Expanded Austenite Layer Research Articles

Functional properties of expanded austenite generated on AISI 316L by plasma nitrocarburizing using different active screen materials

Abstract This study investigates the functional properties of the expanded austenite layers generated on AISI 316L austenitic stainless steel resulting from active screen plasma nitrocarburizing using different active screen materials, i.e. steel or solid carbon. Treatments were conducted at 460 °C for 5 hours in a nitrogen-hydrogen feed gas, whereas for the treatments using a steel active screen, methane was added as carbon precursor. Additionally, the bias plasma conditions applied at the samples were varied between 0 kW and 1.25 kW. Samples were characterized by complementary microstructural and compositional investigations, surface roughness and hardness measurements, pin-on-disk tribological tests as well as potentiodynamic polarization tests in H2SO4 and NaCl electrolytes. The functional properties of the case are discussed based on the contents of nitrogen and carbon in the expanded austenite and on their effective diffusion depths. The results show that the usage of a carbon screen generally produces surfaces with uniform layer thickness, high hardness, improved wear resistance and a delayed tendency to pitting corrosion independent of the bias condition applied to the samples. When applying both screen materials at non-biased condition, the general corrosion resistance is slightly reduced under the conditions used, however, the layers generated using the carbon screen have a wear rate that is 3 times lower. It can be concluded that the carbon screen represents a robust treatment variant for austenitic stainless steels to produce sufficiently thick and wear-resistant surface layers in a short treatment duration, which still have the potential to maintain the corrosion resistance in different environments.

The Interplay Effects between Feed-Gas Composition and Bias Plasma Condition during Active-Screen Plasma Nitrocarburizing with a Solid Carbon Source

Recent technological development of utilizing an active screen made of solid carbon for plasma-assisted thermochemical diffusion treatments opens up new possibilities for control over the in situ generated treatment environment to guarantee reproducible treatment conditions and material responses. Until now, the investigations of active-screen plasma nitrocarburizing (ASPNC) using an active screen manufactured from solid carbon focused on the influence of a single treatment parameter variation on the material response. In this systematic study, experiments were conducted to vary the H2-N2 feed-gas composition while varying the bias plasma power. The experiments served to better understand a simultaneous variation in the mentioned parameters on the resulting treatment environment and material response during ASPNC of AISI 316L austenitic stainless steel. Therefore, nitriding and carburizing effects in the expanded austenite layer can be obtained. It is shown that an increased nitriding effect, i.e., nitrogen diffusion depth and content, was achieved in case of biased conditions and for H2-N2 feed-gas compositions with higher N2 amounts. On the contrary, an increased carburizing effect, i.e., carbon diffusion depth and content, was achieved in nonbiased conditions, independent from the H2-N2 feed-gas composition.

OPTIMASI PARAMETER KARBURISASI TEMPERATUR RENDAH PADA BAJA TAHAN KARAT AUSTENITIK MENGGUNAKAN METODE TAGUCHI

Austenitic stainless steel is a popular material for its corrosion resistant properties, however it has low hardness which limits its application. Low-temperature carburizing can be used to improve the mechanical properties of the austenitic stainless steel by producing expanded austenite layer. In order to get a high-quality layer and an efficient processing operation, the carburizing process must be optimized. In this research, a Taguchi method was utilized to investigate the effect of processing parameters related to the formation of the expanded austenite layer depth in austenitic stainless steel. Four factors were selected to be optimized namely temperature, gas flow rate, time, and gas composition with three levels each. L9(34) orthogonal array was applied with nine experimental tests to get the diffusion depth value of carbon in the expanded austenite layer. S/N ratio was used to determine the optimum factor combination with nominal-the-better quality characteristic and the most significant factor was obtained by applying the Analysis of Variance. Temperature was found to be the most significant factor with 54.91% contribution. The optimum combination was also successfully defined with temperature at 450°C (level 2), gas flow rate at 15 liter per minute (level 2), time at 12 hours (level 3), and gas composition at 15% CH4 – 5% H2 – 80% N2 (level 3). Clearly, in this study the Taguchi method was proven to be appropriately used as one of robust tools in optimizing the thermochemical treatment process parameters.

