Page 10 - Кулик В.В.
P. 10



                         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

                  toughness K  of wheel steels of grade 2 and grade T is negligible. However, it is
                  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
                  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
                  toughness K  is 1.7 times lower. Solid-solution hardening of wheel steels with a

                  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.
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