3.1 STEEL 3.1.1 Iron-carbon compounds 3.1.2 Microstructure of steels 3.1.3 Manufacturing and forming...

3.1 STEEL

• 3.1.1 Iron-carbon compounds

• 3.1.2 Microstructure of steels

• 3.1.3 Manufacturing and forming processes

• 3.1.4 Mechanical properties

• 3.1.5 Steels for different applications

• 3.1.6 Joints in steel

Definitions

•Iron… A chemical element (Fe)

•Ion… A charged particle, e.g. Cl- or Fe++

Iron is an element with the chemical symbol Fe. Steel and cast iron are described as "ferrous" metals and are made from iron with different carbon contents.

Carbon contents

Carbon Content Material

0.02% Wrought Iron - no longer

generally available in the UK

0.15% Low carbon steel

0.15-0.25% Mild and high yield steels

0.5-1.5% High carbon and tool steels

3-4% Cast irons

Ironbridge

3.1 STEEL

• 3.1.1 Iron-carbon compounds

• 3.1.2 Microstructure of steels

• 3.1.3 Manufacturing and forming processes

• 3.1.4 Mechanical properties

• 3.1.5 Steels for different applications

• 3.1.6 Joints in steel

MICROSTRUCTURAL EFFECTS ON STRENGTH

CARBON CONTENT

CONTROL OF GRAIN SIZE• Control by Heating• Control by Working• Control by Alloying

Face centred cubic and Body centred cubic

Volume change on heating steel

The nomenclature is: • FERRITE or Fe: This is the bcc iron which is

formed on slow cooling and may contain up to 0.08% Carbon. Soft, ductile and not particularly strong.

• CEMENTITE: This is iron carbide which contains about 6.67% Carbon.

• PEARLITE: This in the laminar mixture of ferrite and cementite and has an average carbon content of about 0.78%. Hard, brittle and strong 

• AUSTENITE or Fe: This is the fcc iron which is formed at high temperatures and may contain up to 2%C

Phase diagram for

steel (iron/carbon)

Strengths and carbon

contents of steels

High carbon content.

Low elongation

value.Low

impact resistance.

Brittle failure.

MICROSTRUCTURAL EFFECTS ON STRENGTH

CARBON CONTENT

CONTROL OF GRAIN SIZE

• Control by Heating

• Control by Working

• Control by Alloying

Movement of dislocation 1

Grain Boundary

Movement of dislocation 2

Grain Boundary

Movement of dislocation 3

Grain Boundary

Effect of ferrite grain size on the ductile/brittle transition temperature for mild steel

Pore fluid expression die after tensile failure. The inner core has fractured but the outer shell is a less

brittle steel so there was no explosive failure.

MICROSTRUCTURAL EFFECTS ON STRENGTH

CARBON CONTENT

CONTROL OF GRAIN SIZE

• Control by Heating

• Control by Working

• Control by Alloying

Cooling Steel from High Temperatures

• Slow Cooling (annealing) gives large grain size – ductile steel

• Cooling in air (normalising) gives smaller grains

• Rapid cooling in water (quenching) gives hard brittle steel

Part of iron/carbon phase diagram

Cooling Steel from High Temperatures

• Slow Cooling (annealing) gives large grain size – ductile steel

• Cooling in air (normalising) gives smaller grains

• Rapid cooling in water (quenching) gives hard brittle steel

Effect of carbon

content on hardness

for products of rapid cooling

(martensite and bainite)

MICROSTRUCTURAL EFFECTS ON STRENGTH

CARBON CONTENT

CONTROL OF GRAIN SIZE

• Control by Heating

• Control by Working

• Control by Alloying

Early cold worked steel

MICROSTRUCTURAL EFFECTS ON STRENGTH

CARBON CONTENT

CONTROL OF GRAIN SIZE

• Control by Heating

• Control by Working

• Control by Alloying

3.1 STEEL

• 3.1.1 Iron-carbon compounds

• 3.1.2 Microstructure of steels

• 3.1.3 Manufacturing and forming processes

• 3.1.4 Mechanical properties

• 3.1.5 Steels for different applications

• 3.1.6 Joints in steel

Rolling sequence for steel angle

Rolled steel sections

•RSC Rolled Steel Column•UB Universal Beam

•RSA Rolled Steel Angle

• RST Rolled steel T

•RHS Rolled Hollow Section

The RSJ• The UB has parallel flanges. A limited

number of traditional RSJs (Rolled Steel Joists) with tapered flanges are produced in smaller section sizes.

Web

Flange

RSJ Flange UB

3.1 STEEL

• 3.1.1 Iron-carbon compounds

• 3.1.2 Microstructure of steels

• 3.1.3 Manufacturing and forming processes

• 3.1.4 Mechanical properties

• 3.1.5 Steels for different applications

• 3.1.6 Joints in steel

Stress-Strain curves

Stress-Strain curve for steel

Yield

Elastic

0.2% proof stress

Stress

Strain0.2%

Plastic

Failure

Embrittlement at cold temperatures

S-N curves for fatigue

3.1 STEEL

• 3.1.1 Iron-carbon compounds

• 3.1.2 Microstructure of steels

• 3.1.3 Manufacturing and forming processes

• 3.1.4 Mechanical properties

• 3.1.5 Steels for different applications

• 3.1.6 Joints in steel

The properties required of structural steels are:

• Strength. This is traditionally specified as a characteristic value for the 0.2% proof stress

• Ductility to give impact resistance. Ductility increases with reducing carbon content.

• Weldability. (see below).

Steel frame (1)

Steel frame (2)

Steel frame (3)

Steel structure

Light weight steel

Steel Framed housing

HousingDetails

Steel in masonry structure

Steel Bridge

Reinforcing Steels

• Reinforcing steels are tested for strength and must also comply with the requirements of a "rebend" test to ensure that they retain their strength when bent to shape. This limits the carbon content. High yield bars are cold worked.

Bending Reinforcement

Prestressing steels

• Prestressing steels (high tensile steels) are not bent so they can have higher carbon contents that normal reinforcement and have higher strengths. This limits the ductility but is necessary to avoid loss of prestress due to creep

Prestressed slabs

Pre-stressing systems

3.1 STEEL

• 3.1.1 Iron-carbon compounds

• 3.1.2 Microstructure of steels

• 3.1.3 Manufacturing and forming processes

• 3.1.4 Mechanical properties

• 3.1.5 Steels for different applications

• 3.1.6 Joints in steel

The main methods of welding are:• Gas welding. In order to produce a hot enough

flame a combustible gas (e.g. acetylene) is burnt with oxygen. This method is not used for major welding jobs but has the advantage that the torch will also cut the metal.

• Arc welding. In this method a high electric current is passed from the electrode (the new metal for the weld) to the parent metal. The electrode is coated with a "flux" which helps the weld formation and prevents contact with air which would cause oxide and nitride formation.

• Inert gas shielded arc welding. This method uses a supply of inert gas (often argon) to keep the air off the weld so no flux is needed.

Gas and Arc welding

General points about welding.• Do not look directly at a welding process (especially

electric arc). It may damage your eyes.• Always allow for the effect of heating and

uncontrolled cooling of the parent metal. e.g. if high yield reinforcing bar is welded the effect of the cold working will be lost - and with it much of the strength. This heating will also often cause distortion.

• Check the welding rods. If they have become damp the flux will be damaged. Use the correct rods for the steel (e.g. stainless).

• Remember that the welding process cuts into the parent metal and, if done incorrectly, may cause substantial loss of section.

OTHER JOINTING SYSTEMS

• Bolted Joints

• Rivets

Rivets and bolts

Riveting the Empire State Building