Effects of Plasma-Chemical Composition on AISI 316L Surface Modification by Active Screen Nitrocarburizing Using Gaseous and Solid Carbon Precursors

Low-temperature plasma nitrocarburizing treatments are applied to improve the surface properties of austenitic stainless steels by forming an expanded austenite layer without impairing the excellent corrosion resistance of the steel. Here, low-temperature active screen plasma nitrocarburizing (ASPNC) was investigated in an industrial-scale cold-wall reactor to compare the effects of two active screen materials: (i) a steel active screen with the addition of methane as a gaseous carbon-containing precursor and (ii) an active screen made of carbon-fibre-reinforced carbon (CFC) as a solid carbon precursor. By using both active screen materials, ASPNC treatments at variable plasma conditions were conducted using AISI 316L. Moreover, insight into the plasma-chemical composition of the H2-N2 plasma for both active screen materials was gained by laser absorption spectroscopy (LAS) combined with optical emission spectroscopy (OES). It was found that, in the case of a CFC active screen in a biased condition, the thickness of the nitrogen-expanded austenite layer increased, while the thickness of the carbon-expanded austenite layer decreased compared to the non-biased condition, in which the nitrogen- and carbon-expanded austenite layers had comparable thicknesses. Furthermore, the crucial role of biasing the workload to produce a thick and homogeneous expanded austenite layer by using a steel active screen was validated.

Low-temperature carburized high-alloyed austenitic stainless steels in PEMFC cathodic environment

Austenitic stainless steels are promising materials for bipolar plates in polymer electrolyte membrane fuel cells (PEMFC). In this study, we explore the possibility to apply low-temperature carburized high-alloyed (high Cr, Ni, Mo) steels as bipolar plate materials. The electrochemical properties and the microstructural characteristics of the modified surfaces were analyzed by optical microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Auger electron spectroscopy. The carburizing treatment (Kolsterising®, Bodycote) induced the formation of a carbon-stabilized expanded austenite layer. The amount of carbon dissolved in the metal matrix was dependent on the alloy composition, in particular on the amount of Mo. Additionally, surface carbides were found at the surface of low-Ni containing steels. The interfacial contact resistance (ICR) measured in a simulated PEMFC was reduced after carburizing as a consequence of thinner oxide layer and change in oxide composition. Potentiodynamic tests revealed nobilitation of all the steels when carbon was present in solid solution. The occurrence of surface carbides, however, was highly detrimental to the corrosion properties. The ICR of carburized materials in aged condition was higher than the corresponding untreated alloys. The presence of localized corrosion in carburized materials could be responsible for this effect.

Evaluation of the sliding wear and corrosion performance of triode-plasma nitrided Fe-17Cr-20Mn-0.5N high-manganese and Fe-19Cr-35Ni-1.2Si high-nickel austenitic stainless steels

Low-temperature plasma nitriding has been widely studied and applied, in enhancing the wear performance of austenitic stainless steels (ASSs) without losing corrosion resistance. In this work the wear and corrosion behaviours of two specialty ASSs, i.e. Staballoy® AG17 (Fe-17Cr-20Mn-0.5N, in wt%) and RA330® (Fe-19Cr-35Ni-1.2Si, in wt%), were evaluated – and compared to AISI 304 – before and after low-temperature triode plasma nitriding (TPN) at 400 °C and 450 °C. A nitrogen interstitially-supersaturated expanded austenite layer (γN) was developed for all three ASSs after TPN treatment at 400 °C, which led to a) an approximately 4-fold increase in surface hardness, b) a reduction in specific wear rate of at least 2 orders of magnitude in unlubricated dry-sliding, and c) an improved resistance to pitting in 3.5 wt% NaCl aqueous solution. Large numbers of ‘linear defects’ (identified in TEM studies as strips of HCP-εN) were seen in the γN-AG17 layer, that could be correlated to comparatively higher surface hardness and better wear resistance. Several slip/shear bands were also seen in the γN-330 layer, where short-range Cr-segregation could occur, leading to localised corrosion. More importantly, after TPN treatment at 450 °C, alloys AISI 304 and AG17 presented a deterioration in corrosion performance, whereas good corrosion performance was maintained for alloy RA330. Redistribution of Si (in preference to Cr) was revealed in γN-330 after TPN treatment at 450 °C, whereby Si-alloying at a significantly higher level than in the other two alloys investigated appears (in addition to the high Ni content in alloy 330) to be beneficial in delaying CrN precipitation, and thus in maintaining the good corrosion performance of γN after nitriding at low-to-intermediate temperatures.

