Railway Wheels Tribology

Nowadays, the railway is regarded as a reliable, rapid, and secure mode of transportation. The issue of rail and wheel contact is one of the most essential parts of the railway system because improper interactions cause problems like wear and have a detrimental impact on the train’s dynamic performance. In this post, railway wheel durability and factors that influence the safety of trains will be examined.

Rail traffic safety is determined by a variety of factors, including the technical state of the railway infrastructure, rolling stock, traffic, and rail transport organization, professional qualifications, and proper execution of responsibilities by personnel.

In the twenty-first century, cargo and passenger transportation are critical aspects of human existence, and a vast amount of air, overland, sea, and train transportation is carried out every day. The railway system has long been regarded as a reliable, rapid, and secure mode of transit, as well as a significant competitor for air travel in terms of security and speed.

What material is used for railway wheels?

The materials used in wheels and rails were, and still are, those steels with primarily pearlitic structures including strong cementite lamellae to provide high wear resistance in response to established causes of damage.

At the same time, compared to bainitic or martensitic structures, a pearlitic microstructure generated by transformation near to the point of equilibrium guarantees stronger resistance to transformation in operational usage. Despite the fact that UIC Leaflet 812-3 for solid wheels mentions seven varieties of steel that differ primarily in carbon content, heat treatment condition, and hence strength, EN 13262 only lists four.

This illustrates the current state of technology in Europe, since Grade R1 for freight wagon wheels is on the wane in favor of the standard R7 material, while Grades R2/R3 never established themselves in practical practice. By far the most popular grade is R7. All freight wagon wheels, as well as most passenger coaches, are made of this material.

The fracture toughness standards, as well as the normal characteristic mechanical parameters, must be met when wheels constructed of R7 are intended for use in vehicles with tread brakes. The microstructure of the wheel has a high degree of cleanliness and uniformity across its circumference.

This, of course, puts a lot of pressure on the production quality. As a result, these wheels are frequently supplied with lower carbon levels (0.5 percent C), putting them in the lower strength tolerance range, resulting in considerable amounts of pre-eutectoid ferrite in the tread, in addition to pearlite. Although this results in increased toughness, wear resistance is reduced as a result. According to Deutsche Bahn’s (DB) experience, a free (pre-eutectoid) ferrite concentration of 10% or less is beneficial in terms of reducing tread wear.

Railway wheel Durability – Basic Wear

The axle load is generally considered to be carried by only a tiny region where the wheel contacts the rail. However, due to fluctuating vehicle running circumstances, the contact position changes over time, resulting in varying contact area and contact pressure.

There are other sorts of contacts; for example, the wheel tread-rail head contact normally happens on a straight track with lower contact pressure and sliding speed than the wheel flange-rail gauge contact that frequently occurs on a curved track.

Furthermore, even for a single wheel, wear-related profile variations result in varying contact pressure and contact area. Notably, the actual scenario may be more complicated than what has been described here.

Railway wheel durability – Wheel Fatigue

During the 1990s, the problem of rolling contact fatigue in rails became quite visible. After the Hatfield disaster in the United Kingdom in 2000, where roughly 30 rail lengths cracked entirely on the outer rail of a 1500 m radius curve, the railway industry was forced to take RCF problems seriously. According to Federal Railroad Administration records, rolling contact fatigue was the primary cause of 122 derailments between 1995 and 2002, and RCF was suspected of contributing to another 160 derailments.

Due to overstressing of the rail material, rolling contact fatigue (RCF) is a series of rail damages that appear on the surface or near the surface within the rails. Shelling, squats, and gauge corner fractures may occur on the rail surface, while deep-seated shells may emerge beneath the surface. All of these flaws are the consequence of millions of wheel-rails contact cycles repeatedly overstretching the surface or subsurface rail material.

Both tread spalling and wheel out of roundness produce vibrations in the vehicle body, resulting in a reduction in ride comfort. They also result in higher dynamic loadings on the vehicle and track. In addition, rail traffic noise rises dramatically.

For diameter matching reasons, extensive wheel reprofiling, in which a significant portion of the wheel’s diameter is machined away, is required not just on the damaged wheel but on multiple wheels of the affected vehicle. It is obvious that, in addition to growing maintenance costs, this results in a reduction in wheel life to a point where it is sometimes not economically justifiable. In many situations, “artificial” material loss from the wheel caused by turning outweighs “natural” wear caused by rolling contact in service.

Importance of Railway wheel durability

The advent of high-performance rail traffic has increased the necessity for wheel-to-rail contact once again. There is a relationship between this and the common kinds of wheel wear that occur and shorten the life of the wheels.

Out of roundness and tread damage are influenced significantly by the wheel material used and the manufacturing quality of the wheel (poor railway wheel durability). Both traditional laboratory studies and extensive in-service experiments are used to evaluate new wheel materials for railway use. The wheel-rail system test rig allows the pre-testing of wheelsets with new wheel materials under realistic loading conditions.

Last Word

The possibility of the wheel out of roundness and tread damage, as well as the subject of rail wear, can be investigated. Out-of-roundness is influenced by the microstructure and uniformity of mechanical qualities surrounding the wheel rim, according to tests done in recent years. There are alternatives to the wheel materials that have previously been utilized. Furthermore, as the experiences with Shinkansen steel wheels and the bainitic cast iron ADI have revealed, special attention to production quality is required.