steel
DESCRIPTION
hTRANSCRIPT
• 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