Material science chapter 9
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What are the two major groups of alloys?

  • Ferrous alloys and nonferrous alloys (correct)
  • Carbon alloys and non-carbon alloys
  • Ferrous alloys and precious alloys
  • Metallic alloys and nonmetallic alloys

    • All metals used in engineering applications are pure metals.

    False

    What is the primary ingredient in ferrous alloys?

    iron

    Pig iron is produced by mixing iron ore with ______ in a blast furnace.

    <p>coke</p> Signup and view all the answers

    Which of the following metals is considered a nonferrous alloy?

    <p>Copper</p> Signup and view all the answers

    The basic oxygen process uses pig iron to make steel.

    <p>True</p> Signup and view all the answers

    The reaction that occurs when carbon reacts with iron oxide produces ______.

    <p>carbon monoxide</p> Signup and view all the answers

    Match the following terms with their descriptions:

    <p>Ferrous alloys = Alloys containing iron as the main ingredient Nonferrous alloys = Alloys containing little or no iron Pig iron = Intermediate product used to make steel Basic oxygen process = Method to convert pig iron into steel</p> Signup and view all the answers

    What happens to an alloy as the aging time increases?

    <p>It becomes stronger and harder.</p> Signup and view all the answers

    Overaging of an alloy increases its strength and hardness.

    <p>False</p> Signup and view all the answers

    What is the equilibrium phase formed in an Al-4% Cu alloy?

    <p>θ Phase (CuAl2)</p> Signup and view all the answers

    The _____ structure is formed at aging temperatures between 130°C and 190°C in the Al-4% Cu alloy.

    <p>θ’ (theta prime)</p> Signup and view all the answers

    Match the zones or phases with their characteristics:

    <p>GP1 Zone = Copper atom segregated in the supersaturated solid solution GP2 Zone = Tetragonal structure, 10-100 nm diameter θ’ Phase = Nucleates heterogeneously on dislocation θ Phase = Equilibrium phase, incoherent (CuAl2)</p> Signup and view all the answers

    At which temperature does the hardness of an Al-4% Cu alloy reach its maximum?

    <p>130°C</p> Signup and view all the answers

    What type of steel is investigated in isothermal decomposition experiments?

    <p>Eutectoid steel</p> Signup and view all the answers

    What phase is formed when steel is cooled very fast after austenitizing?

    <p>Martensite</p> Signup and view all the answers

    Martensite has a higher hardness than pearlite with the same carbon content.

    <p>True</p> Signup and view all the answers

    The process of isothermal decomposition occurs only at temperatures above the eutectoid temperature.

    <p>False</p> Signup and view all the answers

    GP2 zones decrease hardness by stopping dislocation movement.

    <p>False</p> Signup and view all the answers

    What is the maximum carbon content that results in lath martensite?

    <p>0.6%</p> Signup and view all the answers

    What microstructure forms after 5.8 minutes at 705°C during the isothermal decomposition of austenite?

    <p>Coarse pearlite</p> Signup and view all the answers

    What is the approximate alloy strength of aluminum?

    <p>690 MPa</p> Signup and view all the answers

    Martensite is a ____ phase consisting of a supersaturated interstitial solid solution of carbon in BCC or BCC tetragonal iron.

    <p>metastable</p> Signup and view all the answers

    The transformation of austenite to pearlite is completed after _____ minutes.

    <p>66.7</p> Signup and view all the answers

    Match the types of martensite with their carbon content:

    <p>Lath martensite = Less than 0.6% C Plate martensite = Greater than 1.0% C Mixed martensite = Between 0.6% and 1.0% C</p> Signup and view all the answers

    Which of the following microstructures can be produced by hot quenching to different temperatures?

    <p>Coarse pearlite</p> Signup and view all the answers

    What process involves heating steel alloys to the γ region?

    <p>Austenitizing</p> Signup and view all the answers

    Match the following microstructures with their characteristics:

    <p>Coarse pearlite = Forms at lower cooling rates Fine pearlite = Forms at higher cooling rates Bainite = Intermediate cooling rates Martensite = Very rapid cooling rates</p> Signup and view all the answers

    Increasing the carbon content enhances the strength of materials.

    <p>True</p> Signup and view all the answers

    The transformation from austenite to martensite involves diffusion.

