Untransformed Austenite Research Articles

Chemistry and three-dimensional morphology of martensite-austenite constituent in the bainite structure of low-carbon low-alloy steels

The martensite-austenite constituent (MA) originates from carbon-enriched and untransformed austenite surrounded by bainite and is known to deteriorate the toughness of bainitic steel. The relationship between MA and bainite formation in low-carbon low-alloy steel was investigated with respect to the effects of cooling rate and Nb addition, as well as the relationship between the MA distribution and surrounding bainite. Nb addition enhanced the formation of MA over a wider range of cooling rates, and the fraction of MA became maximum at a cooling rate of 5 K/s. A microstructure consisting of carbide-free coarse bainite with few high angle boundaries is formed near the prior austenite grain boundary, and fine bainite with a high density of carbide and high angle boundaries is formed within the austenite grains. MA tends to be located in the coarse bainite region. Three-dimensional observations of the bainite and MA indicated MA is elongated along bainitic ferrites with the same growth direction or the same habit plane, while MA is blocky when the surrounding bainitic ferrites have different growth directions. Thus, the shape of MA is strongly affected by the crystallography of the surrounding bainite structure. The carbon content in austenite revealed that the carbon content of MA is close to the T0 composition at the transformation temperature, which suggests that MA formation is caused by a reduction of the driving force for transformation by carbon enrichment, which results in incomplete bainite transformation. Nb addition further inhibits the decomposition of austenite and promotes MA formation.

Bainitic reaction and microstructure evolution in two normalized and tempered steels designed for service at elevated temperatures

Abstract In the present work conventional heat treatment like normalizing (bainitic microstructure) and tempering of the alloys has been performed. The materials used in this study were two steels, one the laboratory prepared experimental low alloy Cr-Mo steel in comparison to typical commercial 10CrMo9-10 steel. The determined carbon concentrations of the residual austenite at the different temperatures of bainite transformation supports the hypothesis that the growth of bainitic ferrite occurs without any diffusion with carbon being partitioned subsequently into the residual austenite. It was found that bainitic reaction has stopped when average carbon concentration of the untransformed austenite is close to the T0 line and supports formation of bainitic ferrite by a shear mechanism, since diffusionless transformation is not possible beyond the T0 curve. Normalized samples were air cooled down to room temperature before tempering at various temperatures in the range of 500-750°C. Samples have been austenitized at 980°C for 0.5 hour air cooled and tempered at 500, 550, 600, 650, 700 and 750°C for 1 hour. After heat treatment, the assessment in the microstructure and phase precipitation was made using the samples prepared for metallographic and transmission electron microscope (TEM) on thin foils analysis. Quantitative X-ray analysis was used to determine the retained austenite content after heat treatment like normalizing and tempering and the total volume fraction of the retained austenite was measured from the integral intensity of the (111)γ and (011)α peaks. The changes observed in the microstructure of the steel tempered at the higher temperature, i.e. 750°C were more advanced than those observed at the temperature of 500°C. Performed microstructural investigations have shown that the degradation of the microstructure of the examined steel was mostly connected with the processes of recovery and polygonization of the matrix, disappearance of lath bainitic microstructure, the growth of the size of M23C6 carbides, and precipitation of the secondary M2C precipitates. The magnitude of these changes depended on the temperature of tempering.

Discontinuous precipitation in a nickel-free high nitrogen austenitic stainless steel on solution nitriding

