High alloy Fe-Cr-Ni austenitic stainless steels have become the industry standard alloys for use in ethylene pyrolysis furnaces in the petrochemical industry. Each pyrolysis furnace contains a large array of vertically oriented pyrolysis tubes through which various hydrocarbon feedstocks are flowed and converted into ethylene. The pyrolysis tubes are typically operated at temperatures of 850 – 1100°C, at low pressure (< 0.5 MPa) and short residence times (< 1.0 s). The HP (25%Cr-35%Ni) and ET45 (35%Cr-45%Ni) alloys are the latest in a series of high alloy, heat resisting stainless steels developed in order to provide the strength, ductility, corrosion, and oxidation resistance necessary in the carbon, hydrogen and oxygen rich environment typical of ethylene pyrolysis furnaces. However, the thermal pyrolysis, or cracking, of hydrocarbon molecules creates free carbon, and at the elevated service temperatures the diffusion of carbon into the tube material occurs rapidly. Alloying elements, in particular chromium, are pulled from the matrix to combine with the carbon, resulting in the formation of new carbides and contributing to growth of existing ones. Carburization is the life-limiting mechanism of ethylene pyrolysis tubes, and its effects on tube life manifest in a number of ways, including reducing ductility and the ability of the pyrolysis tube to withstand thermal cycles, and creating internal stresses that result in increases in creep rate. Plant operators prefer to replace tubes at planned outages in order to minimise downtime, but there is currently no reliable end-of-life indicator by which tube replacement decisions can be made. Knowing the level of carburization of an in-situ tube can assist in remaining life estimates based on finite element analysis (FEA) modelling, thermography, and fracture mechanics. There is therefore great interest in the non-destructive detection and monitoring of the level of carburization of in-situ tubes. Due to the changes in microstructure and magnetic properties of the tubes over their service life, the level of carburization can be detected non-destructively using eddy current probes. There is, however, industry demand for automated systems and more accurate remaining life estimation. Quest Integrity Group has recently developed an automated eddy current tube crawler system, and are in the process of developing FEA models for a variety of pyrolysis furnace geometries in order to simulate service conditions over a number of thermal cycles and assist in predicting remaining life. In order to improve the interpretation of the eddy current non-destructive testing results and improve the accuracy of the material property data necessary for the FEA models, fundamental research of carburized tubes is necessary. In the present work, detailed characterization of carburized microstructures has been performed, and the effects of carburized microstructures on the mechanical properties and magnetic response analysed in order to better understand the relationship between the factors that influence the remaining life estimates of ethylene pyrolysis tubes. Extensive detailed characterization of phase fractions and phase distributions in the ex-service tubes demonstrated the wide range of phases present and phase transformations that can occur during service in pyrolysis furnaces. It was determined that the primary chromium-rich carbides are the main contributors to the depletion of chromium in the austenite matrix, but that other phase transformations that produce chromium-containing phases, such as the transformation of NbC or (Nb,Ti)C to η-carbide, and the precipitation of σ-phase, also contribute, particularly in those tubes with minimal microstructural coarsening. The depletion of chromium in the austenite matrix in HP-Nb and HP-Micro alloys was shown to be directly proportional to the increase in the concentration of chromium in the primary precipitate network. The growth of the primary precipitate network was demonstrated to be the main contributor to the reduction in creep resistance of carburized tubes. The steady state creep rate was determined to be dependent on the volume fraction of the primary precipitate network, with highly carburized tubes displaying steady state creep rates an order of magnitude higher than moderately carburized tubes, and up to two orders of magnitude higher than tubes with little to no carburization. The relative importance of phase transformation in determining the differences in creep behaviour was in some case uncertain, due to more than one phase transformation typically occurring. The chromium concentration in the austenite matrix was compared to available data for magnetic permeability in order to predict the amount of ferromagnetic material in each sample, to enable analysis of the eddy current NDT results. An increase in the amount of ferromagnetic material resulted in both an increase in normalised probe inductance and an increase in the range of normalised probe inductance across the test frequencies. The analysis of the eddy current NDT results demonstrated that measurement of the magnetic permeability of Fe-Cr-Ni compositions with chromium concentrations between 15 wt% (in the range of the chromium-depleted austenite matrix material) would enable more accurate interpretation and better insight into the relationship between chromium concentration and magnetic response.