Tackling White Etching Layer (WEL): The Potential Risk to Track Integrity

Railway transportation is six times more efficient and has six times lower CO2, NOx, and PM emissions than air or road travel. Yet, it contributes to just 10% of the global freight and passenger traffic (source: IEA report).

This is changing rapidly as electrification, and high-speed rail networks are developed in India and China. Moreover, the urbanization of smart cities needs metros to enable mass mobility with clockwork schedules and point to point connectivity.

Rapid acceleration-deceleration profile triggers 'white etching layer (WEL)', a unique degradation mode and a potential risk to track integrity. Spallation of these layers leads to the formation of the squat, which increases both vertical impact forces and noise levels, affecting ride quality. (Source: Martensite formation in Mumbai EMU wheels during monsoon in India, 4th Railway Friction Conference).

Ducom twin disc (roller on roller) tribometer with automated loading. 

High speeds in heavier freight axle loads and long-distance trains normally accelerate other failure modes at the rail-wheel interface (Figure 1a, 1b). Reports suggest that millions of Euros are spent in the UK and Netherlands annually for track maintenance due to defects such as squats.

Different failure modes at rail wheel interface that affect safety, maintenance and ride quality

Figure 1A. Different failure modes at rail wheel interface that affect safety, maintenance, and ride quality.

 

Location of different failure modes on a rail head (excessive wear, multiple RCF cracks and WEL)

Figure 1B. Location of different failure modes on a railhead (excessive wear, multiple RCF cracks, and WEL).

Historically, three significant strategies have been employed to address failures:

(1) Advanced materials - high strength and tough bainitic and pearlitic steels. Laser cladding enables higher wear resistance and rolling contact fatigue life (RCF).

(2) Wayside friction modifiers - rail lubrication (top and gage) to control friction at the rail wheel interface to reduce RCF, the intensity of wear, and subsurface stresses. A typical COF between 0.3 to 0.4 is needed for railhead lubrication, and COF < 0.2 is necessary for flange lubrication.

(3) Preventive maintenance - rail grinding to get rid of surface cracks (i.e., controlled wear) and restoring rail profile to extend RCF life and reduce stress hotspots. However, repeated and unscheduled maintenance disrupts urban mobility (e.g., there are 7 million passengers daily on the Mumbai Metro)

White etching layers tend to be less tough and harder compared to the base microstructure. This results in reduced life and premature failures when compared to traditional RCF cracks. How do these failures begin?

Table 2 shows the usual operating conditions for rail wheel contact and shows different rolling-sliding conditions for wheel flange and top of the rail.

Table 2. Typical pressure and sliding conditions at rail-wheel contact. Source: Ducom

Table 2. Typical pressure and sliding conditions at rail-wheel contact.

White etching layers are linked to a zone of transformed nanostructure having deleterious residual stresses, reduced ductility, and higher hardness; cracks can propagate easily within WEL or WEL base material interface. Table 3 summarizes some of the opposing theories involved in the active area of research on the formation of WELs.

Table 3. Mechanical and thermal driven formation of WEL. Source: Ducom

Table 3. Mechanical and thermal driven formation of WEL.

Ducom twin disc (roller on roller RoR) offers a realistic lab platform that can reproduce the characteristic conditions at the rail-wheel interface (Table 4). Contact pressures up to 4 GPa and slip ratios of 100% can accelerate the formation of 'mechanically-driven WEL.' A unique heating facility (temperature of 700 degrees celsius) can augment the formation of 'thermally-driven WEL' (see Figure 5).

Video. Ducom twin disc (roller on roller) tribometer at 8000 N load

Table 4. Contact pressure, %slip and temperature capabilities of Ducom twin disc (RoR). Source: Ducom

Table 4. Contact pressure, %slip and temperature capabilities of Ducom twin disc (RoR).

Ambient temperature twin disc configuration and 600 deg C block on disc configuration of Ducom twin disc (RoR)

Figure 5. Ambient temperature twin disc configuration and 600 deg C block on disc configuration of Ducom twin disc (RoR).

By understanding the conditions and materials which trigger WEL formation, a critical field issue, a better solution can be developed.

Ducom twin disc (RoR) can simulate the contact pressure and the aggressive slips ratios during elevated temperatures and rolling that are thought to trigger the formation of white etching layers in wheels and railheads.

This information has been sourced, reviewed, and adapted from materials provided by Ducom.

For more information on this source, please visit Ducom.

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