Post on 17-Jan-2020
Manufacturing of Steel
Four Steel-Making Processes
1. The Bessemer Process
2. The Oxygen-Furnace Method
3. The Open-Hearth Process
4. The Electric-Furnace Process
Heat Treatments for Steel
• Normalizing consists of heating the steel to a
temperature of about 815ºC or higher,
depending on the type of metal and cooling
several hundred degrees slowly in air. This
process increases uniformity of structure.
• Annealing consists of heating the steel to a
temperature slightly lower than for normalizing
and cooling it several hundred degrees very
slowly, usually in a furnace. It is used to toughen
and reduce brittleness in steel.
Heat Treatments for Steel (Contd)
• Quenching consists of cooling steel very rapidlyin oil, water or brine from a temperature of about815ºC. Quenching increases hardness andstrength, but reduces ductility and toughness.
• Tempering consists of reheating the quenchedsteel to a temperature of 150 to 650ºC andcooling in air to reduce the residual stresses andincrease ductility. Heating to the lowertemperature range produces greater hardness,strength and wear resistance, while higher heatproduces greater toughness.
Forming Processes of Metals
• Hot working breaks up coarse grains andincreases density by closing tiny air holes andforcing the grains closer together.
• Cold working elongates grain in the direction ofthe steel elongation, increases strength andhardness, and decreases ductility. Cold workingresults in more accurately finished products,because there is no cooling shrinkage to beestimated.
Rolling
• Rolling consists of compressing and shaping
an ingot into a useful shape by squeezing it
through a succession of rollers. The piece
being rolled becomes longer and wider as it is
compressed. Variety of cross sections used in
construction can be rolled in long pieces by
means of specially shaped rollers.
Extrusion
• Extrusion consist of forcing a billet of hot, plastic
steel through a die of the desired shape to
produce a continuous length of material of
reduced cross-section in the shape of that die.
The resulting product has the shape of a rolled
product. Most intricate shapes can be formed by
extrusion than by rolling, and the surface is of
higher quality. An extrusion is made in one
operation rather than repetitive operations as in
the case of rolling.
Stainless Steel Extrusions
Steel Products
Structural Steel products consist of the following:
• Standard steel section
• Steel bars reinforcement
• Steel mesh reinforcement
• Alloy steel
• Sheet steel
Standard Steel Section
These consists of hot rolled steel sections,
shapes and plates not less than 3mm thick
necessary to hold a structural frame in
place. Each type of steel used must
conform to various specification as given
in Codes of Practice or Singapore
Standards or British Standards.
Steel Bars Reinforcement
• The steel often comes in standard lengths to suit
the sizes of the delivery vehicle or handling
methods, but the smaller rods can be delivered
in continuous rolls or coils under certain
circumstances. The required lengths are
normally stated on the bending schedule.
Steel Mesh Reinforcement
• Steel mesh reinforcement is manufactured
from plain round, indented or other
deformed rods.The term ‘wire’ is generally
used to denote hand-drawn mild steel and
comes in sizes 5, 6, 7, 8, 10 and 12mm
diameter.
Alloy Steel
• Alloy steel contains substantial quantities of alloy
elements to provide special qualities such as
high strength, corrosion resistance, or heat
resistance. In fact with 14% chromium, it is the
well known ‘Stainless Steel’.
• Alloy steels are usually designated by the
element or elements from which the alloy
derives its particular characteristics.
Sheet Steel
• Steel sheet is available in mild steel, highstrength steel and in stainless steel. The steel isavailable uncoated and with factory appliedcorrosion resistance and decorative pre-finishes,disperse with the need for costly and lesseffective shop and site treatment.
• Sheet pre-finished with paint, PVC coating andlaminates are produced in a wide range ofcolours and textures.
Mechanical Test for Steel
Mechanical tests for steel include:
– Tension
– Bending
– Hardness
– Impact
For structural steel, the tension or tensile test is
the most important.
Tensile Test
Protection of Steel
• Rusting is oxidation or combining of the iron withoxygen, which occurs in the presence ofmoisture.
• It proceeds more rapidly where there isnoticeable dampness, but it occurs in any airwith a relative humidity higher than 70 percent.The rust is formed from the solid metal, reducingits size so that the member becomes weakerand loses any decorative finish it might have.
Methods of Limiting Corrosion
Some general methods of preventing or
limiting corrosion are outlined below:
• Production of corrosion-resistant alloys
• Anodic metal coatings
• Chemical treatments
• Protective paintings
Protective Painting
The protection provided by the paint is due to:
• the presence of constituents calledinhibitive pigments which suppress theanodic and cathodic reactions.
• The slowing down of the movement ofions through the coat. Some cathodicprotection is provided by pigmentedpaints.
