Valevich M. Structure and crack-formation of worked surface of railways weell streugthen truss.

Specific service conditions of railway transport involve problems associated with cracks formed in the wheel tread, the cause of which remains unclear as yet. Cracks may initiate in the process of long-time operation. Also, they may be formed after repair or thermomechanical treatment. The latter includes plasma surface hardening, which has been intensively employed lately. The paper present results of investigations into the effect of surface hardening parameters on structural, thermal and thermally stressed states of railway wheel treads, as well as relationship of these factors with initiation and propagation of cracks. It is shown that tension of a tread may also lead to formation of tensile stresses in sub-surface layers of the tread in excess of the yield stress of a material. These stresses may cause fracture of the tread during cooling of a wheel after tension. Application of comprehensive examinations, including optical metallography, ana-lytical scanning electron microscopy, fractography of the character of fracture, as well as precision methods, which is the method of transmission electron micro-diffraction microscopy, allowed the depth of the cracking zone and causes of the cracks to be de-termined more specifically. It is established that the zones with a high level of internal stresses, as well as the associated crack formation, are localized at a depth of about 100 m from the surface. Also, it is shown that extended non-metallic inclusions (mostly of a sulphide type) directed parallel to the external surface, which are formed under the effect of surface cold-hardening, are one of the causes of formation and propagation of cracks in the sub-surface layers of the wheel tread metal. Another factor is the formation of gradients of dislocation density and local internal stresses (the value of the latter is close to the theoretical strength value) in the zones of co-existence of hard and soft structural components. The strain localized in the field of initiation of cracks is about 40 %. The investigations conducted resulted in working out of the parameters of plasma treatment recommended for the wheel treads, which provide, compared with standard wheelset flanges, a 2.5-3.0 times decrease in the intensity of wear of the plasma treated wheelset flanges, which is required on the basis of conditions of improved contact inter-action of wheels and rails.