Retained Austenite Grains Research Articles

In-situ EBSD study of deformation behavior of retained austenite in a low-carbon quenching and partitioning steel via uniaxial tensile tests

Through using in-situ electron back-scattered diffraction and uniaxial tensile tests, this work mainly focuses on the deformation behavior of retained austenite (RA) in a low-carbon quenching and partitioning (Q&P) steel. In this paper, three different types of RA can be distinguished from different locations, respectively, RA grains at the triple edges, twinned austenite and RA grains positioned between martensite. The results have shown that grains at the triple edges and twinned austenite could transform easily with increasing strain, i.e. are less stable when compared with RA grains distributed between martensite that could resist a larger plastic deformation. Meanwhile, the strain leads to rotations of RA grains distributed at the triple edges and between martensite. Moreover, RA grains with a similar orientation undergone similar rotations with the same true strain. These RA grains rotated along a specific slip plane and slip direction and the grain rotation is taken as a significant factor to improve the ductility of steel. In addition, grain sizes of RA decreased gradually with an increase of true strain and smaller (0–0.2μm) grains were more capable of resisting the deformation. According to kernel average misorientation (KAM) analysis, it can be found that strain distribution is preferentially localized near martensite–austenite phase boundaries and in the interior of martensite. The average KAM values increased continuously with increasing true strain.

Nano-indentation investigation on the mechanical stability of individual austenite in high-carbon steel

Quenching (AQ) and cryogenic treatments (QC) were conducted on the high-carbon SAE 52100 steel to investigate the mechanical stability of individual retained austenite (RA) by nano-indentation. The cross-sections of indented RA region prepared by focused ion beam (FIB) were examined by using transmission electron microscopy (TEM). For the first time, it was directly observed that some parts of RA grain, closest to the indent, in AQ specimen had transformed into strain-induced martensite (SIM). However, not any pop-in or transformation was detected in the indented QC specimen. This clearly indicates that the mechanical stability of RA in QC seems significantly enhanced, which is mainly attributed to the cryogenic treatment resulting in a higher carbon enrichment of RA compared to that in AQ. Furthermore, a higher load of external stress may need to trigger its martensitic transformation in QC specimen.

Effect of microstructure on retained austenite stability and tensile behaviour in an aluminum-alloyed TRIP steel

Microstructures, retained austenite (RA) stability and tensile behaviour were investigated for an aluminum-alloyed TRIP steel. Two distinct microstructural variants (equiaxed and lamellar) were studied: (i) at constant volume percent RA and (ii) at constant UTS. The RA in the lamellar variant transforms more slowly with tensile strain than for the equiaxed variant and stabilizes at ~80% transformed. The higher RA stability for lamellar microstructures is primarily due to the stress shielding effect of RA grains by the surrounding bainite. A secondary contributor is the elongated shape of the RA grains in the lamellar microstructure. The tensile work hardening behaviour correlates with RA transformation behaviour for the two model microstructural variants. The lamellar variant exhibits a relatively constant work hardening rate sustained up to the point of necking. By contrast, the equiaxed variant exhibits a maximum work hardening rate at low strain, followed by a monotonic decrease to the necking point. Comparing the results reported here with those of a previous study on silicon-alloyed TRIP steel shows that for both microstructural variants RA transforms more slowly with tensile strain and the UTS×El values are higher for the aluminum-alloyed steel.

Recrystallization Behavior of a Heavily Deformed Austenitic Stainless Steel During Iterative Type Annealing

