3 - Rail Materials and Metallurgy

Question 1

When were rails first introduced?

Answer 1

Ancient Greeks used grooved stone guides for vehicles (8th to 6th centuries BC)

Question 2

When were rails introduced in the UK?

Answer 2

Wooden rails in the 17th century
1789 cast metal wheels developed to run on cast rails
George Stevenson started to use wrought iron rails, formed in 7 pass rolling process in 1820
By 1825 cast iron rails were causing problems - fractured too quickly
1856 started Bessemer steel making process with first commercial production in Sheffield
Created new rails around Derby station - lasted 16 years with 250 trains per day
Over following 20 years rail profiles developed, similar to those of today

Question 3

Types of rail profiles

Answer 3

Flat-bottom
Bullhead

Question 4

Demands of rails

Answer 4

Support for vertical wheel loads (to avoid excess deformation/bending)
Steers vehicle around bends so must be able to support a side force
Minimal rolling resistance to make movement easy but must also allow for driving and braking traction loads
Usually has to operate without lubrication to achieve driving and braking tractions

Question 5

Why is there both rolling and sliding along a rail?

Answer 5

The wheel will be turning slightly faster than pure rolling to achieve a driving (accelerating) force - vice versa for braking

Question 6

What problems occur on rails due to combined rolling and sliding?

Answer 6

Wear
Rolling contact fatigue
Impact loading at joints, switches and welds

Question 7

Notable point about contact pressure at rail-wheel contact

Answer 7

Exceeds original yield point of steel
Contact patch ~ 20p coin area

Question 8

What does rail’s response to applied loads depend on?

Answer 8

Chemical composition
Steel manufacturing process
Hot forming (rolling) processes
Any subsequent heat or mechanical processes

Question 9

Typical carbon level in steel used

Answer 9

0.6% by weight

Question 10

What is pearlite?

Answer 10

Layered structure of ferrite (alpha-Fe - pure iron, dark in electron microscope image) and cementite (Fe3C - iron carbide, light in EM image)

Question 11

Rail production process

Answer 11

Reheat bloom
7 pass rolling process (British Steel)
Air cooling at controlled rate
Roller straightening - reverse plastic deformation, leaving a straight rail
Acceptance tests (per production run, not per rail)

Question 12

Points about the rail produced

Answer 12

Accurate rail geometry required - many different profiles exist
Production process leaves residual stress in rail
If process is carefully controlled these can help suppress crack growth (i.e. suppress growth of cracks by compressing them)
If cracks grow in tensile residual stress area their growth will be accelerated
Tensile and compressive residual stress must balance one another - no resultant stress in rail

Question 13

What is hypo-eutectoid composition steel?

Answer 13

Less than the eutectoid composition concentration of carbon (0.83% wt)

Question 14

How does the image of pearlite differ on electron micrographs compared to optical light micrographs?

Answer 14

Ferrite and cementite colours are reversed

Question 15

Define interlamellar spacing

Answer 15

Spacing between the ferrite and cementite laths
Usually a finer spacing gives better wear resistance

Question 16

Hardness of pearlitic steel

Answer 16

220-400Hv

Question 17

Rolling element bearing surface taking similar loads and contact pressures to a rail hardness

Answer 17

700-800Hv

Question 18

Key to success of pearlite

Answer 18

Plastic deformation
Strain hardening during first cycles of load application
Changes to structure due to deformation

Question 19

What to think about when considering mechanical properties of rail steel

Answer 19

Tension
Compression
Shear
Bending
(i.e. ordinary tensile data isn’t enough, doesn’t correspond to loading)
Hardness of material is good guide to its properties in compression and shear
Also useful to find shear yield strength k

Question 20

How strain hardening changes properties

Answer 20

Very large increase in yield stress
Initially softer rails harden more, almost reaching same yield values as harder grade
Hardness derived yield is closer to a compression test that tension

Question 21

Structural changes after plastic flow

Answer 21

Pearlite loses clearly layered structure

Question 22

Properties of ferrite

Answer 22

Ductile
Low wear resistance
150Hv

Question 23

Properties of cementite

Answer 23

Brittle
Higher wear resistance
772Hv

Question 24

Structural changes to components of pearlite after plastic flow

Answer 24

Ferrite can flow plastically
Cementite cracks and breaks
Ferrite deformation reduces interlamellar spacing
Increases proportion of surface area covered by hard and wear-resistant cementite lamellae

Question 25

Advantages of welds over bolted joints

Answer 25

Lower or no impact loading as wheels pass over
Lower maintenance (bolts need to be checked and greased)

Question 26

Disadvantages of welds

Answer 26

Often have to be created on-site (large or high-tech equipment not always feasible)
Material at weld is often poor match to surrounding rail
Surface wear differences between rail and weld lead to dips and impact loading
Weld breaks account for ~15% of rail breaks

Question 27

Alumino-thermic welding process

Answer 27

Commonly used to repair weld breaks
Driven by exothermic reaction between iron oxide and aluminium powder
Mould fitted around rail ends, separated by fixed distance and it is all pre-heated
Powders added in proportions to give steel matching the rail type
Fuse is lit, liquid metal forms and pours into mould
Melts ends of rails and cools to solid join
Excess metal is cut away

Question 28

Issues that arise with welds

Answer 28

Welds are castings, so defects tend to be casting related: hot tears (internal cracking caused by solidification shrinkage) or porosity (internal voids)
Can be caused by use of incorrect powder mixtures for rail grade
Incorrect pre-heat, failure of liquid metal to pour correctly
Such defects can be detected with ultrasonic testing in most cases

Question 29

Problems with ‘good’ welds

Answer 29

Heat from welding process changes rail steel microstructure in the heat affected zone (HAZ) either side of weld
For harder rail grades, HAZ is usually softer than rail
Can lead to wearing of ‘dips’ either side of weld
Even very small surface profile variations affect ride comfort
Cause increased dynamic loads on rail and wheel
Increased load can lead to further wear and also fatigue cracking
In many cases the dynamics of vehicle suspension mean the peak rail load and site of fatigue cracking is 1-2m away from weld

Question 30

Points about surface claddings

Answer 30

Laser cladding is additive manufacture of a wear and crack resistant layer on rails
Low cost for bulk of rail
Higher performance where needed
Avoid the substrate to surface interface becoming its own failure location due to peak sub-surface stresses
Currently planned for ‘high value’ track components, not whole rails
Reduces rail surface plastic flow and wear