Page 10 - Кулик В.В.

10

hardening.

The tendency to form flats on the wheel tread surface increases with the

growth of high-temperature (above 500C) plasticity (relative elongation) of steels.

The more rapid increase in plasticity due to the dissolution of vanadium

carbonitride in grade T steel provides more favorable conditions for the formation

of flat-type defects on the tread surface of KP-T wheels as compared to KP-2

wheels. Increased resistance of wheel steels to flat formation is also caused by their

solid-solution hardening with silicon (~ 1%) and manganese (~ 1%), and

precipitation hardening with vanadium (~ 0.17%) and nitrogen (~ 0.013%).

The negative effect of the corrosive environment (distilled water and 3.5%

aqueous NaCl solution) on the fatigue threshold K and the cyclic fracture
th

toughness K of wheel steels of grade 2 and grade T is negligible. However, it is
fc
known that the tendency to low-temperature embrittlement of wheel steel increases

with increasing carbon content: at a temperature of –40°C, the value of K for
fc
grade T steel (0.66% C) is 1.7 times less than for grade 2 steel (0.58% C).

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 grade T steel, compared to grade 2 steel, these processes lead to a

more intensive implementation of low-energy intergranular cleavage fracture

mechanism under cyclic loading and lower fatigue crack growth resistance

characteristics: the fatigue threshold K is 2.4 times lower and the cyclic fracture
th
toughness K is 1.7 times lower. Solid-solution hardening of wheel steels with a
fc

content of 0.58 ... 0.60% carbon in this case causes a decrease in the cyclic fracture

toughness as compared to grade T steel.

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.