ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2023 21 The pieces received to investigate included a bolt of the threaded type (Fig. 4) and a fixing nail (Fig. 5) used in the section of the BAP railroad that connected Mendoza with San Rafael south of the Mendoza province. Both pieces belong to the San Rafael Railway Museum. The bolt would have been used to distribute the efforts (loads) on the rails and the fixing nail helped prevent variations in the gauge width and separation of the railway tracks. In the metallography of the bolt head (Fig. 6), a ferrite-perlite structure is observed. The micrographs show that there is a decrease in the grain size of the constitutive raw material due to the machining work performed on the piece (Fig. 7). The machining that would have been used to create this kind of head is called “drop of bait,” due to its final shape, and is typical of the first era of railways. In the micrograph of the threaded bolt rod (Fig. 8), a cast iron structure of high strength, low alloy, and low carbon content (0.28%) is observed (Fig. 9). It is a structure of the ferritic-perlitic type. This suggests the bolt would have received a hot rolling or forging work with successive tempering treatments, through which the piece is shaped and which gives greater resistance to the material of the piece itself. According to the chemical composition obtained by means of the Spectro Sort Spectrometer with argon spark, it can be said that it is similar to a SAE 1030 steel of those currently used (Table 2). The hardness value of the material obtained was 80 HB[4,5]. At the time of manufacture, standardized or specified threads were not yet established, so the thread comb does not match the sample under study (Fig. 10). However, the thread it most TABLE 3 — CHEMICAL COMPOSITION OF THE FIXING NAIL % C % Si % Mn % P % S % Mo % Co % Cu %Al %Fe 0.083 0.005 0.61 0.026 0.062 0.003 0.014 0.017 <0.003 98.6 Fig. 4 — Threaded bolt, where the letter B (BAP) is seen. Fig. 5 — A fixing or clamping nail. Fig. 6 — Bolt head. Fig. 7 — Metallography obtained, 200x. Fig. 8 — Threaded bolt. Fig. 9 — Metallography of stem zone, 100x. closely resembles is the 9 G 7/8” of the current metric system (Fig. 11). By means of the stereoscopic magnifying glass, the corrosion on the thread can also be observed in greater detail (Figs. 12-13). In this sense, it is observed that the corrosive damage has not been very serious and is mainly due to atmospheric type corrosion. From the metallographic images of the fixation nail (Figs. 14-15), a ferrite- perlite structure is observed, that is, it is low carbon (0.08%) cast iron. The ferric carbide deposited in the limits of the grain edge of the ferrite structure gives it hardness. It would correspond nowadays with a SAE 1008 steel. The hardness value obtained by the tests performed gave 90 HB[4,5]. The chemical composition is described in Table 3. TABLE 2 — CHEMICAL COMPOSITION OF THE THREADED BOLT % C % Si % Mn % P % S % Mo % Ni % Al % Cu % Fe 0.28 0.043 0.83 0.017 0.042 0.003 0.014 0.004 0.03 98.6
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