    <p>False</p> Signup and view all the answers

    What is a key feature of martensite regarding carbon atoms after transformation?

    <p>No change of relative position</p> Signup and view all the answers

    What is the purpose of conducting isothermal decomposition experiments?

    <p>To investigate microstructural changes in eutectoid steel.</p> Signup and view all the answers

    What is the maximum solubility of carbon in the Ferrite phase (α) at 727°C?

    <p>0.02%</p> Signup and view all the answers

    A steel alloy with more than 0.8% C is known as hypoeutectoid steel.

    <p>False</p> Signup and view all the answers

    What is the intermetallic compound formed at 6.67% carbon?

    <p>Cementite</p> Signup and view all the answers

    A eutectoid plain carbon steel contains ____% carbon.

    <p>0.77</p> Signup and view all the answers

    Match the following phases with their carbon solubility characteristics:

    <p>Ferrite phase (α) = Max 0.02% at 727°C Austenite phase (γ) = Solubility of 2.08% at 1147°C Cementite = 6.67% C and 93.3% Fe Ferrite phase (δ) = exists above 1394°C</p> Signup and view all the answers

    In the eutectoid reaction, which of the following products are formed?

    <p>α Ferrite and Fe3C</p> Signup and view all the answers

    The hypoeutectoid steel contains less than 0.8% carbon.

    <p>True</p> Signup and view all the answers

    At what temperature does the eutectic reaction occur in the iron-carbon system?

    <p>1147°C</p> Signup and view all the answers

    What is the process called when eutectoid steel is heated slightly above 727°C?

    <p>Austenitizing</p> Signup and view all the answers

    Pearlite consists of proeutectoid ferrite and cementite.

    <p>False</p> Signup and view all the answers

    What is the carbon content range for hypoeutectoid plain carbon steel?

    <p>greater than 0.02% and less than 0.77%</p> Signup and view all the answers

    If a 0.4% C steel alloy is austenitized and then slowly cooled, it will form proeutectoid ______.

    <p>alpha</p> Signup and view all the answers

    What is formed when a sample of 1.2% C steel alloy is cooled below the eutectoid temperature?

    <p>Both B and C</p> Signup and view all the answers

    What constitutes a lamellar microstructure in steel?

    <p>Alpha ferrite and cementite</p> Signup and view all the answers

    A steel alloy with more than 0.77% carbon is classified as ______.

    <p>hypereutectoid</p> Signup and view all the answers

    Study Notes

    Engineering Alloys

    • Pure or nearly pure metals are not useful in most engineering applications (except for electrical applications).
    • In most applications, especially structural, metal alloys are used.
    • Alloys are made of combinations of metals and sometimes nonmetals.
    • Alloys are classified into two major groups: ferrous and nonferrous alloys.
    • Ferrous alloys contain iron as the main ingredient (90% of the world's production of all metals).
    • Nonferrous alloys contain little or no iron.
    • All steels are ferrous alloys.
    • Titanium and aluminum alloys are examples of nonferrous alloys.

    Metal Cost

    • Metals are selected for applications based on properties and relative cost.
    • A table shows approximate prices of some metals ($/lb) as of May 2001.

    Production of Iron and Steel

    • Iron ore (iron oxide) is mixed with coke inside a blast furnace to produce pig iron.
    • Fe₂O₃ + 3CO → 2Fe + 3CO₂
    • Pig iron is used to make steel through the basic oxygen process.

    Steel Making - Oxygen Process

    • Pig iron and 30% steel scrap are fed into a refractory furnace to which an oxygen lance is inserted.
    • Oxygen reacts with the liquid bath to form iron oxide.
    • Next carbon reacts with iron oxide to form carbon monoxide (FeO + C → Fe + CO).
    • Slag-forming fluxes are added.
    • Carbon content and other impurities are lowered/controlled.
    • Molten steel is continuously cast, cast into ingots, or formed directly into useful shapes.
    • Half of the raw steel is produced by recycling old steel (junk cars or old appliances).