The study describes evolution of the recrystallization microstructure in an austenitic stainless steel during iterative or repetitive type annealing process. The starting heavily cold deformed microstructure consisted of a dual phase structure i.e., strain-induced martensite (SIM) (43 pct in volume) and heavily deformed large grained retained austenite. Recrystallization behavior was compared with Johnson Mehl Avrami and Kolmogorov model. Early annealing iterations led to reversion of SIM to reversed austenite. The microstructure changes observed in the retained austenite and in the reverted austenite were mapped by electron backscatter diffraction technique and transmission electron microscope. The reversed austenite was characterized by a fine polygonal substructure consisting of low-angle grain boundaries. With an increasing number of annealing repetitions, these boundaries were gradually replaced by high-angle grain boundaries and recrystallized into ultrafine-grained microstructure. On the other hand, recrystallization of retained austenite grains was sluggish in nature. Progress of recrystallization in these grains was found to take place by a gradual evolution of subgrains and their subsequent transformation into fine grains. The observed recrystallization characteristics suggest continuous recrystallization type process. The analysis provided basic insight into the recrystallization mechanisms that enable the processing of ultrafine-grained fcc steels by iterative type annealing. Tensile properties of the processed material showed a good combination of strength and ductility.

Effect of Free Surface on the Stability of Individual Retained Austenite Grains in a Duplex Stainless Steel

The present work explored the effect of free surface on the stability of individual austenite grains in a duplex stainless steel. It was found that martensitic transformation took place automatically in the retained austenite grain when a free surface was introduced. This is due to the fact that the martensite nucleation energy barrier can be lowered to a thermally surmountable value as the strain energy induced by martensitic transformation is largely lowered when the matrix constraints were removed.

Unravelling the structural and chemical features influencing deformation-induced martensitic transformations in steels

A combination of Electron Back-Scattered Diffraction (EBSD) and high-sensitivity Electron Probe Micro-Analysis (EPMA) was used to correlate the changes in microstructural features upon deformation with local chemical composition in Transformation-Induced Plasticity steels. A novel cleaning procedure was developed that allows complete monitoring of transformation and deformation processes in relation to the local crystal structure, microstructure and chemical composition. Here we show direct evidence that local variations in manganese content enable a gradual transformation of the retained austenite grains.

The Effect of Intercritical Annealing on the Microstructure and Mechanical Properties in a High Strength TRIP Steel with C‐Partitioning

Direct experimental evidence confirms the partitioning of C to austenite during intercritical annealing is the intrinsic factor that affects the stability of the retained austenite for a newly developed high strength transformation‐induced plasticity (TRIP) steel. The volume fraction of retained austenite and C concentration decreases with increasing intercritical annealing time. Few retained austenite grains and a number of M/A islands were observed in the sample at 830°C annealed for 30 min. When the sample annealed at 830°C for 0.5 min and then isothermally treated at 440°C for 3 min, the investigated low alloy TRIP steel achieved the tensile strength higher than 1000 MPa, the total elongation of 21.8% and the n‐value of 0.32.

Study on Transformation Behavior of Retained Austenite in TRIP Steel

The microstructures of CR600TR steel were characterized using optical metallography, SEM, TEM and EBSD techniques, especially the transformation behavior of retained austenite and fracture mechanism of TRIP steel under stress-strain were studied in detail. It is discovered that there is marked selectivity for those retained austenite grains in their transition sequences: they are even dependent on the extension paths of stress-strain in steel and that curvatures of grain-boundaries which the retained austenite grains lie in.

Nanoindentation investigation on the mechanical stability of individual austenite grains in a medium-Mn transformation-induced plasticity steel

The mechanical stability of individual retained austenite grains is studied by nanoindentation combined with electron backscattered diffraction, scanning electron microscope, energy-dispersive X-ray spectroscopy and transmission electron microscopy. The average hardness of retained austenite and ferrite is 7.7 and 3.4 GPa, respectively. The large scatter of hardness distribution in retained austenite may be due to the varied stability and boundary structure of different austenite grains. The mechanical stability of retained austenite increases with Mn content.

