The effects of metallurgical variables on the mechanical properties of high-chromium cast irons

The.use of high-chromium cast irons for abrasive wear resistance is restricted due to their poor fracture toughness properties. An.attempt was made to improve the fracture characteristics by altering the distribution, size and.shape of the eutectic carbide phase without sacrificing their excellent wear resistance. This was achieved by additions of molybdenum or tungsten followed by high temperature heat treatments. The absence of these alloying elements or replacement of them with vanadium or manganese did not show any significant effect and the continuous eutectic carbide morphology remained the same after application of high temperature heat treatments. The fracture characteristics of the alloys with these metallurgical variables were evaluated for both sharp-cracks and blunt notches. The results were used in conjunction with metallographic and fractographic observations to establish possible failure mechanisms. The fracture mechanism of the austenitic alloys was found to be controlled not only by the volume percent but was also greatly influenced by the size and distribution of the eutectic carbides. On the other hand, the fracture mechanism of martensitic alloys was independent of the eutectic carbide morphology. The uniformity of the secondary carbide precipitation during hardening heat treatments was shown to be a reason for consistant fracture toughness results being obtained with this series of alloys although their eutectic carbide morphologies were different. The collected data were applied to a model which incorporated the microstructural parameters and correlated them with the experimentally obtained valid stress intensity factors. The stress intensity coefficients of different short-bar fracture toughness test specimens were evaluated from analytical and experimental compliance studies. The.validity and applicability of this non-standard testing technique for determination of the fracture toughness of high-chromium cast irons were investigated. The results obtained correlated well with the valid results obtained from standard fracture toughness tests.

Fracture toughness of a series of high chromium abrasion resistant alloys

The fracture properties of a series of alloys containing 15% chromium and 0.8 to 3.4% carbon are investigated using strain fracture toughness testing techniques. The object of the work is to apply a quantitative method of measuring toughness to abrasion resistant materials, which have previously been assessed on an empirical basis; and to examine the relationship between microstructure and K10 in an attempt to improve the toughness of inherently brittle materials. A review of the relevant literature includes discussion of the background to the alloy series under investigation, a survey of the development of fracture mechanics and the emergence of K10 as a toughness parameter. Metallurgical variables such as composition, heat treatment, grain size, and hot working are ???? to relate microstructure to toughness, and fractographic evidence is used to substantiate the findings. The results are applied to a model correlating ductile fracture with plastic strain instability, and the nucleation of voids. Strain induced martensite formation in austenitic structures is analysed in terms of the plastic energy dissipation mechanisms operating at the crack tip. Emphasis is placed on the lower carbon alloys in the series, and a composition put forward to optimise wear resistance and toughness. The properties of established competitive materials are compared to the proposed alloy on a toughness and cost basis.

Siderophore activity of myo-inositol hexakisphosphate in Pseudomonas aeruginosa

Myo-Inositol hexakisphosphate (InsP6), which is found in soil and most, if not all, plant and animal cells, has been estimated to have an affinity for Fe3+ in the range of 10(25) to 10(30) M-1. In this report, we demonstrate that the Fe-InsP6 complex has siderophore activity and is able to reverse the iron-restricted growth inhibition of Pseudomonas aeruginosa by ethylene diamine di(o-hydroxyphenyl)acetic acid. With 55Fe-InsP6 in transport studies, iron uptake is strongly iron regulated, being repressed after growth in iron-replete conditions and inhibited by treatment with potassium cyanide and carbonyl cyanide m-chlorophenylhydrazone. The kinetics of iron transport revealed a Km of 100 nM. Self-displacement of binding of [3H]InsP6 to isolated membranes by InsP6 revealed a single class of binding sites (Kd = 143 +/- 6 nM; Hill coefficient, 1.1 +/- 0.1). The binding of [3H]InsP6 to membranes was not dependent on whether cells had been grown under conditions of high or low iron concentrations. We believe that this is the first report of inositol polyphosphate activity in prokaryotic cells.

Wear of abrasion resisting materials

The wear behaviour of a series of chromium containing white irons has been investigated under conditions of high stress grinding abrasion using a specimen on track abrasion testing machine. The measured abrasion resistance of the irons has been explained in terms of microstructure and hardness and with respect to the wear damage observed at and beneath abraded surfaces. During abrasion material removal occurred by cracking and detachment from the matrix of eutectic carbides as well as by penetration and micromachining effects of the abrasive grits being crushed at the wearing surface. Under the particular test conditions used martensitic matrix structures gave higher resistance to abrasion than austenitic or pearlitic. However, no simple relationship was found between general hardness or matrix microhardness at wear surfaces and abrasion resistance, and the test yielded pessimistic results for austenitic irons. The fine structures of the 15% Cr and 30% Cr alloys were studied by thin foil transmission electron microscopy. It was found that both the matrix and carbide constituents could be thinned for examination at 100 Kv using conventional dishing followed by ion beam thinning. Flany of the rodlike eutectic N7C3 carbides were seen to consist of clusters of scalier rods with individual 117C3 crystals quite often containing central cores of matrix constituent. 3oth eutectic and secondary N7C3 carbides were found to contain stacking faults on planes normal to the basal plane. In the eutectic carbides in the 30A Cr iron there was evidence of an in-situ PI7C3 C. transition which had taken place during the hardening heat treatment of this alloy. In the as-cast austenitic matrix iron strain induced martensite was produced at the wear surface contributing to work hardening. The significance of these findings have been discussed in relation to wear performance.

Microsegregation in nodular cast iron and its effect on mechanical properties

A review of the literature pertaining to the mechanical properties, solidification and segregation effects in nodular cast iron has been made. A series of investigations concerning the influence of microsegregation on mechanical properties of :pearlitic, ferritic and austenitic nodular cast iron have then been reported. The influence of section size on the tensile and impact properties of cornmercial purity and refined ferritic nodular cast iron has been studied. It has been shown. that an increase in section caused a decrease in impact transition temperature of the commercial purity material without greatly affecting the impact transition temperature of the purer material. This effect has been related to increased amounts of segregation effects such as cell boundary carbides in heavier sections of the commercial purity material. Microsegregation studies on the materials used in this thesis have been carried out using an electron probe microanalyser. This technique has shown that concentrations of chromium and manganese and depletions of nickel and silicon occurred at eutectic cell boundaries in nodular cast iron and were often associated with brittle carbides in these areas. These effects have been shown to be more prevalent in heavier sections. The nature of segregation during the solidification of nodular cast iron has been studied by quenching samples of nodular iron during the solidification process. Micro-analysis of such samples has shown that segregation of manganese and chromium occurs by a gradual build-up of these elements at the solid/liquid interface. The microstructures of the quenched specimens revealed carbide filaments connecting graphite nodules and areas of quenched liquid. These filaments have been used as evidence for a revised hypothesis for the solidification of nodular cast iron by a liquid diffusion mechanism. A similar series of experiments has been carried out on two high nickel austenitic irons containing 0.5 per cent manganese and 4 per cent manganese respectively. In both these materials a decrease in elongation was experienced with increasing section. This effect was more drastic in the 4 per cent manganese material which also contained much greater amounts of cell boundary carbide in heavy sections. Micro-analysis of samples of the 4 per cent manganese material quenched during solidification revealed that manganese concentrated in the liquid and that nickel concentrated in the solid during solidification. No segregation of silicon occurred in this material. Carbide filaments appeared in the microstructures of these specimens. A discussion of all the above effects in terms of current concepts is included.