Chromium-rich nitride precipitates in production of nickel-free austenitic stainless steel plates via pressurised solution nitriding of Fe–22.7Cr–2.4Mo ferritic stainless steel at 1473 K (1200 °C) under a nitrogen gas atmosphere was investigated. The microstructure, chemical and phase composition, morphology and crystallographic orientation between the resulted austenite and precipitates were investigated using optical microscopy, X-ray Diffraction (XRD), Scanning and Transmission Electron Microscopy (TEM) and Electron Back Scatter Diffraction (EBSD). On prolonged nitriding, Chromium-rich nitride precipitates were formed firstly close to the surface and later throughout the sample with austenitic structure. Chromium-rich nitride precipitates with a rod or strip-like morphology was developed by a discontinuous cellular precipitation mechanism. STEM-EDS analysis demonstrated partitioning of metallic elements between austenite and nitrides, with chromium contents of about 80 wt.% in the precipitates. XRD analysis indicated that the Chromium-rich nitride precipitates are hexagonal (Cr, Mo)2N. Based on the TEM studies, (Cr, Mo)2N precipitates presented a (1 1 1)γ//(0 0 2)(Cr, Mo)2N, γ//(Cr, Mo)2N orientation relationship with respect to the austenite matrix. EBSD studies revealed that the austenite in the regions that have transformed into austenite and (Cr, Mo)2N have no orientation relation to the untransformed austenite.

Strain partitioning and texture evolution during cold rolling of AISI 201 austenitic stainless steel

Strain partitioning and texture evolution of AISI 201 austenitic stainless steel were investigated upon cold rolling up to a true strain of ε = 0.92. ε-martensite formation is the main work hardening mechanism at low strains (ε = 0.11). With increasing strain, the volume fraction of α’-martensite increases with a sigmoidal-like behavior. Remaining untransformed austenite is intensely fragmented by mechanical microtwins. The in-grain misorientation increases for all phases up ε = 0.51 and then levels off for further strain. Strain partitions evenly between austenite and α’-martensite during cold rolling. X-ray texture measurements revealed that austenite develops Goss, Brass and S texture components up to the largest investigated strain. The presence of Brass component at the highest deformation seems to be assisted by mechanical twinning. The texture components of α’-martensite belong to the α- and γ- fibers. Texture evolution of ε-martensite was followed by electron backscatter diffraction data and results show that texture evolves up to ε = 0.51 and remains nearly unchanged at larger strains, similarly as observed for austenite and α’-martensite.

In Situ Observation of the Lengthening Rate of Bainite Sheaves During Continuous Cooling Process in a Fe–C–Mn–Si Superbainitic Steel

The evolution of lengthening rate of bainite sheaves during continuous cooling process in a Fe–C–Mn–Si superbainitic steel was investigated by in situ observation on high-temperature laser scanning confocal microscope. The lengthening rates of bainite sheaves in three temperature ranges were calculated. The results indicate that the lengthening rate of bainite sheaves continuously decreases with the decrease of temperature during continuous cooling process. The lengthening rate of bainite sheaf depends on undercooling, transformation temperature, the diffusion of carbon atoms and the carbon content in parent austenite etc. The lengthening rate at high temperature is large due to the favorable carbon diffusion, smaller carbon content and less plastic deformation in untransformed austenite. Additionally, the microstructures after different isothermal holding temperatures were analyzed, indicating that the larger lengthening rate of bainite sheaves due to the high isothermal transformation temperature does not mean more amount of bainite transformation. Lower bainitic transformation temperature results in more and finer bainite plates.

X-Ray and Neutron Diffraction Studies of Quenched and Partitioned (Q&P) Multiphase Steel Microstructures

Abstract Novel advanced steel developments in recent years have resulted in many steels with complex multiphase microstructures. Foremost currently are steels that contain significant fractions of untransformed parent austenite phase. “Quenched and partitioned” steels, commonly referred to as Q&P steels, important as potential advanced high-strength (AHS) automotive steels, are an example in which the austenite is chemically stabilized by carbon partitioning from martensite following an interrupted quench above the martensite finish temperature, Mf. It is evident that the progress of microstructural change during this novel quenching and partitioning treatment, the volume of untransformed austenite stabilized during partitioning and the behavior of this austenite according to its carbon content should be understood. In relation to these requirements, recent analysis of X-ray powder diffraction (XRD) and neutron diffraction experiments will be considered. Initially, the effect of sample preparation between the two measurement techniques is considered, followed by observation of the partitioning process in real time allowed by specially prepared Mn-containing experimental alloys, with and without a crucial silicon addition required to enable austenite retention during Q&P treatment.

