The ability of metals and alloys to form and retain protective oxide scales is crucial to their stability at elevated temperatures for extended times. Hence the identification of factors that promote or limit the integrity of oxides on high temperature materials has been the subject of intensive investigations. In the present study the mechanical properties of this chromium-rich scale on 304 stainless steel foil has been investigated in relation to the deformation rates in the substrate. It was shown that heavy cold working (up to 90%) delays the onset of breakaway oxidation and results in a very adherent scale. The cracking behaviour of the scale was found to be strain rate and temperature dependent under slow strain rate conditions when the substrate deforms by creep. No strain rate dependence was observed over the temperature range 700-900°C when faster strain rates (> 10⁻⁵ sec⁻¹) were applied. The transition between these two responses was found to vary only slightly with temperature between 5.0x10⁻⁵ sec⁻¹ and 7.8x10⁻⁵ sec⁻¹, increasing as the temperature is raised. A new method has been described for determining the fracture behaviour of oxide scale by estimating the composite defect size. From a knowledge of the onset of scale cracking, determined in situ using the acoustic emission technique, it was possible to correlate the measured intercrack spacing with the fracture toughness from which the tensile properties of the scale can be evaluated.

Recent work has highlighted unusual effects of vanadium when used in conjunction with other microalloying additions on the hardenability of steels. Positive and negative synergistic effects have been observed, but studies into the mechanisms have been limited. To investigate the effects, vanadium interactions with aluminium, molybdenum, niobium and titanium were studied in low (0.1%) and medium (0.4%) carbon steels, containing normal (0.008%) and enhanced (0.020%) nitrogen. Utilising standard jominy test conditions of 950°C for one hour resulted in classical hardenability responses being obtained, where increasing quantities of microalloying additions in solution increase the hardenability. However, when the jominy test conditions were varied unexpected effects were observed. Extending the austenitising time to eight hours showed that the hardenability was dependent upon kinetic effects such as the rate of solution of the alloy carbides/nitrides and the rate at which the microalloying elements in solution segregated to the austenite grain boundaries. It was also observed that if the austenitising temperature was increased to 1200°C a decrease in hardenability could be obtained by increasing the quantity of vanadium, niobium or titanium. These effects were attributed to a combination of thermal dispersion of microalloying clusters from the austenite grain boundaries, preferrential transformation on large alloy carbides/nitrides and migration of the austenite grain boundaries. Therefore it was considered inadequate to explain hardenablity solely in terms of the carbon concentration, austenite grain size and amount of other alloying elements present. Additional factors such as cluster formation, grain boundary pinning etc., were identified and applied to the results to successfully explain the effects of the alloy interactions on hardenability. Recent studies on vanadium alloyed pearlitic steels showed significant increases in strength could be obtained by precipitation within the pearlitic ferrite. Mechanical property investigations of two steels indicated that a maximum precipitation effect was obtained at an isothermal transformation temperature of 600°C.

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Additive manufacturing (AM) is used to create geometries otherwise impossible to machine. Topology optimization, microstructural texture control, and the use of lattices could be created through AM to increase performance of systems. Currently research focuses on solution aging of printed 17-4 PH, while other heat treatments are not as heavily studied. This study identifies different heat treatments applied to additively manufactured 17-4 precipitation hardened (PH) and the effects on mechanical properties. This study used quasi-static tension, quasi-static compression, and Charpy V-notch testing to analyze the effects of heat treatment as well as the effectiveness of additive manufacturing compared to traditional machining for wrought materials. Data during testing was taken with digital image correlation to identify changes in local strain. The effectiveness of heat treatment was demonstrated in this study and can be used to estimate performance on additively produced 17-4 PH.