Thermal Stability Of Austenite Research Articles

On the austenite stability of cryogenic Ni steels: microstructural effects: a review

Austenite stability is essentially important in improving the cryogenic toughness of cryogenic Ni steels and guiding the development of Ni-saving cryogenic steels. The austenite stability in the cryogenic Ni steels is influenced by many microstructure features, making it a complicated issue which is lack of a systematic discussion. In this article, the microstructural effects on the thermal and mechanical stability of austenite in the cryogenic Ni steels are reviewed and discussed. The thermal stability of austenite (TSA) will be enhanced by the enrichment of austenite-stabilizing elements in the austenite which decreases the martensite-start (Ms) temperature. The grain refinement enhances the TSA by synergistically increasing the nonchemical driving force for the martensite transformation and the concentrations of austenite-stabilizing elements in the austenite. The excessive increase in the volume fraction of austenite weakens the TSA by decreasing the concentrations of austenite-stabilizing elements in the austenite. The film austenite is usually thermally more stable than the block austenite owing to its higher concentrations of austenite-stabilizing elements. The mechanical stability of austenite (MSA) is also influenced by the concentrations of austenite-stabilizing elements which affect the Ms temperature. The reports on the effect of grain size of austenite on the MSA are inconsistent. Both negligible and important effects of the grain size of austenite on the MSA are analyzed. The grain orientation of austenite affects the MSA via changing the Schmid factor and the additional driving force for the martensite transformation. The orientation which yields a larger value of Schmid factor would exhibit a lower MSA. The MSA is affected by the matrix or the neighboring phase due to the stress and strain partitioning among austenite and other constituent phases. The dislocation multiplication could weaken the MSA by assisting the nucleation and growth of martensite embryo and enhance the MSA by hindering the motion of embryo/austenite interfaces when dislocation density is sufficiently large. Austenite with a combination of a high TSA and a moderate or high MSA is considered to be effective strategies to enhance cryogenic toughness of the cryogenic Ni steels.

Austenite stability and deformation-induced transformation mechanism in cold-rolled medium-Mn steel

The austenite stability and tensile properties of intercritically annealed samples of cold-rolled medium-Mn steel (0.15C–5Mn-1.1Al) were investigated. The influence of austenite grain size and element concentration on the mechanical stability of austenite was studied, which demonstrated that the C content in austenite was a critical factor governing the mechanical stability of austenite. The thermal stability of austenite in cold-rolled medium-Mn steel samples annealed for different durations at different temperatures was quantitatively analyzed. The results showed that the effect of annealing temperature on the thermal stability of austenite was stronger than that of annealing time. In addition, the samples subjected to heat treatment under different conditions resulted in different work hardening (WH) behavior and mechanical properties due to variations in the mechanical stability of the retained austenite. It was concluded that these differences were related to the changes in stress- and strain-induced transformation modes of the retained austenite.

Revealing Austenite Stability in Fe–30Mn–5Al–0.5C Twinning‐Induced Plasticity Steel Using Differential Thermal Analysis

The thermal stability of austenite in as‐cast high‐alloyed Fe–30Mn–5Al–0.5C twinning‐induced plasticity (TWIP) steel has been studied by differential thermal analysis (DTA). TWIP steels are subjected to austenization treatments in the temperature range between 800 and 1200 ºC and holding times between 3 and 100 min. Different heating and cooling rate conditions are applied, including cooling rates between 0.1 and 1.0 K s−1 and three heating rates 0.08, 0.33, and 0.83 K s−1. The DTA results show that austenite is stable for all of the heat treatments applied. Microstructural analysis reveals a strong dendritic segregation due to the high percentages of Mn and Al in the composition of the TWIP steel. Finally, the austenitization temperature significantly influences the austenite grain size, unlike the cooling and heating rates that have an insignificant effect. The DTA results are confirmed by the X‐ray diffraction analysis, revealing a single‐phase γ‐austenite crystal structure.

