Iron Carbon

7/28/2019 Iron Carbon

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Ferrous AlloysEutectoid Portion of Fe-C Diagram


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Low Carbon Steel

Greatest bulk tonnage of steel produced and consumed

Never heat-treated to attain a martensitic structure

Preferred material for welding applications

Grain size of low carbon steels can be controlled bycomposition or use of fine-grain practice Hot Working reduces coarse grain structure

Lower finishing temperature (around A1) results in a related lowertemperature or recrystallization, and therefore a finer grain size.

Normalized


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Porcelain Enameled Ware

Steel sheet are formed to desired strength

Coated with a siliceous frit and heated to thetemperature needed to melt the frit to a

glossy state

Two coats, a ground coat and a finish coatwith a pigment for color.


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Automobile Body Stock

Combination of low yield point and high tensile strengthis desirable

Need for a considerable degree of uniform elongation(between yield and tensile). A high work-hardening rateresults in delay of the localized necking whichprecedes fracture.

Ability to make a deep hemispherical indentation (3/4diam.) before fracture

Moderately fine grain size. Too fine impairs forming;

too coarse causes orange-peel defect Low yield-point elongation. High yield point elongation

leads to Luder bands of localized elongation


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Tin Plate

Ordinarily hot- and then cold-rolled to finished gage Process Annealed

Temper-rolled (slight reduction of 0.5 - 1.5% cold-rolling)

Electrolytically plated with tin


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Heavy Steel Plates for Ships

and Tanks Although moderate in yield and tensile strength, does

have proper combination of strength, ductility,toughness, and weldability

Carbon content rarely above 0.25% (reduces toughnessand weldability) and rarely below 0.15%

Manganese is added to increase yield and tensilestrength without reducing ductility

Copper sometimes added to improve corrosion

resistance Given no heat treatment after hot rolling and thus

develop mechanical properties as a result of control ofcomposition and grain size


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Structural Shapes and Pipes

Similar composition for plate products

Microalloyed Steels - small additions of alloyelement (Nb, V, and Ti), good weldability and

high strength


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Low-Alloy High Strength Steel

Appreciable alloy content, up to 10%, and are oftenquenched and tempered to give high levels of bothstrength and impact toughness

Alloy-element selection is based on promoting theformation of martensite or bainite on quenching over arange of section thicknesses with good toughnessdeveloped by tempering at relatively high temperatures

Cr and Mo additions improve high temperature creep andcorrosion resistance (i.e. boilers)

Ni additions improves cryogenic temperature toughness.(Pressure vessels for transportation of liquidifiedpropane)

Because of alloying, weldability decreases


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Welding


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Welding Mild SteelMetallurgical Considerations

The weld material should be low in gas contentand low in oxides or carbon to avoid gas liberationduring weld pool solidification

Weld metal will solidify very rapidly and willtherefore be very fine grain

Metal adjacent to the liquid will be heated into theaustenitic state and usually will be cooled veryrapidly by adjacent cold metal.

Quenching effect on austenitized metal will resultin brittle martensite unless carbon content is low

or the hardenability is low Zone adjacent to austenitized metal which has

been heated to just below A1 temperature. Forinitially cold-rolled steel, this will be annealed andlocally softened zone subject to strain aging and

(if the steel is hardened) tempering


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Potential Weld Defects

Hot Cracking - caused by too high sulfurand/or carbon content or inappropriate alloy-content level.

Cold Cracking - occurs below 300oC onlywhen (1) hydrogen gas, (2) restrain, and (3) ahard martensite microstructure is present

The higher the strength and the alloy (or carbon) contentof a steel, the more likely it is to cold crack on welding


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Carburizing of Low-Carbon Steels

If a hard steel surface is desired, a high-carbonsteel can be used or a much cheaper low-carboncan be heated in a carbonaceous atmosphere toincrease the carbon content of the surface

Factors Affecting Case Depth Temperature and Time: Diffusion controlled mechanism

Carbon Content: Lower carbon content produces higherconcentration gradients, therefore, greater ability to caseharden

Carburized parts are practically always quenched

Other process include Nitriding, Oxide Coating,and Titanium Nitride


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Surface Treatments


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Medium Carbon Steels

Often called engineering alloy steels fractional additions of alloying elements to improved

steels material properties

Widely used for machine parts and highstrength structural component applications

Generally heated treated in three distinctoperations

convert to austenite

quench to form martensite

temper quenched steel to a desired property


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Hardenability

Defined as the depth of useful hardness which can beproduced for a given quenching condition

Does NOT relate to the degree of hardness produced

If hardenability is large relatively large diameter steel will be fully martensitic

steel is termed deep-hardening


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Continuous CoolingTransformation Diagram


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Continuous Cooling

Transformation Diagram


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Jominy Hardenability Test

Heat sample in g region Remove sample, place in jig

Immediately quench one end

with stream of water at aspecific temperature and flowrate.

Effectively, one end is water

quenched and the other endnormalized


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Jominy Hardenability Test

Flat is ground along the side of the cylinder andhardness measurements are taken along its length


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Critical Diameter

quenching a series of long round bars of increasingdiameter in the quench medium of interest.

Bars sectioned transversely in the center

Hardness profile obtained across the diameter

Bar with the center hardness just corresponding tothe critical level of hardening (50% martensite - 50%pearlite) is found.

Critical-diameter Do bar for the steel in that quench


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Ideal Critical Diameter

Hardness profiles can also be developed tocharacterize the quenchant, H value.

1 to 5 for brine

0.8 to 2 for water

0.1 to 0.8 for oil

0.01 to 0.05 for air

Can then find ideal critical diameter

More useful, can calculate actual criticaldiameter based on charts for alloyingelements and H value

Higher Di, better hardenability


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HardenabilityAlloying Elements


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1040: 0.5% Mn, 0.2% Si

2340: 3.0% Ni

4140: 0.80% Mn, 0.8% Cr, 0.25% Mo

4340: 0.8% Mn, 1.7% Ni, 0.8% Cr, 0.3% Mo


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Hardenable Carbon Steels

Examples At times a martensitic surface may be desired, for

wear resistance, with a fine ferritic-pearlitic corestructure for toughness

Can heat complete part followed by quenching

Can also be obtained by heating only surfacelayers of a thicker section by using high-frequency induction or intense flame

Not only can the surface alone be heated forhardening; it is possible to heat just a part of anyassembly (bearing section of crankshaft)

Combination of quenching mediums; waterfollowed by oil (wrenches, pliers, etc.)


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Hardenable Carbon Steels

Examples Maximum martensitic hardness determined

by carbon content

To assure this, normalize to obtain a fine

initial carbide structure After quenching of a carbon steel, the

structure must be tempered

High thermal stresses developed, ultimate

hardness generally not required for mostservice parts


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Tempering Medium Carbon Steels

1340: 1.7% Mn

2340: 3.0% Ni

5140: 1.0% Cr

4340: 1.7% Ni, 0.8% Cr, 0.3% Mo

Iron Carbon

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