Conveyance tube manufacturing via a hot-finished, welded route is an energy-intensive process that promotes the rapid surface oxidation of curved surfaces. Previous studies have used computational and theoretical techniques to assess the oxidation of curved surfaces. However, experimental techniques for assessing the oxidation of curved surfaces, as well as for validating existing computational and analytical studies, have significant limitations that impact their ability to accurately recreate industrial processes. The challenges of thermogravimetric analysis (TGA) for in situ tests for the oxidation of cylindrical geometries were investigated, using an as-welded conveyance tube, and compared to an equivalent tube normalised in industry as well as computational predictions for the same geometry and thermal conditions. A core element of this work was the use of a refractory dummy sample to quantify thermal buoyancy and flow-induced vibration. There was a strong agreement between the oxide mass gain predicted by a computational model compared to that of the TGA sample, with only a 5% discrepancy. However, oxide thickness gain, measured using electron microscopy, showed poor agreement, particularly when comparing industrial and experimental results. This was attributed to the need for further work to account for transient heating, oxide porosity, atmospheric composition variation, and the effect of thermomechanical operations during conveyance tube manufacturing, e.g., hydraulic descaling.

Conveyance tube manufacturing via a hot-finished, welded route is an energy-intensive process which promotes rapid surface oxidation. During normalisation at approximately 950 °C to homogenise the post-weld microstructure, an oxide mill scale layer grows on tube outer surfaces. Following further thermomechanical processing, there is significant yield loss of up to 3% of total feedstock due to scale products, and surface degradation due to inconsistent scale delamination. Delaminated scale is also liable to contaminate and damage plant tooling. The computational thermochemistry software, Thermo-Calc 2023b, with its diffusion module, DICTRA, was explored for its potential to investigate oxidation kinetics on curved geometries representative of those in conveyance tube applications. A suitable model was developed using the Stefan problem, bespoke thermochemical databases, and a numerical solution to the diffusion equation. Oxide thickness predictions for representative curved surfaces revealed the significance of the radial term in the diffusion equation for tubes of less than a 200 mm inner radius. This critical value places the conveyance tubes’ dimensions well within the range where the effects of a cylindrical coordinate system on oxidation, owing to continuous surface area changes and superimposed diffusion pathways, cannot be neglected if oxidation on curved surfaces is to be fully understood.

Conveyance tube manufacturing is a high temperature process which promotes rapid surface oxidation. During heat treatment, an oxide mill scale layer grows on tube surfaces. Inconsistent spalling of this layer from the outer tube surface contributes to yield loss, surface degradation, and tool wear. Experimental and theoretical studies of curved surface oxidation struggle to accurately recreate industrial processes. The computational thermochemistry software, Thermo-Calc©, was used to investigate external oxidation kinetics during conveyance tube manufacturing. A model was developed using the Stefan problem, tailored thermochemical databases, and a numerical solution to the diffusion equation. Particular attention was paid to geometric effects. There was strong agreement between oxide mass gain predictions by the model and equivalent thermogravimetric experiments. However, poor agreement between oxide thickness gain predictions by the model and industrial samples revealed scope for further work addressing transient heating, porosity, and thermomechanical manufacturing operations. Continuous oxide thickness data derived from the Thermo-Calc© model were used to evaluate thermal, growth, and manufacturing-related oxide stress. An Advanced Oxide Scale Failure Diagram (AOSFD),relating critical strain to instantaneous oxide thickness, was developed using these stress data. The AOSFD was used to demonstrate the impact of the transition to induction heating technology on oxide failure, namely improved electromagnetic control and changes to temperature control margins. Overall, this project demonstrates the use of computational techniques to predict geometry-specific oxide kinetics, and how the resulting data can be used to predict the mechanical state of the oxide. The continuous data presented are ideal for the development of an AOSFD which can predict how a given thickness of oxide will fail, if at all, depending on its strain state. The AOSFD is simple to use and therefore offers a practical tool for making informed decisions during conveyance tube manufacturing.