11 - Thermal Aspects of Rail-Wheel Contact Flashcards
Why heat is generated at the rail-wheel interface
Contacts are rarely running in pure rolling
Because of 3D contact between rail and wheel, even a nominally pure rolling situation leaves some parts of the contact sliding
Point on the wheel in centre of contact is in pure rolling
Points either side of this are close to pure rolling, but are likely to have some additional slip in the contact
Sliding and friction force between rail and wheel
Work is done at contact, goes to: heat; noise; metallurgical changes to rail; wear debris generation and growth of cracks
Effects of excessive heat
Most common effect is formation of martensite layers on rail surface
Very hard and brittle form of steel (by heating normal rail steel to over 727C followed by rapid cooling)
Known as ‘white etching layer’ as it appears white when prepared using the Nital chemical etching process to reveal steel structure
Brittle surface layer can easily crack, and once small cracks exists it can grow down into rail
Appearance of white etching layer
Nital (5% nitric acid in ethanol) darkens steel surface, but leaves martneiste white/shiny
Typical effects of excessive heat
Small-looking defects but could be internal cracking
Roughness of rail/wheel surface from defects produces high dynamic loads, accelerating further damage
Extreme effects of excessive heat
Rails melt and rapidly worn
What does an isothermal transformation diagram show?
TTT diagram - time, temperature, transformation
Shows how fast steel must cool to form martensite
What does the TTT diagram show for rail steel?
Zero time is cooling points below 727C, the eutectoid temperature
Martensitic transformation begins at designated horizontal line, M(50%) and M(90%) indicate percentages of the austenite to martensite transformation complete
If material cools more slowly it will transform to the equilibrium pearlite and austenite structures
Moving heat sources - sliding
Example using extreme case of locked (sliding) wheel
Rail sees heat source moving along its surface
Each point on rail will be briefly heated to same temperature as the wheel and will then rapidly cool
Rail will only be heated over a very shallow depth
Forms white etching layer on rail
Wheel will receive continuous heat input at a single point
Although wheel is sliding along, it is stationary relative to heat source
Much larger depth of wheel will become hot and will therefore cool slower
Wheel thermal damage is likely but not formation of white etching layer on wheel
Moving heat sources - spinning
Spinning wheel on stationary train
Relative to rail the heat source (rail-wheel interface) is stationary
Single point on rail will get continuous heat input and will be heated over a large depth - will remain hot and cool slowly when train eventually moves away
Wheel will see a heat source moving relative to its surface
Any point on wheel will be briefly heated and all then cool
White etching layer will form on wheel, eventually bulk of wheel will become hot
Partitioning of heat input
Assume rail and wheel are in such close contact that they are at the same temperature within the contact patch
Temperatures are equal, but energy flow will be determined by prior temperature of the surfaces as they enter contact
Initially when both are cold, energy flow is approximately 50% into each
Situation at the start of a wheel slide or spin
After a few seconds, system approaches steady state
For cold rail surface constantly meeting already hot wheel, in steady state most energy will flow into the rail (and vice versa)
For contact combining rolling and sliding, results are not so clear but work the same - equal temperatures of rail and wheel within contact patch, majority energy flow into cooler body in steady state
What is the Peclet number?
Ratio of speed of movement and size of heat source (contact patch) to thermal diffusivity of material
Static solution - P«1
Moving heat source - P > 5
Effect of heat input - rail with an existing defect
Modelled as a simple 2D longitudinal rail cross-section
Includes Hertzian pressure profile and uniform temperature
Deformed rail surface
Deformed geometry plots combine contact and thermal load
Compressive stress caused by thermal loading is relieved by thin heated layer rising up
Crack opening stress is generated
Crack opening effect is very similar to crack opening effect of fluids trapped in surface breaking inclined RCF cracks
Deformed crack surfaces
Combined contact and thermal load (1000C cases)
‘Double’ appearance of crack indicates it’s open
Shear deformation of crack walls relative to one another can be seen (just as would be expected for fluid filled cracks)
Thermal input has ‘unlocked’ the crack, just as fluid pressure or lubrication would
Micro-view of cracks and steel microstructure
Standard RCF crack propagation
Defined by fluids penetrating surface cracks and assisting growth
