• Steel Classification

• Production

•Properties

•Types of steel used in the construction industry

STEEL

INTRODUCTION

Steel and steel alloys are used widely in civil engineering

applications

Wrought iron still used on smaller scale for pipes and general

blacksmith work

Cast iron is used for pipes, hardware and machine part not

subject to tensile or dynamic loading

Steel usually have existing product from supplier

However, it is important to understand how steel is

manufactured and treated and how it responds to loads and

environmental conditions.

The Disney Concert Hall in Los Angeles. The

cladding is all stainless steel

Eiffel tower

Sydney Harbour Bridge

Steel Classification

Steel products that are used in civil engineering can be

classified as follows:

• Structural steel for use in plates, bars, pipes, structural shapes

etc.

• Reinforcing steel bar (rebars) for use in concrete

reinforcement

• Miscellaneous steel applications as forms and pans

Steel Manufacturing

3 basic phase of production

Reduction of iron ore to pig iron

Iron ore is mined from earth and melted in blast furnace to produce pig

iron

Pig iron : impure product, weak in tension

Refining pig iron to steel

Removal of excess carbon and other impurities in three types of furnace

namely;

Open heath furnace

Electric-arc furnace

Basic oxygen furnace

Forming the steel into products

Alloying elements are added to molten steel cast into large blocks or cast

continuously into desired shapes

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Basic forms of iron

Forms of iron % of carbon

Cast iron 2% – 5%

Steel Up to 1.5%

Wrought iron 0.05% to 0.15%

Dividing line of cast iron and steel is the presence of free graphite

If there is free graphite cast iron

Otherwise steel

Steel becomes harder and more brittle with higher carbon content

Steel and wrought iron can be distinguished by putting a drop of

nitric acid on the material grey stain on steel will be produced

due to higher carbon content

IRON-CARBON ALLOYS Different forms in which iron can be present

Forms of iron Description

Ferrite (α-iron) • Pure iron, soft, ductile and malleable

• Wrought iron is about 90% ferrite

Cementite • Iron carbide, very hard and brittle,

• White cast iron has a good amount of cementite

Pearlite • 0.85% carbon

• An aggregate made of alternate layers of small crystals of ferrite and

cementite

•Steel in this composition is the strongest because of the absence of large

crystals of ferrite and cementite

Austenite (γ-iron) • Solid solution of ferrite and cementite

•Non-magnetic and resist wear but is not brittle

Martensite • chief constituent of rapidly-cooled steel

•Strong, hard and brittle

• Imperfectly preserved austenite

Sorbite • Imperfectly resolved pearlite

OTHER FACTORS IN MAKING IRON

PRODUCTS

Effect of rate of cooling

Strong cooling coarse pearlite structure

Rapid cooling fine pearlite

Rapid cooling from high temperature little time for

austenite to transform to pearlite and new material called

martensite is formed

Martensite hard and brittle but can be modified by

subsequent heat treatment – used in the manufacturing of

Termo Mechanically Treated (TMT) bars

MECHANICAL WORKING (TREATMENT)

OF STEEL

One of the very desirable qualities of steel as different from

cast iron is that it can be:

hot worked into different shape

cold worked

heat treated

to give desirable properties

HOT WORKING OF STEEL

Steel ingots are heated to the required temperature and the

operations involved are the following:

Rolling

Carried in specially-prepared rolling milss

Red hot ingots are passed through different rollers to get desired shape

Forging

Pressing

Drawing

Metal is drawn through different dies and specially-shaped tools

This process to prepare the reinforcement rods

COLD WORKING STEEL If a steel bar is stressed beyond its yield point and unloaded, in the

next cycle of loading it will found that the yield point has been

raised. This is due to strain hardening of steel.

However, this higher yield point is lost and the steel is restored to

normal steel if we heat the bar to a high temperature after cold

working

Cold twisted deformed (CTD) bars for concrete reinforcement

are produced by this principle the cold working is carried by

twisting the bar beyond the yield point.

