Loads in contact between wheel and rail
To materials, which work in the contact zone between the wheel and the rail, there are serious demands. In the contact zone between the wheel and the rail, the following processes occur:
- clean rolling under permanent normal and side loads causes high shear stresses similar to a wheel, so in the rail
- relative movements between the wheel and the rail create slippage in the contact zone, which leads to mechanical and thermal loading of materials.
As a result, various phenomena occur on the rolling surface of the wheels and the working surfaces of the rails., which lead to the formation of various defects in the wheel and rail:
- adhesive wear
- plastic deformation
- rolling contact fatigue and
- thermal fatigue.
European wheel steels
To counteract the above mechanisms of wheel and rail damage, steels have been and are being used in Europe, which have a predominantly pearlite structure. This structure contains solid cementite plates, which guarantee high wear resistance. At the same time, the pearlite microstructure, which is formed near the equilibrium point, provides higher resistance to metallurgical transformations when heated in service, than, eg, bainitic or martensitic structure.
Wheel Steel Standards
UIC standard 812-3 for solid wheels includes seven steel grades, which differ mainly in carbon content, heat treatment and, so, strength level, a European standard EN 13262 includes only four brands (table 1).
Table 1 - Requirements for wheel steels according to UIC standards 812-3 и IN 13262
As the R1 brand for freight car wheels is increasingly being replaced by the R7 brand, and R2 / R3 grades were rarely used in operational practice, then it means, what EN 13262 reflects the current state of the technology for the production of railway wheels from Europe.
R7 is the most commonly used steel grade. It applies to all wheels of freight cars and most passenger cars.. Since the wheels made of steel grade R7 are intended for use in wagons with block braking, along with the requirements for the usual mechanical properties, the requirements for fracture toughness must be met (KIC).
Experience has shown, what, if the carbon content exceeds 0,5 %, fracture toughness KIC majestic 80 MPa · m1/2 can only be achieved then, when the steel has a small grain size (fine grain), high purity and high homogeneity of the microstructure along the entire perimeter of the wheel. This places heavy demands on the quality of the wheel..
For this reason, these wheels are usually supplied with reduced carbon content. (< 0,5 %). This puts them on the lower strength limit., so besides pearlite in the working layer of the rim there is a large amount of hypoeutectoid ferrite.
Although this results in higher fracture toughness, wear resistance decreases accordingly. Based on the experience of German Railways 10 % free (hypoeutectoid) ferrite is favorable for minimizing wheel wear on the rolling surface.
Steel grade R8 is increasingly used for traction wheels of locomotives and motor cars.. The use of the R9 brand is limited to the niche of special cars for construction and mixed transport systems.
In this way, materials, which are used for solid wheels in Europe, mostly limited to unalloyed steels with maximum carbon content 0,56 % after appropriate heat treatment of the rolling surface (processing for dispersed perlite) strength from 820 to 980 MPa.
- Materials used for Wheels on Rolling Stock / K. Mädler, M. Bannasch – Deutsche Bahn AG, Technical Centre, Brandenburg-Kirchmöser, GERMANY