    The Iron-Carbon Binary Alloy System

    • Plain carbon steel contains 0.03% to 1.2% C; 0.25 to 1.0% Mn and minor amounts of other impurities.
    • Ferrite phase (α): Very low solubility of carbon (max 0.02% at 727°C).
    • Austenite phase (γ): Solubility of C is 2.08% at 1147°C and 0.8% at 0°C.
    • Ferrite phase (δ): Exists above 1394°C and below melting (limited value).
    • Cementite (Fe₃C): Intermetallic compound, 6.67% C and 93.3% Fe.

    Invariant Reactions in the Fe - Fe₃C Phase Diagram

    • Peritectic reaction: 1495°C; Liquid (0.53%C) + δ (0.09% C) → γ (0.17% C)
    • Eutectic reaction: 1147°C; Liquid (4.3% C) → γ austenite (2.08%C) + Fe₃C (6.67%C)
    • Eutectoid reaction: 727°C; γ Austenite (0.8%C) → α Ferrite(0.02%C) + Fe₃C (6.67%C)

    Slow Cooling of Plain Carbon Steel

    • A steel alloy containing 0.77% C is called the eutectoid plain carbon steel.
    • If the eutectoid steel is heated slightly above 727°C and held there, the structure becomes homogeneous austenite (γ)- known as austenitizing.
    • If cooled below the eutectoid temperature, a lamellar microstructure of ferrite (light) and cementite (dark) is formed called pearlite.

    Slow Cooling of Plain Carbon Steel, Cont.

    • A steel alloy containing greater than 0.02% and less than 0.77% C is called hypoeutectoid plain carbon steel.
    • If a sample of 0.4% C steel alloy is austenitized (heated to point “a”) and then slowly cooled to point “b,” proeutectoid α (or primary α) is formed.
    • Further cooling to point “d” produces pearlite which consists of eutectoid α and Fe₃C.

    Slow Cooling of Plain Carbon Steel, Cont.

    • A steel alloy containing greater than 0.77% C is called hypereutectoid plain carbon steel.
    • If a sample of 1.2% C steel alloy is austenitized (heated to point "a") and then slowly cooled to point "b," proeutectoid cementite (or primary cementite) is formed.
    • Further cooling to point "d" produces pearlite which consists of eutectoid α and Fe₃C.

    Heat Treatment of Plain Carbon Steels

    • By heating steel alloys to the γ region (austenitizing), more C atoms are absorbed by the FCC crystal.
    • This, followed by cooling at different rates, produces alloys with different mechanical properties.
    • If after austenitizing, the alloy is cooled very fast or quenched, martensite is formed.
    • Martensite is a metastable phase consisting of super-saturated interstitial solid solution of C in BCC or BCC tetragonal iron.
    • Extensive amount of carbon is trapped in the martensite microstructure due to quenching.

    Microstructure of Martensite

    • If the steel alloy contains less than 0.6% C, the resulting martensite will consist of domains of lath of different orientation (lath martensite).
    • If the steel alloy contains greater than 1.0% C, the resulting martensite will consist of a fine structure of parallel twins (plate martensite).
    • Between 0.6% and 1.0% C, the resulting martensite will be mixed.

    Martensite, Cont.

    • Upon quenching, the transformation from austenite (γ) to martensite is diffusionless—there is no time for diffusion.
    • No change in the relative position of carbon atoms after the transformation.
    • Martensite is much stronger and harder than pearlite with the same carbon content.
    • These properties are further enhanced with increases in carbon content.
    • Enhanced strength is due to high dislocation concentration and interstitial solid solution strengthening.

    Isothermal Decomposition of Austenite

    • Isothermal decomposition experiments are used to investigate microstructural changes in eutectoid steel.
    • Steel samples are first austenitized, then rapidly cooled to a specific temperature below the eutectoid temperature in a salt bath.
    • Finally, the samples are quenched in water at various time intervals.
    • Each sample's microstructure is then analyzed.

    Isothermal Decomposition of Austenite, Cont.

    • The microstructural changes during isothermal decomposition of eutectoid plain carbon steel are shown.
    • Samples are hot quenched to 705°C in a salt bath and kept at that temperature for various time intervals and subsequently quenched to room temperature.
    • Note: after 5.8 minutes at 705°C, coarse pearlite appears in the austenitic sample and after 66.7 minutes all of the austenite has transformed to pearlite.

