Ferrous Alloys

Ferrous Alloys

출처 : The Science and Engineering of Materials, 4th edby Donald R Askeland Pradeep P Phulé

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by Donald R. Askeland – Pradeep P. Phulé

Designations and Classification of SteelsSimple Heat TreatmentsSimple Heat TreatmentsIsothermal Heat TreatmentsQuench and Temper Heat TreatmentsQ pEffect of Alloying ElementsApplication of HardenabilitySpecialty SteelsSurface TreatmentsWeldability of SteelStainless SteelsC ICast Irons

General Categories of Ferrous gMetals and Alloys

Carbon and alloy steelsStainless steelTool and Die steelCast IronsCast IronsCast Steels

**Ferrous tools first appear about 4000 to 3000 BC, ppmade from meteoritic iron. Real ironworking started in about 1100 BC in Asia Minor, and started the Iron Age.g

Production of Iron and Steel

Raw Materials for Production

Iron Ore

Limestone ----------

Coke

Raw Materials Pig IronRaw Materials Pig Iron

The three raw materials are dumped into a blast furnace.

Hot air (2000*F) is blasted into the furnace, which helps drive the chemical reaction The coke forms CO helps drive the chemical reaction. The coke forms CO and the CO reduces the iron oxide to iron.

The slag floats to the top and the metal is transferred to molds and cools. IT IS NOW PIG IRON, ready for more iron work or steelmaking more iron work or steelmaking.

Blast Furnace

Tuyeres

Iron Ore + Coke + Limestone + Air (1.93) (0.96) (0.48) (3.93)

(Same height as a 10 story building)

Pig Iron + Slag + Gases + Flue Dust(1.0) (0.55) (5.68) (0.09)

(a)In a blast furnace, iron ore is reduced using coke (carbon) and reduced using coke (carbon) and air to produce liquid pig iron. The high-carbon content in the pig iron is reduce by introducing oxygen y g yginto the basic oxygen furnace to produce liquid steel. An electric arc furnace can be used to produce liquid steel by melting scrap.

(b) Schematic of a blast furnace (b) Schematic of a blast furnace operation. (Source: www.steel.org. Used with permission of the American Iron permission of the American Iron and Steel Institute.)

Pig Iron SteelPig Iron Steel

To make steel you are simply removing more impurities, such as, manganese, silicon, carbon…, from the pig iron from the pig iron.

Impurities are removed by re-melting the metal and Impurities are removed by re melting the metal and adding carbon, steel scrap, and more limestone.

The metal can be melted using one of three methods.Open-Hearth furnaceElectric furnaceBasic Oxygen furnace. (BOF)

Open-Hearth Furnace

Uses a fuel to generate heat, and melt the metal.

Basic-Oxygen Furnace

Fastest steelmaking process – can make 250 tons of steel / hourMelted pig iron and scrap are poured Melted pig iron and scrap are poured (charged) into a vessel.Fluxing agents are added, like limestone.The molten metal is blasted with pure oxygen. This produces iron oxide which then reacts with carbon to produce CO and CO2 The slag floats to the top of and CO2. The slag floats to the top of the metal.Higher steel quality than open hearth. Used to make plate, sheet, I-beam, t bing nd h nneltubing and channel.

Electric FurnaceElectric Furnace

Uses electric arc from electrode to metal to heat and melt it.Can produce 60-90 tons of steel per day.Can produce 60 90 tons of steel per day.Steel is higher quality than open-hearth and BOF

Vacuum Furnace

Uses induction furnaces.Air is removed from the furnace, this removes the gaseous impurities from the molten metal. Produces very high-quality steel.

Killed – Semi-Killed – Rimmed SteelKilled – Semi-Killed – Rimmed Steel

Killed Steel – This is a fully deoxidized steel, and thus, has no porosity.

hi i li h d b i l likThis is accomplished by using elements like aluminum to de-oxidize the metal. The impurities rise and mix with the slag.gIt is called killed because when the metal is poured it has no bubbles, it is quiet.Because it is so solid, not porous, the ingot shrinks considerably when it cools, and a “pipe” or “shrinkage cavity” forms. This must be cut off and g yscrapped.

Killed – Semi-Killed – Rimmed Steel

Semi-Killed Steel: This is practically the same as killed steel, h d ffwith some minor differences. It is only partially de-oxidized, and therefore, is a little more porous than killed steel.Semi-Killed does not shrink as much as it cools, so the pipe is much smaller and scrap is reduced.It is much more economical and efficient to produce.p

Rimmed Steel: This is produced by adding elements like aluminum to the molten metal to remove unwanted gases aluminum to the molten metal to remove unwanted gases. The gasses then form blowholes around the rim.

