In a two-part research, the degradation process of a commercially utilised NiCoCrAlYRe coating on an IN738LC substrate was both simulated and experimentally validated. The first part of the study delved into the microstructural characteristics of the overlay coating system and analysed the oxidation and interdiffusion mechanisms. This second part extends the analysis to simulations of the coating degradation at temperatures of 900 °C and 1000 °C. The interdiffusion between the substrate and coating was modelled utilising thermodynamic and kinetic mobility data, while the oxidation aspect was addressed using two different established models from Whittle and Meier et al., along with an adapted approach specifically tailored for MCrAlY overlay coatings devised in this research. The simulation results were validated experimentally for up to 7000 h, with the degradation state evaluated based on the depletion of β-NiAl in the coating. Good agreement was found between simulated and experimental data. The initial coating and substrate microstructures were sufficiently predicted based on thermodynamic data, and the simulations closely matched the observed degradation pattern. The most precise alignment between simulated and experimental β-NiAl depletion was achieved with the newly proposed oxidation modelling approach for MCrAlY systems. While more optimisation is required to accurately predict the precipitation of chromium phases and carbides and to account for interdiffusion-obstructing effects, particularly at 1000 °C, the results demonstrate the potential of thermodynamic-kinetic modelling for estimating the lifespan of MCrAlY systems under diffusion-driven degradation conditions.

Using dilatometer experiments, numerical data can be collected during the sintering process and used to develop a constitutive model for shape changes during sintering. The dilatometer chamber is closed and small compared to a normal industry oven used for sintering. Depending on the final application of the sintered product, different temperatures and heat cycles may be used during the process. It is therefore important for a constitutive model to be robust, giving valid results even when changes are made to the sintering procedure. This is done by optimizing the constitutive parameters with respect to different sintering cycles. Variations of initial density after compaction has also been considered in the constitutive model. The deviatoric part of the model is investigated for experimental results in nonhomogeneous sintered specimens to complete the constitutive model. The model presented is for the full sintering process and relevant physical aspects have been accounted for.

This article considers recent progress in modelling the degradation of Fe9Cr steels exposed to high-temperature CO2. Computational modelling is used to rationalise the mechanism of the so-called breakaway oxidation, which is shown here to be associated with the carburisation of the underlying Fe9Cr substrate of finite dimensions. Oxidation kinetics, non-steady-state carburisation kinetics, and the mass transport mechanisms are covered. The theoretical and numerical/analytical challenges are discussed, with possible ways forward being suggested. Thus, we demonstrate that the software systems built on Prof. Hillert’s legacy are maturing rapidly towards engineering tools which can be used to anticipate the degradation of complex multicomponent alloys in engineering situations of relevance and significant complexity. 

By performing a dilatometer experiment, measured shrinkage can be used to determine the adjustable parameters in a constitutive model for sintering. While the dilatometer machine is excellent at collecting data, its conditions differ from those in an industrial sintering oven. Therefore, the constitutive model must be robust and in the present study, different sintering time cycles have been used to optimize the adjustable parameters in the model. Investigations on initial density has also been performed to better understand the sintering process. Before sintering of particles begins, during the debinding process, densification can be detected, which is dependent on the initial density. The constitutive model for sintering was improved to include this phenomenon by adding particle rearrangement based on the theoretical packing of spheres.

During sintering, a green body of powder particles is heated to high temperatures, fusing the particles together. In cemented carbide production, the sintering process generally results in substantial densification of the material. By using a dilatometer, shrinkage during the sintering process can be measured. For a green body of lower density, early particle rearrangement has been observed. This is investigated here using different initial densities using the same powder, leading to a suggested addition to the constitutive model. The environment in the dilatometer and the sintering furnace differs, especially with respect to heating and temperature during holding. This effect can be minimized by creating robustness in the model, making it independent of the heating cycle. Here, this is done by optimizing the constitutive parameters towards four heating cycles for a specific powder.

