Page 43 - Міністерство освіти і науки України

41

Sciences of Ukraine, Lviv Polytechnic National University, Ministry of Education and
Science of Ukraine Lviv, 2019.
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 K and the cyclic fracture toughness K of standard wheel steels grade 2 and
fc
th
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
(ΔK ) and Mode II fracture (ΔK ІІ fc ). They can be considered decisive for the process of
І fc
damage, in contrast to the fatigue thresholds ΔK and ΔK ІI th .
І 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
4
([V‧N]‧10 = 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 (M ) and the end (M ) of the martensitic transformation.
f
s
Keywords: high-strength railway wheel steels, defects of the tread surface, fatigue
crack growth resistance, solid-solution and precipitation hardening, operational reliability,
repair of the tread surface.