The Effects of Long-Term Isothermal Ageing on the Microstructure of HP-Nb and HP-NbTi Alloys
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
High alloy Fe-Cr-Ni-C austenitic stainless steels have become the principal alloys for use in steam-methane reforming furnaces within the petrochemical industry. Each furnace contains a large array of vertically oriented centrifugally cast tubes through which a mixture of methane and steam is flowed across a nickel-oxide catalyst in order to obtain a mixture of hydrogen, carbon monoxide and carbon dioxide and water commonly known as synthesis gas (or syngas). Generally, the tubes operate at temperatures between 850-1050°C, internal pressures between 1-3.5MPa and are expected to withstand service lives in excess of 100,000 hours. The combination of high temperatures and moderate stresses causes creep to be the dominant failure mechanism experienced by these tubes in service.
The HP austenitic alloys are the latest in a series of heat resisting (H-series) stainless steels developed to provide high temperature strength, ductility, and corrosion resistance in the oxygen, carbon, and sulphur rich environments typical of these furnaces. Extensive work has been carried out to optimise HP alloys’ microstructure in order to maximise the alloy’s creep resistance. Strength increases have largely been realized through the use of niobium and/or titanium additions, which modify the primary precipitates (formed during solidification) and secondary precipitates (formed during exposure to the service temperatures). These strength increases have typically been observed during laboratory accelerated creep testing of the ‘modified’ HP alloys where the temperature and/or stress is increased to achieve failure of the material within reasonable time period (typically between 1000-2000 hours). However, since the duration of typical accelerated creep tests often represent less than 2% of the tubes’ actual service life, uncertainty surrounds the validity of using this testing method as the basis to predict the tubes actual service life. This uncertainty has largely arisen due to the significant microstructural evolution that occurs within these alloys during prolonged service exposure and is not captured within the typical accelerated testing time-frame.
In the present work, the microstructures of HP alloys modified with a single addition of niobium (HP-Nb) and dual additions of niobium and titanium (HP-NbTi) have been characterized in the as-cast condition and after long-term (10,000 hours) isothermal laboratory ageing at 1000, 1050 and 1100°C. The main focus of this study is to provide further insight into the microstructural features that increase the HP-NbTi alloy’s creep resistance in comparison to the HP-Nb alloy when performing accelerated creep testing and determine if these microstructural features remain stable during long-term ageing. The microstructure and crystallography of the primary and secondary precipitates in each alloy have been studied in detail using light optical microscopy, high resolution scanning electron microscopy, transmission electron microscopy, various electron diffraction methods (EBSD, SAD and CBED), Powder X-ray Diffraction and energy dispersive X-ray spectroscopy. Specific attention has been paid to the niobium-rich and niobium-titanium-rich phases that form as a direct result of HP alloy’s modification with niobium and titanium.
The current research is part of a wider project conducted in collaboration with Quest Integrity Group Ltd. (Wellington, New Zealand) that aims to characterize the microstructural and mechanical properties of the HP-Nb and HP-NbTi alloys during long-term service exposure. The microstructural characterization presented in this thesis will subsequently be used by Quest Integrity Group to build a comprehensive understanding of the relationship between HP-Nb and HP-NbTi alloy’s microstructure and creep properties. This understanding will allow Quest Integrity Group to more accurately predict the service life of HP-Nb and HP-NbTi alloy tubes within steam-methane reforming furnaces.