Kristian Vinter Dahl

Nitrogen diffusion and nitrogen depth profiles in expanded austenite: experimental assessment, numerical simulation and role of stress

Abstract The present paper addresses the experimental assessment of the concentration dependent nitrogen diffusion coefficient in stress free expanded austenite foils from thermogravimetry, the numerical simulation of nitrogen concentration depth profiles on growth of expanded austenite into stainless steel during gaseous nitriding, a qualitative discussion of the role of stress on local equilibrium conditions of growing expanded austenite and a discussion of the erroneous concentration dependent diffusivity of nitrogen in expanded austenite as obtained from applying the Boltzmann–Matano method to composition–depth profiles.

Application of Complementary Techniques for Advanced Characterization of White Etching Cracks

Abstract White etching crack (WEC) networks were characterized in heavy loaded bearings for wind turbines. Both conventional techniques as reflected light microscopy and (scanning and transmission) electron microscopy as well as electron backscatter diffraction and ion channelling contrast imaging were applied. The complementary use of the techniques in unravelling the complicated failure mechanisms is explored in the present work.

Thermodynamic and kinetic modelling: creep resistant materials

Abstract The use of thermodynamic and kinetic modelling of microstructure evolution in materials exposed to high temperatures in power plants is demonstrated with two examples. Precipitate stability in martensitic 9–12%Cr steels is modelled including equilibrium phase stability, growth of Laves phase particles and coarsening of MX, M23C6 and Laves phase particles. The modelling provided new insight into the long term stability of new steels. Modelling of the detrimental precipitation of Z phase Cr(V,Nb)N is described, which points to new approaches in alloy development for higher temperatures. Predictions of interdiffusion between a MCrAlY coating and an IN738 bulk alloy by multicomponent diffusion calculations provide a highly versatile tool for life assessment of service exposed gas turbine components as well as for the development of improved coatings.

Interdiffusion Between Ni-Based Superalloy and MCrAlY Coating

Interdiffusion at the interface between a Co-36.5Ni-17.5Cr-8Al-0.5Y, MCrAlY coating and the underlying IN738 superalloy was studied in a large matrix of specimens isothermally heat treated for up to 12,000 hours at temperatures 875°C, 925°C or 950°C. Modelled results using the finite difference software DICTRA was compared with experimental average composition profiles measured across the interface using a new experimental approach.

Simulation of Nitrogen Concentration Depth Profiles in Low Temperature Nitrided Stainless Steel

Abstract. A numerical model is presented, which simulates nitrogen concentration-depth profiles as obtained with low temperature gaseous nitriding of stainless steel. The evolution of the calculated nitrogen concentration-depth profiles is compared with experimental nitriding kinetics. It is shown that the evolution of the surface concentration is likely to play a dominant role under experimental conditions.

Grey‐scale conversion X‐ray mapping by EDS of multielement and multiphase layered microstructures

A procedure for grey‐scale conversion of energy dispersive spectroscopy X‐ray maps has been developed, which is particularly useful for the plotting of line composition profiles across modified layered engineering surfaces. The method involves (a) the collection of grey‐scale elemental maps, (b) the calculation of mean grey‐scale levels along strips parallel to the layered microstructure and (c) the conversion of grey‐scale line profiles into composition line profiles. As an example of the grey‐scale conversion method and its advantages for multielement and multiphase layered microstructures, the procedure has been applied to a layered microstructure that results from a plasma‐sprayed metallic MCrAlY coating onto a nickel‐superalloy turbine blade. As a further demonstration of the accuracy and amount of compositional data that can be obtained with this procedure, measured compositional profiles have been obtained for several long‐term isothermal heat treatments in which significant interdiffusion has taken place. The resulting composition profiles have greatly improved counting statistics compared to traditional point‐by‐point scans for the same scanning electron microscope time and may be considered as a rapid alternative to energy dispersive spectroscopy spectrum imaging. The composition profiles obtained may be conveniently compared with results of multicomponent thermodynamic modelling of interdiffusion.