    Isothermal Decomposition of Austenite, Cont.

    • If the isothermal decomposition process is repeated by cooling to progressively lower temperatures, an isothermal transformation (IT) diagram may be developed; the S-shaped blue lines in the figure.
    • The first S-shaped curve indicates the time needed for the transformation of austenite to begin, and the second curve indicates the time required to complete it.
    • Hot quenching to different temperatures produces different microstructures, including coarse pearlite, fine pearlite, bainite, and martensite.

    Isothermal Decomposition of Austenite, Cont.

    • If hot quenching temperature is between 550°C to 250°C, an intermediate structure bainite is produced.
    • Bainite contain nonlamellar eutectoid structure of a ferrite and cementite.
    • Upper bainite: Between 550°C and 350°C
    • Lower bainite: Between 350°C and 250°C

    IT Diagrams for Non-eutectoid Steels

    • The S curves of IT diagrams for non-eutectoid steels are shifted to the left.
    • Note that the transformation-begin curve is discontinuous.
    • Therefore, it’s not possible to hot quench from austenitic region to produce 100% martensite.
    • The extra transformation line (top black line) indicates the start and formation of proeutectoid ferrite.
    • IT diagrams are used only for understanding the transformation process; in industry, isothermal transformation or heat treatments is not used.

    Continuous Cooling Transformation Diagram

    • In industrial heat treatment, continuous cooling from the austenitic state to room temperature is performed in one step.
    • For plain carbon steel, the transformation from austenite to other phases takes place over a range of temperatures (as opposed to isothermal transformation).
    • Note that all dotted lines indicate the path of transformation at various cooling rates.
    • The solid blue region indicates the region of transformation.
    • Note that the start and finish lines are shifted to the right compared to IT.
    • No bainite transformation path occurs.

    Continuous Cooling Transformation Diagram

    • In continuous cooling of eutectoid steel at various cooling rates, different microstructures may be produced.
    • Water quenching produces a rapid cooling rate to 100% martensite.
    • The blue path is the critical or lowest possible cooling rate to produce 100% martensite.
    • Oil quenching produces a slower cooling rate leading to a mixture of martensite and pearlite.
    • If the slower cooling paths do not intersect the red line, no martensite will be produced.
    • Successive slow cooling rates produce fine and coarse pearlite.

    Annealing and Normalizing

    • A plain carbon steel sample is fully annealed when it is heated to temperatures 40°C above the austenite-ferrite boundary, held for necessary time, and cooled slowly to room temperature.
    • A plain carbon steel sample is process annealed (or stress relief) when it is heated to temperatures below eutectoid temperature and cooled slowly to room temperature.
    • Process annealing is normally applied to hypoeutectoid steel.
    • A plain carbon steel sample is normalized fully when it is heated to temperatures 40°C above the austenite-ferrite and austenite-cementite boundaries and cooled slowly to room temperature.

    Annealing and Normalizing, Cont.

    • The full anneal heat treatment softens the steel to its lowest strength and grows large equiaxed grains.
    • The microstructure mostly consists of proeutectoid ferrite/cementite (depending on carbon content) and pearlite.
    • The process anneal heat treatment relieves the internal residual stresses through realignment of dislocations.
    • The normalization heat treatment process refines the grain structure, produces a stronger steel, and creates a more uniform composition.
    • The microstructure consists of proeutectoid ferrite and pearlite.

    Tempering of Plain Carbon Steel

    • Quenching the austenitized plain carbon steel results in the formation of a strong yet brittle martensitic steel.
    • The ductility of the martensitic steel may be improved through a heat treatment process called tempering.
    • After quenching, the formed martensite is heated at temperatures below eutectoid for a period of time.
    • The dark line in the figure shows the tempering path.
    • The tempering temperature influences the strength, ductility, and microstructure of the steel.
    • For instance, tempering between 400°C and 700°C produces a spherodite microstructure.

    Martempering and Austempering

    • Martempering (Marquenching) refers to the process of austenitizing, quenching at around Ms, holding in quenching media until the temperature is uniform; removing before bainite forms; and cooling at a moderate rate.
    • Austempering refers to a similar process as martempering but held in quenching media till austenite to bainite transformation takes place.

