Fitting on back panel of a military carriage - Thomas steel - Modern Times - Switzerland

Marianne. Senn (EMPA, Dübendorf, Zurich, Switzerland) & Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

Stratigraphic representation: none 

Fig. 4: Stratigraphic representation of the object in cross-section using the MiCorr application. This representation showing for the metal part a welding seam (M2) can be compared to Fig. 10.

Analyses performed:
Metallography (nital etched after etching with Oberhoffer’s reagent), Vickers hardness testing, LA-ICP-MS, SEM/EDX.

The remaining metal is a Mn-rich soft steel (C content around 0.1 mass%) containing manganese sulphide inclusions with a varying Fe content (Tables 1 and 2). The numerous inclusions form parallel rows (Fig. 4). This orientation is typical for hot rolled metal. After etching with Oberhoffer’s reagent, three main welding seams become visible (Fig. 5). Near the surface they are also outlined by corrosion (Figs. 4 and 10). After nital etching, the metal shows a ferritic structure with tertiary cementite and lamellar pearlite at the grain boundaries (Figs. 6 and 7). The grains are small with an ASTM grain size of 10 and are recrystallized due to annealing after hot rolling. The average hardness of the metal is HV1 165. The hardness is slightly high for such a structure and this is due to the Mn content of the metal. The chemical composition, especially the Mn content and the presence of carbo-nitrides (not analysed here), is typical for Thomas steel.

 

Elements Ni/Co Al P Ti V Cr Mn Co Ni Cu As Mo Ag Sn Sb W C* mass%
Median (mg/kg) 2.4 < 300 < < 140 3600 200 480 140 700 10 < 10 10 < <0.1
Detection Limit (mg/kg)   5 82 10 2 13 2 1 3 1 3 3 1 1 1 4  
RSD % 1 - 8 - - 3 7 2 1 6 3 7 - 7 8 -  

*visually estimated

Table 1: Chemical composition of the metal. Method of analysis: LA-ICP-MS. Lab Inorganic Chemistry, ETH.

 

Elements

S Mn Fe Cu Total
Inclusions 26 46 31 2.2 105

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

The average thickness of the corrosion products is about 80µm (Figs. 4 and 8). In bright field they appear grey, marbled and heavily cracked (Fig. 8). Under polarised light, the corrosion products appear orange to dark-brown (Fig. 9). At the metal - corrosion products interface they are dark-brown. The middle part is red-orange and the outer part is bright orange. The elemental mapping of the corrosion layers shows no distinctive stratification, but areas near the metal - corrosion crust interface as well as the top surface of the corrosion layer seem to have a lower O content (Fig. 10). The O content indicates the presence of iron hydroxides (Table 3). Soil materials (rock fragments, dust) are found in the welding seams near the surface.

Elements

Location

O S Mn Fe Total
In welding seam 34 < < 67 102
Inner corrosion layer 38 0.7 0.8 66 106

Table 3: Chemical composition (mass %) of the corrosion layer (from Fig. 10). Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.

Corrected stratigraphic representation: none

The fitting was produced from Mn-containing Thomas steel. It was forged out of four strips, hot rolled and annealed. The corrosion contains only few external markers such as sand grains and dust particles in the outermost layers. The presence of soil materials in the welding seams near the surface could be due either to the corrosion progress (by diffusion through the corrosion crust) or to the manufacturing process.

References on object and sample

References object

1. Degrigny, C. (2011) Protection temporaire d’Objets métalliques base fer et cuivre à l’aide d’Inhibiteurs de corrosion Non Toxiques : application aux objets patrimoniaux techniques et scientifiques de grandes dimensions exposés en atmosphère non contrôlée, rapport interne HE Arc CR.

 

References sample

2. Rumo, L. (2009) Rapport Empa.

References on analytic methods and interpretation