Adnan Ozekcin

Carburization of high chromium alloys

Iron-nickel-chromium based heat resistant alloys are designed to operate at high temperatures in corrosive gaseous environments. Under mixed oxidizing-carburizing conditions, the microstructure of such materials changes progressively during service and the physical and mechanical properties are altered. One instance where such microstructural changes are encountered is in furnace tubes for pyrolysis applications. In the present study, kinetic experiments in the laboratory are combined with microstructural observations on alloys which have undergone long service times to develop an understanding of the fundamental processes that induce property changes in the material. Based on this study, four distinct stages are identified. These include: Initial oxidation, Oxidation in a carburizing environment, Direct carburization and Internal oxidation. Each of these stages is described. Questions are posed with respect to the sudden alteration in process stream chemistry or temperature. For instance, how does a drop in the oxygen partial pressure to levels where a chromium oxide film is unstable affect a preformed film? What beneficial effects are provided by inhibitors such as H2S especially under conditions where an oxide film cannot form?

Research Progress on Friction Stir Welding of Pipeline Steels

Friction Stir Welding (FSW) has been widely commercialized to join aluminum alloys, but is yet to be broadly applied to structural steels. The primary difficulty in welding steels relates to severe loads and temperatures experienced at the interface between the FSW tool and the base material. These conditions are challenging even for the most advanced and expensive tool materials. However, within the last five years, tool advancements have begun to enable FSW of steels. Polycrystalline boron nitride (PCBN), tungsten-rhenium alloys, and mixtures thereof appear to be capable of producing sound welds in steel. This paper describes the results of a continuing study on the FSW of pipeline steels. Pipe grades from API X65 to X120 were subjected to FSW. Strength and toughness measurements using the crack tip opening displacement test were performed. The weld microstructure was evaluated using optical, scanning electron, and transmission electron microscopy. A computational fluid dynamics model was developed to better understand the effect of process parameters on thermal cycles, strain rates and strain experienced by material in the weld stir zone. The results indicate that the microstructure and properties of the welds have little dependence on the tool material, while significant variations in properties were observed between steels produced by different manufacturers. In general, obtaining high levels of toughness on par with gas metal arc mechanized girth welds appears difficult when using the FSW process. The results emphasize the need for a better understanding on the role of process parameters on microstructural evolution and weld quality during FSW of pipeline steels. As a full-scale demonstration of FSW on pipeline steels, several circumferential girth welds were produced in 762 mm (30 inch) diameter X80 pipe. The results of these efforts are discussed.Copyright

Transport of sulfur through preformed spinel films on low alloy Fe--Cr steels

Abstracts are not published in this journal

Corrosion of wire screens in sulphur recovery units

The chrome-nickel austenitic stainless steels (SS) including the widely used 18Cr-8Ni (type 304) have performed well in sulphur containing environments encountered in the petrochemical industries. However, during a planned turnaround 304 steel components (support screens) used in a sulphur recovery unit showed corrosion and scaling ranging from 10 to 750 ,urn thick. The gaseous environment in the sulphur recovery unit contains species such as H2S, CS2, SO2 and water vapour, and during turnaround oxygen is injected into the unit through the screens. The oxygen injection results in a temperature rise, which was not recorded. The aim of this study was to provide an understanding of the mechanism of degradation of the 304 SS wire screens by characterization of scale and the underlying alloy. Sorrel and Hoyt [1] investigated the corrosion resistance of 304 SS to sulphur attack and found this material to be at least ten times as resistant as carbon steel. Backensto et al. [2] also studied the sulphidation of various grades of SS and stated that extra low carbon grade and the stabilized grades of SS sulphidized at the same rates as the plain 304 SS. Sorrel and Hoyt [1] questioned the validity of their statement and pointed out that the effect of sensitizing treatment was not investigated by Backensto et al. [2]. Natesan and Chopra [3] sulphidized 304 SS under low sulphur pressures and suggested that an alloy with equal amounts of Fe, Cr and Ni is suitable for mixed sulphidation/oxidation environments. Verma [4] has indicated that depletion of chromium content in austenitic 310 SS matrix by internal sulphidation lowers the resistance of the alloy to sulphur attack. In fact, it has been recommended [5] that steels for coal gassification units, operating in mixed oxidation/sulphidation environments should have a minimum of 25% chromium for sulphidation resistance. Thus, it is evident from the above different studies that the chromium content of an alloy is crucial to provide resistance against sulphidation. Consequently, the present investigation was directed at correlating the bad performance of the support screens with the chromium distribution in the scale as well as in the alloy. Four wire screen samples, designated A2, A3, B2 and C1, were investigated. These had colours ranging from brown to reddish brown to black and showed visual evidence of scaling. Sample C1 was hardly corroded while sample A2 showed maximum corrosion. In many regions of sample A2, the loss of wire thickness was as large as 50%. Detailed investigations were therefore carried out on sample A2. A cross-section of the wire in the most corroded region, shown in Fig. la, reveals a compact corrosion scale of about 750/~m. An expanded scanning electron microscope (SEM) image of the corrosion scale is shown in Fig. lb. A multilayer structure is evident in the corrosion scale, as shown in Fig. lb. The different layers of the scale, representing sulphides of different composition, are listed in Table I. The iron content in the overall scale decreases progressively from the outer layer to the inner layer whereas the chromium content increases. The subsurface region beneath the corrosion scale