    Classification of Plain Carbon Steel

    • Four-digit AISI-SAE code indicates a plain carbon steel.
    • If the first two digits are 10, this indicates a plain carbon steel.
    • Last two digits indicate carbon content in 100th wt%.
    • Example: 1030 steel indicates plain carbon steel containing 0.30 wt% carbon.
    • In addition to carbon, other impurities, such as Mn, may also exist in plain carbon steels but in minute amounts.

    Limitations of Low Alloy Steels

    • Cannot be strengthened beyond 690 MPa without losing ductility and impact strength.
    • Not deep hardenable.
    • Low corrosion resistance.
    • Rapid quenching leads to cracking and distortion.
    • Poor impact resistance at low temperatures.
    • To overcome these deficiencies, alloy steels containing significant amounts of impurities, other than carbon (like manganese, nickel, chromium, molybdenum, and tungsten), are developed.
    • These steels are often used in automotive and construction.

    Classification of Alloy Steels

    • First two digits indicate the principal alloying element.
    • Last two digits indicate the percentage of carbon.

    Distribution of Alloying Elements

    • Distribution depends upon the compound and carbide forming tendency of each element.

    Effects of Alloying Element on Eutectoid Temperature

    • Mn and Ni lower the eutectoid temperature.
    • They act as austenite stabilizing elements.
    • Tungsten, molybdenum, and titanium raise the eutectic temperature.
    • They are called ferrite stabilizing elements.

    Hardenability

    • Hardenability determines the depth and distribution of hardness induced by quenching.
    • Hardenability depends on composition, austenitic grain size, and structure before quenching.
    • Jominy hardenability test: A cylindrical bar is austenitized and one end is quenched. The Rockwell C hardness is measured up to 2.5 inches from the quenched end.

    Hardenability, Cont.

    • For 1080 plain carbon steel, the hardness value at the quenched end is 65 HRC, while it is 50 HRC at 3/16 inch from the quenched end.
    • Alloy steel 4340 has high hardenability and has hardness of 40 HRC 2 inches from the quenched end.
    • In alloy steel, decomposition of austenite to ferrite is delayed.
    • Cooling rate depends on bar diameter, quenching media, and bar cross-section.

    Mechanical Properties of Low Alloy Steels

    • Table presents the mechanical properties and applications of typical low-alloy steels.

    Aluminum Alloys

    • Precipitation strengthening heat treatment creates a fine dispersion of precipitated particles in the metal which hinder dislocation movements.
    • Basic steps: solution heat treatment, quenching, and aging

    Decomposition Products Created by Aging

    • Super-saturated solid solution is in an unstable condition.
    • Alloy tends to seek a lower energy state by decomposing into metastable or equilibrium phase.
    • Super-saturated solid solution as the highest energy state.
    • Equilibrium precipitate has the lowest energy state.

    Effects of Aging on Strength

    • Aging curve: Plot of strength or hardness versus aging time.
    • As aging time increases, the alloy becomes stronger, harder, and less ductile.
    • Overaging decreases strength and hardness.

    Example - Al 4% Cu Alloy

    • Al-4% Cu is solutionized at about 515°C.
    • Alloy is rapidly cooled in water.
    • Alloy is artificially aged in 130–190 °C.
    • Structures formed:
      • GP1 zone: Copper atoms segregate in supersaturated solid solution at lower aging temperature.
      • GP2 zone: Tetragonal structure, 10–100 nm diameter.
      • θ' phase: Nucleates heterogeneously on dislocations.
      • θ phase: Equilibrium phase, incoherent (CuAl₂).

    Correlation of Structure and Hardness

    • GP1 and GP2 zones increase hardness by stopping dislocation movement.
    • At 130°C when θ' forms, hardness is maximum.
    • After θ' forms, GP2 zones are dissolved, and θ' gets coarsened, reducing hardness.

    General Properties of Aluminum

    • Low density, corrosion resistance.
    • High alloy strength (about 690 MPa).
    • Nontoxic and good electrical properties.
    • Production: Aluminum hydroxide is precipitated from aluminum solution.
    • Aluminum hydroxide is thickened and calcined to Al₂O₃, which is dissolved in cryolite and electrolyzed.
    • Metallic aluminum sinks to the bottom and is tapped out.

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