Kulyk V. Development of the concept for high-strength wheel steels creating

In the thesis, from the standpoint of structural mechanics of fatigue fracture, recommendations in the field of materials science are formulated and substantiated, which provide a solution to the important scientific and technical problem of increasing the reliability and durability of high-strength railway wheels. A new concept of steels creating is proposed that provides high resistance to wear and damage of the tread surface of the wheels. Requirements for the complex of the mechanical characteristics of such steels and the reasonably chemical composition of a steel for railway wheels of the new generation, as well as the heat treatment mode for their repaired surface have been developed. It was revealed that the negative effect of the corrosive environment on the fatigue threshold Kth and the cyclic fracture toughness Kfc of standard wheel steels grade 2 and grade T is insignificant, and the tendency to low-temperature embrittlement increases with increasing carbon content in the steel. It has been established that under the influence of thermo-force factors in the contact zone during braking, when the pearlite structure is transformed into martensitic, the output compressive residual stresses of the II type change to tensile strength the stronger, the higher the carbon content in steel and its cooling rate. It is shown that the damageability of the rolling surface of model wheels during contact-cyclic loading of a wheel-rail pair increases with increasing strength (hardness), which is effected by high carbon content in the wheel steel. At the same time, the damage is definitely correlated with the cyclic fracture toughness of the steel with Mode I fracture (ΔKІ fc) and Mode II fracture (ΔKІІ fc). They can be considered decisive for the process of damage, in contrast to the fatigue thresholds ΔKІ th and ΔKІI th. To assess of serviceability of wheel steels, it was proposed to use high-temperature (500-800ºС) values of their relative elongation, which characterizes the tendency to form slides on the tread surface, and the operational reliability diagram – the relationship between damage resistance and wear resistance, which comprehensively characterize the tendency of steel to spall formation on wheels tread surface. Based on the structural strength and operational reliability diagrams, it was found that the optimal combination of strength characteristics and cyclic crack resistance, resistance to wear and damage is provided by complex alloyed steels with a combination of solid-solution (with increased Si i Mn content) and precipitation (after micro-alloying V i N) hardening. Recommended complex-alloyed steel with solid solution (~ 1% Si and ~ 1% Mn) and precipitation ([V‧N]‧104 = 20…25%) hardening for low carbon content (0.52 ... 0.53%) for experienced industrial inspection in order to manufacture railroad wheels of a new generation. It is shown that the increase in cyclic crack resistance of the heat affected zone of repair tread surface of railway wheels is achieved by the formation in this zone of a mixed structure of upper and lower bainite (~ 66%) and martensite (~ 34%) and holding time of 2-3 h, interrupting the cooling of steel at 100ºС, that is, at a temperature between the points of onset (Ms) and the end (Mf) of the martensitic transformation.