Download - 5-Lecture 2b, October 1st, 2013
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Sherif A. Mourad
Professor of Steel Structures and Bridges
Faculty of Engineering,
Cairo University
STR654:
INSPECTION, MAINTENANCE
and REPAIR of STEEL
STRUCTURES
Lecture 2b, October 1st, 2013
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TOPICS
Basic Metallurgy.Welding Procedure.
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SOURCE & MANUFACTURING
Metals come from natural deposits ofore.
Ores are contaminated with
impurities. Impurities are removed by
mechanical or chemical processes.
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SOURCE & MANUFACTURING
Primary (or virgin) metal is extracted from
purified ore. Secondary metals are extracted from scrap.
Mining for metals is either open pit orunderground methods.
Selective mining works on small veins or beds
of high grade. Bulk mining works on large quantities of low
grade ore to extract a high grade portion.
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SOURCE & MANUFACTURING
There are two types of ores: Ferrous (iron)and nonferrous.
There is approximately 20 times the
tonnage of iron in the earths crustcompared to all other non-ferrous productscombined.
Some of the chemical processes that occurduring steel making are repeated during
welding operations.
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BLAST FURNACE IRON
Utilizes chemical reaction between a solid fuelcharge and the resulting column of gas.
Three different materials are used for the
charge:
Ore (mainly iron oxide).
Flux (limestonecalcium oxide + carbon dioxide) Coke (primarily carbon).
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BLAST FURNACE IRON
Coke reduces iron oxide to iron metal. Lime reacts with impurities and floats them to
the surface (slag).
Resulting iron (pig iron) is used as a startingpoint for further purification.
Elements such as carbon, silicon, phosphorous,sulfur and nitrogen are removed or reducedusing different types of furnaces (open hearth,electric, basic oxygen, ).
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CAST IRON INGOT
After passing through the refining furnace, themetal is poured into cast iron ingot molds.
The ingot is a rather large square column of
steel.At this point, the metal is saturated with
oxygen.
A substantial amount of oxygen must beremoved (deoxidation) using additives to tie upthe oxygen into gases or in slag.
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COMMON INGOTS
Rimmed Steel (least oxidation). Capped Steel (more uniform core).
Killed Steel (complete removing of oxygen). Semi-killed Steel (small amount of deoxidization
to kill any rimming action).
Vacuum Deoxidized Steel (removal of oxygenwithout producing nonmetallic incursion)
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CLASSIFICATION OF STEEL
Carbon Steel.
Low Alloy Steel.
High Alloy Steel.
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CARBON STEEL
Basically an alloy of iron and carbon Low Carbon (up to 0.15% carbon).
Mild Carbon (0.15 to 0.29% carbon).
Medium Carbon (0.3 to 0.59% carbon).
High Carbon (0.6 to 1.7% carbon).
Most of the production is low and mild,because of their relative strength and ease of
welding.
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LOW ALLOY STEEL
Having 1.5% to 5% total alloy content.Alloys are added to improve strength and
toughness, retard corrosion, and modify
response to heat treatment.
Alloy elements are manganese, silicon,
chromium, nickel, molybdenum & vanadium. Low alloy steels have higher tensile strength and
yield strength than carbon steel
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HIGH ALLOY STEEL
Expensive and specialized steels with alloy levels thatexceed 10%.
Because of the high levels of alloying elements, special
care and practices are required in welding. Austenitic manganese steel (high carbon & manganese levels)
has great toughness and hardens while undergoing cold work.
Stainless steel (high chromium and Nickel) has high resistanceto corrosion.
Tool steel ( chromium, tungsten, molybdenum & vanadium)is used in making tools, dies, punches, extruding dies, forging.
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STEEL SPECIFICATIONS
Egyptian Code 260/71.American sources: AISC, AISI, ASTM.
Most commonly used steel in structural works areA36-77 & A242-79.
Prefix A is for ferrous metals.
36 & 242 are just index numbers. 77 & 79 are the years the standard was originally
adopted.
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CRYSTALLINE STRUCTURE OF
METALS
On cooling, atoms assemble into a regular
crystal pattern (liquid solidifies orcrystallizes).
In a crystal, the atoms & molecules are
fixed and not free to move (crystallinelattice).
When temperature increases, thermalenergy is absorbed by the atom andmovement increases.
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CRYSTALLINE STRUCTURE OF
METALS
As distance between atoms increase, the
lattice breaks down and crystal melts. If the crystal contains one type of atom, it
melts at a single temperature.
If the crystal contains two or more types ofatoms, it starts to melt at one temperaturebut not completely molten until a highertemperature.