Improvements in machinability by alloying of the workpiece often adversely impact the end user properties of a material. For example, the common use of non-metallic inclusions can lead to improved tool life during turning or milling, but often adversely affects weldability, corrosion, and wear resistance. A cutting tool material meets kilometers of workpiece material during a machining operation. Hence elements in small quantities in the workpiece may insignificantly affect the end user properties but may have large effects on tool wear. One such effect is the formation of refractory and wear resistant reaction products between the workpiece and tool. Such reaction products forming on tool surfaces may lead to improved machinability. This paper proposes the use of small amounts of alloying to induce such a Tool Protection Layer. Additionally, the paper develops a computational framework for designed alloying which balances formation of Tool Protection Layers, its in-process retention, and the functional properties of the alloy. The method has been validated for a case of manganese steel. The calculations were validated first by a wide range of diffusion experiments. Then by industrial turning of cast alloys, by comparing one reference and two newly designed alloys based on the alloying concept. The alloy with 0.003 mol fractions of Al resulted in more than 3 times increase in tool life, due to in-operando formation of Al2O3 Tool Protection Layer. The designed manganese steel maintained its functional properties with respect to abrasive wear resistance and retained its ability to work harden.

Gas-blowing technology is widely used in converter steelmaking to homogenize liquid steel and accelerate chemical reactions, with Argon oxygen decarburization (AOD) being the dominant process for stainless steelmaking. Due to the harsh environment, it is advisable to study the phenomenon using small-scale physical models and numerical simulations before conducting industrial-scale trials. This paper presents a practical computational fluid dynamics (CFD) approach for simulating the AOD process, with chemical reactions considered. This approach can simulate the entire process in a reasonable time using a standard workstation. The simulation employs a Finite Volume Method CFD approach to handle mass, momentum, and energy transfer, and a local equilibrium assumption is utilized. The study shows that a practical approach can be used to model the initial stage of decarburization in the AOD process with a reduced accuracy in mass transport calculations. The accuracy of the simulation is validated using industrial data, and good agreement is found.

A phase field model is developed to study the discontinuous (cellular) precipitation (DP) reaction in a hypothetical A-B miscibility gap system. Unlike previous treatments, the model employs an interaction energy term which couples the composition and the phase field variable to enable the necessary grain boundary movement. The influence of factors such as interaction energy, interfacial mobility and grain boundary diffusivity on the transformation rate and overall microstructure evolution are discussed.

Breakaway corrosion of stainless steels remains a challenge for many industrial applications operating in harsh conditions. The lifetimes of metallic components are often determined by the corrosion propagation after breakaway. Nevertheless, studies on the protective properties of the Fe-rich oxides formed after breakaway are scarce. This study investigates the influence of Ni on the protection after breakaway on a broad range of Fe18CrxNi model alloys, by systematically inducing breakaway of the initially formed protective, Cr-rich oxides. The results clearly demonstrate an improved protection after breakaway for higher Ni-contents, explained by the alloys’ ability to avoid internal oxidation involving Fe-rich BCC and spinel.

Chemical interactions that drive crater wear in turning are often studied using diffusion couples where the tool and workpiece are fixed. In contrast, in actual turning, there is a constant supply of new workpiece material at the tool-chip interface. In this work, diffusion simulations of a WC-Co(6%) and Ti-5Al-5V system were conducted, with constant replenishment of titanium at the interface (open system) and a fixed amount of material (closed system). The simulations showed that the formation of W(bcc), ry-phase, and TiC is dependent on the activity of C and the permeability of Co and C in titanium. Scanning and transmission electron microscopy-based techniques were used to analyse a Ti-5Al-5V-5Mo-3Cr and WC-Co(6%) diffusion couple and a worn WC-Co(6%) insert. The sequence of phases in the closed system simulation was similar to that observed in the diffusion couple. The open system simulation indicated that W(bcc) can form at WC-WC boundaries (where Co is low) within the subsurface of a WC-Co(6%) that has adhered titanium, and at the WC/Ti interface. Additionally, high densities of stacking faults and dislocations were found within subsurface WC grains, indicating a significant reduction of the tool's integrity.