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P92 steels are commonly used in advanced steam generating power plants with welding being a main method of component fabrication. Such welds are typically post-weld heat treated at temperatures up to 780°C, which is designed to be as high as possible without the danger of the formation of any austenite during the heat treatment itself. Moreover, it is recognised that, in practice, there will be differences between the specified heat treatment temperature and that which is actually achieved. There are a variety of weld consumables available that have been designed for joining P92 steels; these consumables are heavily alloyed but have ranges over which the compositions of each element in the consumable are allowed to vary. While reaustenitisation of P92 base metals during post-weld heat treatment in the normal temperature range is not likely, it is known that weld metals have much lower A1 temperatures than parent metals. As such, concerns have been raised that, although weld consumables may exist w...

Metallographic and fractographic studies were undertaken to investigate weld metal microcracking in a 2·25Cr–1 M o steel fabrication. Numerous small microcracks were found in single-pass TIG welds in the as-welded condition. An unusual form of brittle fracture on prior austenite grain boundaries was observed, associated with a dense array of intergranular submicrometre sulphide inclusions. The observations indicated that sulphide precipitation had occurred by a solidstate mechanism as the weld metal cooled through the austenite range, while elemental sulphur segregation promoted grain-boundary faceting and embrittlement. The microcracks appeared to have formed on the previously faceted and embrittled grain boundaries by a low-temperature quench cracking mechanism. The factors responsible for the severe sulphur segregation are reviewed, and the implications are discussed in the context of recent interest in the role of grain-boundary sulphides in weld stress relief cracking.

When a fracture surface is exposed to high temperatures, its original features may be smoothed by surface diffusion, and new features such as thermal facets and grain-boundary grooves may form. Obs...

Metallographic studies of weld solidification in the submerged-arc strip-Cladding process are described. It is shown that different solidification sequences took place in different parts of the same weld bead. The primary or leading phase was austenite in the lower part of the bead, but δ-ferrite in the upper regions, while in the central area of the head austenite and δ-ferrite grew simultaneously as primary phases. This unexpectedly complex behaviour is discussed in the context of recent It work on the solidification of austenitic steel welds, It with reference to effects on solidification cracking and ultrasonic inspection.

While the effects of welding parameters on the shapes of single weld beads have been extensively studied, less is known about how bead shape and penetration alter in multipass welding. In this paper are described studies on the shapes of overlapping beads in manual metal arc cladding of ferritic steel by austenitic stainless steel. Different deposition techniques were used to vary bead positioning and the degree of overlap, and the effects on bead height, width, penetration, and dilution were determined. It is shown that various changes in these parameters occur when weld beads are overlapped. The most important effect is a reduction in penetration and dilution, which is especially marked when the position of arc impingement is on the previous weld bead rather than the base plate. The implications for ultrasonic inspection of clad components, and for weld modelling, are discussed.

The evaluation of greenhouse and field screening methods for the identification of resistance to bacterial blight (Xanthomonas vignicola Burkh.) amongst germplasm and breeding lines of cowpea (Vigna unguiculata(L.) Walp.) is described. A stem injection technique and a leaf infiltration method were found reliable under greenhouse conditions while foliar spray inoculation was appropriate for use in both greenhouse and field experiments. A seed inoculation technique for evaluating seedling mortality may prove useful although only one susceptible cowpea accession was used in this study. Since distinct syndromes of bacterial blight (seedling mortality, stem canker and foliar blight) are recognised, it may be advantageous to adopt a different inoculation method for each syndrome to establish the relationship between the resistance of different plant parts within cowpea genotypes. Results confirm the existence of at least two distinct mechanisms of resistance to bacterial blight in cowpea. Hypersensitiv...

A zebra disease resistance factor has been incorporated into the high-yielding Agave hybrid no. 11648 by crossing this hybrid with A. lespinassei. The progeny of three crosses have been screened for resistance to Phytophthora nicotianae, the chief pathogen of zebra disease and results show that 56.7% of the plants have sufficient resistance to be of economic value. The two screening techniques used are described, and the nature of the resistance factor discussed.