Rail materials & Metallurgy

Question 1

What is the history of rails?

Answer 1

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

Wooden rails were in use in the UK in the 17th century

1789 cast metal wheels developed to run on cast rails

George Stevenson started to use wrought iron rails, formed in a 7 pass rolling process, developed 1820

By 1825 cast iron rails were giving problems – they fracture too easily

1856 – Bessemer steel making process with first commercial production in Sheffield

Used to create new rails around Derby station – they lasted 16 years with 250 trains/day

Over the following 20 years rail profiles developed, similar to those used today

Question 2

Draw the flat-bottom & bullhead rail profiles with all their parts

Answer 2

Rails consist of

1. The Head
2. The Web
3. The Foot

Question 3

What are the demands placed on rails?

Answer 3

1. Rail acts as a beam between the wooden or concrete sleepers
2. Support for the vertical wheel loads is essential – avoidance of excess deformation / bending
3. The rail also steers the vehicle around bends – ability to supply a side force is essential
4. Minimal rolling resistance to make movement easy, but, must also allow for driving and braking traction loads
5. Usually has to operate without lubrication to achieve the driving and braking tractions
6. Within the rail-wheel contact patch both rolling and some sliding contact take place

Question 4

Why both rolling and sliding?

Answer 4

The wheel will be turning marginally faster than pure rolling to achieve a driving (accelerating) force

Question 5

Combined rolling and sliding contact leads to…

Answer 5

…wear, and ‘rolling

contact fatigue’

Question 6

Does Impact Loading also occur on rails?

Answer 6

Yes

Question 7

How big is the contact patch between rail and wheel?

Answer 7

20p coin area

Question 8

The rail’s response to all the applied loads depends on:

Answer 8

1. Its chemical composition
2. the steel manufacturing process
3. hot forming (rolling) processes
4. any subsequent heat or mechanical processes

Question 9

What is the most widely used steel in rail?

Answer 9

The form of steel called pearlite has been found very successful for rails, and is the most widely used

Question 10

Why is pearlite the most commonly used rail material?

Answer 10

1. Good mechanical properties

2. Low cost

Question 11

What is classified as carbon level steel?

Answer 11

Carbon level is the steel is typically around 0.6% by weight

Question 12

What does pearlite consist of?

Answer 12

Pearlite is a layered structure of:

1. Ferrite - α-Fe (pure iron, dark in electron microscope image)
2. Cementite – Fe3C (iron carbide, light in e-microscope image)

Question 13

What are the steel manufacturing steps?

Answer 13

1. Liquid steel production by blast furnace & basic oxygen converter process, or by electric arc remelting
2. Secondary steel making–melt is further refined to reduce non-metallic inclusions (vacuum degassing) reduce excess carbon, and alloying elements are added
3. Continuous casting, typically 330x254mm blooms, slow cooled from 600C

Question 14

What is the rail manufacturing process?

Answer 14

1. Re-heat bloom
2. 7 pass rolling process (Tata)
3. Air-cooling at a controlled rate
4. Roller straightening– reverse plastic
   deformation, leaving a straight rail
5. Acceptance tests (per production run, not per rail)

Question 15

Why residual stress is actually good for rails?

Answer 15

The production process leaves residual stress in the rail

If the process is carefully controlled these can help suppress crack growth, i.e. suppress growth of cracks by compressing them

Question 16

When can crack growth be accelerated?

Answer 16

If cracks grow in a tensile residual stress area their growth will be accelerated

Question 17

What should residual compressive and tensile stresses do?

Answer 17

Tensile and compressive residual stress must balance one another – there’s no resultant stress in the rail

Question 18

How much carbon content do rails have?

Answer 18

Rails are typically 0.6% wt carbon

Question 19

Where does pearlite come from?

Answer 19

Pearlite is a Hypo-eutectoid composition steel, i.e. less than the eutectoid composition concentration of carbon.

The eutectoid composition is 0.83% weight carbon.

Rails are typically 0.6% carbon.

As the steel cools the structure becomes a mixture of ferrite (trace or no carbon), and cementite (6.67% carbon).

Question 20

What is pearlite?

Answer 20

1. A layered structure of ferrite and cementite (Fe3C)
2. On electron micrographs the ferrite is dark, the cementite light
3. The colours are reversed on optical light micrographs

Question 21

What are the important features of pearlite?