HEAT TREATMENT OF STEEL

Process by which the steel is heated and cooled under

controlled conditions to change the structural or physical

properties of the steel

Common heat treatment process are as follows:

Annealing

Case hardening

Quenching

Tempering

Normalizing

Annealing

Steel is heated to the required temperature which depends

on the percentage carbon content of steel.

Temperature is maintained for specified time and then,

allowed to cool slowly in a furnace

Purpose of annealing:

Relieving any strain produced in the steel during welding

Restoring, the grain structure to that minute size which gives

best quality

Softening the steel after hardening

Temperature used for true annealing is 700 – 1000 C

Case Hardening

Case hardening is one of the surface-hardening process.

Increase carbon content at the surface

Many industrial applications require steels with a hard,

wear-resistant surface, and a relatively soft, tough

inside.

Examples: gear, cam shaft, roller, etc

Type of steel used : low carbon steel

A carbonizing mixture (charcoal, cyanide) is used

Case Hardening Process

The article is held in the mixture and heated to 900 C –

950 C when the carbon diffuses into the surface of the

article

**For critical components as for use in lifting very heavy

weights, it is better to use special strength alloy steel than

increase hardness by case hardening. It is because it is

difficult to get reliable case hardening operations carried

out locally and any mishap can lead to very serious

consequences

Quenching for hardening

quenched steel - steel that has been hardened by immersing it

in water or oil to cool it

If steel is heated to high temperature and suddenly quenched

by water or dipping it in oil, then the steel becomes harder as

well as stronger and more brittle

This principle is applied in tile manufacturing of TMT steel

bars for reinforced concrete

Tempering

Hardened steel produced by quenching is too brittle to be

used without some tempering

Tempering is a process done subsequent to quench

hardening. Quench-hardened parts are often too brittle. This

brittleness is caused by a predominance of Martensite. This

brittleness is removed by tempering. Tempering results in a

desired combination of hardness, ductility, toughness,

strength, and structural stability. Tempering is not to be

confused with tempers on rolled stock-these tempers are an

indication of the degree of cold work performed.

Normalizing

Normalizing is a type of heat treatment applicable to ferrous

metals only. It differs from annealing in that the metal is

heated to a higher temperature and then removed from the

furnace for air cooling.

In this process, rate of cooling is faster than in annealing.

Hence, it gives finer pearlite structure

Steel is relatively soft and ductile but harder than the

annealed material.

TYPES OF STEEL

MILD STEEL AND OTHER STEELS

Many type of steel – e.g: mild steel (structural steel), tool

steel, machinery steel, high tensile steel and special steel

Differ in their carbon content, ultimate strength, yiled point,

% elongation at ultimate failure, hardness etc.

Distinguishing property of mild steel can be taken as the

nature of stress strain curve

Yield strength : 250N/mm2

Strain hardening strength : 1.6 x yield strength (340N/mm2)

Ultimate failure strength : 340N/mm2

% of elongation of mild steel at ultimate failure is as much as

30%

Thus, mild steel is very ductile compared to other types of

steel

Ductility is measures by the percentage of elongation at

failure

WEOUGHT IRON

Contains less that 0.15% carbon

Made from white pig iron by removing most of the carbon,

manganese, silicon, phosphorous and sulphur by the puddling

process in a reverberatory furnace

Steel nowadays replaced replaced wrought iron in common

use – equally strong in tension and compression

CAST IRON

3 – 5 times stronger in compression than in tension

Compression strength : 560 N/mm2

Tension strength : 560 N/mm2

Carbon lowers the melting point of iron, so, cast iron melts

at lower temperature than steel – easier to make castings of

cast iron even in small factories

Grey cast iron – extensively used for casting

Cast iron is still used widely in industry where the forces in

compression and the structural part does not have to

withstand shock, bend and tension

Cast iron – cheap – capable of manufacture with simple tools

in small factories

Cast iron is very resistant to corrosion and extensively used

in rainwater pipes, flush water sytem etc.