Examination on dry sliding wear behavior of AISI 304 stainless steel treated with salt bath nitriding process

The wear behaviour of AISI 304 stainless steel was investigated at consistent load under salt bath nitriding. In this salt bath nitriding process, the work pieces are drenched into the liquid shower, the nitride salts contained in it diffuses into the surface of the steel and forms Fe3N compounds. Then the specimens were immersed in salt bath furnace for nitriding at 570 °C for different timing parameters like salt-bath nitride1 (SN1) 60 min, salt-bath nitride2 (SN2) 120 min and salt-bath nitride3 (SN3) 180 min so that the nitrogen diffuses into the steel surface and then it is air cooled. After heat treatment wear test was done to find out the wear loss in the material. The pin of 40 mm length and 12 mm diameter was held on rotating disc of 150 mm diameter. The wear test for all specimens was conducted under the normal loads of 20 N and 30 N respectively. Metallographic tests like optical microscopic results, SEM is done to specimens for comparing physical characteristics before and after heat treatments. To compare the mechanical characteristics of the heat treated specimens before and after heat treatments they are subjected to pin on disc experiment and Vickers hardness test to determine wear resistance and hardness of the material. From SEM analysis, it was observed that a white layer is formed called “S-Phase” an expanded austenite layer formed on the salt bath specimen nitrided for (120 min, 180 min). It was concluded that nitride distribution increases with increase in nitriding time. The hardness of salt bath nitride3 (180 min) was found to be 835 HV.

Nano-Sized Cr-N Clustering in Expanded Austenite Layer of Low Temperature Plasma Nitrided Fe-35Ni-10Cr Alloy

Nano-Sized Cr-N Clustering in Expanded Austenite Layer of Low Temperature Plasma Nitrided Fe-35Ni-10Cr Alloy

Diffusional Model of Expanded Austenite Layer Growth on AISI 316L Stainless Steel During Low-Temperature Hybrid Thermochemical Treatment

A simultaneous nitrogen and carbon diffusional model in AISI 316L stainless steel is presented in this numerical study to simulate expanded austenite layer growth developed during hybrid thermochem...

Influence of the Active Screen Plasma Power during Afterglow Nitrocarburizing on the Surface Modification of AISI 316L

Active screen plasma nitrocarburizing (ASPNC) increases the surface hardness and lifetime of austenitic stainless steel without deteriorating its corrosion resistance. Using an active screen made of carbon opens up new technological possibilities that have not been exploited to date. In this study, the effect of screen power variation without bias application on resulting concentrations of process gas species and surface modification of AISI 316L steel was studied. The concentrations of gas species (e.g., HCN, NH3, CH4, C2H2) were measured as functions of the active screen power and the feed gas composition at constant temperature using in situ infrared laser absorption spectroscopy. At constant precursor gas composition, the decrease in active screen power led to a decrease in both the concentrations of the detected molecules and the diffusion depths of nitrogen and carbon. Depending on the gas mixture, a threshold of the active screen power was found above which no changes in the expanded austenite layer thickness were measured. The use of a heating independent of the screen power offers an additional parameter for optimizing the ASPNC process in addition to changes in the feed gas composition and the bias power. In this way, an advanced process control can be established.

Plasma Nitrocarburizing of AISI 316L Austenitic Stainless Steel: a First Step for Treatment of Components with Complex Geometries

Abstract The surfaces of AISI 316L austenitic stainless steel components with complex shapes and geometries can be subjected to extreme loads with intensive wear stress resulting a short lifetime. Low temperature active screen plasma nitrocarburizing (ASPNC) can be applied to form a thin duplex layer known as expanded austenite, improving the hardness and wear resistance with acceptable corrosion resistance. However, ASPNC of shaped components and components with different aspect ratios may yield non-uniform expanded austenite layer and/or reproducibility problems limiting their applications for specific cases. In this study, first, ASPNC of AISI 316L treated at different temperatures was investigated using active screen made of carbon-fibre reinforced carbon (CFC) to determine a reliable treatment temperature without CrN precipitation. In addition, several non-uniformity effects related to geometry and shape of structured samples were investigated during ASPNC treatment at different biased conditions. It was shown that in case of structured samples, a weak bias power at the samples was an essential process parameter to guarantee the formation of thick and homogenous expanded austenite layer while in non-biased conditions, thin and inhomogeneous expanded austenite is formed.

