Archaeologic analogues: Microstructural changes by natural ageing in carbon steels

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Abstract

When discussing the container material for highly active radionuclear waste, carbon steel is one of the materials most frequently proposed by the international scientific community. Evidently, security with respect to the container behaviour into deep geological deposits is fundamental. Among other parameters, knowledge about material mechanical properties is essential when designing the container. Time ageing of carbon steel, apart from possible alterations of the chemical composition (e.g. corrosion) involves important microstructural changes, at the scale of centuries and millenniums. The latter may cause variations of the mechanical properties of carbon steel storage containers, with the corresponding risk of possible leakage. In order to properly estimate such risk and to adjust the corresponding mathematical models to reality, the microstructural changes observed in this study on archaeologic samples are evaluated, comparing ancient and modern steels of similar chemical composition and fabrication processes.

Introduction

Archaeologic analogues have potential to provide valuable information about the physico-chemical and mechanical behaviour of metallic containers destined for storage of highly active radionuclear waste. These containers have to be secure during thousands of years, when definitely confined to deep geological deposits. Among other proposals, carbon steel presents one of the most accepted solutions when designing this kind of containers. The contribution of archaeologic carbon steel analogues in this field has three different aspects: corrosion rate associated to the geo-chemical properties of the ground surroundings, microstructural changes by ageing during large periods of time and diffusion models for different elements through the oxide layer present in such samples [1], [2], [3], [4]. This investigation is focused on the quantification of microstructural changes in archaeologic carbon steel samples caused by ageing during centuries and millenniums.

The changes on mechanical properties can be associated to modifications in the microstructure of the steel, which have to be taken into account when designing containers for deep geological deposits. Based on a metallographic study of the various archaeologic samples under investigation, there is evidence of a change in the initial morphology of the carbon steel microstructure because of a natural ageing process. Though the mechanism of these processes is still not very well understood [4], it is relatively easy to detect modifications in the microcomponents of carbon steels.

In the present research, we have compared ancient steel structures with equivalent structures of nowadays manufactured steels. In all cases, we have studied hypoeutectoid carbon steels with approximately 0.15 wt% carbon content.

Ancient steels have suffered from thermal treatment as a result from the hot forging manufacturing process and normalisation. Modern steels are manufactured by hot drawing and normalizing.

The selection of all archaeologic samples has been made attending to the chronologic reliability of the archaeological deposits they belong and the extension of metallic areas without any trace of corrosion. Each archaeologic sample stems from excavation sites of the Iberian Peninsula.

All archaeologic pieces under study have a high reliability respecting their chronology. They have been extracted from safe and trustworthy layers in systematic archaeologic excavations and restoration works (Table 1). Due to intense prehistory and history at the Iberian Peninsula, and, therefore, to its richness in archaeologic artefacts all of the studied archaeologic sites belong to the Peninsula, chronology varying from millenniums to hundreds of years (Fig. 1). The age of different layers of the excavated sites is accurately defined by relevant archaeologic studies. This is important when trying to establish a quantification of structural changes suffered from the samples over the time, using the variation of mechanical properties as an indicator.

Section snippets

Experimental

The number of studied samples was high, but, in order to simplify the conclusion of the study, only the clearer structures, showing differences with modern similarly manufactured steels, will be discussed.

The samples belong to three different excavation sites (Table 1). One is the Roman thermal spring of Cerro Muriano, Cordoba, which belongs to the Republic and High Empire period, more precisely, during the regency of the emperors Augustus and Tiberius. The excavation is located about 16 km

Results and discussion

The pearlite phase of a hypo or hypereutectoid hot-rolled normalized steel consists of a laminar structure of cementite and ferrite, as demonstrated in Fig. 2, Fig. 3. The cementite lamellae exhibit, as shown in Fig. 4, a somewhat sinuous (wavy) morphology. In the case of hypoeutectoid steels, the matrix consists of ferrite crystals (Fig. 2), while in hypereutectoid steel the grains are of pearlitic structure and their boundaries are filled with continuous pro-eutectoid cementite (Fig. 3). In

Conclusions

Ancient carbon steels that were hot-forged and cooled down at air (normalized process) show microstructure changes over the time. This is proved by studying different archaeologic samples of different chronology. Changes in the morphology, due to a very long and slow ageing process, during centuries and even millennia, demonstrate this. Possibly the most adequate indicator for these changes can be found in the cementite layers from pearlite colonies and equally, in the pro-eutectoid cementite

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