Oxidation of austenitic and duplex stainless steels during primary processing

During manufacturing, stainless steels will experience high temperatures at many steps: continuous casting, reheat, rolling and annealing. After continuous casting, it is usual for the ascast metal to cool to ambient temperatures. In order to achieve successful hot forming, the metal must then be reheated to a suitable temperature. This process is called "reheat", and commonly takes place in large, propane-fired walking beam furnaces at temperatures in excess of 1200°C for times of 2 or more hours. During reheat, a thick oxide scale will form which, if left in place during cold working, may cause damage to the surface of the final cold rolled product. As a result, manufacturers generally apply a mechanical descaling procedure immediately prior to rolling. In order to study the effect of reheat conditions on the oxide scale formation, austenitic grades (Types 316L and 304) and duplex grades (Types S32101 and S32205) were oxidised. The temperatures used ranged from 700°C through to 1300°C for times up to, and including, 5 hours. The results of the heat treatments were investigated using a range of analytical techniques including: scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy and site specific cross-section preparation using focused ion beam for both SEM investigation and transmission electron microscopy analysis. The use of these techniques has provided a thorough description of oxides that are expected to form at a variety of temperatures and conditions. On austenitic grades a number of sub-surface oxide structures form, such as dendritic internal oxide particles and sub-surface oxide layering, which may pose great challenges to the mechanical descaling processes. External oxidation has not been neglected in this study and complex chemical and phase distributions within external oxide scales have been characterised. The presence of an as-cast oxide scale during reheat does not have a substantial effect on the reheat oxide formed, however, the as-cast microstructure may contribute to the formation of subsurface oxide banding. Duplex stainless steels produce oxides which are highly dependant on the phase distribution and surface finish of the substrate. Large scale oxide nodules have been found to form only on the austenite with an apparently protective thinner oxide present on the ferrite. The formation of these nodules has been studied from the initial stages of oxidation by the use of environmental SEM and site-specific preparation of important features to reveal that nodules form where the thin, protective chromium rich layer chemically fails during oxidation enabling local breakaway regions to form

O-mannosylation in Candida albicans enables development of interkingdom biofilm communities

Peer reviewedPublisher PD

Effects of food-borne nanomaterials on gastrointestinal tissues and microbiota

Ingestion of engineered nanomaterials is inevitable due to their addition to food and prevalence in food packaging and domestic products such as toothpaste and sun cream. In the absence of robust dosimetry and particokinetic data, it is currently challenging to accurately assess the potential toxicity of food-borne nanomaterials. Herein, we review current understanding of gastrointestinal uptake mechanisms, consider some data on the potential for toxicity of the most commonly encountered classes of food-borne nanomaterials (including TiO2 , SiO2 , ZnO, and Ag nanoparticles), and discuss the potential impact of the luminal environment on nanoparticle properties and toxicity. Much of our current understanding of gastrointestinal nanotoxicology is derived from increasingly sophisticated epithelial models that augment in vivo studies. In addition to considering the direct effects of food-borne nanomaterials on gastrointestinal tissues, including the potential role of chronic nanoparticle exposure in development of inflammatory diseases, we also discuss the potential for food-borne nanomaterials to disturb the normal balance of microbiota within the gastrointestinal tract. The latter possibility warrants close attention given the increasing awareness of the critical role of microbiota in human health and the known impact of some food-borne nanomaterials on bacterial viability. For further resources related to this article, please visit the WIREs website.</p

The effect of surface preparation on the precipitation of sigma during high temperature exposure of S32205 duplex stainless steel

Although the formation of sigma phase in duplex stainless steels is reasonably well documented, the effect of surface finish on its formation rate in surface regions has not been previously noted. The growth of the sigma phase precipitated in the subsurface region (to a maximum depth of 120 μm) has been quantified after heat treatment of S32205 duplex stainless steel at 1073 K (800˚C) and 1173 K (900˚C) after preparation to two surface finishes. Here, results are presented that show that there is a change in the rate of sigma phase formation in the surface region of the material, with a coarser surface finish leading to a greater depth of precipitation at a given time and temperature of heat treatment. The growth rate and morphology of the precipitated sigma has been examined and explored in conjunction with thermodynamic equilibrium phase calculations

Microstructural characterisation of creep tested 9CR welds for MarBN steel

Creep properties of 9Cr heat resistant steels can be improved by the addition of boron and nitrogen to produce martensitic boron-nitrogen strengthened steels (MarBN). The joining of this material is a crucial consideration in the material design since welds can introduce relatively weak points in the structural material. In the present study, creep tests of a number of MarBN weld filler metals have been carried out to determine the effect of chemistry on the creep life of weld metal. The creep life of the weld metals was analysed, and the evolution of creep damage was investigated. Significant differences in the rupture life during creep have been observed as a function of boron, nitrogen and molybdenum concentrations in the weld consumable composition. Although the creep lives differed, the particle size and number in the failed creep tested specimens were similar, which indicates that there is a possible critical point for MarBN weld filler metal creep failure