Plasma Nitrocarburizing of AISI 316L Austenitic Stainless Steel: a First Step for Treatment of Components with Complex Geometries

Abstract The surfaces of AISI 316L austenitic stainless steel components with complex shapes and geometries can be subjected to extreme loads with intensive wear stress resulting a short lifetime. Low temperature active screen plasma nitrocarburizing (ASPNC) can be applied to form a thin duplex layer known as expanded austenite, improving the hardness and wear resistance with acceptable corrosion resistance. However, ASPNC of shaped components and components with different aspect ratios may yield non-uniform expanded austenite layer and/or reproducibility problems limiting their applications for specific cases. In this study, first, ASPNC of AISI 316L treated at different temperatures was investigated using active screen made of carbon-fibre reinforced carbon (CFC) to determine a reliable treatment temperature without CrN precipitation. In addition, several non-uniformity effects related to geometry and shape of structured samples were investigated during ASPNC treatment at different biased conditions. It was shown that in case of structured samples, a weak bias power at the samples was an essential process parameter to guarantee the formation of thick and homogenous expanded austenite layer while in non-biased conditions, thin and inhomogeneous expanded austenite is formed.

Effect of the roughness produced by plasma nitrocarburizing on corrosion resistance of AISI 304 austenitic stainless steel

In this work, we studied the influence of surface roughness produced by sputtering during plasma nitrocarburizing treatments on corrosion resistance of AISI 304 austenitic stainless steel in an aqueous solution of sodium chloride. Plasma nitrocarburizing treatments were carried out in a gaseous atmosphere containing 80% N2+2% CH4+18% H2, at 400Pa, for 2h and at temperatures of 375°C, 430°C and 475°C. Surface roughness after plasma nitrocarburizing was characterized using 3D confocal microscopy. Corrosion susceptibility of the plasma nitrocarburized surfaces was investigated by electrochemical impedance spectroscopy, potentiodynamic polarization, and scanning electron microscopy. The results showed that when increasing the temperature of the plasma nitrocarburizing treatment, both thickness and amount of precipitates increased, as expected. Analyses by 3D confocal microscopy also showed increased surface roughness with increasing plasma nitrocarburizing temperature. When comparing two plasma nitrocarburized specimens with similar precipitate-free structure, it was found that thinner thickness of the expanded austenite layer and lower surface roughness resulted in higher corrosion resistance.

Abrasive resistance and corrosion properties of AISI 316 sieve via low-temperature gaseous nitriding

AISI 316 austenitic stainless steel samples were treated via gaseous nitriding at low temperature (430 °C) to obtain a single expanded austenite layer (S-phase). The structural phases were characterized via both optical microscopy and X-ray diffraction. Microhardness and dry-sliding wear behavior were investigated via scanning electron microscopy, electron probe of microanalysis, Vickers indentation tester, and ball-on-flat wear tester, respectively. The wear of the substrate AISI 316 steel was severe and characterized by strong adhesion, abrasion, and spallation, while the wear of the S-phase layer was mild and dominated by slight abrasion. Furthermore, the corrosion properties were investigated via potentiodynamic polarization testing in 3.5% NaCl solution. The results show that the S-phase layers was produced at low nitrided temperature with improved wear resistance as well as improved corrosion resistance.

Solid carbon active screen plasma nitrocarburizing of AISI 316L stainless steel: Influence of N2-H2 gas composition on structure and properties of expanded austenite