Malleable cast Iron

Used to made small parts – eg. Concrete sleeper fastening for

rails

Manufactures as small casting by the process called ‘

malleable cast iron process

Can be machined easily and gets deformed without rupture

Can endure many battering without cracking and has better

corrosion qualities

Ductile Iron

Less brittleness

Commonly used instead of malleable cast iron in railway

sleepers insert – easier to manufacture

Large articles also can be made from ductile iron

Traditionally, cast iron pipes used for carrying water and

sewage as they are more corrosion resistant compared to

steel pipes

Now, these pipes being replaced by ductile cast iron pipes –

high strength, ductility and corrosion resistance

Cast iron pipes – weak in tension – liable to be damaged

when there is settlement along its length

Ductile cast iron – can stand small settlements without

damage

Can be cast into any shape – used extensively instead of steel

for making items of complex machining operations

Ductile iron also high corrosion resistance like cast iron

Corrosion resistance of cast iron

Grey cast iron pipes buried in soil have been found to not been

corroded even after a period of 150years. In the corrosion of grey

iron at the surface, say of a buried pipe, the graphite present will be

left as a residue within the corrosion products which adheres very

firmly to the unattached metal substrata. This graphite containing

corrosion product provided a barrier against further corrosion attack

if it remain undisturbed as in buried pipes. Thus, the graphitic

corrosion residue in cast iron can limit the rate at which further

attack can occur. However, this residue is much less in strength than

the than the original cast iron. But, if left undisturbed, the pipe can

work for a very long time.

MARKET FORM OF STEEL

INTRODUCTION

Important market forms of steel used in building

construction:

Steel bars of many shape and grades or strength (For RC and

fabrication of grills, gates etc.)

High tensile steel for prestressed concrete works

Various shape of I, channel, angle, plates and other rolled

sections for structural fabrication

Stainless steel for special uses.

TYPES OF STEEL REINFORCEMENT

Steel rods for RC work should be of specified tensile strength

and they should develop good bond strength with concrete

Types : mild steel, torsteel, TMT bars

Different diameter used for RC works – only standard size

are used

Types of bar :

Hot rolled bar

Cold-twistwed deformed bar (CTD-bars torsteel)

Therm-mechanically treated reinforcement bars

TEST FOR STEEL

Tension test for reinforcement bars

Using testing machine to measure:

Yield strength

Ultimate tensile strength

ROLLED SECTIONS – STRUCTURAL

STEEL 2 main families of structural steel members: Conventional hot-rolled steel section Cold-formed steel section

Hot rolled steel section Used for fabrication of trusses, columns, beams etc of building Made by rolling hot steel into various shapes in specially-designed

rolling mills Famous available hot rolled steel sections Angle section

Channel section

I-section

T-section

Other rolled section

Cold-formed light-gauge (thin walled) steel sections

Structural member which is cold-formed to the desired

structural shapes for carbon or ally steel (strips or flats) by

press-brake operations

Thickness range : 0.38mm to 6.35mm

Made of alloy steel and cold formed higher strength than

hot-rolled sections

ROLLED SECTIONS – STRUCTURAL

STEEL

Advantage of cold formed sections over

hot rolled sections

Cold-formed sections are:

Thinner more length of material from same weight of steel more economical design for light loads and and moderate spans can be made from these sections

Effective shapes or configuration of steel sections can be produces by cold-formed operations more favorable strength-to-weight ratio can be achieved through these sections

Aesthetically pleasing sections like box section

Higher strength

Extensively used in fabrication of roof trusses

STAINLESS STEEL

General term given to certain alloys of iron, chromium and

nickel

High resistance to corrosion

Designated by the percentage of chromium and nickel

Eg. 18-8 stainless steel indicates 18% chromium and 8%

nickel