Answer 21

1. The spacing between the ferrite and cementite laths is the ‘interlamellar spacing’
2. Different grades of rail steel have different spacings
3. Usually a finer spacing gives better wear resistance

Question 22

What is the definition of interlamellar spacing?

Answer 22

The spacing between the ferrite and cementite laths is the ‘interlamellar spacing’

Question 23

What is the hardness of pearlitic steel?

Answer 23

Depending on grade, pearlitic steel

has a hardness in the range 220-400Hv

Question 24

What is the hardness of a rolling element bearing surface?

Answer 24

A rolling element bearing surface taking similar loads and contact pressures has a hardness of 700-800Hv

Question 25

What is the key to the success of pearlite?

Answer 25

The key to the success of pearlite is plastic deformation:

1. Strain hardening during the first cycles of load application
2. Changes to the structure due to deformation

Question 26

What are the types of loading that rail steel is subjected to?

Answer 26

1. Compression
2. Shear
3. Bending

Question 27

Where severe plastic flow takes place the pearlite…

Answer 27

…loses its clearly layered structure.

To understand what’s happening we need to look at the constituent phases within the pearlite:
• Ferrite – ductile, low wear resistance, 150Hv
• Cementite - brittle, high wear resistance, 772Hv

Yield points of the ferrite and cementite are quite different – while the ferrite can flow plastically the cementite breaks and cracks

Question 28

Why is the breaking and cracking of cementite a good thing?

Answer 28

Deformation of the softer ferrite matrix produces a reduction in the interlamellar spacing

Looking down on the rail surface, the effect is to increase the proportion of the surface area covered by the hard and wear resistant cementite lamellae

CEMENTITE IS WEAR RESISTANT UNLIKE FERRITE

Question 29

Why do we usually weld the rails instead of bolting them?

Answer 29

Some bolted joints are used, but most modern rail uses welded joints to give a continuous rail surface

1. Lower or no impact loading as wheel pass over
2. Lower maintenance (bolts need to be checked and greased)

Question 30

Are welds trouble free?

Answer 30

Nope.

Question 31

What are the limitations of welds?

Answer 31

1. They often have to be created on site – large or hi-tech equipment is not always feasible
2. The material at a weld is often a poor match to the surrounding rail
3. Surface wear differences between rail and weld lead to dips and impact loading
4. Weld breaks can account for around 25% of rail breaks (2006 data from Institute of Rail Welding)

Question 32

What process is most commonly used to repair rail breaks?

Answer 32

Alumino-thermic welding

Question 33

Describe the process of Alumino-thermic welding

Answer 33

1. Based on the generation of superheated liquid metal, on-site, using simple equipment
2. Driven by an exothermic reaction between iron oxide and aluminium powder
3. A mould is fitted around the rail ends, separated by a fixed distance, and it is all pre-heated
4. Powders are added in the proportions which will give steel matching the rail type
5. A fuse is lit and the liquid metal forms, and pours into the mould. It melts the ends of the rails and cools to a solid join
6. Excess metal is cut away

Question 34

What are the issues that arise with welds?

Answer 34

The weld is a casting, so defects tend to be casting related:

1. Hot tears (internal cracking caused by solidification shrinkage)
2. Porosity (internal voids)

A. These can be caused by use of incorrect powder mixtures for the rail grade
B. Incorrect pre-heat, failure of liquid metal to pour correctly
C. Such defects can be detected with ultrasonic testing in most cases

Question 35

Why do defects in welds occur?

Answer 35

1. caused by use of incorrect powder mixtures for the rail grade
2. Incorrect pre-heat, failure of liquid metal to pour correctly

Question 36

How can we detect the defects in welds?

Answer 36

Such defects can be detected with ultrasonic testing in most cases

Question 37

Why do even “good” welds can cause problems?

Answer 37

1. Heat from the welding process can actually change the steel microstructure in the HAZ (heat affected zone) in either side of the weld!
2. For harder rail grades, the HAZ is usually softer than the rail
3. This can lead to wearing of ‘dips’ either side of the weld

Question 38

What is the HAZ?

Answer 38

The Heat Affected Zone, referring to the affected zone on the rails caused by excess heat due to welding

Question 39

Do small surface profile variations have an impact?

Answer 39

1. Even very small surface profile variations affect ride comfort
2. They cause increased dynamic loads on rail and wheel
3. Increased load can lead to further wear, and also fatigue cracking
4. In many cases the dynamics of the vehicle suspension mean the peak rail load (and site of fatigue cracking) is 1-2 metres away from the weld