G. Palombarini

Compatibility tests on steels in molten lead and lead-bismuth

Abstract The compatibility of steels with liquid lead and liquid lead–bismuth is a critical issue for the development of accelerator-driven system (ADS). In this work the results of a set of preliminary tests carried out in stagnant molten lead at 737 K and in lead–bismuth at 573, 673 and 749 K are summarised. The tests were conducted for 700, 1200, 1500 and 5000 h. Three steels were tested: two martensitic steels (mod. F82H and MANET II) and one austenitic steel (AISI 316L). The martensitic steels underwent oxidation phenomena at the higher testing temperature, due to oxygen dissolved in the melts. At a lower test temperature (573 K) and higher exposure time (5000 h) the oxidation rate of the martensitic steel seems to be lower and the developed oxide layer protective against liquid metal corrosion. The austenitic steel, in turn, exhibited an acceptable resistance to corrosion–oxidation under the test conditions.

Mechanism of thermochemical growth of iron borides on iron

The mechanism of thermochemical growth of iron borides on iron was studied by analysing the products which form on samples placed in contact with B4C-base powder mixtures with different boronising potential. The analyses were carried out by means of metallographic and X-ray diffraction techniques. Three subsequent stages of growth were observed which explain the occurrence of typical properties of the thermochemically grown boride coatings, such as preferred crystallographic orientations and textures, as well as mechanical compactness and hardness, which play a determining role in the tribological behaviour of borided components.

Behaviour of materials for accelerator driven systems in stagnant molten lead

The behaviour of a modified F82H martensitic steel (both uncoated and aluminised) and tungsten was evaluated by immersion tests in stagnant, oxygen-saturated liquid lead at 793 K under an argon atmosphere, for exposure times up to 3700 h. The F82H steel is a candidate structural material for Accelerator Driven Systems (ADS), while tungsten is potentially suitable for a beam window working immersed in liquid lead. Layers of Me3O4 consisting of an inner sublayer with Me=Fe and Cr and an outer chromium-free sublayer of Fe3O4, formed on the surface of the uncoated steel while the aluminised steel showed no significant alteration. On tungsten, a compact and adherent layer of WO3 formed and reacted with liquid lead leading to the formation of friable Pb–W–O ternary compounds. A two-step mechanism is proposed for interactions of metals and alloys exposed to oxygen-saturated molten lead: the first step is the solid phase oxidation of the base metal; the second is the interaction between liquid lead and the oxide layer formed on the metal surface.

On the morphology of thermochemically produced Fe2B/Fe interfaces

Etude de la formation des interfaces Fe 2 B/Fe dans le cas de systemes differant fortement en reactivite thermochimique. On peut considerer differents cas suivant la reactivite fer-bore et la morphologie revetement-substrat, qui dependent de la composition du milieu de boruration

Mssbauer and metallographic analysis of borided surface layers on Armco iron

Armco iron samples boronized at 850 and 1000° C in crystalline boron powder have been studied. Scattering and transmission Mössbauer measurements, supported by optical and electron scanning metallography and by X-ray diffraction analysis, enabled the surface phases to be identified, the multi-layer structure of the coatings to be defined and the average thickness of each layer to be measured. In addition to an inner Fe2B layer and to an intermediate FeB layer, the presence of an outer layer of a third phase richer in boron that FeB has been ascertained in the boride coatings. The morphology of the reaction products and their mechanical consistency have also been examined and discussed.

Surface structure of boride layers grown on Fe-C-Ni alloys

Three Fe-C-Ni synthetic alloys differing in Ni content have been produced, powder borided for 15 h at 850° C with a B4C-base mixture and then characterized by using surface Mössbauer spectroscopy, X-ray diffraction, metallography and microhardness measurements. The nature and disposition of the interaction products, boriding depths and hardness values of the predominant boride Fe2B have been determined. The role of the Ni content in the alloy on the boriding process has been outlined.

Composition and structure of boride layers grown on low-manganese ternary iron alloys

Three Fe-C-Mn alloys containing 0.30, 0.60 and 1.35 wt % Mn, respectively, were laboratory cast and homogenized, borided for 4, 8 and 15 h at 850° C in contact with a B4C-base powder mixture and then studied by using surface Mössbauer spectroscopy. X-ray diffraction, metallography and microhardness measurements. The boriding depth and the properties of the reaction products were determined and discussed with regard to the manganese content in the alloys. The comparison with the results previously obtained for borided Fe-C-Ni and Fe-C-Cr alloys allowed some general features to be determined with regard to the coatings which can grow on iron alloys in contact with high power boronizing media.

Phase composition and structure of boride layers grown on laboratory-cast low-chromium alloys

Three Fe-C-Cr alloys containing up to 5.65 wt% Cr have been laboratory cast and homogenized, borided for 15 h at 850° C with a B4C-base powder mixture and then characterized by using surface Mössbauer spectroscopy, X-ray diffraction, metallography and microhardness measurements. The boriding depth and the nature, disposition and relative amounts of the reaction products have been determined and discussed with regard to the chromium content in the alloys. The mechanical compactness and hardness of the innermost Fe2B single-phase layers, which are quantitatively predominant in the boride coatings, have been related to the extent of crystallographic order.

Surface melting of a medium carbon steel by laser treatment

A 0.4wt% C normalized carbon steel has been graphite-coated and irradiated in air with a cw-CO2 laser beam at 1.3 kJ cm−2 to melt a near-surface layer of material. The solidified and the solid-state transformed regions have been investigated by means of surface Mössbauer spectroscopy, X-ray diffraction, metallography and microhardness measurements. Nature and morphology of the phases have been determined in each region, and their origin discussed in relation to the conditions of the treatment.

Electron probe microanalysis of nickel and chromium in Fe-C-Ni and Fe-C-Cr alloys borided at 850° C

Laboratory-cast iron-carbon alloys containing relatively low contents of nickel and chromium, respectively, were powder borided at 850° C for 4, 8 and 15 h. The redistribution of nickel and chromium between boride coatings and substrates was studied by metallography and electron probe microanalysis. It was shown that both the elements enter iron borides, substituting for iron. Chromium, however, concentrates in the coatings depleting the underlying unborided matrix, while nickel preferentially concentrates beneath the boride coatings, allowing low-Ni iron borides to be formed.