Painting of Steel
• Painting of steel prevents rust. However painting
is expensive and paint must be replaced
periodically at additional cost.
SPECIAL PURPOSE COATING SYSTEMS
• Intumescent Paint are examples of special
purpose coating systems. They can provide fire
ratings for exposed steel for up to three hours.
• Hot-Dip Galvanizing - There are several
reasons for selecting galvanizing as a coating
system. For light fabrication and some medium
structural applications, galvanizing can be the
lowest cost coating system.
Intumescent Paint
• Solve constraints between design and fire-
resistance.
• Smooth and pleasant feature in various colours
provide both fire-protection and decorative
functions.
Hot-Dip Galvanizing
• Hot-dip galvanizing is a special form of cathodic
protection in which sacrificial zinc is chemically
applied onto the bare steel to be protected.
Mill Certificate
• Name of Mill, Country
• Steel Standard, Grade
• Heat Number
• Tensile, elongation, chemical properties
• Other delivery conditions agreed with supplier
• Check material delivery against mill certificate
Delivery Condition
• Material
- standard, grade
• Dimension
- shape, length
• Tolerance
- straightness of beam
- flatness of plate
• Other delivery conditions
- witness testing
Material Testing
• Inspection and Testing ensures compliance withstandards and specifications
• Acceptance by lot or cast
• Test Unit
- same form
- same grade
- same quality
Material Storage
• Avoid ponding
• Avoid support settlement
• Avoid member distortion
• Avoid rain
• Avoid corrosion
• Allow inspection, stock-keeping, handling
• Observe safety, stability of stacking
Steelwork Specification
• National Productivity and Quality Specifications
(NPQS)
- common spec for building works
- performance based
• Chapters on Structural Steelwork
- C5-10 Structural Steelwork
- C5-20 Protective works for Structural Steelwork
• Project Specific Data (PSD)
Steel Fabrication
• From material take-off to final erection and
handover
• Includes material planning, cutting, fitting,
welding, protective coating and assembly
• Best done in covered workshop with proper
equipment and working environment
Steel Erection
• Simple lifting operation using mobile crane
• Heavy lifting operation of entire structure using
strand jack system
• Improper handling can cause additional
distortion to delivery condition
Steelwork Quality Assurance
• To ensure steelwork quality:
- employ qualified welder
- engage accredited fabricator
- appoint independent bodies
* accredited inspection body for inspection
* accredited laboratory for testing (NDT)
Importance of Weldability
• Generally all steel can be welded
• Different procedures will be required, if hydrogencracking is to avoided
- absoprtion of hydrogen into the weld causing
brittleness
• Can be avoided by using:
- clean joints
- baked electrodes
- slow cooling
Fire Resistance
• Reduction of strength and stiffness of steel
structure in elevated temperature leading to
possible failure and collapse
• Fire resistance design is required for steel
structure to maintain structural safety in fire
condition.
Fire Resistance Design
• Fire resistance design assesses the ability of a
steel structure to withstand the effect of fire and
provides designs to maintain structural safety.
• Three issues involved in fire protection include:
- life safety
- protection of the structure, and
- fire suppression
Reinforcing Steel
• Reinforced concrete is the combination ofconcrete and steel.
• Concrete is strong in compression but lessstrong in tension. Steel is equally strong in bothtension and compression.
• Hence steel is reinforced into concrete in orderto take up the tensile stress produced in astructure.
Reinforcing Bar
• Steel bars for reinforced concrete are termed
“reinforcing bar” and are commonly referred to
as “rebar”.
• There are only two grades of steel involved,
namely mild steel and high yield steel. The
term mild steel refers to low carbon steel having
nominal carbon and manganese contents.
Stress- Strain Relationship
STAINLESS STEEL
• Stainless steel which are known as high-alloy
steels, have chromium and nickel in their chief
alloying elements.
• They contain 16 – 28 percent chromium and
may contain up to 22 percent nickel.
• Stainless steel are available is rolled, extruded
and drawn forms.
Classification of Stainless Steel
Stainless steel is classified as:
1. Martensitic - contain about 13 per cent
chromium
2. Ferritic - contain usually about 17 per cent
chromium
3. Austenitic contain usually 16 – 19 percent
chromium together with 6 – 14 percent nickel.
Advantages of Stainless Steel
1. Stainless steel is compatible with non-metalsand the metal, or washing from it, will not stainadjacent materials.
2. Thermal movement is low and stainless steelis also resistant to high temperatures.
3. It is very hard and strong and has good, mirrorlike, appearance.
4. It is corrosion resistant.
Advantages of Stainless Steel (Contd)
4. In service, the surface of stainless steel does
not hold dirt readily, and light accumulations of
non-greasy dirt may often be washed off with
plain water.
5. Stainless steel does not have magnetic
properties.
6. They have good weldability.