Comparison of the effects of a conventional heat treatment between cast and selective laser melted IN939 alloy

Additive manufacturing (AM) is a process where, as the name suggests, material is added during production, in contrast to techniques such as machining, where material is removed. With metals, AM processes involve localised melting of a powder or wire in specific locations to produce a part, layer by layer. AM techniques have recently been applied to the repair of gas turbine blades. These components are often produced from nickel-based superalloys, a group of materials which possess excellent mechanical properties at high temperatures. However, although the microstructural and mechanical property evolution during the high temperature exposure of conventionally produced superalloy materials is reasonably well understood, the effects of prolonged high temperature exposure on AM material are less well known. This research is concerned with the microstructures of components produced using AM techniques and an examination of the effect of subsequent high temperature exposures. In particular, the paper will focus on the differences between cast and SLM IN939 as a function of heat treatment and subsequent ageing, including differences in grain structure and precipitate size, distribution and morphology, quantified using advanced electron microscopy techniques

Microstructural evolution in a nickel based superalloy for power plant applications as a consequence of high temperature degradation and rejuvenation heat treatments

The microstructural evolution of the Ni-based superalloy CMSX-4 including the change in gamma prime size and distribution and the degree of rafting has been examined in detail using field emission gun scanning electron microscopy (FEGSEM) and transmission electron microscopy (TEM) after high temperature degradation and rejuvenation heat treatments. The relationship between the microstructure, mechanical properties and the applied heat treatment procedures has been investigated. It is shown that there are significant differences in the rafting behaviour, the size of the ‘channels’ between the gamma prime particles, the degree of rafting and the size of the tertiary gamma prime particles in each of the different microstructural conditions studied. Chemical segregation investigations were carried out to establish the cause of reduced mechanical properties of the rejuvenated sample after high temperature degradation compared to an as-received sample after the same degradation procedure. The results indicate that although the microstructure of as-received and rejuvenated samples were similar, the chemical segregation was more pronounced in the rejuvenated samples, suggesting that chemical segregation from partitioning of the elements during rejuvenation was not completely eliminated. The aim of this research is to provide greater understanding of the suitability of rejuvenation heat treatments and their role in the extension of component life in power plant applications

Microstructural and chemical rejuvenation of a Ni-based superalloy

The microstructural evolution of the Ni-based superalloy CMSX-4 including the change in gamma prime morphology, size and distribution after high temperature degradation and subsequent rejuvenation heat treatments has been examined using field emission gun scanning electron microscopy (FEGSEM) and transmission electron microscopy (TEM). In this paper it is shown that there are significant differences in the size of the ‘channels’ between gamma prime particles, the degree of rafting and the size of tertiary gamma prime particles in each of the different microstructural conditions studied. Chemical analysis has been carried out to compare rejuvenated and pre-service samples after the same subsequent degradation procedure. The results indicate that although the microstructure of pre-service and rejuvenated samples are similar, chemical differences are more pronounced in the rejuvenated samples, suggesting that chemical segregation from partitioning of the elements was not completely eliminated through the applied rejuvenation heat treatment. A number of modified rejuvenation heat treatment trials were carried out to reduce the chemical segregation prior to creep testing. The creep test results suggest that chemical segregation has an immeasurable influence on the short-term mechanical properties under the test conditions used here, indicating that further work is required to fully understand the suitability of specific rejuvenation heat treatments and their role in the extension of component life in power plant applications

Microstructural evolution of boron nitride particles in advanced 9Cr power plant steels

B and N can be used to increase the creep strength of advanced 9Cr power plant steels by means of microstructural stabilization and precipitation strengthening; however, the formation of boron nitride (BN) particles removes B and N from solution and reduces the strengthening effect of B and N simultaneously. In the current study, the BN precipitation/dissolution conditions in 9Cr-3W-3Co-V-Nb steels have been investigated to understand how to prevent the formation of BN. A series of austenitizing heat treatments have been designed using thermodynamic predictions as a guide in an attempt to dissolve the BN present after the production of 9Cr-3W-3Co-V-Nb type steels and to prevent also the precipitation of BN during the subsequent heat treatments. Advanced electron microscopy has been carried out to investigate the evolution of the BN particles in relation to the austenitization temperature. Energy Dispersive X-ray spectroscopy (EDS) has been used to identify the B-containing phases, and a method has been developed using secondary electron images to quantify the amount of BN present within the microstructure. It has been found that BN solubility is sensitive to the B and N levels in the steel composition, as indicated by thermodynamic calculations. However, it is proposed that austenitizing heat treatments at temperatures ranging from 1448 K to 1473 K (from 1175 °C to 1200 °C) with durations from 1 to 7 hours can effectively prevent the precipitation of BN as well as dissolving most of the BN particles formed during initial steel manufacture