Results in little or no piping.HOWEVER, impurities also tend to collect in the center of the ingot, so products or rimmed steel need to be inspected and tested.

Continuous Casting Continuous Casting

-Molten metal skips ingot step and goesingot step, and goes directly the furnace to a “tundish”

-Metal solidifies in the mold-The metal descends @ about 1”/sec-The solidified metal then goes through‘pinch rollers’ that determine the final form.

B fit f C ti C tiBenefits of Continuous Casting

Costs less to produce final productMetal has more uniform composition and properties

than ingot processing.

(a)In a blast furnace, iron ore is reduced using coke (carbon) and reduced using coke (carbon) and air to produce liquid pig iron. The high-carbon content in the pig iron is reduce by introducing oxygen y g yginto the basic oxygen furnace to produce liquid steel. An electric arc furnace can be used to produce liquid steel by melting scrap.

(b) Schematic of a blast furnace (b) Schematic of a blast furnace operation. (Source: www.steel.org. Used with permission of the American Iron permission of the American Iron and Steel Institute.)

Designations and Classification Designations and Classification of Steels

Designations - The AISI (American Iron and Steel Institute) and SAE (Society of Automotive Engineers) Institute) and SAE (Society of Automotive Engineers) provide designation systems for steels that use a four- or five-digit number.Cl ifi i S l b l ifi d b d h i Classifications - Steels can be classified based on their composition or the way they have been processed.

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Electron micrographs of (a) pearlite (b) bainite and (c) Electron micrographs of (a) pearlite, (b) bainite, and (c) tempered martensite, illustrating the differences in cementite size and shape among these three microconstituents (×7500). (From The Making, Shaping, and Treating of Steel, 7500). (From The Making, Shaping, and Treating of Steel, 10th Ed. Courtesy of the Association of Iron and Steel Engineers.)

Simple Heat Treatments

Process Annealing — Eliminating Cold Work: A low-g gtemperature heat treatment used to eliminate all or part of the effect of cold working in steels.Annealing and Normalizing Dispersion Strengthening: Annealing and Normalizing — Dispersion Strengthening: Annealing - A heat treatment used to produce a soft, coarse pearlite in steel by austenitizing, then furnace

li N li i A i l h t t t t bt i d cooling. Normalizing - A simple heat treatment obtained by austenitizing and air cooling to produce a fine pearlitic structure.Spheroidizing — Improving Machinability: Spheroidite - A microconstituent containing coarse spheroidal cementite particles in a matrix of ferrite, permitting excellent particles in a matrix of ferrite, permitting excellent machining characteristics in high-carbon steels.

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

Schematic summary of the simple heat treatments for (a) hypoeutectoid steels and (b) hypereutectoid steels(a) hypoeutectoid steels and (b) hypereutectoid steels.

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The microstructure of spheroidite, with Fe C particles dispersed in a ferrite matrix Fe3C particles dispersed in a ferrite matrix (× 850). (From ASM Handbook, Vol. 7, (1972), ASM International, Materials Park, OH 44073 )OH 44073.)

Isothermal Heat TreatmentsIsothermal Heat Treatments

Austempering - The isothermal heat treatment by which austenite transforms to bainite.Isothermal annealing - Heat treatment of a steel by austenitizing, cooling rapidly to a temperature between the A1 and the nose of the TTT curve, and holding until 1 , gthe austenite transforms to pearlite.


Th t i d i th l l h t t t t i The austempering and isothermal anneal heat treatments in a 1080 steel.

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The TTT diagrams for (a) a 1050 and (b) a 10110 steel.

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Dark feathers of bainite surrounded by light martensite, obtained by interrupting the isothermal transformation process (× 1500). (ASM Handbook, Vol. 9 Metallography and Microstructure (1985) )

Quench and Temper Heat TreatmentsQuench and Temper Heat Treatments

Retained austenite - Austenite that is unable to transform into martensite during quenching because of the volume expansion associated with the reactionthe volume expansion associated with the reaction.Tempered martensite - The microconstituent of ferrite and cementite formed when martensite is tempered.Quench cracks - Cracks that form at the surface of a steel during quenching due to tensile residual stresses that are produced because of the volume change that that are produced because of the volume change that accompanies the austenite-to-martensite transformation.Marquenching - Quenching austenite to a temperature just above the M and holding until the temperature is just above the MS and holding until the temperature is equalized throughout the steel before further cooling to produce martensite.