Microstructure of tool steel upon combined semi-solid processing and thermomechanical treatment

Abstract Semi-solid processing can lead to microstructures with a high proportion of metastable austenite embedded in carbide–austenite network even in ordinary steels. This type of microstructure is a result of non-uniform distribution of alloying elements in a partially melted material. The paper deals with semi-solid processing and heat treatment of the ledeburitic steel with 1.8% carbon and 11% chromium. Its initial microstructure typically consists of ferrite matrix with primary chromium carbides and globular cementite particles. Upon semi-solid processing, the material comprised 95% metastable austenite, predominantly in the form of polyhedral grains with the size of 19 μm. Previous research has demonstrated the grain refinement effect of plastic deformation applied immediately after solidification. In follow-up experiments, the influence of thermomechanical treatment upon subsequent evolution of microstructure was studied. In this experiment, particular attention was devoted to the decomposition of metastable austenite and its contribution to microstructure refinement. Upon the transition through the semi-solid state, the material was cooled down below its solidus temperature and subjected to deformation. This was followed by several cooling schedules. Some of the schedules included a dwell at 600 °C to allow metastable austenite to decompose. Cooling schedules were immediately followed by heating to austenitizing temperature and cooling to room temperature at an equal rate. These procedures produced various types of microstructures. The treatment with the dwell produced mixed-type microstructures with an exceptional form of martensite and partially decomposed carbide network. Processing without the dwell led to the development of microstructures with polyhedral retained austenite grains embedded in the original carbide network. Their hardness ranged from 445 to 850 HV30.

Quantitative assessment of the effects of microstructure on the stability of retained austenite in TRIP steels

The stability of retained austenite (RA) in transformation-induced plasticity steels (TRIP) is affected by many factors, including chemical composition, RA grain size, neighboring microconstituents of RA and temperature. The composition and microstructure factors are interrelated, so it is difficult to separate out the influence of each individually. In this investigation, methods were developed to study the effects of RA grain size and neighboring microconstituents in a silicon-alloyed low-carbon (0.2C–1.6Si–1.6Mn) TRIP steel. Uniaxial tensile tests, performed at −20°C, room temperature (20°C) and 40°C, were interrupted at several strain levels. Scanning electron micrographs were obtained from each condition, and RA was quantified using a novel technique called the categorical chord-length distribution (CCLD), which enables microstructural quantification based on specific neighboring microconstituents. The results show that RA adjacent to bainitic ferrite (BF) and fine RA grain size are correlated with higher RA stability. A modified Burke–Matsumura–Tsuchida stability model was developed to kinetically analyze the effects of microstructure on RA transformation. The CCLD and kinetic modeling analysis indicates that the stability of RA inside BF is less sensitive to the testing temperature than RA inside polygonal ferrite (PF), and thermodynamic analysis of the driving force for transformation as a function of temperature and carbon content implies that there is a higher carbon content in RA inside BF. Additionally, nanohardness tests showed that the hardnesses of BF and PF are not significantly different after moderate amounts of deformation. Thus, the enhanced stability of RA inside BF compared to PF is more strongly related to the elevated carbon content of RA inside BF rather than stress partitioning differences for RA adjacent to BF or PF.

Deformation-induced austenite grain rotation and transformation in TRIP-assisted steel

Uniaxial straining experiments were performed on a rolled and annealed Si-alloyed TRIP (transformation-induced plasticity) steel sheet in order to assess the role of its microstructure on the mechanical stability of austenite grains with respect to martensitic transformation. The transformation behavior of individual metastable austenite grains was studied both at the surface and inside the bulk of the material using electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD) by deforming the samples to different strain levels up to about 20%. A comparison of the XRD and EBSD results revealed that the retained austenite grains at the surface have a stronger tendency to transform than the austenite grains in the bulk of the material. The deformation-induced changes of individual austenite grains before and after straining were monitored with EBSD. Three different types of austenite grains can be distinguished that have different transformation behaviors: austenite grains at the grain boundaries between ferrite grains, twinned austenite grains, and embedded austenite grains that are completely surrounded by a single ferrite grain. It was found that twinned austenite grains and the austenite grains present at the grain boundaries between larger ferrite grains typically transform first, i.e. are less stable, in contrast to austenite grains that are completely embedded in a larger ferrite grain. In the latter case, straining leads to rotations of the harder austenite grain within the softer ferrite matrix before the austenite transforms into martensite. The analysis suggests that austenite grain rotation behavior is also a significant factor contributing to enhancement of the ductility.

