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

500C) 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