Plasma nitrocarburizing based on active screen technology using a carbon-fiber reinforced carbon active screen was applied in an industrial-scale unit for thermochemical surface treatment of austenitic stainless steel. This concept is based on the use of a solid-carbon-source for the generation of highly reactive process gases directly in the active screen plasma. In this work, plasma nitrocarburizing of AISI 316L stainless steel was performed by means of the variation of the precursor gases H2 and N2 in the range of 0% N2 up to 100% N2 without the use of any additional carbon bearing gas. For a set of H2-N2 gas mixtures, the resulting reaction gas was monitored using infrared laser absorption spectroscopy (IRLAS). The four main stable species HCN, CH4, NH3 and C2H2 were detected. A detailed analysis of the surface microstructure resulting for each specific H2-N2 precursor gas mixture was performed. This included glow discharge optical emission spectroscopy (GDOES), optical microscopy, micro hardness measurements, as well as X-ray diffraction analysis.The concentrations of hydrocarbons CH4 and C2H2 were most abundant in case of pure hydrogen plasma, and a carbon-expanded austenite layer with hardness values up to 600 HK0.01 and smooth hardness gradient resulted. The admixture of N2 to the precursor gas significantly increased the concentrations of HCN and NH3. Due to this, a duplex structure of nitrogen-expanded austenite γN and carbon-expanded austenite γC formed. With increasing content of N2 up to 50% in the precursor gas, the resulting layer thickness increased and the hardness reached values up to 1300 HK0.01. At strong nitrogen excess in the precursor gas, the nitrogen concentration in the expanded austenite significantly increased, the Fe2–3(N, C) phase was formed, and simultaneously the layer thickness decreased. Structure and properties of the expanded austenite layer significantly changed by only varying the H2-N2 ratio of the precursor gas mixture.

Effect of C2H2/H2 Gas Mixture Ratio in Direct Low-Temperature Vacuum Carburization

The effect of the acetylene and hydrogen gases mixture ratios in direct low-temperature vacuum carburization was investigated. The gas ratio is an important parameter for producing free radicals in carburization. The free radicals can remove the natural oxide film by strong reaction of the hydrocarbons, and then thermodynamic activity can be increased. When the gas ratio was below one, carbon-supersaturated expanded austenite layers were formed on the surface of the AISI 316L stainless steel, which had a maximum carbon solubility up to 11.5 at% at 743 K. On the other hand, when the gas ratio was above one, the carbon concentration of the layers was low even if the process time was increased enough to reach the maximum carbon solubility. As a result, the carbon concentration underneath the surface was determined to be highly dependent on the gas mixture ratio of acetylene and hydrogen. In conclusion, it is necessary to restrict the ratio of acetylene and hydrogen gases in the total mixture of gases to form an expanded austenite layer with high carbon concentration in direct low-temperature vacuum carburization.

Gradient Elastic–Plastic Properties of Expanded Austenite Layer in 316L Stainless Steel

The gradient mechanical properties, variation of stress with strain and surface cracking behavior of expanded austenite developed on 316L austenitic stainless steel were investigated by nanoindentation tests, X-ray residual stress analysis and scanning electron microscope observation in four-point bending tests. The results show that the plastic properties of the carburizing layer including true initial yield strengths and strain hardening exponents increase significantly from substrate to surface, while the true elastic modulus just improves slightly. Due to the onset of plastic flow, the residual stresses are almost equivalent to the true initial yield strengths from surface to the depth of ~ 10 μm. The results of four-point bending tests show that surface stress increases linearly with the increase in strain until the strain reaches ~ 1.0%, after that the plastic yield happens. The expanded austenite surface layer is brittle, and the cracks will be created at the strain of ~ 1.4%. The cracking stress is about ~ 2.4 GPa.

Thermal stability of expanded austenite formed on a DC plasma nitrided 316L austenitic stainless steel

Expanded austenite formed during low temperature plasma nitriding of austenitic stainless steels is known for its excellent wear and corrosion resistance. These wear and corrosion properties may degrade by exposure of the surface hardened steel to high temperatures, between 400°C and 700°C. DC low temperature plasma nitrided 316L austenitic stainless steel (400°C for 20h) was heated up to investigate the stability of the 3μm thick expanded austenite layer in the range 400°C<T<700°C. Time-resolved X-ray diffraction experiments were undertaken in a thermomechanical simulator coupled to a synchrotron light source. Two series of experiments were carried out: a) isothermal treatments and b) a continuous heating experiment from room temperature up to 700°C. The results show that during the isothermal heating experiments, expanded austenite remained stable up to 400°C. Although no precipitated phases (ferrite or nitrides) were detected at this temperature, nitrogen diffusion towards the matrix occurred, promoting thickening of the expanded austenite layer. The coefficients of thermal expansion of expanded austenite, determined by linear regression, in the range 80 to 400°C, resulted in different values for different crystallographic directions. These differences can be attributed to an anisotropic relief of residual stresses in different directions, which compensate a “non-stressed” thermal expansion of expanded austenite. Very fine and dispersed CrN nanosized precipitates are formed in the nitrogen rich region of the expanded austenite layer, after exposure to temperatures higher than 400°C. A lamellar product – called nitrogen pearlite – forms, in the nitrogen poor region of the expanded austenite layer, near the matrix, after exposure to temperatures higher than 500°C. The higher is the temperature, the coarser is the nitrogen pearlite formed. Nitrogen diffuses to the matrix along austenite grain boundaries, leading to nucleation and growth of nitrogen pearlite in regions relatively far from the nitrided layer. Thermodynamic modeling of the Fe-16.4Cr-9.8Ni-2.0Mo system, taking into account the incorporation of nitrogen during nitriding, shows that the eutectoid temperature decreases steadily with increasing nitrogen content. Based on this result it is possible to explain different precipitation behaviors in the nitrogen rich and nitrogen poor regions of the expanded austenite layer.

