The Investigation of Strain-Induced Martensite Reverse Transformation in AISI 304 Austenitic Stainless Steel
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This paper presents a comprehensive study on the strain-induced martensitic transformation and reversion transformation of the strain-induced martensite in AISI 304 stainless steel using a number of complementary techniques such as dilatometry, calorimetry, magnetometry, and in-situ X-ray diffraction, coupled with high-resolution microstructural transmission Kikuchi diffraction analysis. Tensile deformation was applied at temperatures between room temperature and 213 K (−60 °C) in order to obtain a different volume fraction of strain-induced martensite (up to ~70 pct). The volume fraction of the strain-induced martensite, measured by the magnetometric method, was correlated with the total elongation, hardness, and linear thermal expansion coefficient. The thermal expansion coefficient, as well as the hardness of the strain-induced martensitic phase was evaluated. The in-situ thermal treatment experiments showed unusual changes in the kinetics of the reverse transformation (α′ → γ). The X-ray diffraction analysis revealed that the reverse transformation may be stress assisted—strains inherited from the martensitic transformation may increase its kinetics at the lower annealing temperature range. More importantly, the transmission Kikuchi diffraction measurements showed that the reverse transformation of the strain-induced martensite proceeds through a displacive, diffusionless mechanism, maintaining the Kurdjumov–Sachs crystallographic relationship between the martensite and the reverted austenite. This finding is in contradiction to the results reported by other researchers for a similar alloy composition.
The formation of martensite during deformation is a very common phenomenon in austenitic stainless steels which, if susceptible to such transformations, are called metastable. The transformation from FCC (Face-Centered Cubic, γ) austenite to BCC (Body-Centered Cubic, α′) martensite may happen in a direct manner (γ → α′) which usually occurs on intersections of shear bands, twins, or via HCP (Hexagonal Close Packed, ε) martensite (γ → ε → α′). However, ε martensite fully transforms into α′ when a higher strain is applied. The maximum volume fraction of ε martensite is strongly dependent on the deformation temperature and usually takes place at strains lower than 0.15.[2,3] As reported by Tavares et al., it is possible to obtain a different transformation succession, i.e., γ → α′ → ε when applying high pressure, for instance by HPT (High Pressure Torsion) processing at the highest possible pressure and deformation conditions. The influence of BCC martensite and/or its formation on the materials properties such as mechanical,[5,6] corrosion resistance,[7,8] formability,[9, 10, 11, 12] hydrogen cracking susceptibility,[13, 14, 15] and fracture propagation have been investigated. Mathematical models for strain-induced transformation in austenitic stainless steels have also been already developed.[17, 18, 19]
BCC as well as HCP martensite undergoes a reverse transformation during the heating process. Previous dilatometric studies revealed ε → γ reverse transformation between RT and 473 K (200 °C) and α′ → γ reverse transformation in the range of 773 K to 1073 K (500 °C to 800 °C).[3,20] Both (ε → γ, α′ → γ) transformations are shifted to higher temperatures with the increase of heating rate (at the heating rate higher than 0.1 K/s). Tomimura et al. described two types of possible reverse transformation (α′ → γ) mechanisms: 1st diffusive by nucleation and growth of new austenite grains and 2nd diffusionless shear reversion. The reverse transformation plays a significant role in grain refinement in metastable austenitic stainless steels. UFG (Ultrafine-grained) as well as NG (nano-grained) materials have been produced by reversion annealing of AISI 304, AISI 304L, and less stable AISI 301LN and AISI 201 metastable stainless steels, where the material strength is enhanced with a moderate decrease in plasticity.[23, 24, 25, 26, 27, 28, 29] Grain refinement via reversion annealing still draws attention, i.e., Sun et al. reported diffusive reversion of strain-induced martensite during annealing in the range of 823 K to 923 K (550 °C to 650 °C).
