Railway Track & Structures - June 2008 - (Page 19)

TTCI R&D grains), using optical and electron microscopy. Figures 2a through 2c show randomly selected examples of the constituents of this premium rail microstructure. This study allowed a better understanding of the effect microstructure constituents have on mechanical properties. The gained knowledge has been used to conduct an advanced design of the next generation premium rails with higher fatigue resistance. It is well known that as steel approaches the eutectoid composition (100 percent pearlite), pearlite becomes the major contributing factor to the yield strength, which is mainly controlled by interlamellar spacing. This is the case of rail steels as per the the American Railway Engineering and Maintenance-of-Way Association specification; rail steels should be fully pearlitic. The yield strength for typical premium rails is between 100 to 120 ksi. This strength can be increased for comparable steel chemistries through a higher cooling rate, which will result in finer interlamellar spacing, and, in general, finer microstructure. Rail’s microstructure optimization is possible by adding low amounts of strategically selected elements, but, most importantly, the use of advance designed thermomechanical treatment. This can result in significant improvements in fatigue performance of rails, without affecting, as possible, rail’s price. The table at top presents the improvements in hardness, mode of failure and the hardness improvements. Figure 2 shows examples of the microstructure of randomly-selected premium rails showing a) pearlite, interlamellar spacing, b) pearlite colony size, pro-eutectoid cementite and c) prior austenite grain and d) RCF crack growth; horizontal and vertical arrows indicating primary and secondary crack growth, respectively. Note the inclusion in the secondary crack growth. Nonmetallic inclusion analysis Higher-strength rail steels have a tighter control of nonmetallic inclusions, primarily oxides and sulfides with comparable volume fraction of the inclusions. There are significant differences, as far as type, composition and shape of the nonmetallic inclusions and this can contribute to potential reductions in mechanical properties together with the differences in proeutectoid cementite. The majority of the inclusions found on the microstructure of the investigated rails are MnS and Al2O3 and some complex inclusions. Preliminary results of the inclusions analyzed indicate that their number or volume, including voids, have a secondary effect on fatigue. In contrast, parameters such as size, type of inclusion, distribution and shape are more detrimental factors that can reduce rail’s life due to excessive www.rtands.com rolling contact fatigue (RCF). It is important to mention that while it is well understood that nonmetallic inclusion can have a deleterious effect on the initiation and growth of defects, it appears that in rail steels, the presence of nonmetallic inclusions is not the major contributing factor to initiate the occurrence of fatiguerelated problems. The microstructural analysis of RCF cracks showed that some of the cracks initiated at the surface and are associated with the prior austenite grain boundaries. Furthermore, the secondary crack path during RCF is often at the prior austenite grain boundaries, some of which contain pro-eutectoid cementite (Figure 2d). Therefore, the analysis shows that detrimental phases, e.g., pro-eutectoid cementite, are the most detrimental microstructural fixture that causes crack initiation. Furthermore, hard inclusions, e.g., Al2O3 can sponsor crack growth through secondary cracking. To minimize the nucleation and propagation, cleaner steels with low amounts of detrimental phases are required. Research and development The preliminary results of this study show that the major metallurgical factors contributing to the strength of fullypearlitic steels are: (a) solid solution, (b) pearlite colony size, (c) interlamellar spacing, and to a lesser extent (d) the austenite grain size. In this rail development program, the goal is to control the transformation of austenite and the presence of proeutectoid cementite, which should be minimized or eliminated. The final rail will be pearlitic with fine interlamellar spacing and thin cementite lamellae to improve wear performance. Additionally, this rail steel will possess superior microcleanliness to reduce the level of inclusions that will improve fatigue resistance. In the development of the next generation of rail steel, Figure 3 shows the process followed to address the most Railway Track & Structures June 2008 19 http://www.aar.com http://www.rtands.com

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Railway Track & Structures - June 2008 Contents On Track Industry Today Supplier News AREMA News NRC News TTCI R&D Grade Crossings Provide Interface Between Railroads, Public Grade-Crossing Equipment & Materials Guide Fastener Suppliers Getting a Grip Grinding Plays Part in Rail Maintenance Big Picture Products and Literature People Calendar Sales Representatives Website Directory Advertisers Index Professional Directory Classified Advertising Chicago Perspective

Railway Track & Structures - June 2008

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