5-lecture 2b, october 1st, 2013

8/13/2019 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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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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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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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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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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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.

5-lecture 2b, october 1st, 2013

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