Low-Temperature Nitriding of Deformed Austenitic Stainless Steels with Various Nitrogen Contents Obtained by Prior High-Temperature Solution Nitriding

In the past decades, high nitrogen steels (HNS) have been regarded as substitutes for conventional austenitic stainless steels because of their superior mechanical and corrosion properties. However, the main limitation to their wider application is their expensive production process. As an alternative, high-temperature solution nitriding has been applied to produce HNS from three commercially available stainless steel grades (AISI 304L, AISI 316, and EN 1.4369). The nitrogen content in each steel alloy is varied and its influence on the mechanical properties and the stability of the austenite investigated. Both hardness and yield stress increase and the alloys remain ductile. In addition, strain-induced transformation of austenite to martensite is suppressed, which is beneficial for subsequent low-temperature nitriding of the surface of deformed alloys. The combination of high- and low-temperature nitriding results in improved properties of both bulk and surface.

Microstructural investigations of pure nickel exposed to KCl induced high temperature corrosion

Abstract Oxidation of 99·99% pure nickel was studied with and without 0·10 mg cm−2 KCl(s) in an environment containing 5 vol.-%O2, 40 vol.-%H2O and 55 vol.-%N2 at 600°C for up to 168 h. Oxide microstructure was investigated by X-ray diffraction (XRD), focused ion beam (FIB), broad ion beam (BIB) and SEM/EDX. Oxidised nickel shows an approximately parabolic oxide growth rate. The oxide scale is dense with some pores at the oxide/metal interface. Adding small amounts of KCl does not result in a faster corrosion rate of nickel. However, the surface morphology changes and small oxide crusts were observed in the vicinity of former KCl particles. This is proposed to be the result of a NiCl2–KCl eutectic on top of the oxide scale formed above 514°C. The oxide scale formed in the presence of KCl contains more and differently distributed voids than the scale formed without KCl.

Modelling Cr depletion under a growing Cr2O3 layer on austenitic stainless steel: the influence of grain boundary diffusion

The oxidation behaviour of austenitic stainless steels in the temperature range 723–1173 K is strongly influenced by the grain size of the oxidizing alloy. In this work the evolution of the concentration profiles of Cr, Ni and Fe in the substrate below a growing Cr2O3 layer is simulated with a Fisher-type numerical model, which takes both volume and grain boundary diffusion into consideration. The model is based on a two-dimensional control volume-based solution of Ficks 2nd law for multicomponent diffusion and includes cross-term diffusion coefficients. The oxide layer is assumed to grow according to a parabolic rate law as a consequence of rate limiting diffusion of Cr cations through the oxide layer; the retraction of the oxide/alloy interface associated with the removal of Cr atoms from the substrate is included in the calculations. Numerically, the movement of the oxide/alloy interface is formulated such that the initial mesh can be used throughout the calculation.The calculated concentration profiles of the alloying elements emphasize the importance of grain boundaries in supplying Cr from the alloy to the growing oxide layer. For temperatures of 823 and 923 K the simulations predict a significantly lower concentration of Cr atoms in the alloy at the oxide/alloy interface than that predicted by the conventional one-dimensional analytical Wagner solution, where an effective diffusion coefficient at the interface is assumed.

Characterization of pack cemented Ni2Al3 coating exposed to KCl(s) induced corrosion at 600 °C

Abstract Oxidation of Ni2Al3 produced by pack aluminizing of pure nickel was studied with and without 0.10 mg cm−2 KCl(s) deposit in an environment containing 5% O2, 40% H2O and 55% N2 at 600 °C for up to 168 h. Oxide microstructure and composition was investigated by SEM/EDX, BIB, TEM and GDOES. Oxidised Ni2Al3 shows minimal weight gain, while adding KCl results in a small weight loss consistent with evaporation of KCl. On the surface of samples exposed to the gas environment only, a 30 nm oxide of Al oxide was present, but where KCl was present as deposit, 50–250 nm thick nodules form that are enriched in K, O and Al.