Effect of microstructure on retained austenite stability and work hardening of TRIP steels

Retained austenite is a metastable phase in transformation induced plasticity (TRIP) steels that transforms into martensite under local stress and strain. This transformation improves sheet formability, allowing this class of higher strength steels to be used for applications such as automotive structural components. The current work studies two distinct TRIP steel microstructures, i.e. equiaxed versus lamellar, and how microstructure affects the austenite transformation during uniaxial tensile loading. Different heat treatments were employed to obtain the two microstructures, and the bainite hold times of the treatments were varied to change the volume fraction of retained austenite. Based on uniaxial tensile response and magnetic saturation measurements, the bainite hold time of 100 s was determined to produce the best results in terms of largest strain at the ultimate tensile strength and highest volume fraction of retained austenite. The work hardening of the samples with a 100 s bainite hold was evaluated by calculating the instantaneous n value as a function of strain. It was found that the lamellar microstructure has a lower maximum instantaneous n value than the equiaxed microstructure, but has higher work hardening values for strain levels greater than 0.05 and up to the ultimate tensile strength. This difference in work hardening behaviour corresponds directly to the transformation rate of retained austenite in the two microstructures. The slower rate of transformation in the lamellar microstructure allows for work hardening to persist at high strains where the transformation effect has already been exhausted in the equiaxed microstructure. The different rates of transformation can be attributed to the location, carbon content, and size of the retained austenite grains in the respective TRIP microstructures.

Research and development of 780 MPa cold rolling TRIP-aided steel

As an industry-university cooperative project, an experimental research was conducted to develop a new cold-rolled transformation-induced plasticity (TRIP) steel with a tensile strength of 780 MPa and an elongation of 25% under the conditions that some processing parameters were preestablished according to the actual conditions of factory production lines. The optimal heat treatment conditions for obtaining excellent strength-ductility balance, specifically in intercritical annealing temperature and isothermal treatment temperature, were investigated by means of thermal simulation tests. Ultimately the desirable mechanical properties were attained successfully, and the microstructure and retained austenite stability were studied by optical microscopy (OM) and electron back-scattered diffraction (EBSD). It has been discovered that the sizes of retained austenite grains are generally proportional to the sizes of carbon-clear space (ferrite and bainite) around them, and there is marked selectivity in their transition sequences under stress-strain.

Effect of the bainitic transformation temperature on retained austenite fraction and stability in Ti microalloyed TRIP steels

TRIP properties in four hot-rolled microalloyed steels were studied as a function of bainitic domain entrance temperature. It was found that this temperature affects the size and morphology of the retained austenite grains. Size and morphology was shown to play a role on the stability of the retained austenite. This stability was shown to be the major factor affecting the mechanical properties of these steels. In the case of the lowest bainitic temperature, the TRIP effect is hindered due to the premature transformation of austenite into martensite probably due to an insufficient carbon enrichment of the small austenite grains. Moreover the bainitic treatment performed at the highest temperature contained a very low volume fraction of retained austenite that does not transform even under deformation. Using TEM and the Kikuchi-line method, it was shown that the sample that did not undergo a phase transformation during FIB milling had a carbon concentration in its retained austenite grains of approximately 1.7%.

Evaluation of the Crystallographic Orientation Relationships between FCC and BCC Phases in TRIP Steels

The crystallographic orientation relationships that are active during the transformation of austenite to bainite are studied for two TRIP steels by means of Electron BackScatter Diffraction (EBSD). A detailed evaluation of about 360 retained austenite grains and their BCC neighbours was performed. Three relationships were considered, namely Kurdjumov-Sachs, Nishiyama-Wassermann and Pitsch. It was found that the majority of the austenite grains had at least one neighbour that could be related with one of the three orientation relationships. The Kurdjumov-Sachs relationship appeared to be dominant and no strong indication for variant selection could be retrieved from the studied data. It was, however, also demonstrated that some precautions need to be made since a clear distinction between the evaluation of a small region of the microstructure and conclusions made for the complete material is necessary.

