LECTURE NO. 19(Handout)STEEL AND ALUMINUM

Objectives:• To explain the engineering properties and usages of

steel and aluminum

STEEL: Introduction

• Steel is combination of iron, carbon (0.01-1%), manganese ( 1.6%), phosphorus, sulfur, and silicon (each 0.6%), together with about 20 other alloys.

• The alloys are added to molten steel to produce steel of different characteristics, such as hardness, tensile strength, and toughness

• In civil engineering construction, steel is mainly used as reinforcing bars in reinforced concrete structures and as beams, column, and truss & frame members in steel structures

STEEL PRODUCTION• Steel production includes following three phases:

– Production of pig iron– Refining pig iron into steel– Forming the steel into products

STEEL PRODUCTION: Production of pig iron

Raw materials include:• The iron with impurities (i.e., iron ore):

– Mostly iron oxides (Hematite, Fe2O3, and magnetite)

– Iron carbonates, silicates, and sulfides

• Coal

• Limestone to remove impurities, such as silica and alumina

STEEL PRODUCTION: Production of pig iron-----contd.

Pig iron is produced using a blast furnace in the following steps:

• The ore is crushed, soaked and the iron is magnetically extruded from the waste. The processed ore contains about 65% iron.

• It is heated in the presence of carbon in a blast furnace. Oxygen in the ore reacts with carbon to form gases.

• A flux (limestone) is used to help remove alumina and silica impurities from the ore.

• The molten iron, with an excess of carbon in the solution, collects at the bottom of the furnace (pig iron). The excess carbon and slag must be removed to produce high-quality steel.

• The impurities (slag) float on the top of the molten pig iron

STEEL PRODUCTION: Refining pig iron into steel

• Following three types of furnaces are used for refining pig iron into steel:

– Open Hearth Furnace has been used since the early 1900's

– Basic Oxygen Furnace is more efficient than open hearth, can refine 300 tons of steel in 25 minutes

– Electric Arc Furnace uses an electric arc between carbon electrodes to melt and refine steel. It requires high amount of energy• Primarily used to recycle scrap steel• Alloying elements and agents are added

STEEL PRODUCTION: Forming the steel into products

The molten steel from the furnace with additives is either: o Cast into ingots (i.e., large blocks of

steel) or o Cast continuously into semi-finished

products of varying widths, lengths, and thicknesses

The semi-finished products come in three types: o Blooms (square or rectangular in

cross-section with area 36 in2 or 232 cm2)

o Plates and slabs (flat and wide) o Billets (generally, square lengths of

steel 4-6 in or 10-15 cm in cross-section)

EFFECT OF CARBON CONTENT ON MECHANICAL PROPERTIES OF STEEL

The effects of carbon on mechanical properties of steel are shown in the following figure:

From the above figure, following may be noted:

•Tensile strength and yield point both increase with increase in carbon content up to about 1%.

•Percent elongation decreases with increase in carbon content indicating that steel with high carbon content is found to have low ductility and vice-versa.

HEAT TREATMENTS OF STEEL

The properties of steel may be improved by applying the following heat treatments: Annealing Annealing consists of heating steel to a defined temperature and then

cooling slowly It removes internal stresses, gases from steel It increases ductility of steel It changes electrical and magnetic properties of steel

Normalizing Normalizing consists of heating steel to a defined temperature

(higher than annealing temperature) then cooling by ambient temperature

It produces a uniform fine-grained microstructure to steel It is adopted to make plates with high toughness

HEAT TREATMENTS OF STEEL----contd.

Hardening Hardening consists of heating the steel in a similar manner as in

case of normalizing and then cooled very rapidly, called "quenching"

It hardens the steel

It locks the steel in permanent state of strain called residual strain

However, cooling/contracting faster can cause immediate cracking

MECHANICAL TESTING OF STEEL• Steel should be tested as fabricated because fabrication affects mechanical properties significantly

• Following are the various tests conducted on the samples of steel:– Tensile test– Torsion test– Charpy V-Notch impact test– Bending test– Hardness test

MECHANICAL TESTING OF STEEL:Tensile test

• Tensile test on steel samples is conducted basically to study the normal stress-strain behavior of steel

• A typical stress-stress diagram is shown below:

Using the stress-strain curve, the following important mechanical properties of steel are determined:

Yield point and yield strength Ultimate and fracture strengths Elongation and area reduction of

steel E and ν

MECHANICAL TESTING OF STEEL:Tensile test---contd.

