the grain size.

Residual stress formation and stability in bearing steels due to fatigue induced retained austenite transformation

In this work, for development and sustainability of compressive residual stresses (RS) in bearing steel materials is examined. Of particular interest is the role of retained austenite (RA) transformation in fatigue performance. Cyclic torsional loading was performed for a prescribed number of cycles at specific stress levels. Specimens were then examined using X-ray diffraction techniques to observe RS values and RA volume fraction. Based on the experimental results, an empirical model incorporating RA transformation and material relaxation was developed to predict RS formation. The results obtained from the model corroborate well with experimental measurements. Experimental results indicate that much of the RA present is not useful for generating compressive RS. However, by controlling applied loading, compressive RS can be generated and maintained throughout fatigue life. These findings were then translated into rolling contact fatigue (RCF) scenarios to identify loading conditions to maximize the benefits associated with RA transformation. The experimental and modeling results highlight the utility of RA transformation in increasing compressive residual stresses within the material, but emphasize the limitations of RA in the system.

Ultra-low-temperature process effects on microscale abrasion of tool steel AISI D2

ABSTRACTUltra-low-temperature process treatments could raise tool steel wear resistance through microstructural change that occurs on the material, enhancing, that way, tools and dies lifetime. To investigate the tool steel wear resistance impact, micro-abrasive wear tests were carried out and an analysis based on the Archard’s law was considered, evaluating specimen mass loss by laser interferometry. Micro-hardness, X-ray diffractometry, scanning and optical microscopy and carbides quantitative evaluation were carried out aiming to material characterisation. Results demonstrated a micro-hardness improvement, ranging from 0.9–4.7% for the cryogenically treated specimens, when compared to the bulk material. This effect is related, mainly, to the retained austenite transformation and to the increase of fine secondary carbides dispersed amount in the martensitic matrixes cryogenically treated.

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.

Retained austenite phase detected by Mössbauer spectroscopy in ASTM A335 P91 steel submitted to continuous cooling cycles

Samples of ASTM A335 P91 steel submitted to continuous cooling at different rates were analyzed in the form of foils and powders by means of Mössbauer spectroscopy. The Continuous Cooling Transformation (CCT) diagram of steel ASTM A335 P91 displays two basic microstructural domains at low temperatures – ferritic and martensitic – whose limits depend on the austenite holding temperature, the precise chemical composition and the cooling conditions from the austenite mother phase. Under certain conditions, the martensitic transformation may not be completed, leading to a final microstructure with a non-negligible percentage of the austenite phase retained in a metastable state. This retained austenite could be detrimental for the mechanical properties of the steel.Mössbauer analysis suggested that powdering process promotes the retained austenite transformation to martensite; in particular, in the present case, all the austenite transformed into martensite during powdering. Foil samples instead displayed retained austenite whose relative fraction was determined as a function of the cooling rate. At the same time, the carbon content of retained austenite was estimated for the faster cooled samples; a preliminary explanation for the observed trends is given.

Effect of strain path on forming limits and retained austenite transformation in Q&P 1180 steel

Forming limits and retained austenite (RA) transformation in Q&P 1180 steel are quantified as a function of plastic strain levels for three different strain paths. In-plane uniaxial tension testing was performed in a standard test frame, while limiting dome height tooling was employed for out-of-plane biaxial and plane strain tension experiments. Sheet specimens were tested incrementally for each strain path, and the RA content at each level of strain was measured using electron backscatter diffraction (EBSD). The biaxial tension strain path resulted in the greatest effective strain prior to necking at 0.355, compared to 0.123 for plane strain and 0.142 for uniaxial tension. EBSD measurements for various levels of plastic strain reveal a clear dependence of RA rate of transformation on strain path for the three linear strain paths that were employed in this work. Thinning strains appear to provide a slightly better correlation to RA transformation than effective strain levels, where biaxial tension achieved the greatest level just prior to necking, followed by plane-strain tension, and then uniaxial tension.