Railway Track & Structures - December 2008 - (Page 23)

results from FAST. Test zones with elastic fasteners exhibit higher-gauge strength than test zones with cut spike fasteners. There have been no major performance problems in any of the test zones under the accumulated tonnage. Thermite welds The performance of thermite welds continues to be a major concern for railroads and for the FRA. Thermite welds fail at rates that are disproportionately high compared to track length they comprise. The most recent tests of thermite welds with design modifications intended to improve weld performance were completed at FAST in January 2008 after 117 mgt. The improvements included increased weld gap (1.5-inch vs. the standard 1.0-inch gap), improved weld geometry and increased weld hardness in the railhead. Thermal modeling has shown that increasing the weld gap should reduce the criticality of preheat, thus increasing the likelihood of good rail end melt back and proper fusion. The optimized geometry reduces stress risers at critical areas where fatigue cracking is likely to initiate. The increased weld hardness in the railhead should reduce crushing and spalling, while retaining the toughness of current welds in the web and base. Forty-one test welds were installed in the high rail of a five-degree curve and its spirals at FAST. Ten each of the three types of welds described above, along with 11 standard one-inch welds were tested. Shelling of weld material was the primary failure mode during the test (Figure 3). The majority of the 1.5-inch gap welds began to develop shelling between 65 and 100 mgt; the one-inch gap welds began experiencing shelling after 100 mgt. While the wider gap welds began to shell sooner than the standard gap welds, overall failure rates were similar. Table 1 shows failure types by percentage for each weld type. (Note that a 10 percent increase in failure percentage for the three types of improved welds is one additional failed weld.) Shelling is related to weld hardness and strength. The head-hardened welds were expected to be more resistant to shelling than standard metallurgy welds. But the average hardness of the head-hardened welds was only about 20 HB harder than the standard welds. Continuing increases in rail hardness is a compounding factor. When premium rail hardness was 360 HB, a typical thermite weld of 340 HB was only 20 HB softer than the rail and the welds did not batter significantly faster. Now, with some rails having hardness exceeding 400 HB, the differential in hardness and deformation has increased. With the 315,000-pound cars at FAST, harder and stronger welds may be needed. Thirty-two wide-gap welds (2.75-inch gap) from two suppliers are being tested at the eastern site of the revenue service test. The first set of welds to be installed has accumulated 153 mgt; the second set has accumulated 96 mgt. The welds with the most tonnage are beginning to show early signs of shelling. Some of the welds in the second set are developing the type of metal flow that often precedes the onset of shelling. Many more tests than are described in this article are being conducted at FAST and in revenue service. These tests provide results that facilitate the safe and economical operation of HAL traffic. www.rtands.com Railway Track & Structures December 2008 23 http://www.rtands.com

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Railway Track & Structures - December 2008

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