Steel & CostIron Making

7/30/2019 Steel & CostIron Making

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Steel


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Metal Alloys

Most engineering metallic materials are alloys.Elemental metals are generally very soft and not very

usable.

Metals are alloyed to enhance their properties, such as

strength,

hardness or

corrosion resistance,

and to create new properties, such as

superconductivity and

Engineering metal alloys can be broadly divided into

Ferrous alloys and

Non-ferrous alloys


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Metal

Non-ferrousFerrous

Carbon Low Alloy High Alloy

Cast ironsSteels

Low-C

Medium-C

High-C

Tool (Mo,V,W,Cr,Ni)

Stainless (Cr, Ni)

High-

strengthlow-alloy

Grey iron

Nodular iron

White ironMalleable iron

Alloy cast irons

Classes of Metals


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Steel

Structural framing

Roofing / Cladding

Interior products


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Steel-making

Since the mid-1800s, a number of processeshave been developed for refining pig iron into

steel

Today, the two most important processes are Bessemer converter

Basic oxygen furnace(BOF)

Electric furnace Both are used to produce carbon and alloy

steels


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Making Steel

(lowering the carbon and removing impurities)

In 1856, H. Bessemerpatented the converter for

purifying pig iron.

Hot air is forced through

the molten metal in a

pear-shaped vessel

Si, Mn, C and other

impurities are oxidized

and removed as slag.


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The Bessemer converter

Much smaller furnace

More impurities removed

(oxidised)

Calculated amount of carbon

added to make steel!, ifrequired

Poured into molds to form

ingots Entire cycle time (tap-to-tap time)

takes 25 to 30 min
http://upload.wikimedia.org/wikipedia/commons/7/7c/Bessemer_Converter_Sheffield.jpghttp://upload.wikimedia.org/wikipedia/commons/f/f1/Bessemer_Converter_(PSF).jpg


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Basic Oxygen Furnace (BOF)

Accounts for 70% of steel production in U.S Adaptation of the Bessemer converter

Bessemer process used airblown up through

the molten pig iron to burn off impurities

BOF uses pure oxygen

Typical BOF vessel is 5 m inside diameter &

can process 150 to 200 tons per heat

Entire cycle time (tap-to-tap time) takes 45 min


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Basic Oxygen Furnace


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Basic Oxygen Furnace - Stages

(1) Charging

(2) Pig iron(3) Blowing(4) Tapping(5) Pouring


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Electric Arc Furnace

Accounts for 30% of steel production in U.S.

Scrap iron and scrap steel are primary raw materials

Capacities commonly range between 10 and 100

tons per heat Complete melting requires about2 hr; tap-to-tap

time is 4 hr

Usually associated with production of alloy steels,tool steels, and stainless steels

Noted forbetter quality steel but higher cost per ton,

compared to BOF


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Electric Arc Furnace


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The whole spectrum of steel products!


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Alloy Designation


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Carbon Steels and Low Alloy Steels

Alloy Designation Alloy Designation AISI: American Iron and Steel Institute

SAE: Society of Automotive Engineers

ASTM: American Society for Testing and Materials

UNS: Unified Numbering System

AISI Grade X1X2X3X4

Older, butstill widely

used

Primaryalloying

elements

Carboncontent

10, 11, 12 plain C steel13 Mn steel

2x Ni steel, x=%Ni3x Ni-Cr Steel, x=%Ni+Cr4x Mo Steel, x=%Mo5x Cr steels, x=%Cr6x Cr-V Steels, x=%Cr+V7x W-Cr Steels, x=%W+C

9x Si-Mn Steels, x=%Si+Mn

X1X2

eg. 15 = 0.15%C

5195 =?

1040

Fe-0.4%C

2520Fe-5%Ni-0.2%C

Fe-1%Cr-0.95%C


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What is a steel and alloy of?

Iron (Fe) and Carbon (C)


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Plain Carbon Steels

An alloy of Fe & C whose properties depends only

upon the %age of Carbon present in it.


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Plain Carbon Steel vs. Alloy Steel

3 Classifications

Low Carbon Steel

Medium Carbon Steel

High Carbon Steel


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Plain Carbon Steels: General Properties

Yield strength: 300MPa (mild steels) - 700MPa (high C steels) Tensile strength: 400-1000 MPa

Ductility: EL% 15-30

Youngs modulus: 210 MPa.