Relative Influences of Aluminium and Silicon on the Kinetics of Bainite Formation from Intercritical Austenite

Bainite formation from intercritical austenite is of great practical importance for the production of TRIP-assisted steels. Silicon and aluminium play important roles during this transformation by delaying carbide precipitation, thus favouring the carbon enrichment of untransformed austenite, which makes its stabilisation down to room temperature possible. Previous studies have shown a strong dependence of bainite formation kinetics on both chemical composition and transformation temperature. In the present work, the effect of silicon and aluminium contents on bainite formation kinetics is investigated experimentally using dilatometry combined with microscopical observations. The experimental results are analysed by comparison with thermodynamic parameters, such as the activation energy G* for nucleation of bainite and the carbon content C-To corresponding to the T-o-curve. It is shown that the faster transformation kinetics induced by the substitution of silicon by aluminium can be ascribed (i) to a higher driving force for nucleation, (ii) to a higher carbon content C-To at the To-curve and (iii) to the precipitation of carbide in austenite in steels with a low At content.

Comparative study on microstructure and mechanical properties of a C-Mn-Si steel treated by quenching and partitioning (Q&P) processes after a full and intercritical austenitization

A C-Mn-Si steel with lean microalloy elements was treated by a series of quenching and partitioning (Q&P) processes after a full or intercritical austenitization (hereinafter referred as F-QP and I-QP process, respectively). The discrepancy in microstructure and mechanical properties of the samples under these two heat treatment processes was investigated. The results indicate that the austenite grains are mainly film-like in the F-QP samples, while blocky austenite grains account for the majority in the I-QP samples. And the later contains a larger amount of retained austenite than the former. In addition, the reason why the austenite fraction decreases with the increasing of partitioning time after the peak value can be related to the bainite transformation due to the dynamic change of phase region in the continue cooling transformation (CCT) diagram of untransformed austenite with carbon enrichment. The I-QP samples show higher product of strength and elongation (PSE) but relatively lower strength, while the F-QP samples are just the reverse. Notably, the one-step I-QP samples show an excellent strength–ductility balance, of which the tensile strength and total elongation are 1140–1280MPa and 17–22%, respectively. The exponential function and Olson-Cohen (O-C) model were utilized to fit the variation of retained austenite fraction with strain. By comparison, retained austenite in the I-QP sample is more sensitive to the increasing strain, i.e., lower mechanical stability.

Direct observation of martensitic reversion from lenticular martensite to austenite in Fe-Ni alloy

Martensitic reversion from lenticular martensite to austenite was investigated in Fe-29%Ni alloy. Martensitically reversed austenite has essentially the same crystal orientation as untransformed austenite, which means that the crystallographic reversibility appears even in non-thermoelastic martensitic transformations. On the other hand, a small amount of the reversed austenite forms having a specific twin orientation with respect to the untransformed austenite. The formation of twined austenite reveals that reversed austenite nucleates at the untransformed austenite/martensite interface and grows into the interior of lenticular martensite. In addition, it was found that martensitically reversed austenite contains high-density dislocations leading to a significant strengthening.

Microstructure and partitioning behavior characteristics in low carbon steels treated by hot-rolling direct quenching and dynamical partitioning processes

In this work, a new process and composition design are proposed for “quenching and partitioning” or Q&P treatment. Three low carbon steels were treated by hot-rolling direct quenching and dynamical partitioning processes (DQ&P). The effects of proeutectoid ferrite and carbon concentration on microstructure evolution and mechanical properties were investigated. The present work obtained DQ&P prototype steels with good mechanical properties and established a new notion on compositions for Q&P processing. Microstructures were characterized by means of electro probe microanalyzer (EPMA), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD), especially the morphology and size of retained austenite. Mechanical properties were measured by uniaxial tensile tests. The results indicated that introducing proeutectoid ferrite can increase the volume fraction of retained austenite and thus improve mechanical properties. TEM observation showed that retained austenite included the film-like inter-lath austenite and blocky austenite located in martensite/ferrite interfaces or surrounded by ferrites. It was interesting that when the carbon concentration is as low as ~0.078%, the film-like inter-lath untransformed austenite cannot be stabilized to room temperature and almost all of them transformed into twin martensite. The blocky retained austenite strengthened the interfaces and transformed into twin martensite during the tensile deformation process. The PSEs of specimens all exceeded 20GPa.%.