Thermal and Mechanical Stability of Austenite in Metastable Austenitic Stainless Steel

The roles of grain size, texture, strain, and strain rate on the thermal and mechanical stability of austenite in AISI 321 metastable austenitic stainless steel were studied. Ultrafine grain (UFG), fine grain (FG), and coarse grain (CG) specimens with average grain sizes of 0.24, 3, and 37 µm sizes, respectively, were investigated. To determine the thermal stability of austenite (TSA), samples were soaked in liquid nitrogen (− 196 °C) for varying times between 0.5 and 24 hours. On the other hand, the mechanical stability of austenite (MSA) was studied by subjecting cylindrical specimens to both quasi-static (4.4 × 10−3 s−1) and dynamic loading conditions (between 1300 and 8800 s−1). Thermally-induced α′-martensite was only observed at an incumbent time in AISI 321 to suggests an isothermal martensitic transformation occurred. Both Kurdjumov-Sachs ($$ \{ 111\}_{\gamma } ||\{ 110\}_{{\alpha^{\prime}}} $$ and $$ \langle \bar{1}01\rangle_{\gamma } ||\langle 1\bar{1}1\rangle_{{\alpha^{\prime } }} $$) and Nishiyama–Wasserman ($$ \{ 111\}_{\gamma } ||\{ 110\}_{{\alpha^{\prime}}} $$ and $$ \langle 112\rangle_{\gamma } ||\langle 011\rangle_{{\alpha^{\prime } }} $$) orientation relationships existed between the untransformed γ and thin-plate α′-martensite. The thermally-induced phase transformation was highly suppressed in UFG specimens. While TSA decreased with an increase in grain size, MSA decreased with a decrease in grain size. While thin-plate α′ predominantly formed in the thermally-treated AISI 321 steel (FG and CG specimens only), lath and irregularly-shaped α′ formed in the specimens deformed under quasi-static and dynamic loading conditions, respectively. Irrespective of strain rate, deformation-induced α′ in UFG specimens inherited the morphology of the deformed austenite grain that is equiaxed. Irrespective of grain size, MSA also decreased with increase in strain (up to a critical strain for specimens deformed under dynamic loading condition) and decrease in strain rate. In the event of adiabatic shear band (ASB) formation in a specimen deformed at high strain rate, MSA increased as the ASB was approached due to the temperature rise in the ASB region. Electron backscattered diffractometry examination revealed that the evolution of both thermally- and deformation-induced martensite is orientation-dependent in FG and CG specimens. The instability (thermal and mechanical) of the austenite phase is highest in the RD/CD||[100]-oriented grains (RD and CD are rolling and compression directions, respectively), followed by grains oriented near RD/CD||[110] and RD/CD||[111], in that order. These findings could open a new window of engineering the initial texture of metastable austenitic stainless steel to either aid thermally and/or mechanically-stable austenite phase or promote both isothermal and deformation-induced martensitic phase transformation.

Difference between carbon and nitrogen in thermal stability of metastable 18%Cr-8%Ni austenite

The effects of carbon and nitrogen addition on the athermal α′-martensitic transformation in metastable 18%Cr-8%Ni austenite are investigated. The thermal stability of austenite in nitrogen-added steel is higher than that in carbon-added steel, and the difference between the effects of carbon and nitrogen addition becomes remarkable as the amount of both elements increases. As the two-step transformation (γ → ε → α′) always occurs in metastable 18%Cr-8%Ni austenite, the suppression of ε-martensite results in the thermal stabilization of austenite. Nitrogen is more effective than carbon for increasing the stacking fault energy, leading to a higher thermal stability of the nitrogen-bearing 18%Cr-8%Ni steels.

Decomposition characteristic of austenite retained in GCr15 bearing steel modified by addition of 1.3 wt.% silicon during tempering

The decomposition characteristic of austenite retained in a GCr15 bearing steel modified by the addition of 1.3wt.% silicon during tempering was investigated by microstructural observation, X-ray determination, and dilatometric experiment. The addition of 1.3wt.% silicon in the modified GCr15 bearing steel significantly increases the amount of remaining austenite. After tempering at 300°C for 96h, 18vol.% of austenite with 1.6wt.% carbon remained. Austenite decomposition during the tempering is a bainitic transformation, and occurs via the displacive mechanism, following by carbon partitioning into the remaining austenite. The bainite transformation becomes slower as the carbon enrichment in austenite improves. In contrast, carbide precipitation accelerates the bainite transformation kinetics. However, the carbon enrichment in austenite associated with carbon partitioning and the precipitation of carbides are competitive processes, with their relative rates depending on temperature. Consequently, the improvement in the thermal stability of austenite is ascribed to the combined effects of the partitioning of carbon into austenite and the suppression of carbide precipitation.

