How does the heat treatment of railway wheels work?

Railway wheels are among the most heavily loaded components of a rail vehicle. They carry heavy loads, guide the vehicle through points and curves, and transmit braking and acceleration forces. Temperatures on the wheel surface can reach several hundred degrees Celsius.

The different functional areas of a wheel (Figure 1), such as the rim (1) with its wear allowance (2), flange (3), web (4) and hub (5), perform different tasks. They therefore also require different material properties, which are largely determined by the heat treatment of the wheels.

Austenitising: preparing for microstructural transformation

Heat treatment begins by heating the wheel to the austenitising temperature. Railway wheels are generally made from unalloyed or low-alloy carbon steels, which are austenitised at around 800 to 900 °C.

At these temperatures, the atomic lattice of iron changes from a body-centred cubic structure (ferrite) to a face-centred cubic structure (austenite). At the same time, carbides dissolve and carbon becomes evenly distributed throughout the material.

The red-hot wheel is then removed from the furnace and placed in a cooling machine for controlled cooling. Figure 2 shows a wheel in the cooling equipment just before the actual cooling process begins.

Controlled cooling of the wheel rim

The wheel begins to rotate in the cooling machine. At the same time, the wheel rim is selectively cooled with water and/or air (Figure 3). The quantities of coolant, cooling times and cooling media are precisely controlled by the system’s computer.

This controlled cooling produces the required mechanical properties, such as strength and toughness, and an optimum microstructure in the wheel rim. A pearlitic-ferritic microstructure, a mixture of iron and iron carbide phases, has proved particularly suitable for wheel treads.

Residual stresses during the cooling process

Once the wheel rim has cooled sufficiently (Figure 4), the wheel is removed from the cooling machine and cooled in still air. At this stage, the wheel rim is under tensile stress: it tends to contract as it cools, but the still-hot web and hub prevent it from doing so. These areas are, in turn, under compressive stress at this point.

Controlled cooling, particularly of the wheel rim, is the decisive stage of heat treatment, during which the microstructure and mechanical properties are produced within a few minutes. The challenge is to cool the wheel rapidly enough to achieve the required strength while controlling the process sufficiently to avoid unwanted microstructural constituents such as martensite.

Desired stress states in the finished wheel

As cooling in still air continues, the web and hub eventually cool completely as well (Figure 5). Their slower cooling produces the mechanical properties required in these areas.

At the same time, a characteristic residual stress state develops: parts of the web are under tensile stress, while the wheel rim is under compressive stress. This state is specifically desired because compressive residual stresses in the wheel rim counteract the formation of cracks that can arise from wheel–rail contact or tread braking.

Tempering to reduce stresses

Finally, the wheels are tempered once more at around 450 to 550 °C. Unlike conventional quenching and tempering, however, this heat treatment stage no longer changes the microstructure. Its sole purpose is to reduce internal stresses.

The microstructure and mechanical properties of the railway wheel are thus produced entirely during controlled cooling, particularly in the wheel rim.

Material properties of railway wheels

The material properties of a railway wheel that are influenced by heat treatment and must be verified through prescribed materials testing include strength, toughness, fracture toughness and fatigue strength, as well as microstructure (microstructural constituents and grain size) and residual stress state.

Research and development in modern heat treatment

The performance of a railway wheel does not depend on a particularly exotic material. On the contrary, comparatively ‘simple’ unalloyed carbon steels are generally used, albeit with a very high degree of purity. The decisive factor is heat treatment, which produces optimum material properties and reliable performance in service.

Together with its partners MSA Chemnitz and ITW Chemnitz, Werkstoff Service is developing new heat treatment technologies for the railway sector as part of a publicly funded research project (Figure 6).

Modern cooling machines are now highly automated CNC systems with production capacities of several hundred thousand wheels per year, in which railway wheels are manufactured fully automatically and with consistently high quality.

W.S. Werkstoff Service – expertise in the railway sector

Whether damage analysis of a railway component or non-destructive testing of a rail vehicle is required, the experts at our W.S. RailwayCenter support operators, manufacturers and maintenance providers with quality assurance, materials and component testing, and approvals in the railway sector.

An overview of our services is available at https://www.werkstoff-service.de/en/w-s-railwaycenter/.