This paper reports the formation of SIM during the tensile test at different temperatures, and the reverse transformation analysis by the use of DSC, in-situ X-ray diffraction, and dilatometry. Moreover, to determine the reverse transformation sequence, samples were annealed at chosen temperatures and the resultant microstructure was investigated using a novel, high-resolution transmission Kikuchi diffraction technique. The main objective of this study was to gather more information about the influence of SIM formation on mechanical and physical properties and to find the SIM reverse transformation mechanism in the AISI 304 steel.
The investigated material was an AISI 304 (1.4301, X5CrNi18-10) stainless steel bar. For further investigations, the bar was cut into tensile samples (φ 3 × 110 mm) and solution heat treated from 1323 K (1050 °C) with soaking time of 1 hour in air. The oxide layer was removed by electropolishing in the Struers A2 solution.
The deformation was done using INSTRON 5982 universal testing machine equipped with a cooling chamber. Samples were tested in 296 K, 273 K, 263 K, 253 K, 243 K, 223 K, and 213 K (23 °C, 0 °C, −10 °C, −20 °C, −30 °C, −50 °C, and −60 °C) with the initial strain rate of 5.0 × 10−4 s−1. Soaking time of 10 minutes was applied for temperature stabilization. All further investigations were performed on the uniform elongation zones of the deformed samples. Hardness measurements of fractured specimens (5 on each sample) were performed using Tukon 2500 by Wilson Hardness applying the Vickers method with the applied force of 9.8 N (HV 1). The uncertainty of mechanical properties was calculated based on the standard deviation.
Metallographic examination has been done on longitudinal sections of uniform plastic deformation zones of fractured specimens. The samples were ground, polished, and etched using 30 g NH4F + 50 mL HNO3 (65 pct) + 20 mL H2O reagent. The observation was carried out in bright-field and differential interference contrast (DIC) to reveal deformation features in samples.
Transmission Kikuchi diffraction (TKD) was used in order to investigate microstructure evolution during the reverse transformation. A standard technique was used for the preparation of the thin, electron transparent foil. Samples cut at a cross-section perpendicular to the tensile axis were mechanically ground and electropolished using A2 reagent in a Twin-jet polishing apparatus Tenu-pol-5 by Struers. TKD analysis was performed by means of scanning electron microscope Versa 3D by FEI equipped with Hikari EDAX camera and a custom-designed thin foil holder. Orientation and phase maps were collected at the 20 kV of acceleration voltage, beam current of 16 nA, step size of 10 nm, and acquisition rate of 100 to 200 patterns per second. The camera binning was set at 4 × 4 with the binned pattern resolution of 160 × 120 pixel. BCC (for strain-induced martensite) and FCC (for austenite) phases were chosen for phase analysis, where 8 bands on each Kikuchi pattern were analyzed. The average pixel fraction, with the confidence index higher than 0.1, was at least 0.80. More experimental details can be found in our previous paper. In order to avoid any image distortions, map cleaning/post-processing was not used.
