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9 CHAPTER 3: MICROSTRUCTURAL ANALYSIS The research team investigated rail from various transit authorities using nondestructive and light optical stereoscopy and microscopy. The following sections summarize microstructural analysis and provide detailed descriptions of each method used. 3.1 Rail Used in Transit Service According to the survey and several personal communications with various transit systems, most transit systems use 115-lb RE rail and to a lesser extent 100-lb ARA. Several transit agencies are in the process of upgrading their tracks from 110-lb ARA to 115-RE rail. The MTA-NYCT uses different types of standard carbon rail of 100 lb (ARA-B, OH, FT, HH). The site visit showed that only Amtrak (New York South tunnel and Baltimore station tunnel) uses 136-lb RE rail. 3.2 Description of Test Samples Nine samples of 115-lb rail and 10 sections of 136-lb rail showing corrosion were donated by different transit authorities. Some of these samples had experienced failures due to corrosion. Random samples of 115- and 136-lb rail were selected, as indicated in Figure 3, for the metallographic analysis. These samples were obtained from the base of the rail close to the corrosion region as well as from the head of the rail in order to compare the microstructures of corroded and noncorroded areas. NDE was performed on two sections of the 115-lb rail, which were selected based on the severity of the corrosion. The two most corroded samples were used to identify the effects of corrosion in the microstructure of the sample. The metallographic samples were prepared using standard grinding and polishing procedures. The phases present in the microstructure were revealed using 2% Mital as the etching agent. The surfaces of the NDE samples were mechanically ground. Figure 3 shows a series of macro- images of a sample containing corrosion at the base of the rail.
10 Figure 3. Macro pictures of a 115-lb rail showing corrosion at the base of the rail. Different views of a section of rail donated by the St. Louis, Missouri, Metro System: (a) 3-D image,( b) front (c) side views, (d) and (e) corroded section of the rail. Figure 3 (continued). Different views of a section of 100-lb rail showing severe corrosion at the base of the rail. The rail was donated by Port Authority Trans-Hudson, New Jersey. This rail was used for the magnetic particle evaluation. (b) (d) (a) (g) (c) (e) (h)(f)
11 Figure 3 (continued). Sections of 136-lb rail showing different types of corrosion at the base of the rail. The presented macro-images of the rails show the rail donated by Amtrak. All of these rails failed due to corrosion in various locations along the east coast. The samples were used in the following way: (i) metallographic analysis,(j,k) numerical FEA simulations,(l,m,n) other examples. Note, view (i) shows one of the most severe corrosion conditions. (k) (i) (j) (l) (m) (n)
12 Other sections of rail donated by the Toronto Transit Commission will be presented in the FEA subsection 3.5 of this report. For this project, all rails used were donated by the following transportation systems: ⢠Amtrak ⢠St. Louis, Missouri, Metro System ⢠Port Authority Trans-Hudson (PATH), New Jersey ⢠Toronto Transit Commission ⢠Light Rail, Sistema de Transporte Colectivo Metro, México City 3.3 Nondestructive Evaluation Two sections of rail were selected for the magnetic particle evaluation. One sample was a 115-lb rail section donated by the St. Louis, Missouri, Metro System and the other sample was a 100-lb rail section donated by PATH, New Jersey. These samples were selected because they exhibited two different types of significant damage at the rail base. Figure 4 shows pictures of the rail samples used for the NDE. The magnetic particle technique was used on both samples to determine if any cracks were present. After applying the particles and magnetic field to the samples, there was no evidence of cracks growing into the parent material from the rail base. Under these circumstances, micro- cracking cannot be detected using magnetic particle inspection. Therefore, a more sensitive analysis, using an optical microscope, was performed to inspect for micro-cracking from the rail base into the parent material. It was found that rail base corrosion does not accelerate the crack formation or propagation to the parent rail. Therefore, it is concluded that there is no compromise of the railâs integrity due to the presence of micro-cracks; nonetheless, corrosion, in particular, pitting is usually an accelerator of fatigue-corrosion conditions. The details of this analysis are discussed in more detail in the following section including Figures 5-8.
13 Figure 4. Pictures of the magnetic particles evaluation of the rail base for the Port Authority Trans-Hudson (a,b) and St. Louis, Missouri, Metro System (c,d). Note that there is no apparent evidence of cracks growing from the base of the rail to the parent material. (b) (d) (a) (c)
14 3.4 Metallurgical Analysis of Rails 3.4.1 100- and 115-lb Rail The St. Louis, Missouri, Metro System donated the rail for this analysis. Figure 5 shows the microstructure of both the as-polished and as-etched conditions at different magnifications for a section of the corroded 115-lb rail. It is clear that the microstructure of both samples corresponds to the typical fully pearlitic microstructure of the standard or high strength rails. Note the amount of inclusions, coarse pearlite, interlamellar structure (typical of pearlite), and grains observed on the rail. Figure 5. Microstructure of the 115-lb rail donated by the St. Louis, Missouri, Metro System. The sample in as-polished conditions at various magnifications (a) 100 X, ( b) 1000 X and the sample in as-etched conditions( c) 100 X and (d) 1000 X. Note the large amount of inclusions on the as- polished (a, b) samples and the pearlitic microstructure (c, d). This sample was extracted in close vicinity of the corroded area. (a) (b) (c) (d)
15 Figure 6 shows the microstructure of both the as-polished and as-etched conditions at different magnifications for a noncorroded railhead section from the 115-lb rail. Comparing the microstructure of the rail in close vicinity to the corroded area (Figure 5) with the microstructure of a location free of corrosion (Figure 6), no significant change in the microstructure of the material is shown. This means that corrosion has no effect on the microstructure, except for the section that reacts with oxygen and water or other elements or compounds forming other phases. Figure 6. Microstructure of the 115-lb rail donated by the St. Louis, Missouri, Metro System. The sample in (a) as-polished, 500X and (b) as-etched conditions 1000 X, respectively. 3.4.2 136-lb Rail The 136-lb rail was sectioned as shown in Figure 7(a) to conduct the metallographic analysis. The bottom edge of the rail shows the most severe corrosion damage. Independent of the corrosion rate in the track system (e.g., rail, tie plates, and clips), combining corrosion with cyclic stresses can considerably accelerate the risk of catastrophic failure. It is well documented that tracks with both cyclic traffic and corrosion create stress concentrators. This type of damage is usually caused by diffusion and is more commonly referred to as fatigue corrosion. The corrosion growth rate is relatively slow and uniform under these conditions, but if it is not detected in time, it can end in an abrupt catastrophic failure. The separate sections of the rail were polished and analyzed using the stereoscope and optical microscope under polished and etched conditions. The rail section shown in Figure 7a was used to identify the effect of corrosion (i.e., micro-cracks) on the parent rail. As discussed earlier, micro-cracking can cause catastrophic failures, so a more in-depth study was completed during the metallographic analysis. Figure 7b confirms the magnetic particle inspection results, which determined that there is no evidence of micro-cracking in the sample. Figure 7c-e displays the microstructure of the 136-lb rail. These images do not show significant differences in microstructure when compared with the images of the 115-lb rail in Figures 6 and 7. (a) (b)
16 Figure 7. Macro- (a,b) and micro- (c-f) pictures of the 136-lb rail donated by Amtrak. (a) Cross section of the rail showing how the samples were extracted for analysis, (b) magnified section of the base of the rail showing no micro-cracks, microstructure (a) as-polished (1000 X) and as- etched (d) 500 X,( e) 1000 X and( f) difference between parent rail and corroded sections typical of rust. (b) (d) (a) (c) (e) (f) Parent Rail Layer 1 Layer 2