Divided into low (


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Low Carbon Steel

Carbon < 0.3wt% Used wherever soft,

deformable materials

are needed

E.g., structural sections,rivets, nails, wire, pipe.


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Medium Carbon Steels

Carbon = 0.3 - 0.6wt%

Used where higher strength is

required

E.g., gears, shafts, axles, rods,

etc.


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High Carbon Steels

Carbon = 0.6 - 1.2wt%

used where high hardness

is required

Eg. hammers, chisels, drill,

springs.


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Mild steel panelsfor easy shaping

Medium-carbon steelchassis for strength and

toughness

high-carbon steelsprings


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Alloy Steel

Alloy steel may be defined as one whose

characteristics properties are due to some

elements other than Carbon. Although all

Plain-Carbon steels contain moderate

amounts of Mn & Si, but they are not

considered alloy steels because the principalfunction of Mn & Si is to act as de-oxidizer

during steel manufacturing process.


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Why alloying is necessary?


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Why alloying is necessary?

Many purposes, some of the most important are:-

increase hardenability,

reduce danger of warpage

improve strength & toughness at high & low temperatures,

resist grain growth at elevated temperature,

improve wear, corrosion, fatigue & creep resistance.

improve machine-ability,

improve magnetic properties


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Alloying Elements used in Steel

Nickel (Ni) (2xxx)

2% to 5%

Increases toughness

Increases impact resistance

12% to 20% with low amounts of C possessgreat corrosion / scaling resistance

universal grain refinerin alloy steels

unfortunately is a powerful graphitiser.


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Chromium (Cr) (5xxx)

Usually < 2%

increases hardenability and strength

5 % Cr steels used for making forging dies

typically used in combination with Ni and Mo

10.5% < Cr < 27% = stainless steel

used for corrosion resistance

Improves non-scaling properties

Causes grain growth

Reduces toughness


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Molybdenum (Mo) (4xxx)

Usually < 0.3%

increases hardenability and strength

Mo-carbides help increase creep resistance at

elevated temps improves the tensile strength & sp. heat resistance

has favourable influence on the welding properties.

Steel with higher contents tend to be difficult to forge

typical application is hot working tools


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Alloying Elements used in SteelManganese (Mn)

acts as de-oxidizerduring steel manufacturing

combines with sulfur (MnS) to prevent brittleness& improves machining

>1%

increases hardenability

improves strength, wear resistance of steel 11% to 14%

increases hardness

good ductility

high strain hardening capacity excellent wear resistance

Ideal forimpact resisting tools


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Vanadium (V)

Usually 0.03% to 0.25%

stabilities martensite and increases hardenability.

induces resistance to softening at high temperatures once

the steel is hardened

increases hot hardness properties in High Speed & Tool

steels by increasing cutting properties.

increases strengthwithout loss of ductility

Like Nickel it restrains grain growth


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Tungsten (W)

increases hot hardness

used as cutting tool steels


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Sulfur (S) (11xx)

Imparts brittleness

Improves machining Some free-machining steels contain

0.08% to 0.15% S


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Boron (B) (14xx)

for low carbon steels, can drastically increase

hardenability

improves machinablity and cold forming capacity

Aluminum (Al)

deoxidizer

0.95% to 1.30% produce Al-nitrides during nitriding


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Copper (Cu)

0.10% to 0.50%

increases corrosion resistance

Reduces surface quality and hot-working ability

used in low carbon sheet steel and structural

steels

Silicon (Si)

About 2%

increases strength without loss of ductility

enhances magnetic properties


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Alloy Steel

> 1.65% Mn, > 0.60% Si, or >0.60% Cu

Most common alloy elements:

Chromium, nickel, molybdenum, vanadium,

tungsten, cobalt, boron, and copper.


