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

Abstract

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 lowtemperature nitrocarburizing treatment using a mixture of NH 3 , N 2 , H 2 , and C 2 H 4 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 6 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 lm sublayer has a constant lattice parameter. The lattice expansion is analyzed as a function of the Boltzmann-variable (g ¼ 0.5 Â t À1/2). 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. Published by AIP Publishing.

<i>In Situ</i> XRD Experiments on the Growth of Expanded Austenite Using Different Process Gases

“…AISI 361L (or 1.4402) type stainless steel specimens were obtained from L. Klein SA. Glow discharge optical emission spectrometry and X-ray fluorescence measurements have been done to test the composition and the measured values were in agreement with the industrial standard [7]. The alloy itself was homogenized at 1050 °C temperature and cold drawn to 3 meter long rods.…”

“…2. The deviations from the pure parabolic growth were observable [7], however their extent was not large enough for a quantitative study of the interface control. After the transition the diffusion controlled part was stable and we did not experience a significant time dependence of the growth rate coefficient unlike in Ref.…”

“…To this system we attached a purpose built reaction chamber, this chamber allowed gas-phase nitriding, carburizing and nitrocarburizing up to 600 °C processing temperatures (the details of the reaction chamber are described in Ref. [7]).…”

“…The composition dependence of the diffusion coefficients [9] also favors sharper composition gradients at the low concentration side. This indeed allows the determination of the thickness of the growing expanded austenite via the Lambert-Beer attenuation law [7,10].…”

“…In this work we summarize our first results on the processing AISI 316L stainless steel specimens with a purpose built reaction chamber attached to a Bruker D8 diffractometer [7]. We carried out nitriding and nitrocarburizing treatment on ex-situ depassivated and nickel coated specimens as well as in-situ depassivation of cold drawn steel specimens.…”