Development and microstructure characterization of single and duplex nitriding of UNS S31803 duplex stainless steel

In this work, the development of a duplex nitriding (DN) surface treatment combining High Temperature Gas Nitriding (HTGN) and Low Temperature Plasma Nitriding (LTPN) is reported. The microstructure and hardness variation of the duplex treated steel is compared with the properties obtained during single HTGN and single LTPN of UNS S31803 stainless steel. Single LTPN of UNS S31803 was carried out in an Active Screen Plasma Nitriding reactor, at 400°C for 20h, in a 75% N2+25% H2 atmosphere. Single HTGN of UNS S31803 was carried out at 1200°C, under a 0.1MPa high purity N2 gas atmosphere, during 8h. The developed duplex nitriding (DN) surface treatment consists of a combination of both, HTGN and LTPN treatments, carried out in the same conditions described above.The microstructure of the as received material was composed by ferrite and austenitic stringers, aligned in the rolling direction. The results showed that LTPN of the UNS S31803 duplex stainless steel promotes the formation of a duplex modulated structure composed by 2.5μm thick, 1510±52HV hard, expanded ferrite (αN) regions, and 3.0μm thick, 1360±81HV hard, expanded austenite (γN) regions on ferrite and austenite grains, respectively. Intense coherent ε-Fe3N nitride precipitation inside expanded ferrite was observed. ε-Fe3N nitrides precipitated with an orientation relationship [111] αN//[120] ε-Fe3N, leading to increased microhardness of the expanded ferrite regions. After the first step of the duplex nitriding treatment (HTGN) a 550μm thick, 330HV hard, nitrogen rich, fully austenitic layer formed at the surface of the specimens, by transformation of ferrite stringers into austenite. The second nitriding step (LTPN) led to the formation of a homogeneous expanded austenite layer, 1227±78HV on top of the thick fully austenitic layer, formed during the first step. The duplex treatment resulted in a more homogeneous, precipitate-free, microstructure and a better transition between the mechanical properties of the hardened outermost layer and the softer substrate.

In-situ kinetics study on the growth of expanded austenite in AISI 316L stainless steels by XRD

The formation of expanded austenite in Cr-Ni austenitic stainless steels like AISI 316L is not completely understood despite its technological relevance. In this work, we present an in-situ X-ray diffraction study on the growth kinetics of the expanded austenite. We applied a low-temperature nitrocarburizing treatment using a mixture of NH3, N2, H2, and C2H4 gases at atmospheric pressures in a novel and custom built chamber attached to a Bruker D8 Advance diffractometer. The nitrocarburizing temperature was varied between 340 and 440 °C, and the possible effects of the gas amount were also tested. The thickness of the growing layer was determined from the shrinkage of the unmodified austenite peak. The growth rate coefficient was calculated using the linear-parabolic equation. The resulting coefficients follow the Arrhenius law with the activation energy of 165 ± 12 kJ/mol. This value is in good agreement with the diffusion activation energy for heavy interstitials like carbon and nitrogen. The expanded austenite peak was modelled by a multilayer approach, where each 0.5 μm sublayer has a constant lattice parameter. The lattice expansion is analyzed as a function of the Boltzmann-variable (η = 0.5 × t−1/2). The expanded austenite layer in this metric has a constant width. Furthermore by rescaling with the lattice expansion of the first sublayer, it is possible to create a scale-independent master curve. These findings indicate that thickening of the expanded austenite is purely diffusion controlled, while the extent of strain is set by the uptake rate of the gas atoms.