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High Strength Low Alloy Steels

Low alloy = alloying elements


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Tool Steels

A class of (usually) highly alloyed steels

designed foruse as industrial cutting tools,

dies, and molds

To perform in these applications, they must

possess

high strength, hardness, hot hardness, wear

resistance, and toughness under impact

Tool steels are heat treated


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AISI Classification of Tools Steels

T, M High-speed tool steels - cutting tools in machining

H Hot-working tool steels - hot-working dies for

forging, extrusion, and die-casting

D Cold-work tool steels - cold working dies for

sheet metal press-working, cold extrusion, andforging

W Water-hardening tool steels

S Shock-resistant tool steels - tools needing high

toughness, as in sheet metal punching and

bending

P Mold steels - molds for molding plastics and rubber

T l St l


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Tool Steels Carbon tool steels: 0.8~1.2%C

High alloy tool steels are oftenalloyed with Mo, V, W, Cr

and/or Ni

E.g., HSS, W-Cr-V (18-4-1)

Yield strength: 1000-1500+

MPa

Tensile strength: up to

2000MPa Ductility: EL% 5-15

Youngs modulus: 200 MPa

(alloying generally reduces

Youngs Modulus)

T l St l


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Tool Steels

Uses

Used where extremehardness is required.

Ductility/toughness

usually sacrificed Eg. Moulds and dies,

saws, cutting tools,

punches

Stainless Steel (SS)


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Stainless Steel (SS)Highly alloyed steels designed for corrosion resistance

Principal alloying element is chromium, usually greater than

11.5% Cr forms a thin impervious oxide film that protects surface

from corrosion

Nickel (Ni) is another alloying ingredient in certain SS to

increase corrosion protection

Carbon is used to strengthen and harden SS, but high C

content reduces corrosion protection since chromium carbide

forms to reduce available free Cr, therefore Carbon content is

kept very low - < 0.1% to avoid Cr3C2 formation


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Properties of Stainless Steels

In addition to corrosion resistance, stainlesssteels are noted for theircombination of

strength and ductility

While desirable in many applications, these

properties generally make SS difficult to

work in manufacturing

Significantly more expensive than plain C or

low alloy steels


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Types of Stainless Steel

Classified according to the predominant

phase present at ambient temperature:

1. Austenitic stainless - typical composition

18% Cr and 8% Ni

2. Ferritic stainless - about 11.5% to 27%

Cr, low C (0.25% max), and no Ni

3. Martensitic stainless - as much as 18%

Cr but no Ni, higher C content (0.15-0.75%) than ferritic stainless

Stainless Steels Typical Mechanical


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Stainless Steels - Typical Mechanical

Properties

Yield strength : 200-1600 MPa Tensile strength : 300-1800MPa

Ductility : EL% 2-20

Youngs modulus:~170 MPa (alloyingreduces Youngs Modulus)


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Cast irons Abraham Darbys Ironbridge

Ductile iron used

in drain grids


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Overview of cast iron Iron with 1.7 to 4.5% carbon and 0.5 to 3% silicon

Lower melting point and more fluid than steel (better

castability)

Low cost material usually produced by sand casting

A wide range ofproperties, depending on composition &

cooling rate

Strength

Hardness

Thermal conductivity

Damping capacity


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Composition of Cast Iron

A typical cast iron contains

1.7 to 4.5% carbon,

0.5 to 3.0% silicon,

less than 1.0% manganese, and

less than 0.2% sulfur.


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Production of cast iron Raw material

Pig iron,

scrap steel,

limestone and

carbon (coke) Cupola

Electric arc furnace

Electric induction furnace Usually sand cast, but can be gravity die

cast in reusable graphite moulds

finished by machining


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Cupola Melting of Gray Cast Iron


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Dependence of Types of cast iron

Various types of cast iron can be produced,

depending on the

Chemical composition,

Cooling rate, and

the type andamount of Inoculants used.


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Effect of cooling rate Slow cooling favours the formation ofgraphite &

low hardness

Rapid cooling promotes carbides with high

hardness

Thick sections cool slowly, while thin sections cool

quickly

Sand moulds cool slowly, but metal chills can be

used to increase cooling rate & promote white iron


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Types of cast iron Gray cast iron - carbon as graphite

White cast iron - carbides, often alloyed

Ductile cast iron

nodular, spheroidal graphite

Malleable cast iron

Graphite nodules are irregular clusters / tempered graphite

Compacted graphite cast iron CG or Vermicular Iron


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Microstructures of cast iron

Nodular iron

aFe and

graphite spheres

Gray iron

aFe and

graphite flakes

White iron

cementite and

pearlite

Malleable iron

aFe and

tempered

graphite flakes

low melting point, castable, cheap; however, can be brittle.