Self-stabilization of untransformed austenite by hydrostatic pressure via martensitic transformation

For improving the understanding of austenite stability in steel, hydrostatic pressure in untransformed austenite that is generated via martensitic transformation was evaluated from macro- and micro-viewpoints, and its effect on austenite stability was investigated in a Fe-27%Ni austenitic alloy. X-ray diffractometry revealed that the lattice parameter of untransformed austenite is continuously decreased via martensitic transformation only when martensite becomes the dominant phase in the microstructure. This suggests that the untransformed austenite is isotropically compressed by the surrounding martensite grains, i.e., hydrostatic pressure is generated in untransformed austenite dynamically at a later stage of martensitic transformation. On the other hand, microscopic strain mapping using the electron backscatter diffraction technique indicated that a finer untransformed austenite grain has a higher hydrostatic pressure, while a high density of dislocations is also introduced in untransformed austenite near the austenite/martensite interface because of lattice-invariant shear characterized by non-thermoelastic martensitic transformation. Furthermore, it was experimentally demonstrated that the hydrostatic pressure stabilizes the untransformed austenite; however, the austenite stabilization effect alone is not large enough to fully explain a large gap between martensite start and finish temperatures in steel.

High-temperature functional behavior of single crystal Ni51.2Ti23.4Hf25.4 shape memory alloy

In this work the functional behavior of the new Ni51.2Ti23.4Hf25.4 high temperature shape memory alloy is investigated along three crystal orientations in compression and compared with the polycrystal behavior. Transformation temperatures were measured following specific aging treatments with the aim to optimize the shape memory and pseudoelastic behaviors. Two aging treatments were then selected in order to fulfill the optimum pseudoelastic behavior (500°C/4 h), and the maximum transformation temperatures (550°C/10 h) without compromising the alloy functionality. High temperature X-ray diffraction was utilized to determine the crystal orientations in the austenite phase for the single crystal specimens. The martensite structure was found to be the B19 orthorhombic, in contrast with monoclinic structure for lower Hf contents. Digital image correlation was successively used during isobaric strain-temperature experiments on the single crystal and polycrystal specimens. Large strain heterogeneities were found for the single crystals which pair with X-ray diffraction data that show un-transformed austenite below the martensite finish temperature. This reveals the heterogeneity of the austenite–to–martensite transformation for the present NiTiHf alloy which explains the divergence between the theoretical and experimental transformation strains. In addition, using digital image correlation it is possible to capture the local strain fields associated with fully transformed specimen regions. The transformation strains calculated locally are in close agreement with the theoretical transformation strains calculated with lattice deformation theory for the cubic austenite to the orthorhombic martensite phase transformation.

The effect of microstructure on stress-induced martensitic transformation under cyclic loading in the SMA Nickel-Titanium

A combined experimental and analytical study to determine the configurations of transforming martensite during ambient temperature cyclic deformation of superelastic Nickel-Titanium has been conducted. Full-field, sub-grain-size microscale strain measurements were made in situ during cycling using distortion-corrected Digital Image Correlation combined with Scanning Electron Microscopy (SEM-DIC). Using grain orientation maps from Electron Backscatter Diffraction analysis, possible configurations of martensite formed during cyclic deformation were identified by matching the calculated and measured strain fields. This analysis showed that the inclusion of Correspondence Variants (CVs) in addition to Habit Plane Variants (HPVs) of transformed martensite was necessary to provide a robust fit between calculated and measured strain fields. The approach also provided evidence that there was a more rapid accumulation of residual strain in CV regions and that a correlation existed between residual strain accumulation and the loss of actively transforming martensite in later cycles. It was also found that regions of CVs could coexist with untransformed austenite and Habit Plane Variants (HPVs) in individual grains throughout the microstructure, and that these regions of CVs formed before the end of the macroscopic stress plateau. The CV structure that forms during the initial superelastic deformation of Nickel-Titanium plays a critical role in shaping and stabilizing subsequent martensite recovery during cyclic loading.