A methodology suitable for TEM local measurements of carbon concentration in retained austenite

Carbon concentration in retained austenite grains is of great importance determining the mechanical properties of hot-rolled TRansformation Induced Plasticity steels. Among the different techniques available to measure such concentrations, Kikuchi lines obtained in Transmission Electron Microscopy provide a relatively easy and accurate method. The major problem however is to be able to locate an austenitic grain in the observed Transmission Electron Microscopy thin foil. Focused Ion Beam in combination with Scanning Electron Microscopy was used to successfully prepare a thin foil for Transmission Electron Microscopy and carbon concentration measurements from a 700 nm retained austenite grain.

On the microstructure and phase identification of plasma nitrided 17-4PH precipitation hardening stainless steel

Systematic microstructure characterisation of plasma nitrided (350–500 °C for 10 to 30 h) 17-4PH alloy was carried out using SEM, XRD and TEM. Experimental results have shown that the microstructure and phase constituents of the plasma surface alloyed cases are highly treatment temperature dependent. When treated at low-temperatures (≤ 420 °C), the microstructure is dominated by nitrogen supersaturated martensite (α' N-expanded martensite); Nitrogen S-phase grains can be formed from the pre-existent retained austenite by converting the retained austenite grains in 17-4PH but no continuous S-phase layer was found. When treated at high-temperatures (above 420 °C), a surface γ′–Fe 4N compound layer was formed, CrN precipitated and S-phase was decomposed.

Characterization of individual retained austenite grains and their stability in low-alloyed TRIP steels

In situ three-dimensional (3-D) X-ray diffraction experiments have been performed at a synchrotron source on low-alloyed multiphase TRIP steels containing 0.25 wt.% Si and 0.44 wt.% Al and produced with different bainitic holding times, in order to assess the influence of the bainitic transformation on the thermal stability of individual austenite grains with respect to their martensitic transformation. A detailed characterization of the austenite grain volume distribution at room temperature was performed as a function of the prior bainitic holding time. In addition, the martensitic transformation behaviour of individual metastable grains was studied in situ during cooling to a temperature of 100 K. Both the carbon content and the grain volume play a key role in the stability of the austenite grains below 15 μm 3, while the carbon content exerts the dominant effect in the stability of the bigger grains. Measurements also suggest that the tetragonality of the thermally formed martensite is suppressed.

Study on the Formation of Ferrite-Cementite Microstructure by Strain Induced Dynamic Transformation in Medium Carbon Steels

Cold heading quality (hereafter CHQ) steels are widely used in automobile and construction industries. The various processes involved in the production of wire rod from CHQ steels are drawing, spheroidization annealing, forming, and quenching/tempering. Especially, the spheroidization annealing is inevitably used to soften the wire rod before severe forming. However, due to the economical constraints, many efforts have been made to shorten or avoid the spheroidization annealing process. One of the possible ways is to produce the ferrite-cementite (FC) microstructure directly after the drawing (rolling) process without performing any subsequent spheroidization annealing. In the present study, the effect of SIDT (Strain Induced Dynamic Transformation) on the microstructure of medium carbon steels was investigated to develop spheroidized annealing-free steel wire rods. When 0.45%C steels were hot deformed by heavy reduction at low temperatures conditions, deformed microstructures were quite different from conventional ferrite-pearlite structure. For example, the microstructure containing 57 vol. % of ferrite, which is far beyond the equilibrium ferrite fraction of 43 vol. % of 0.45%C steel was obtained by deformation at 700°C. Moreover, as deformation temperature decreased to 675°C, the pearlite structure disappeared completely and new microstructure composed of ferrite and cementite appeared. It was considered that this FC microstructure was obtained owing to small retained austenite grains existing between fine SIDT ferrites prefer to transform to cementite and ferrite instead of pearlite during cooling. Finally, the optimum conditions for producing the ferrite-cementite microstructure have been identified.