Railway wheels are among the most stressed elements of railway vehicles.. Wheelset load (two wheels and an axle) up to 25 tons and more, guide the train along all bends in the track, subject to wear when driving on rails. Wheels transfer acceleration and deceleration from the car to the rails, are experiencing thermal stresses when slipping on the rail and when braking with brake pads.
Railway wheel elements
Various functional elements of the wheel, such as hub, disk, rim or flange (picture 1) perform different tasks and therefore have different requirements for mechanical properties their materials. These properties depend mainly on the chemical composition of the wheel steel, modes of hot deformation processing of each element and - to a significant extent - from the mode of heat treatment of wheel elements.
Railway Wheels Standards
Railroad wheels are usually made from high-purity unalloyed or low alloyed steels.. Leading national and international standards, which define the wheel steel grades and partially the wheel manufacturing technology are the following: UIC 812-3V, GUEST 10791, AAR M107 и IN 13262.
Wheel steel requirements of the European EN standard 13262
European standard EN 13262 defines four steel grades, the carbon content in which does not exceed 0,60 %; manganese content - up to 0,80 % and silicon - up to 0,40 %.
Depending on the steel grade, the standard imposes different requirements on the mechanical properties of the wheels, eg:
1) Strength limit rim in tension - from 780 to 1050 MPa.
2) Impact strength of the rim - not less 9 J for U-notch at room temperature.
3) Residual stresses – compressive stresses in the rim of the finished wheel.
4) Tensile strength of the disc - by 100-130 MPa below, than in the rim.
5) Fatigue strength of a machined disc - not less 450 MPa at 107 test cycles.
Microstructure of wheel steel
The microstructure of steel is the "carrier", which determines the above mechanical properties. Fine-grained steels with a finely dispersed ferrite-pearlite structure provides an optimal combination of mechanical properties, wear resistance and thermal stability. Therefore, in Europe, railway wheels are mainly used precisely ferrite-pearlite steels. In recent years, true, there is an intensive research of bainitic steels with the aim of their application in railway wheels.
Heat treatment of railway wheels
Heat treatment of railway wheels is the most important technological operation, which provides them with the necessary mechanical properties. For almost all railway wheels, heat treatment is the process of heat hardening of the rim.. After austenitization at a temperature of about 900 ° C rapid cooling to a temperature of about 300 ° C only the rim is exposed. This is usually done with water.. After the end of the rim hardening, the disc and hub are still at a temperature close to the lower critical temperature. Further cooling of the wheel is carried out in air, which provides the rim with high strength and compressive residual stresses. The final operation is to release the entire wheel at a temperature of about 500 ° C. This tempering is done to reduce the overall level of residual stresses., but without significant changes in the microstructure and mechanical properties of the wheel material.
Thermokinetic transformation diagram austenite in wheel steel
On the picture 2 presents a thermokinetic diagram of the transformation of austenite in unalloyed steel with a carbon content of about 0,50 %. This steel matches well with typical EN wheel steel 13262.
Picture 2 – Thermokinetic diagram of transformations of austenite in unalloyed steel
with a carbon content of about 0,50 % with different cooling modes (1, 2 and 3) and
the types of steel microstructure corresponding to these modes.
In the picture diagram 2 three modes of steel cooling and the corresponding steel microstructure are shown. The desired cooling mode for hardening the wheel rim is somewhere to the left of the mode 2. The cooling curve crosses the pearlite region with a minimum fraction of free ferrite and does not enter bainite and martensitic areas.
Different controlled cooling rates in different functional elements of the wheel lead to different mechanical properties. This makes it possible, in principle, to control the properties of the material in the wheel elements, taking into account the future operating conditions of the wheel..
The purpose of the heat treatment of the railway wheel
The main goal heat treatment of railway wheel high homogeneity of the microstructure in the rim, moreover, as in radial, and in the tangential direction. This is necessary to ensure uniform wear on the rolling surface and prevent the formation of wheel out-of-roundness.. Besides, such defects in wheel operation as fatigue damage to the rolling surface are minimized.
Railroad wheel rim hardening
Controlled cooling of the wheel is the most important step in the heat treatment.. It is at this stage that the microstructure and mechanical properties are formed, and, hence, and wheel quality.
Hardening process – accelerated cooling – controlled by varying:
- cooling medium,
- cooling medium consumption and
- the duration of exposure to the cooling medium (picture 3).
Heat treatment of wheels should ensure:
- fulfillment of the specified requirements for mechanical properties in the rim, disc and wheel hub;
- fulfillment of requirements for the type of microstructure in various elements of the wheel (eg, pure ferrite-pearlite structure without any other structural components);
- creating in the rim near the rolling surface compressive tangential (district) residual stresses.
Source: AFT Advanced Forging Technology, Germany