Chisel point - Eutectoid to hypereutectoid steel - Early medieval times - Switzerland

Complementary information

Nothing to report.

Stratigraphic representation: none. 

Fig. 4: Stratigraphic representation of the object in cross-section using the MiCorr application. This representation can be compared to Fig. 11, credit MiCorr_HE-Arc CR.

Complementary information

Nothing to report.

Analyses performed:
Metallography (nital etched), Vickers hardness testing, LA-ICP-MS, SEM/EDX.

The remaining metal is a eutectoid to hypereutectoid steel (C 0.8-1 mass%) with cracks and porosity (Figs. 5 and 6). Its composition is given in Table 1 (except Fe). Of the trace elements, only Ni reaches a medium concentration (Ni 0.07 mass%). The composition and the high Ni/Co ratio are atypical for iron worked in smithies of the settlement at Develier-Courtételle JU, CH. The few slag inclusions and their chemical composition confirm this (Table 2). The slag is rich in CaO whereas the pisolithic or bean ore worked in the Central Jura smelting district is rich in Al2O3. The slag inclusions cannot be smithing slag because the artefact is formed from one piece of metal. The slag probably incorporated while refining the bloom or during the bloom smelting process. The ore from which this metal was smelted cannot yet be identified. Intergranular corrosion has developed, penetrating into the metal structure (Fig. 6). The etched metal shows a dominant martensitic structure, typical of quench hardened steel (Figs. 5, 7, 8 and 9). The characteristic forms of the martensite are non-organised needles (Fig. 8). In restricted areas bainite nodules occur (Figs. 7 and 8). In the centre of the point, hypereutectoid steel is composed of cementite in the grain boundaries and in needle like shapes within the grains (Fig. 10). Hypereutectoid regions also occur in the centre of the shaft and on its left surface (Fig. 5). Here the hypereutectoid steel is composed of martensite and cementite in the grain boundaries and in needle form within the grains (Figs. 6 and 9). Nevertheless, most of the chisel is dominated by eutectoid steel. The average hardness of the quench hardened eutectoid steel is about HV1 690. In the bainite nodules the average hardness drops to about HV1 350, the martensite has an average hardness of HV1 750.

 

Elements V Cr Mn P Co Ni Cu As Ag Ni/Co C* mass%
Median mg/kg 2 20 2 300 40 680 100 190 < 17 0.8-1
Detection limit mg/kg 1 8 1 70 1 1 1 3 0.2 1 6
RSD % 50 36 15 21 5 13 55 10 -    

*visually estimated

Table 1: Chemical composition of the metal. Method of analysis: LA-ICP-MS, Laboratory of Analytical Chemistry, Empa (for details see Devos et al. 2000).

 

Structure Location Na2O MgO Al2O3 SiO2 K2O CaO TiO2 FeO Total

SiO2/Al2O3

Glass and? Tip 0.6 2.4 7.8 50 3.9 15 < 20 100 6.4
Glass and? Tip 0.6 2.5 8.9 65 4.7 18 0.7 4.2 104 7.3
Glass with needles Middle 0.7 2.6 10 59 4.6 21 0.7 2.5 101 5.9
Glass with needles Middle 0.7 2.5 8.7 58 4.7 21 0.6 3.0 100 6.7
Glass with needles Middle 0.8 2.3 8.5 51 4.1 24 0.7 3.6 95 6.0

Table 2: Chemical composition of the slag inclusions (mass%). Method of analysis: SEM/EDS, Laboratory of Analytical Chemistry, Empa.

Complementary information

Nothing to report.

The metal is heavily corroded and the thickness of the corrosion crust is irregular, but averages about 0.5mm. The corrosion products enclose different ghost structures: the corrosion crust has preserved the shape of polygonal grains (Figs. 11 and 13) and cementite needles (Fig. 12). Under polarised light, the corrosion products at the metal - corrosion crust interface are dark-red and form an inner S-rich layer (CP3, Figs. 11 and 13). Cl occurs locally (CP3) and C is present in layers CP2 and CP3 (Fig. 13). The outer layer of the corrosion products (CP1) is orange and contains more O than the inner layer CP3 (Figs. 11 and 14).

Elements

O S Cl Fe Total
Inner red/light-grey layer (CP3) 27 1.2 1.3 72 103
Inner dark-red layer (average of 2 similar analyses) (CP2) 31 0.9 < 66 99
Inner orange grain zone 36 < < 63 101
Outer orange sub-layer (average of 2 similar analyses) (CP1) 35 < < 63 98

Table 3: Chemical composition (mass %) of the corrosion crust (from Figs. 11-14). Method of analysis: SEM/EDS, Laboratory of Analytical Chemistry, Empa.

Complementary information

Nothing to report.

Corrected stratigraphic representation: none.

The tool is made of hard, eutectoid to hypereutectoid steel and was, as a last manufacturing step, entirely quenched. In this state it is too hard and brittle to be used. It is likely that the quenched hypereutectoid structure provoked the breakage of the tool when it was first used. The absence of a tempering process at the end is unclear. The chemical composition of the metal and the slag inclusions indicate that the artefact was imported to the hamlet of Develier-Courtételle JU. The corrosion has, in areas, replaced the metal structure retaining a ghost structure; the outer layer has been mostly removed during surface cleaning. Extensive pitting corrosion has occurred, typical for steel with an elevated C content. The presence of S in the inner layer can be explained by putrefaction processes in the soil. Chlorides are still present at the metal-corrosion interface as no desalination process was attempted. Iron carbonates are present in the centre of a corrosion pit. The corrosion is of terrestrial type.

References on object and sample

References object

1. Eschenlohr, L., Friedli, V., Robert-Charrue Linder, C., Senn, M. (2007) Develier-Courtételle. Un habitat mérovingien. Métallurgie du fer et mobilier métallique. Cahier d'archéologie jurassienne 14 (Porrentruy), 314-315.

 

References sample

2. Eschenlohr, L., Friedli, V., Robert-Charrue Linder, C., Senn, M. (2007) Develier-Courtételle. Un habitat mérovingien. Métallurgie du fer et mobilier métallique. Cahier d'archéologie jurassienne 14 (Porrentruy), 275.

References on analytic methods and interpretation