Quenching Process and Surface Hardness of National Standard Rails

Quenching Process and Surface Hardness of National Standard Rails

• What are the common quenching processes for national-standard steel rails? What are their characteristics?​

The common quenching processes for national-standard steel rails include integral quenching and surface quenching. Integral quenching involves heating the entire rail above the critical temperature and then rapidly cooling it, which can improve the overall hardness of the rail and is suitable for heavy-haul railways, but the toughness of the rail decreases slightly after treatment. Surface quenching only quenches the surface of the rail head, forming a hardened layer on the surface while the core remains tough, suitable for mixed passenger and freight lines, which can balance wear resistance and impact resistance.​

• What is the relationship between rail surface hardness and wear resistance?​

Generally, the higher the rail surface hardness, the better the wear resistance. When the surface hardness is in the range of HB300-HB380, as the hardness increases, the rail's ability to resist wheel wear enhances, and its service life is prolonged. However, higher hardness is not always better. If it exceeds HB400, the rail becomes more brittle and prone to spalling, which affects wear resistance and safety.​

• What are the differences in requirements for surface hardness of national-standard steel rails in different lines?​

High-speed railways require the rail surface hardness to be HB340-HB380 to withstand the high-frequency impact of high-speed trains and reduce wear. Heavy-haul railway rails need a surface hardness of HB360-HB400 to cope with severe wear caused by large axle loads. The surface hardness of ordinary trunk railway rails is generally HB320-HB360, which can meet daily passenger and freight needs. The hardness of rails for factory and mine special lines can be appropriately reduced, usually HB300-HB340.​

• How to test the surface hardness of national-standard steel rails?​

A common testing method is using a Brinell hardness tester, which presses a hard alloy ball of a certain diameter into the rail surface and calculates the hardness value based on the indentation diameter. During testing, samples need to be taken from different parts such as the rail head tread and rail web to ensure representative results. For laid rails, a portable hardness tester can be used for on-site testing, which is easy to operate and does not damage the rail. The testing frequency is 10% sampling of each batch of rails; if unqualified, double sampling is required, and the entire batch is rejected if still unqualified.​

• What effects can improper quenching processes have on rail performance?​

Excessively high quenching temperature will lead to coarse grains in the rail, uneven surface hardness, and local excessive wear during use. Insufficient cooling rate cannot achieve the expected hardness, reducing the rail's wear resistance and shortening its service life. If tempering is not performed after quenching, excessive internal stress in the rail will easily cause cracks, which may break under train loads, seriously affecting driving safety.