All rail freight cars – and most passenger – braking by pressing the brake pads against the rolling surface of the wheels. If you have ever seen, how the stop-crane is ripped off in the car, then at this moment all the compressed air is released from the braking system and strong springs press the brake pads against the wheel rims.
Thermal cracks in railway wheel rims
Even with normal pad braking along the wheel rim, the smallest thermal cracks appear on the tread surface. They arise as a result of cyclic heating and cooling of the rim during full braking or during deceleration to reduce speed..
Usually, the higher the ability wheel steel to heat hardening, the more thermal cracks occur. Therefore, in terms of preventing thermal cracks, steel with a reduced carbon content is more preferable..
Thermal cracks and residual stresses
Moreover, known, that the use of hard braking, eg, for an hour on a long mountain descent, can lead to the formation of hazardous tensile residual stresses. These tensile stresses promote the growth of surface thermal cracks into the rim., what can lead – and sometimes leads – to brittle destruction of the rim and the entire wheel. The larger the crack in the rim and the higher the residual tensile stresses in it, the more easily brittle fracture of steel occurs.
Fracture toughness of wheel steel
Under the same conditions, the occurrence of brittle fracture depends on the properties of the material, in the case of a railway wheel - wheel steel. This property of a material to resist brittle fracture is called fracture toughness.. The behavior of materials during brittle fracture is dealt with in a separate section of material mechanics - fracture mechanics.
Picture 1 - View of the specimen for fracture toughness tests and the place of its cutting from the rim of a railway wheel according to EN 13262. Six samples are cut from one wheel. The test result is the average value of fracture toughness for six samples.
In general, steels with a lower carbon content have a higher resistance to brittle fracture.. The figure below shows the dependence of the fracture toughness of the wheel rim on the carbon content of the wheel steel.
Low carbon steel has better thermal cracking resistance and higher fracture toughness, but lower wear resistance. Therefore, the choice of wheel steel for the wheels of a particular train on a particular railway is the choice of the appropriate carbon content, taking into account:
- axial loads;
- travel speeds;
- braking conditions and
- wheel maintenance level in operation.
Fracture toughness of wheel steels according to EN 13262
New edition of the European standard EN 13262 from 2011 year introduces the minimum value of fracture toughness in wheels, which are braked by brake pads on the rolling surface. Such a technical requirement for the fracture toughness of the rim in the standard for railway wheels is set in the world for the first time.
The figure above shows the permissible fracture toughness range for ER7 wheel steel according to EN 13262 by the same standard. It turns out, that according to this standard, wheels made of steel with a carbon content of more 0,50 % cannot be used for wheels with rim braking.
On the other hand, in Japan, a large number of wheels made of steel with a carbon content of more 0,60 % braking on the rim without any problem. This is because, that Japan pays great attention to maintenance of the brake system to prevent abnormal (abnormal) braking conditions, which can lead to a change in the residual stresses in the rim from the original compressive to tensile. Somebody think, that this is based on a purely Japanese system of thinking, which Europeans do not have and already, sure, you and me. 🙂