The effect of tempering p gtemperature on the mechanical properties of a 1050 steel.

Retained austenite (white) trapped between martensite needles (black) (× 1000). (From ASM Handbook, Vol. 8, (1973))

Increasing carbon reduces the Ms and Mftemperatures in plain-carbon steels.


Formation of quench cracks caused by residual stresses produced during o at o o que c c ac s caused by es dua st esses p oduced du gquenching. The figure illustrates the development of stresses as the austenite transforms to martensite during cooling.

The marquenching heat The marquenching heat treatment designed to reduce residual stresses ands quench crackingands quench cracking.

The CCT diagram (solid lines) for a 1080 steel compared with the TTT diagram (dashed lines).

The CCT diagram for a low-alloy, 0.2% C Steel.

Effect of Alloying ElementsEffect of Alloying Elements

Hardenability - Alloy steels have high hardenability.Effect on the Phase Stability - When alloying elements are added to steel the binary Fe Fe C stability is are added to steel, the binary Fe-Fe3C stability is affected and the phase diagram is altered.Shape of the TTT Diagram - Ausforming is a thermomechanical heat treatment in which austenite is plastically deformed below the A1 temperature, then permitted to transform to bainite or martensite.permitted to transform to bainite or martensite.Tempering - Alloying elements reduce the rate of tempering compared with that of a plain-carbon steel.

(a)TTT Curve(b) CCT curves for a 4340 steel.

The effect of 6% manganese on the stability ranges of the h i h id phases in the eutectoid

portion of the Fe-Fe3C phase diagram.

When alloying elements introduce a bay region into the TTT diagram, the steel can be ausformed.

The effect of alloying elements on the phases formed during the tempering of steels. The air-hardenable steel shows a secondary hardening peak. hardening peak.

Application of HardenabilityApplication of Hardenability

Jominy test - The test used to evaluate hardenability. An austenitized steel bar is quenched at one end only, thus producing a range of cooling rates along the barproducing a range of cooling rates along the bar.Hardenability curves - Graphs showing the effect of the cooling rate on the hardness of as-quenched steel.Jominy distance - The distance from the quenched end of a Jominy bar. The Jominy distance is related to the cooling rate.cooling rate.

The set-up for the Jominy test used for determining the hardenability of a steel.

The hardenability curves for several curves for several steels.

The Grossman chart used to determine the hardenability at the center of a steel bar for different quenchants.

The hardenability curves for several steels.

Specialty SteelsSpecialty Steels

Tool steels - A group of high-carbon steels that provide combinations of high hardness, toughness, or resistance to elevated temperaturesto elevated temperatures.Secondary hardening peak - Unusually high hardness in a steel tempered at a high temperature caused by the

f ll b dprecipitation of alloy carbides.Dual-phase steels - Special steels treated to produce martensite dispersed in a ferrite matrix.martensite dispersed in a ferrite matrix.Maraging steels - A special class of alloy steels that obtain high strengths by a combination of the martensitic and age hardening reactionsmartensitic and age-hardening reactions.

Microstructure of a dual-phase steel Microstructure of a dual-phase steel, showing islands of light martensite in a ferrite matrix (× 2500). (From G. Speich, ‘‘Physical Metallurgy of Dual-Phase Steels,’’ Fundamentals of Dual-Phase Steels, The Metallurgical Society of AIME, 1981.)

Surface Treatments

Selectively Heating the Surface Rapidly heat the Selectively Heating the Surface - Rapidly heat the surface of a medium-carbon steel above the A3temperature and then quench the steel.Case depth - The depth below the surface of a steel at which hardening occurs by surface hardening and carburizing processes.g pCarburizing - A group of surface-hardening techniques by which carbon diffuses into steel.C idi H d i th f f t l ith b Cyaniding - Hardening the surface of steel with carbon and nitrogen obtained from a bath of liquid cyanide solution.Carbonitriding - Hardening the surface of steel with carbon and nitrogen obtained from a special gas atmosphere.atmosphere.

(a) Surface hardening by localized heating. (b) Only the surface heats above the A1 temperature and is quenched to martensite.

Carburizing of a low-carbon steel to produce a high-carbon, wear-resistant surface.