Thermally Stable Ni-rich Austenite Formed Utilizing Multistep Intercritical Heat Treatment in a Low-Carbon 10 Wt Pct Ni Martensitic Steel

Austenite reversion and its thermal stability attained during the transformation is key to enhanced toughness and blast resistance in transformation-induced-plasticity martensitic steels. We demonstrate that the thermal stability of Ni-stabilized austenite and kinetics of the transformation can be controlled by forming Ni-rich regions in proximity of pre-existing (retained) austenite. Atom probe tomography (APT) in conjunction with thermodynamic and kinetic modeling elucidates the role of Ni-rich regions in enhancing growth kinetics of thermally stable austenite, formed utilizing a multistep intercritical (Quench-Lamellarization-Tempering (QLT)-type) heat treatment for a low-carbon 10 wt pct Ni steel. Direct evidence of austenite formation is provided by dilatometry, and the volume fraction is quantified by synchrotron X-ray diffraction. The results indicate the growth of nm-thick austenite layers during the second intercritical tempering treatment (T-step) at 863 K (590 °C), with austenite retained from first intercritical treatment (L-step) at 923 K (650 °C) acting as a nucleation template. For the first time, the thermal stability of austenite is quantified with respect to its compositional evolution during the multistep intercritical treatment of these steels. Austenite compositions measured by APT are used in combination with the thermodynamic and kinetic approach formulated by Ghosh and Olson to assess thermal stability and predict the martensite-start temperature. This approach is particularly useful as empirical relations cannot be extrapolated for the highly Ni-enriched austenite investigated in the present study.

Thermal and mechanical stability of retained austenite surrounded by martensite with different degrees of tempering

The mechanical and thermal stability of austenite in multiphase advanced high strength steels are influenced by the surrounding microstructure. The mechanisms underlying and the relations between thermal and mechanical stability are still dubious due to the difficulty of isolating other factors influencing austenite stability. In this work, martensite/austenite microstructures were created with the only significant difference being the degree of tempering of the martensite matrix. Hence, the effect of tempering in martensite is isolated from other factors influencing the stability of austenite. The thermal stability during heating of retained austenite was evaluated by monitoring phase fractions as a function of controlled temperature employing both dilatometry and magnetometry measurements. The mechanical stability was studied by performing interrupted tensile tests and determining the remaining austenite fraction at different levels of strain. The thermal stability of this remaining austenite after interrupted tests was studied by subsequent reheating of strained specimens. The results are evidence for the first time that thermal recovery of martensite during reheating assists austenite decomposition through shrinkage and softening of martensite caused by a reduction of dislocation density and carbon content in solid solution. This softening of martensite also leads to a subsequent reduction of austenite mechanical stability. Additionally, remaining austenite after pre-straining at room temperature was thermally less stable than before pre-straining, demonstrating that plastic deformation has opposing effects on thermal and mechanical stability.

Effect of annealing temperature on microstructural modification and tensile properties in 0.35 C–3.5 Mn–5.8 Al lightweight steel

The microstructural modifications occurring during annealing treatment of an Fe–0.35 C–3.5 Mn–5.8 Al ferrite-based lightweight steel and its effects on the tensile properties were investigated with respect to (α+γ) duplex microstructures. Steels annealed above the dissolution finishing temperature of κ-carbides (795°C) were basically composed of ferrite band and austenite band in a layered structure. As the annealing temperature was increased the tensile strength increased, while the yield strength and elongation decreased. This could be explained by a decrease in the mechanical as well as thermal stability of austenite with increasing size and austenite volume fraction. In the 980°C annealed steel in particular, whose mechanical stability due to austenite was lowest, cracks were readily formed at ferrite/austenite (or martensite) interfaces with little deformation, thereby leading to the least tensile elongation. In order to obtain the best combination of strength and ductility the formation of austenite having an appropriate mechanical stability was essentially needed, and could be achieved when 22–24 vol.% fine austenite was homogeneously distributed in the ferrite matrix, as in the 830°C or 880°C annealed steels.