Effect of intercritical deformation on microstructure and mechanical properties of a low-silicon aluminum-added hot-rolled directly quenched and partitioned steel

Here, we applied hot-rolling in conjunction with direct quenching and partitioning (HDQ&P) processes with different rolling schedules to a low-C low-Si Al-added steel. Ferrite was introduced into the steel by intercritical rolling and air cooling after hot-rolling. The effect of intercritcal deformation on the microstructure evolution and mechanical properties was investigated. The promotion of austenite stabilization and the optimization of the TRIP effect due to a moderate degree of intercritical deformation were systematically explored. The results show that the addition of 1.46wt% of Al can effectively promote ferrite formation. An intercritical deformation above 800°C can result in a pronounced bimodal grain size distribution of ferrite and some elongated ferrite grains containing sub-grains. The residual strain states of both austenite and ferrite and the occurrence of bainite transformation jointly increase the retained austenite fraction due to its mechanical stabilization and the enhanced carbon partitioning into austenite from its surrounding phases. An intercritical deformation below 800°C can profoundly increase the ferrite fraction and promote the recrystallization of deformed ferrite. The formation of this large fraction of ferrite enhances the carbon enrichment in the untransformed austenite and retards the bainite transformation during the partitioning process and finally enhances martensite transformation and decreases the retained austenite fraction. The efficient TRIP effect of retained austenite and the possible strain partitioning of bainite jointly improve the work hardening and formability of the steel and lead to the excellent mechanical properties with relatively high tensile strength (905MPa), low yield ratio (0.60) and high total elongation (25.2%).

Acicular ferrite formation during isothermal holding in HSLA steel

Microstructural observation and high-resolution dilatometry were employed to investigate acicular ferrite formation during isothermal holding in the HSLA steel. A decrease in isothermal temperature suppresses formation of polygonal ferrite and promotes formation of acicular ferrite. Island-like martensite/austenite constituents are dispersed inside of the acicular ferrite grain. The displacive model assuming autocatalytic nucleation was developed to describe the incomplete transformation phenomenon well. Decrease of isothermal temperature lowers the activation energy, and thus enhances the formation of acicular ferrite. Increase of amount of polygonal ferrite and acicular ferrite both results in decrease of M s. Formation of polygonal ferrite causes diffusion of solute alloying atoms into the untransformed austenite, which lowers M s. Acicular ferrite transformation suppresses martensitic transformation by diffusion of solution atoms during isothermal holding, and introduction of internal stress resulting from volume expansion during FCC → BCC transformation.

Revealing ultralarge and localized elastic lattice strains in Nb nanowires embedded in NiTi matrix.

Freestanding nanowires have been found to exhibit ultra-large elastic strains (4 to 7%) and ultra-high strengths, but exploiting their intrinsic superior mechanical properties in bulk forms has proven to be difficult. A recent study has demonstrated that ultra-large elastic strains of ~6% can be achieved in Nb nanowires embedded in a NiTi matrix, on the principle of lattice strain matching. To verify this hypothesis, this study investigated the elastic deformation behavior of a Nb nanowire embedded in NiTi matrix by means of in situ transmission electron microscopic measurement during tensile deformation. The experimental work revealed that ultra-large local elastic lattice strains of up to 8% are induced in the Nb nanowire in regions adjacent to stress-induced martensite domains in the NiTi matrix, whilst other parts of the nanowires exhibit much reduced lattice strains when adjacent to the untransformed austenite in the NiTi matrix. These observations provide a direct evidence of the proposed mechanism of lattice strain matching, thus a novel approach to designing nanocomposites of superior mechanical properties.