Weldability of Steel

Th d l t f th The development of the heat-affected zone in a weld: (a) the structure at the (a) the structure at the maximum temperature, (b) the structure after cooling in a steel of low h d bilit d ( ) th hardenability, and (c) the structure after cooling in a steel of high hardenability.y

Stainless SteelsStainless Steels

Stainless steels - A group of ferrous alloys that contain at least 11% Cr, providing extraordinary corrosion resistanceresistance.Categories of stainless steels:• Ferritic Stainless Steels• Martensitic Stainless Steels• Austenitic Stainless Steels

P i i i H d i (PH) S i l S l• Precipitation-Hardening (PH) Stainless Steels• Duplex Stainless Steels

(a) The effect of 17% (a) The effect of 17% chromium on the iron-carbon phase diagram. At low-carbon contents, ferrite is stable at all temperatures. (b) A section of the iron-chromium-nickel-carbon phase diagram at a constant 18% Cr-8% Ni At constant 18% Cr-8% Ni. At low-carbon contents, austenite is stable at room temperatures.

(a) Martensitic stainless steel containing large primary carbides and small carbides formed during tempering (× 350) (b) Austenitic stainless steel (×carbides formed during tempering (× 350). (b) Austenitic stainless steel (×500). (From ASM Handbook, Vols. 7 and 8, (1972, 1973))

Cast IronsCast Irons

Cast iron - Ferrous alloys containing sufficient carbon so that the eutectic reaction occurs during solidification.Eutectic and Eutectoid reaction in Cast IronsEutectic and Eutectoid reaction in Cast IronsTypes of cast irons:• Gray cast ironGray cast iron• White cast iron• Malleable cast iron• Ductile or nodular, cast iron• Compacted graphite cast iron

Schematic drawings of the five types of cast iron: (a) gray iron, (b) white iron, (c) malleable iron, (d) ductile iron, and (e) compacted graphite iron.

The iron carbon phase diagram showing the relationship between the stable The iron-carbon phase diagram showing the relationship between the stable iron-graphite equilibria (solid lines) and the metastable iron-cementite reactions (dashed lines).

The transformation diagram for austenite in a cast iron.


( ) Sk t h d (b) h t i h f th fl k hit i (a) Sketch and (b) photomicrograph of the flake graphite in gray cast iron (x 100).

The effect of the cooling rate or casting size on the tensile properties of two gray cast irons.

The heat treatments for ferritic and pearlitic malleable irons.

(a) White cast iron prior to heat treatment (× 100). (b) Ferritic malleable iron with graphite nodules and small MnS inclusions in a ferrite matrix (× 200) (c) Pearlitic graphite nodules and small MnS inclusions in a ferrite matrix (× 200). (c) Pearlitic malleable iron drawn to produce a tempered martensite matrix (× 500). (Images (b) and (c) are from Metals Handbook, Vols. 7 and 8, (1972, 1973)) (d) Annealed ductile iron with a ferrite matrix (× 250). (e) As-cast ductile iron with a matrix of ferrite (white) and pearlite (× 250). (f) Normalized ductile iron with a pearlite matrix (× 250).

The CCT diagram for a low-alloy, 0.2% C steel.

The hardenability curves for several steels.

(b) A section of the iron-chromium-nickel-carbon chromium nickel carbon phase diagram at a constant 18% Cr-8% Ni. At low-carbon contents, austenite i bl is stable at room temperature.

Classification of Ferrous AlloysMetal Alloys

Ferrous Nonferrous

Steels Cast Irons

Low Alloy Gray iron

White iron

Malleable i

Ductile iron

Low-carbon Medium-carbon High-carbonHigh Alloy

iron iron ironiron

Low carbon Medium carbon High carbon

HighPlain

High strength, low alloy

Heat treatablePlain ToolPlain Stainless

Classification of Ferrous Alloys• Based on carbon content

– Pure iron (< 0.008wt% C)

From the phase diagram, it is composed almost exclusively of the ferriteexclusively of the ferrite phase at room temperature.

Steels (0.008 ~ 2.14wt% C)I t t l th i t t i t f b th d F CIn most steels the microstructure consists of both α and Fe3C phases.Carbon concentrations in commercial steels rarely exceed 1.0Carbon concentrations in commercial steels rarely exceed 1.0 wt%. Cast irons (2.14 ~ 6.70wt% C)Commercial cast irons normally contain less than 4.5wt% C

Ferrous Alloys — Steels

The carbon content is normally less than 1.0 wt%.

Plain carbon steels: containing only residual concentrations of impurities other than carbon and a little manganesemanganese

About 90% of all steel made is carbon steel.

Alloy steels: more alloying elements are intentionally added in specific concentrations.

