Page 13 - Кулик В.В.
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which is based on the diagrams of structural strength and operational reliability
taking into account the fracture mechanisms of Mode I and Mode II, and provides
the optimal combination of their characteristics of resistance to wear and contact
fatigue damage, due to the reduced carbon content.
2. For the first time, it is stated that the tendency to form flats on the
wheel tread surface is intensified with an increase in high-temperature (above
500C) plasticity (relative elongation) of steels. It is shown that the increased
resistance of wheel steels to flat formation is causes by their solid-solution
hardening (with silicon and manganese) and precipitation hardening (with
vanadium and nitrogen).
3. It is established that due to the influence of thermo-force factors in the
contact zone during braking, when the pearlite structure is transformed into
martensitic one, the initial residual compressive stresses of the II type change to
tensile. The higher is the carbon content in the steel and the rate of its cooling, the
higher is the intensity of this change. In steel with increased (0.65-0.70%) carbon
content, these processes cause a more intensive implementation of low-energy
intergranular cleavage fracture mechanism under cyclic loading and a decrease of
fatigue fracture characteristics: the fatigue threshold K is 2.4 times lower and the
th
cyclic fracture toughness K is 1.7 times lower.
fc
4. For the first time, it is shown that the damageability of the tread
surface of model wheels during cyclic contact loading of a wheel-rail pair is
intensified with increasing strength (hardness), which is effected by high carbon
content in the wheel steel. In this case, the damage is definitely correlated with the
cyclic fracture toughness of the wheel steel of Mode I fracture (ΔK ) and Mode II
І fc
fracture (ΔK ІІ fc ). They can be considered the determining parameters of this
damage process, in contrast to the fatigue thresholds ΔK and ΔK ІI th .
І th
5. For the first time, it was found that the optimum combination of
strength and fatigue crack growth resistance of the precipitation-hardened wheel
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steel is ensured by the content of vanadium and nitrogen [V‧N]‧10 = 22% and the