MECHANICAL TESTING OF STEEL:Torsion, Charpy, Bending, and Hardness Tests

• Torsion test to determine the shear modulus (G) of structural materials used in design of members subjected to torsion, e.g. rotating shafts, helical compression springs

• Charpy V-Notch impact test to determine toughness

• Bending test to evaluate the ability of steel to resist cracking during bending (ductility)

• Hardness test to evaluate the ability of steel to resist localized plastic deformation (i.e. small dent or scratch)

USES OF STEEL

• There are two major applications of steel in civil engineering construction:

– Reinforcing steel (steel bars of different diameters)

– Structural steel (steel sections of different shapes and sizes used as columns, beams, bracings, frames, etc)

USES OF STEELReinforcing Steel

Forms:Reinforcing steel is manufactured in three forms:• Plain bars• Deformed bars (mostly used in almost all types of reinforced

concrete construction)• Plain and deformed wire fabric (used as nominal reinforcement

in slabs and pipes)

USES OF STEELReinforcing Steel----contd.

Grades:Deformed or plain steel bars are produced in following four principal grades: Grade 40 (300): σy = 40,000 psi (300 MPa) Grade 50 (350): σy = 50,000 psi (350 MPa) Grade 60 (400): σy = 60,000 psi (400 MPa) Grade 75 (500): σy = 75,000 psi (500 MPa)

USES OF STEELReinforcing Steel----contd.

USES OF STEELStructural Steel

Sectional shapes:Shapes of structural steel are designated by letters: W, HP, M, S, C, MC, and L

• W is a doubly symmetric wide flange with parallel flanges (most common as beams)

• HP is like the w shape but the flanges and webs have the same thickness and the depth and width are the same (used as bearing piles)

• S is a doubly symmetrical wide-flange with an inside flange slope of 16.67% (used as beams or girders)

• M shape is doubly symmetrical shapes which do not fit in W, HP, or S shapes

• C shape is channel with inside flange slope of 16.67% • MC shape is a channel that cannot be classified as a C shape

• L are angle shapes with equal or unequal legs

USES OF STEELStructural Steel-----contd.

Some common sections of structural steel are shown in the following figure:

USES OF STEELStructural Steel-----contd.

Thickness of flanges (tf) and webs (tw) of some commonly used sections of structural steel with their shape categories are indicated in the following figure :

USES OF STEELStructural Steel-----contd.

Grades of structural steel:• They are six grades produced in the US: A36, A529, A572, A242,

A588, and A514 • Carbon content ranges from 0.15% to 0.27%. All have a small

amount of copper and manganese alloys except A36, which only has copper

• A36 has the lowest tensile strength (250 MPa). It is most commonly used in steel buildings, bridges, and transmission towers because it does not experience brittle fracture at low temperatures

• Grades A242 and A588 are corrosion-resistant, high-strength, low-alloy structural steels. No protection is needed.

• A242 and A588 can be used un-coated in most atmospheres and when coated produce longer lives than any other grade. They have initial cost but low maintenance

• High-strength steels result in lighter sections

USES OF STEELStructural Steel-----contd.

USES OF STEELStructural Steel-----contd.

USES OF STEELStructural Steel-----contd.

USES OF STEELStructural Steel-----contd.

ALUMINUMIntroduction

• Most plentiful metal on Earth (8% of the Earth's crust), although primarily found as oxides

• 2-3% of electricity consumed in the U.S. is used in aluminum production.