Heating Parameters for the Applied Techniques
Averaged Heating Rate (K/min)
In-situ magnetometric studies during heating and at room temperature were performed using vibrating sample magnetometer (VSM) 7407 by LakeShore. The applied magnetic field during room-temperature and heating experiments was equal 1 T. Dilatometric investigations were carried out using the R.I.T.A. L78 induction heating high-resolution dilatometer by Linseis on 10-mm cylindrical samples with the diameter in the range of 2.5 mm (diameter dependent on the elongation during the tensile test). The applied heating rate was 0.05 °C/s and the samples were heated up to 727 K (1000 °C). Differential scanning calorimetry measurements were carried out using the SDT Q600 by TA Instruments, where the sample purge gas was argon with the flow of 80 mL/min. X-ray diffraction (XRD) measurements for fractured specimens were performed by means of PANalytical’s Empyrean diffractometer using Co Kα (λ = 1.7890 Å) in the parallel beam (equipped with Goebel mirror) configuration. The X-ray tube operating parameters were 40 kV and 40 mA. In-situ X-ray diffraction (XRD) measurements were done by means of PANalytical’s Empyrean diffractometer using Cu Kα (λ = 1.5418 Å) equipped with the Anton Paar’s HTK1200 high-temperature chamber in the temperature range of 273 K to 973 K (0 °C to 700 °C). During the experiment, the sample was maintained under vacuum conditions (pressure lower than 6 × 10−7 mbar). The sample’s position was corrected with respect to temperature displacements during the measurements (tungsten powder of 99.9999 pct purity was used as a standard). The temperature step was set at 323 K (50 °C), where each temperature was approached by a ramp of 10 K/min, then the sample’s temperature was stabilized for 20 minutes. Afterwards, diffraction patterns were collected in the range of 30 to 85 deg for the 2Θ angles. The single diffraction pattern’s acquiring time was 4:30 hours. The obtained data were analyzed using the Rietveld-type FullProf Suite package.
3.1 Formation of Strain-Induced Martensite
In order to elucidate the work hardening change during the tensile experiment, derivatives dσ/dε were calculated (Figure 1(b)). The curves exhibit two characteristic features: the so-called “softening” effect at low strains and work hardening effect at strains above 0.2. Both aspects are well-known features of metastable stainless steel materials related to the kinetics of γ → ε and ε → α′ transformations.
3.2 Reverse Transformation of the Strain-Induced Martensite
In order to investigate the reverse transformation of SIM, a multi-method approach was applied. Three different in-situ methods: DSC, dilatometry, and VSM were used to elucidate the start and finish temperatures of the transformation and to show its kinetics. For the material deformed at 243 K (−30 °C), in-situ X-ray analysis was performed during the heating experiment. The same sample was also subjected to isothermal annealing at temperatures of 873 K, 973 K, and 1073 K (600 °C, 700 °C, and 800 °C) (soaking time 4 hours) and investigated by means of TKD technique.
A thorough microstructure investigation allowed us to put a hypothesis that this effect is related to the internal strain relaxation. A detailed description and discussion is presented further in this study.
The Magnetic Saturation and Volume Fraction of α′ Martensite in Deformed AISI 304 Stainless Steel Before Heating up to 1103 K (830 °C) and After the Cooling Process
Deformation Temperature (°C)
Solution Heat Treated
296 K (23 °C)
273 K (0 °C)
263 K (−10 °C)
253 K (−20 °C)
243 K (−30 °C)
223 K (−50 °C)
213 K (−60 °C)
Magnetization saturation (emu/g)
Volume percent of α′ martensite (pct)
3.3 Reverse Transformation Sequence—Microstructure Investigations
The reverse SIM transformation is completed in temperatures of about 923 K (700 °C) (Figure 9(a)); however, an additional effect can be seen on the dilatometric curves up to 1123 K (850 °C). In order to elucidate microstructural evolution, isothermal annealing of the as-deformed in 243 K (−30 °C) specimen was employed. Selected annealing temperatures of 873, 973, and 1073 K (600 °C, 700 °C, and 800 °C) were chosen and correspond to characteristic features of stages I and II (Figure 7). The application of TKD technique allowed for clear identification of α′ and γ phase distribution and crystallographic relationships. The data are presented in the form of PM (Phase Maps) and IPF (Inverse Pole Figures) maps. Additionally, Image Quality maps (IQ) are included as separate images to show qualitative information about grain boundary distribution in the microstructure.
The deformation behavior of the 304 alloy is governed by the combined effect of strain hardening of the austenitic phase and formation of the strain-induced martensite. The latter phenomenon depends on the deformation temperature and, as it drops, SIM formation is greatly intensified. The increased SIM formation kinetics during the temperature decrease can be observed on the derivative curves based on the peak height increase as well as the peak shift toward lower values of strain. Interestingly, the peak position is linearly dependent on the deformation temperature (Figure 3).