Thermal Stability of Austenite and Properties of Quenching & Partitioning (Q&P) Treated AHSS

Abstract A Fe-0.2C-1.87Mn-1.42Si-0.0405Al steel subjected to an appropriate Quenching & Partitioning treatment (Q&P) exhibits the combination of high tensile strength (1311 MPa) and high elongation (13.6%). The thermal decomposition of retained austenite in the as-treated steel has been studied at an elevated temperature of 500oC by means of differential scanning calorimetry (DSC). Activation energy has been obtained by performing a Kissinger analysis method. The DSC results show that the activation energy of thermal decomposition of the retained austenite in this Q&P steel is 221.3KJ/mol, which is in a good agreement with the result of retained austenite in similar chemical composition steel subjected to a TRansformation Induced Plasticity (TRIP) treatment. This investigation helps to investigate the stability of retained austenite in Q&P steels upon cooling or under external stress.

Evolution and thermal stability of retained austenite in SAE 52100 bainitic steel

In this work, the evolution of retained austenite during isothermal bainitic heat treatment in SAE 52100 steel, 1.01C–1.36Cr–0.32Mn–0.25Si (wt.%), is investigated with optical microscopy, X-ray diffraction and thermo-magnetic measurements. A significant amount of austenite is retained in the material. The dependence of the volume fraction of retained austenite on bainitic holding time at 503 K shows a maximum at 45 min. It has been demonstrated that the increase of volume fraction of retained austenite occurs as a result of the increased carbon concentration. The thermal stability of austenite is investigated. The temperature at which retained austenite starts to decompose to ferrite and carbides upon heating varies with bainitic holding time. The transformation of austenite to martensite during cooling till 10 K is found to be not complete, and a large amount of austenite remains untransformed.

Thermal stability of metastable austenite in rapidly solidified chromium–molybdenum–vanadium tool steel powder

Abstract Thermal stability of metastable austenite in a Cr–Mo–V tool steel of ledeburite type was investigated by tempering rapidly solidified (RS) particles at temperatures from 100 up to 700 °C and by continuous heating during differential thermal analysis. A rapid increase in microhardness was observed after the tempering at temperatures over 400 °C. According to Mossbauer effect measurements, only non-magnetic phases were observed in the RS particles after atomization, as well as after the tempering at temperatures below 540 °C. Above this temperature, the metastable austenite gradually transformed into martensite during cooling from the tempering temperature. The secondary hardening peak corresponding to 1220 HV appears at 600 °C. This temperature is higher than the temperature of the secondary hardening peak for this steel after conventional heat treatment. The thermal stability of austenite was determined and the mechanisms of phase transformations responsible for the achievement of secondary hardness in this steel following rapid solidification are described.

Thermal stability of metastable austenite in rapidly solidified chromium?molybdenum?vanadium tool steel powder

Thermal stability of metastable austenite in a Cr–Mo–V tool steel of ledeburite type was investigated by tempering rapidly solidified (RS) particles at temperatures from 100 up to 700 °C and by continuous heating during differential thermal analysis. A rapid increase in microhardness was observed after the tempering at temperatures over 400 °C. According to Mössbauer effect measurements, only non-magnetic phases were observed in the RS particles after atomization, as well as after the tempering at temperatures below 540 °C. Above this temperature, the metastable austenite gradually transformed into martensite during cooling from the tempering temperature. The secondary hardening peak corresponding to 1220 HV appears at 600 °C. This temperature is higher than the temperature of the secondary hardening peak for this steel after conventional heat treatment. The thermal stability of austenite was determined and the mechanisms of phase transformations responsible for the achievement of secondary hardness in this steel following rapid solidification are described.

Microstructure of martensite–austenite constituents in heat affected zones of high strength low alloy steel welds in relation to toughness properties

Microstructure and fracture properties relationships have been investigated in the heat affected zones (HAZs) of a high strength low alloy steel used for offshore applications. Metallographic examinations of simulated HAZ microstructures were conducted to investigate the detailed microstructure of the martensite–austenite (M–A) constituents. Using scanning and transmission electron microscopy, various microstructures were found for the M–A constituents. In mixed particles, retained austenite was located at the periphery of the islands. Chemical and/or mechanical effects could possibly account for the stabilisation of this austenite phase. In situ cooling experiments in the transmission electron microscope showed that stacking faults play an important role in the thermal stability of austenite. Impact properties of simulated HAZ microstructures are strongly affected by both the bainitic microstructure and M–A constituents. In particular, freshly transformed high carbon martensite was shown to be much more deleterious than retained austenite.