Micro and macro-mechanical behavior of a transformation-induced plasticity steel developed by thermomechanical processing followed by quenching and partitioning

A low-alloyed transformation induced plasticity steel was subjected to thermomechanical processing (TMP) followed by quenching and partitioning treatment to achieve a desired combination of the strength and ductility. The developed microstructures were precisely analyzed, and the mechanical responses of individual micro-constituents were studied by nanoindentation using a reliable scanning probe microscopy. The results indicate that the characteristics of the constituent phases (i.e., the lath martensite, blocky fresh martensite and the retained austenite) were dictated by the prior TMP. The occurrence of dynamic recrystallization and the formation of equiaxed-shape fine austenite grains during TMP would provide fast diffusion track for carbon to diffuse through the untransformed austenite. The carbon partitioning from martensite to the surrounding austenite ensures the austenite stabilization and was identified as the major factor for martensite softening at micro-scale level. The room temperature mechanical properties were studied via shear punch and tensile testing methods. The obtained superior mechanical properties, the ultimate tensile stress of ~1400MPa and shear elongation to fracture of ~19% were justified considering the proper work hardening behavior of the material.

Influence of hot working parameters on microstructure evolution, tensile behavior and strain aging potential of bainitic pipeline steel

The microstructure evolution, tensile properties and aging behavior were studied in a low-carbon pipeline steel by performing a number of physical simulations in a thermo-mechanical simulator. The austenite status, before entering the finishing stage, was varied by varying parameters in the austenitization and the roughing stages. The finishing parameters and the subsequent cooling strategy were kept fixed throughout all applied simulations. It was found that starting the finish rolling stage with pancaked austenite is more effective in motivating the formation of acicular ferrite and refining martensite/austenite phase than refining the prior austenite grains. The former effect resulted in improving both of ultimate tensile strength and proof stress without significant ductility decrease. Increasing the deformation during roughing stage resulted in stimulating the transformation kinetics during subsequent processing and consequently decreasing the untransformed austenite that forms the martensite/austenite microconstituent. Furthermore, increasing the austenitization temperature is found to enhance both of strength and ductility when starting the finishing stage with pancaked austenite. On the other hand, attaining aging effect in the studied steel was only possible after creating new dislocations by pre-straining. An increase up to 63MPa in the yield strength is recorded due to aging after 2% pre-straining.

Laser confocal observation on the formation of martensite and test on the mechanical properties of steels treated by Q-P-T process

The advanced high strength steels(AHSS) for auto manufacturing have been developed to the 3rd generation. The typical one is steel treated by Q-P-T process, which consists of the prior martensite, fresh martensite and retained austenite. In this paper, the non-uniform distributive formation of martensite and untransformed austenite during the one-step quenching treatment was observed by laser scanning confocal microscope. For the Q-P-T process, the non-uniform distributions of untransformed austenite, prior martensite and segmented austenitic grains are the main reasons to form the multi-scale martenite. Moreover, the fresh martensite possesses three highlights, such as microstructure of finer laths, same orientation in a blocky area considered as a “packet” and higher mechanical properties. The research result of this paper reveals the microstructure evolution of Q-P-T treated steel, and provides the accurate theoretical basis for improving the Q-P-T process and the mechanical properties.

Effect of Quenching and Partitioning with Hot Stamping on Martensite Transformation and Mechanical Properties of AHSS

Two-step quenching and partitioning treatment with hot stamping was applied to advanced high-strength steel (AHSS). The newly treated steel possesses a fine microstructure and typically curved micromorphology. The martensite start temperature of the newly treated steel is increased through the effect of plastic deformation on austenitic microstructure. However, the martensite volume fraction of this steel is deceased because of the enhanced stability of the untransformed austenite after plastic deformation. Consequently, the fraction of retained austenite is increased. The newly treated steel also shows excellent mechanical properties. The volume fraction of retained austenite reaches the highest value of 17.2% when hot stamping is performed at 750 °C. Hence, the steel displays favorable plasticity with an elongation of 14.5%. Moreover, the highest hardness value of 426 HV is obtained when hot stamping is performed at 650 °C. The newly developed process may be employed to develop a new generation of AHSSs.