High-temperature TEC [1173 K to 1273 K (900 °C to 1000 °C)] has the same value of 25 µm/(m °C) for all samples as a result of a fully austenitic structure after a full reverse SIM transformation.
In-situ kinetics of the reverse SIM transformation was analyzed by four different measurement techniques: calorimetry, dilatometry, magnetometry, and X-ray diffraction. Temperatures of about 723 K and 973 K (450 °C and 700 °C) were found for the start and finish temperatures of the reverse SIM transformation, respectively. It is important to mention that similar values were obtained for all the techniques despite one order of magnitude speed differences in heating speeds during measurements. It is the first sign for the diffusionless transformation type as the SIM transformation is independent of the time variable.
An additional effect was revealed during the reverse transformation by means of calorimetry, dilatometry, and magnetometry experiments around 823 K (550 °C) where the kinetics of the reverse transformation is altered. Splitting is observed in all of the curves’ peaks irrespectively of the amount of martensite present in the material microstructure. According to the authors’ knowledge, no explanation of such an effect can be found in the literature. This “two-stage” process can be explained with the help of the in-situ XRD experiment. The lattice parameter change (Figure 9(b)) shows substantial deviation from linearity in both austenitic and martensitic phase at the temperature of 673 K (400 °C) while at 773 K (500 °C), the lattice expansion follows a linear curve during cooling caused by thermal expansion. The beginning of this phenomenon corresponds to the start of the SIM reversal transformation.
It is proposed that for the given region of temperatures, significant release of residual strains is observed. Those strains inherited from the deformation process or created by different expansion coefficient of austenitic and martensitic phases are the driving force for increased kinetics of the reverse transformation at the early stages of the process. After strains release, kinetics of transformation slows down and its further increase (second part of the peak) is related to temperature rise.
The microstructure TKD analysis confirms the above-mentioned results. In all of the investigated cases, the nucleation and growth of austenitic phases was not observed. Such behavior would create various orientations in separate regions of the reversed austenite and should signal a diffusive type of reversion transformation, according to Tomimura et al. Contrarily, all regions (reverted austenite within one grain) possess a similar orientation with some misorientation caused by the rearrangement of dislocations during the recovery process. Moreover, the Kurdjumov–Sachs crystallographic orientation relationship between SIM and reverted austenite was found (Figure 11). Based on Tomimura et al. widely accepted SIM reverse transformations definitions, we found diffusionless shear transformation. It is in contradiction to the work of Sun et al., where based on TEM analysis, the authors found that a similar alloy was transformed by a diffusion-type mechanism. Based on current knowledge, it is hard to find an explanation to this discrepancy as it can be a result of a slightly different chemical composition or materials’ processing history. A detailed analyses of the Fe-Cr-Ni system and the SIM formation and reverse transformations are still needed.
Transformation-induced plasticity gave the highest increase of elongation at the temperature of 263 K (−10 °C), where ~50 pct of the SIM was found after deformation.
Maximum reaction rate of ε and α′ formation was found to have a linear correlation to temperature; as temperature decreases maxima are shifted toward lower strains.
The amount of SIM increases with temperature decrease; the highest increase of SIM is observed up to the temperature of maximum elongation.
The reverse transformation of SIM takes place in the range of 723 K to 973 K (450 °C to 700 °C) and it is not dependent on the heating rate (in investigated range of heating rates).
The kinetics of the SIM reverse transformation reveals two distinct stages; in-situ XRD suggests that first stage can be stress assisted.
The Kurdjumov–Sachs crystallographic orientation relationship between SIM and reverted austenite was found.
On the basis of the microstructural TKD measurements, diffusionless (shear) reversion was found; shear reversion is in agreement with the theory of stress-assisted reversion of SIM.
This research was supported by the Polish National Science Centre (NCN) under Project No. 2016/23/N/ST8/01252.
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