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GRAINS AND GRAIN
BOUNDARIES
As the metal cools into freezing point, a small
group of atoms begins to assemble intocrystalline form.
These crystals are scattered throughout thebody with no specific orientation.
As crystallization continues, crystals begin to
touch one another, stopping their free growth. Grain boundary defines the edge of crystals.
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GRAINS AND GRAIN
BOUNDARIES
Initial grain size is influenced by the rate
of cooling and temperature.
In a fillet weld, the initial crystal
formation takes place at the point wherethe molten metal meets the solid base
metal.As the metal continues to solidify, grains
in the center are smaller and finer.
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GRAINS AND GRAIN
BOUNDARIES
Grain size has an effect on the soundness
of the weld.
Smaller grains are stronger and more
ductile than larger grains. If cracks occur, the tendency is for it to
start in the area where the grains arelargest.
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HEAT TREATMENT
The temperature that the metal is heated, lengthof time it is held at that temperature, and the
rate that it is cooled have an effect on the
metals crystalline structure (microstructure).This microstructure determines the properties
of the metal. This microstructure can be
manipulated by heat treatment.
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PREHEAT Heat from welding disperses through the metal
and radiates to the atmosphere causingrelatively rapid cooling.
Preheating the weldament may slow the rate of
cooling of the metal. Preheat temp. is commonly 330 to 400oF
Thick weld metal will require preheat, as theheat is conducted away from the weld zonerapidly as the mass increases.
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STRESS RELIEVING
The metal closest to the weld is subject to thehighest temperature, which decreases as thedistance from the weld zone increases.
This nonuniform heat causes nonuniformexpansion and contraction.
These stresses may be relieved by uniformly
heating the structure after it has been welded. Metal is heated to a temperature just below the
point where microstructure changes.
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HARDENING
Hardness of steel may increase by heating it upto 50oF to 100oF above the temperature that a
microstructure change occurs, then placing the
metal in a liquid solution that rapidly cools.This rapid cooling (quenching) locks in place
microstructures that contribute to hardness.According to the speed they cool the metal, oil
is fast, water is faster and salt brine is fastest.
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TEMPERING
Tempering takes place usually after quenching. Metal is reheated and held for a length of time
to about 1335oF, then cooled at room
temperature.
Tempering reduces brittleness and produces a
balance between high strength and toughness.
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ANNEALING
A metal is heated up to 50o
F to 100o
F abovethe temperature that a microstructure change
occurs, then cooled at a very slow rate (usually
in a furnace).The main aim of annealing is to soften steel and
create a uniform fine grain structure.Welded parts are seldom annealed as high
temperatures may cause distortion.
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NORMALIZING
Normalizing is similar to annealing but with adifferent method of cooling.
Normalized steel is cooled in still air, rather
than in a furnace.
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HEAT TREATMENT SUMMARY
Various ways of controlling the heating andcooling of metals can improve certain
properties, but often at the expense of other
properties. Increasing strength or hardness may at the same
time reduce ductility and make the metal more
brittle.
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EFFECT OF ALLOYINGELEMENTS
Carbon: up to 1.7% - steel, above 1.7% cast iron.High carbon steel and cast iron require special
care for welding.
Sulfur: normally undesirable as it causesbrittleness and can create welding difficulties. It
may improve machinability of steel as it causes
machine chips to break rather than curl and clog
the machine.
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EFFECT OF ALLOYINGELEMENTS
Manganese: up to 1% is usually present.Deoxidizer and desulphurizer. It also increases
the tensile strength and hardenability.
Chromium: is a hardening alloying element, alsoincreases corrosion resistance and strength at
high temperatures.
Nickel: Improves ductility and toughness. Added
with chrome to form austenitic stainless steel.
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EFFECT OF ALLOYINGELEMENTS
Molybdenum increases the depth of hardeningcharacteristics of steel.
Silicon usually contained in steel as a deoxideizer.
It increases strength and reduce ductility. Phosphorous greatly reduces ductility and
toughness.
Aluminum is mainly used as a deoxideizer. Copper improves corrosion resistance. High
levels can cause welding difficulties.
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EFFECT OF ALLOYINGELEMENTS
Columbium used in austenitic steel as a stabilizer,reacting with carbon and leaving chromium.
Tungsten provides strength at high temperatures.
Vanadium keeps steel in fine-grain condition. Nitrogen is sometimes used to reduce the amount
of nickel in austenitic stainless steel.
Alloying elements may affect the allotropiccharacteristics or affect crystalline changes at hightemperature.
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ELECTRICITY FORWELDING
Electric source.
Power required (watts).A/C to D/C.