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Gray Cast Iron

R M t i l d f G C t I


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Raw Materials used for Gray Cast Irons

Iron Sources

Iron Scrap

Internal Returns

Machined Chip Briquettes

External Purchased Scrap Steel Scrap

Pig Iron

Coke

Graphite and Silicon Carbide Ferro-silicon and Ferro-manganese

P ti f G t i


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Excellent compressive strength (compressive

strength is typically 3-4 times tensile strength),

Excellent machinability (graphite acts to break up the

chips and lubricate contact surfaces), Excellent wear resistance (graphite flakes self-

lubricate), and

Outstanding sound and vibration-damping capacity(graphite flakes absorb transmitted energy).

Properties of Gray cast irons

Properties of Gra cast irons


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Good corrosion resistance and the enhanced fluidity

due to high silicon contents

Thermal conductivity high

The formation of the lower-density graphitereduces

the amount of shrinkage, making possible the

production of more complex iron castings

The pointed edges of the flakesact as preexisting

notches or crack initiation sites, giving the material a

characteristic brittle nature resulting low impact

resistance

Ductility is low (0.6%)



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The size, shape, and distributionof the graphite flakes

have a considerable effect on the overall properties ofgray cast iron.

For maximum strength, small, uniformly distributed

flakes are preferred.



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Applications of Gray cast irons

Engines Cylinder blocks, liners,

Transmission housing

Brake drums, clutch plates

Pressure pipe fittings, Machinery beds

Furnace parts, ingot and glass moulds


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Ductile or SG iron


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Ductile or SG iron

Also known as spheroidal graphite (SG),

and nodular graphite iron

Inoculation with Ce or Mg or bothcauses graphite to form as spherulites,

rather than flakes

Farbetter ductility than gray cast iron

P d ti f SG i


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Production of SG iron Composition similar to gray cast iron except

for higher purity

Melt is added to inoculant (Mg) in ladle.

Magnesium as wire, ingots or pellets is

added to ladle before adding hot iron

Mg vapour rises through melt, removing

sulphur.

P d ti f SG i


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Production of SG iron

Prior to solidification, graphite forms assmooth-surface spheres.

This addition is known as a nodulizing,

and the product becomes ductile or

nodular cast iron


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Properties of SG iron good ductility, high strength

Strength higher than gray cast iron

Ductility up to 6% as cast or 20% annealed

Low cost

Simple manufacturing process makes complex shapes

Machineability better than steel

Good toughness, wear resistance,

low-melting-point castability, up to a 10% weight reduction compared to steel

makes ductile iron an attractive engineering material


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Applications of SG iron

Automotive industry 55% of ductile iron in

USA

Crankshafts, front wheel spindle supports,steering knuckles, disc brake callipers

Pipe and pipe fittings (joined by welding)


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Malleable iron


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Malleable iron Produced by heat treatment ofwhite cast iron

Graphite nodules are irregular clusters

Similar properties to ductile iron

starting white iron structure restricts the sizeand thickness of malleable iron products such

that most weigh less than 5 kg

depending on the type of heat treatment,

various types of malleable iron can be

produced


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Properties of Malleable iron

Similar to ductile iron

Good shock resistance

Good ductility

Good machineability

corrosion resistance


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Applications of Malleable iron Similar applications to ductile iron

Malleable iron is better for thinner castings

Ductile iron better for thicker castings >40mm

Vehicle components

Power trains, frames, suspensions and wheels

Steering components, transmission and

differential parts, connecting rods

Railway components Pipe fittings

products such as door keys, gear wheel, and crank

levers


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White cast iron

White cast iron


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White cast iron White fracture surface

No graphite, because carbon forms Fe3C or more complexcarbides. Features promoting the formation of cementite

over graphite are:

low Carbon equivalent (1.8 to 3.6% carbon),

0.5 to 1.9% Silicon,

0.25 to 0.8% Manganese, &

rapid cooling

very hard, brittle & abrasion

resistant

Often alloyed


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Uses ofWhite cast iron

Products such as

gates,

fences,

parts of stove are manufactured by

using white cast iron.

In addition it is also used tomanufacture malleable cast iron

Fracto graphs of cast irons


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Fracto-graphs of cast irons


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Thanks


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Welding

Weldability of cast iron is low and

depends on

the material type,

thickness,

complexity of the casting, and

on whether machinability is

important


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Weldability

White cast iron - not weldable

Small attachments only

Grey cast iron - low weldability

Welding largely restricted to salvage and

repair

Ductile and malleable irons - goodweldability (inferior to structural

steel) Welding increasingly used during

manufacture

Steel & CostIron Making

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