• Recycled aluminum is used widely (25%)

• Pure aluminum is not suitable for structural applications; alloying elements are almost always added for producing structural aluminum

ALUMINUMUses

• 25% used for containers

• 20% used for building applications (doors, windows, siding)

• 10% used for electrical conductors

• Balance is used for industrial goods, consumer products, aircraft and vehicles

ALUMINUMAdvantages

• Light weight, has 1/3 density of steel

• Has a high strength to weight ratio

• Surface can be hardened by anodizing/hard coating

• Has good thermal and electrical conductivity

• Has alloys that are weldable

• Will not rust

• Has good formability: die cast, easily machined

• Is nonmagnetic, and nontoxic

ALUMINUMProduction

• Uses what is called "Bayer" process, developed in 1880's • Sodium-aluminate leaching process to produce alumina (Al2O3). The

electrolytic process (Hall and Heroult) is used to reduce alumina to pure aluminum

• Production starts with the mining of aluminum ore, bauxite. Bauxite is 45-60% alumina

• Bauxite is crushed, and washed to remove minerals and then dried to remove water

• Bauxite is mixed with soda ash and lime, passed through a digester, pressure reducer, and settling tank to produce a concentrated solution of sodium aluminate (few impurities)

• Solution is seeded with hydrated alumina crystals. • These attract other alumina crystals and settle out of solution • Crystals are washed to remove remaining impurities and then reduced

using the Hall-Heroult process. Molten aluminum (99% pure) is the result

• Aluminum is placed in furnace to further remove impurities and to add any alloy elements

• Aluminum is either shipped to a foundry for casting or casts into ingot• Powder metallurgy can be used to produce high strength aluminum. A

powdered aluminum is packed in a mold and heated until it is fused.

ALUMINUMMetallurgy

• Aluminum has a FCC lattice structure that is very malleable. Typical elongation is over 40%

• Pure aluminum (more than 99% aluminum) is typically used for non-structural applications such as electrical conductors and sheet metal

• Aluminum is typically very weak, although by adding alloys, it can become up to 15 times stronger

• Alloys used for Aluminum - Copper, Manganese, Silicon, Magnesium, and Zinc

• Cold working increases aluminum's strength

• Typical heat treatment for aluminum alloys include annealing, hardening, aging, and stabilizing

ALUMINUMAlloy designation (grading) system

• Wrought aluminum alloys are used for products fabricated by deforming to it's final shape. They are given a 4-digit code – The first digit is the alloy series – The second digit indicates a modification to the basic alloy if

different from 0 – The third and fourth digits identify the specific alloy used

• Cast aluminum alloys are formulated to flow directly into the mold. They are given a 3 digit code and 1 digit after a decimal point – The first digit is the alloy series – The second and third digits identify special alloys – The digit after the decimal point indicates if the alloy

composition is for the final casting

ALUMINUMTemper treatments

• Specification of an aluminum includes how it was tempered

• Aluminum alloys for structural applications are classified as either heat treatable or not

• Heat treatment does not influence the mechanical properties

• The properties of these alloys are dependent on the amount of cold working

ALUMINUMTesting and properties

• Tensile test is similar to that of steel – Stress strain curve is linear up to the proportional limit and smooth

curve up to the ultimate strength – No upper and lower yield point – Elastic modulus (69 GPa or 10,000 ksi). Not affected by alloying or

temper treatments – Yield strength (based on 0.2% offset)

• 28 - 255 MPa (Non-heat treatable)• 55 - 538 MPa (Heat treatable)• 69 - 276 MPa (Cast)

– Ultimate strength: 69 - 1799 MPa – Percent elongation: 5% - 43% (1/16 inch) – Fatigue endurance limit: 21-165 MPa

• Coefficient of thermal expansion is 0.0000023 / C (0.000013 / F), twice as large as steel and concrete

• Strength values are considerably affected by temperature: temperature above 150 C (300 F), tensile strengths are reduced considerably. Depends on alloy.

ALUMINUMFastening with bolts or rivets

Bolts and rivets can join aluminum members 1. Bolts

To prevent galvanic corrosion, they must be aluminum, aluminized steel, galvanized steel, cadmium plated steel, or stainless steel

2. Rivets Usually aluminum and cold-driven

Both bolted and riveted joints are designed based on the shear strength of the fasteners and the strength of the members around the fasteners