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Feeling The Strain: Rail Steel Performance Under The Spotlight

SINCHOLD Port Engineering Co.,Ltd | Updated: Mar 28, 2017

TRANSPORTATION Technology Center Inc’s (TTCI) evaluation of track components and structures is an important part of the Association of American Railroads’ (AAR) research programme. Experiments at the Facility for Accelerated Service Testing (Fast) in Pueblo, Colorado, are regularly updated to ensure that the programme meets the needs of the industry.


railway accessories

New and untried products, as well as structures that are over 100 years old, are evaluated under controlled conditions that would be difficult or impossible to replicate in revenue service. Testing at Fast provides important information on the performance of various components and structures to the railway industry, without exposing railways to potential risks that could be associated with revenue service testing.

For example, during the first half of 2016, 79 million gross tonnes (MGT) were accumulated on track components and structures at Fast. This brought total tonnage at Fast to approximately 4.3 billion gross tonnes, with approximately 3.3 billion of that being under 39-tonne axleloads.

There are four riveted steel bridge spans installed on the High Tonnage Loop (HTL) at Fast. Three of the four spans are over 100 years old, with tonnage on one of those spans now reaching 980 MGT and the other two accumulating 250 MGT. The fourth span is 62 years old and has accumulated 420 MGT. All spans are performing acceptably, even though the equivalent Cooper’s Loading of the Fast heavy axleload train exceeds the normal rating of some of the spans.

Tonnage on the concrete spans tested has now reached 1774 MGT. The benefits of vertical track stiffness modifications (adding or increasing cushioning layers between the rail and the bridge deck) in reducing ballast and track geometry degradation has been demonstrated. Premium rails installed for testing in 2014 have reached 421 MGT, and tonnage on the intermediate strength test rails was 598 MGT when the test concluded. Sleeper and fastener tests continue with 1145 MGT on most of the wood and concrete sleepers. Tonnage on 200 plastic-composite sleepers and 100 wood sleepers installed during summer 2015 has reached 141 MGT, and a further 100 plastic composite sleepers were also installed in summer 2016.

Evaluation continued for electric flash butt (EFB) welds with two methods of post-weld treatments intended to reduce the soft heat affected zones associated with EFB welds with tonnage reaching 255 MGT for those treated with an arc weld overlay and 140 MGT for those treated with induction heating. Special track work tests include evaluations of two Number 20 turnouts with optimised switch point geometries, frog bolts in a high-angle crossing diamond, and frog steels in a flange-bearing panel. Fast is also used as a test bed for the development and evaluation of inspection or monitoring technologies such as machine vision car inspection, buried fibre optic cable for detection of rail breaks and flat wheels, and automated cracked wheel detection.

Tonnage at Fast is accumulated with an 18,000-tonne train, with most of the cars loaded to 142,880kg. The typical train operating speed is 64km/h, resulting in an overbalance of about 43.2mm in the five or six-degree curves. Traffic is bi-directional, with running split roughly equally in each direction.

High-strength rail

The current high-strength rail test started in 2014 when six rail types were installed in a non-lubricated, 304.8m-long, five-degree curve with 10.12cm of superelevation. Rails from JFE Steel (JFE-C) Voestalpine (UHC), Tata Steel (MHH), Nippon Steel (HEX), ArcelorMittal (AHH), and Panzhihua (PZH) have accumulated 421 MGT. The rails were cut into 12.2m lengths, and these rails were installed in interspersed segments throughout the curve. Each rail type was installed in at least four separate locations and have been evaluated for rolling contact fatigue (RCF) damage, wear, and fatigue related rail failures.

The RCF assessment method used at Fast is a subjective, visual assessment based on the 0 to 3 rating scale shown in Figure 1. The rails with the least RCF were rated 0, and those with the most were rated 3. Each rail type was assessed by the same engineer at each sleeper in the test section.

rails

Three assessments were made: first, before rail profile grinding after 247 MGT (primarily to remove RCF); second, soon after the rail was ground; and third, after total tonnage accumulation of 343 MGT.

Figure 2 shows the results of the assessments; the sleeper-by-sleeper ratings are averaged by rail type with the rails labelled A to F due to the preliminary nature of the findings. The rail grinding removed most, but not all, of the RCF and re-development of RCF has been fairly slow and consistent since grinding, with relative rankings remaining the same. The rail type judged to have the most RCF before grinding (type E) had the least RCF after grinding, and continues to have the least.

Rail profiles are measured periodically with five measurements taken per 12.2m rail section. The most recent measurements were taken at 384 MGT. Results for the high rail of the curve are shown in Figure 3, with the same A to F designations used in Figure 2. The total area loss is plotted versus railhead hardness. As expected, the harder rails are generally wearing less. Note that the rail with the most wear is also the rail currently with the least RCF. The differences between rails are relatively small at this point, and total area loss includes one corrective rail grind. There have been no fatigue-related rail breaks or rail shells in the test section.

The second test of intermediate-strength (IS) rails started in 2012, and was concluded in summer 2016 after 598 MGT. (The first test ended in early 2012 because 18 gauge-corner shells developed during the period from 340 to 380 MGT.) Both IS rail tests were conducted in the same 244m segment of a lubricated, five-degree curve with 10.12cm of superelevation.

chart

The second test included intermediate strength 136 RE rails from Evraz Rocky Mountain Steel; Lucchini; ArcelorMittal US; Steel Dynamics; and Tinecké zelezárny/Moravia Steel; along with a standard-strength (SS) rail from Evraz Rocky Mountain Steel. The IS rails ranged in hardness from 330HB to 360HB, while the hardness of the SS rail was 320HB. Rails from each of the rail types were again cut into 12.2m lengths that were welded in interspersed segments throughout the string, and then installed in the curve. Each rail type was installed in at least four separate locations.

Rail wear was measured in a manner similar to the measurements of the premium rails. The SS rail wore more than any of the IS rail. However, wear on all rails in the well-lubricated curve was low and was not the determining factor in rail life.

The primary difference between the first and second tests was rail profile grinding. The rail in the first test was lightly ground once at installation, then not ground again. The rail in the second test was ground approximately every 60 MGT in an attempt to suppress the development of gauge corner shells.

Shelling during the second test began in the high rail after approximately 280 MGT. The first (and subsequent) shell(s) initiated in approximately the same location in the gauge corner as most of the shells in the previous test - 7.6mm down from the top of the rail, and 10.2mm in from the gaugeface of the rail.

chart

With the first shell (as with most shells) there was longitudinal growth (parallel with the running surface of the rail). However, in addition, a transverse defect (TD) initiated from the shell surface. This TD then grew perpendicular to the rail running direction (see photo above left), and it was not discovered until a service failure occurred. (None of the shells in the previous test resulted in transverse defects.) Another shell and transverse defect occurred at 310 MGT (see photo above right). The mechanism of this TD was similar to the first shell and TD. Again, this defect was not discovered until a service failure occurred. Shell defects can mask TDs that develop directly below the shell during ultrasonic inspection.

During the subsequent 288 MGT, 13 more shells developed. There were no TDs detected in association with any of these shells. Intermediate strength rails from two suppliers did not develop gauge corner shells.

Mild, preventive grinding of the rails did not eliminate the development of the shells. However, it reduced or delayed the occurrence of such shells on some types of rails and prevented shell development on other rails. Optimal grinding profiles and intervals may have eliminated shell development in all IS rail types.

A new test to study the effects of broad and narrow track gauge on the performance of IS rail will start during the autumn of 2017.