Rim wear resistance directly affects service life railway wheel. Therefore, it is wear resistance that is a very important characteristic of wheel steel from an economic point of view., namely – rolling stock operating cost.
Carbon in wheel steels
The main factor, which affects the wear resistance of steel, is its carbon content. The higher carbon content in steel, the higher its wear resistance. However, an increase in carbon content increases the tendency of wheel rims to thermal damage and therefore standards for railway wheels are usually set by several grades of steel - from medium carbon to high carbon..
The table below shows the carbon content and hardness of steel in railway wheel rims according to:
- European standard EN 13262,
- American standard AAR M-107 / M-208,
- Japanese standard JIS E 5402-1, as well as
- interstate standard GOST 10791-2011.
Japanese wheel steels
You can see in the table, that the Japanese standard specifies for wheel steel only one grade with a carbon content of 0,60 to 0,75 %. This is due to the following circumstances. Near 90 years ago in Japan there were big problems with excessive wear of railway wheels. Since railway technology was borrowed from Europe, then wheel steel had a low steel content at that time (near 0,5 %), as it was then in Europe.
It was clear, that increasing the carbon content could be effective in increasing the life of railway wheels. However, this could shorten the life of the rails.. Therefore, in Japan, scientific research was carried out to find the optimal ratio of carbon content in wheel and rail steels..
Higher carbon in wheels – less rail wear
The result of these studies was unexpected.. Was found, that increasing the carbon in the wheel steel reduces not only wheel wear, but also rails. On the picture 1 shows the results of model wear tests for various combinations of wheel and rail steel grades: each of three grades of wheel steel with each of three grades of rail steel.
The explanation for this result was as follows. The smallest steel particles of steel, which formed on the wheel-rail contact surface acted as an abrasive material between the rolling surface and the rail head. Therefore, they contributed to wear and tear like wheels, and rail. It means, that less wheel wear reduces the amount of this abrasive material and, hence, also reduces rail wear. Based on this, a revolutionary conclusion was made, that with an increase in the carbon content in the wheel, the service life of the rail is in no way reduced, and, may be, on the contrary increases. Later, based on these research results, the carbon content of Japanese wheels was gradually increased., until it reached the current level in the JIS E standard 5402-1 (0,60-0,75 %).
Japanese wheels on the German railroad
German railway (Deutsche Bahn) for a long time had problems with the out-of-roundness of the wheels of ICE high-speed trains. Since there were no such problems with the wheels of the Japanese high-speed trains Shinkan-sen, it was decided to carry out comparative tests on existing German trains of Japanese wheels, manufactured according to Japanese JIS E standard 5402-1 and standard European wheels made of ER7 steel (carbon content no more 0,52 %).
After six years of testing, beginning with 2003 of the year, Japanese steel showed significantly higher wear resistance - in 1,5 times less, than ER7 wheel steel.
In parallel with the comparative tests of wheel steels during the operation of wheels on an operating railway, comparative tests of the wear of rails were carried out on a full-scale test equipment. In this test, full-size wheels were subjected to cyclic loading from straight rails.. Particular attention was paid to the fact, so that the above-mentioned wear particles remain on the wheel-rail contact surface and act as an abrasive.
On the picture 2 shows the results of comparative tests using two grades of wheel steel, ER7 and JIS.
1) Rail material: R260 according to EN 13674-1;
2) Three tests of each material under the same conditions
Picture 2 - Wear of rails depending on the brand of wheel steel
These results do not show that, that the wear of the rail is reduced with an increase in the carbon content in the wheel steel. This is where they differ from Japanese experimental data., shown in the figure 1. However, based on their, at least, we can conclude, that the higher carbon content in the wheel steel has no harmful effect on the wear of the rails.
Source: Okagata Y, Nippon Steel & Sumitomo Metal Technical Report No. 105, 2013