Stainless steels

Classification of Steels A di t Th i C b C t tAccording to Their Carbon Contents

Low-carbon steels

L th 0 25 t%CLess than 0.25 wt%C

Medium-carbon steels

0.25 ~ 0.60 wt%C

High-carbon steels

0.60 ~ 1.4 wt%C

The Designation of SteelsA four-digit number:

th fi t t di it i di t th ll t tthe first two digits indicate the alloy content;

the last two, the carbon concentration

For plain carbon steels, the first two digits are 1 and 0;alloy steels are designated by other initial two-digit combinations (e.g., 13, 41, 43)

The third and fourth digits represent the weight percent carbon multiplied by 100

For example, a 1040 steel is a plain carbon steel containing 0.40 wt% C.

The Designation of Steels

A four-digit number: the first two digits indicate the alloy content; the last two, the carbon concentration; ,

4141 40 40

Identifies P tIdentifies major alloying element(s)

Percentage of carbon

( )

Table 11.2a AISI/SAE and UNS Designation Systems

• AISI: American Iron and Steel Institute • SAE: Society of Automotive Engineers• SAE: Society of Automotive Engineers • UNS: Uniform Numbering System

Steel AlloysSteel Numerical Name Key AlloysSteel Numerical Name Key Alloys

10XX, 11 XX Carbon only 13XX Manganese

23XX 25 XX Ni k l23XX, 25 XX Nickel31XX, 33XX, 303XX Nickel-Chromium

40XX Mo41XX Cr-Mo

43XX & 47XX Ni-Cr-Mo 44XX Mn-Mo44XX Mn-Mo48XX Ni-Mo

50XX, 51XX, 501XX, 521XX, Cr 61XX Cr-V

81XX, 86XX, 87XX, 88XX Ni-Cr-Mo 92XX Si-Mn

93XX, 98XX Ni-Cr-Mo 94XX Ni-Cr-Mo-Mn

XXBXX BoronXXBXX BoronXXLXX Lead

94XX Ni-

Less than 0 25 wt%CLow-Carbon Steels

Less than 0.25 wt%CUnresponsive to heat treatments intended to form martensite; strengthening is accomplished by cold workstrengthening is accomplished by cold workMicrostructures: ferrite and pearliteRelatively soft and weak, but having outstanding ductility y , g g yand toughnessTypically, σy = 275 MPa, σUT = 415~550 MPa, and ductility = 2 %E25%ELMachinable, weldable, and, of all steels, are the least expensive to produceexpensive to produceApplications: automobile body components, structural shapes, and sheets used in pipelines, buildings, bridges, etc. p p p g g

TTT Diagram of Some Hypoeutectoid Alloys

Table 11.1aCompositions of Five Plain Low-Carbon Steels

Table 11.1bMechanical Characteristics of Hot-Rolled Material and

Typical Applications for Various Plain Low-Carbon Steels

Medium-Carbon Steels0.25 ~ 0.60 wt%C

May be heat treated by austenitizing, quenching, and then tempering to improve their mechanical properties

Stronger than low-carbon steels and weaker than high-carbon steels

T i l T il P ti f Oil Q h d d T d Pl i C bTypical Tensile Properties for Oil-Quenched and Tempered Plain Carbon

a Classified as high-carbon steels

High-Carbon Steels0.60 ~ 1.4 wt%C

Used in a hardened and tempered conditionUsed in a hardened and tempered condition

Hardest, strongest, and yet least ductile; especially wear resistant and capable of holding a sharp cutting edgep g p g g

Containing Cr, V, W, and Mo; these alloying elements combine with carbon to form very hard and wear-resistant carbide compounds (e.g., Cr23C6, V4C3, and WC)

Applications: cutting tools and dies for forming and h i t i l k i h k bl d ishaping materials, knives, razors, hacksaw blades, springs,

and high-strength wire

Table 11.3 Designations, Compositions, g , p ,and Applications for Six Tool Steels

Comparison of the Advantages Off d b C b St l d All St l

Carbon Steel Alloy Steel

Offered by Carbon Steels and Alloy Steels

Lower cost Higher strength Greater availability Better wear y

Toughness

Special high temperature behavior

Better corrosion resistance Special electrical Special electrical properties

94XX Ni-

Alloy steel is more expensive than carbon steel; it shouldAlloy steel is more expensive than carbon steel; it should be used only when a special property is needed.

Table 11.2a AISI/SAE and UNS Designation Systemsg y

Stainless SteelsStainless steels are selected for their excellent resistance to corrosion.

Stainless steels are divided into three classes: martensitic, ferritic, or austenitic

The predominant alloying element is chromium; a concentration of at least 11 wt% Cr is required

It permits a thin, protective surface layer of chromium oxide to form when the steel is exposed to oxygen.

The chromium is what makes stainless steel stainless!stainless!

Ferrous Alloys

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