Heat Treatment of Steel
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Questions and Answers

What is the primary hardening element in carbon tool steel?

  • Iron
  • Martensite
  • Carbon (correct)
  • Pearlite carbon

    • At what temperature does the pearlite carbon change to martensite or hardening carbon?

  • At the critical point
  • At the decalescence point
  • At room temperature
  • At a certain temperature during heating (correct)

    • What happens to the steel when it is heated to the decalescence point?

  • It changes color
  • It absorbs heat without appreciably rising in temperature (correct)
  • It cools rapidly
  • It becomes brittle

    • What is the purpose of tempering or stress relieving hardened materials?

    <p>To improve dimensional stability and toughness</p> Signup and view all the answers

    What is the difference between full annealing and normalizing heat treatment?

    <p>Both temperature and cooling rate</p> Signup and view all the answers

    What is the result of the hardening process?

    <p>A tougher, more durable item</p> Signup and view all the answers

    What is the primary goal of heat treatment of steel?

    <p>To alter the physical and mechanical properties</p> Signup and view all the answers

    Why is annealing done to steel?

    <p>To remove residual stresses and restore ductility</p> Signup and view all the answers

    What is a prerequisite for hardening steel?

    <p>Sufficient carbon and alloy content</p> Signup and view all the answers

    What is the result of softening steel?

    <p>Decreased strength and hardness</p> Signup and view all the answers

    What is the purpose of heat treatment in manufacturing?

    <p>To enable other manufacturing processes</p> Signup and view all the answers

    Why are steels particularly suitable for heat treatment?

    <p>They respond well to heat treatment</p> Signup and view all the answers

    What is the primary goal of heat treatment in materials?

    <p>To modify the behavior of the material to maximize service life</p> Signup and view all the answers

    What is the purpose of solution annealing?

    <p>To put precipitates back into solution</p> Signup and view all the answers

    What is the purpose of thermo-mechanical treatments in heat treatment?

    <p>To modify the component shape and structure</p> Signup and view all the answers

    What is the result of quenching in heat treatment?

    <p>Non-equilibrium structures are produced</p> Signup and view all the answers

    What is the result of stress relieving heat treatment?

    <p>A decrease in internal stresses</p> Signup and view all the answers

    What is the goal of austempering?

    <p>To create bainite</p> Signup and view all the answers

    What is the purpose of normalizing in heat treatment?

    <p>To produce small grain size and improve toughness and strength</p> Signup and view all the answers

    What is the purpose of tempering in heat treatment?

    <p>To relieve the brittleness of fully hardened steels</p> Signup and view all the answers

    What is the difference between austempering and martempering?

    <p>The goal of the process</p> Signup and view all the answers

    What is the advantage of austempering over other heat treatments?

    <p>It produces a metal with superior impact resistance</p> Signup and view all the answers

    What is the result of spheroidising in heat treatment?

    <p>The material's strength and hardness are reduced</p> Signup and view all the answers

    What is the final step in the austempering process?

    <p>Air-cooling</p> Signup and view all the answers

    What is represented by the CCT diagram?

    <p>Various transformation products at different cooling rates</p> Signup and view all the answers

    What is the primary advantage of CCT diagrams over TTT diagrams?

    <p>CCT diagrams are more practical for continuous cooling</p> Signup and view all the answers

    What is hardenability?

    <p>The ability of the material to be hardened by forming martensite</p> Signup and view all the answers

    What is the Jominy end-quench test used for?

    <p>To measure the hardenability of a material</p> Signup and view all the answers

    What is the effect of fast cooling on the material?

    <p>It increases the risk of warping and crack formation</p> Signup and view all the answers

    What is precipitation hardening also known as?

    <p>Age hardening</p> Signup and view all the answers

    What is the purpose of the solution heat treatment in precipitation hardening?

    <p>To form a single-phase solution</p> Signup and view all the answers

    Why is the supersaturated solution stable at low temperatures?

    <p>Because of the low diffusivity of the solute</p> Signup and view all the answers

    What happens if the precipitation heat treatment is continued for a very long time?

    <p>The hardness decreases</p> Signup and view all the answers

    What is the role of heat capacity in determining the cooling rate?

    <p>It is the energy content of the heated mass</p> Signup and view all the answers

    What is the result of embrittlement during tempering?

    <p>An increase in hardness and a reduction in ductility</p> Signup and view all the answers

    What is the purpose of recrystallization in metals processing?

    <p>To reduce the strength and hardness of a material and increase ductility</p> Signup and view all the answers

    What is the main difference between TTT and CCT diagrams?

    <p>The method of cooling</p> Signup and view all the answers

    What is the result of austempering?

    <p>Transformation of austenite to bainite</p> Signup and view all the answers

    What is the advantage of using CCT diagrams in engineering applications?

    <p>They are more economic than TTT diagrams</p> Signup and view all the answers

    What is the result of continuous cooling in CCT diagrams?

    <p>Formation of martensite or pearlite</p> Signup and view all the answers

    Study Notes

    Heat Treatment of Steel

    • Heat treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape.
    • Steels are particularly suitable for heat treatment, since they respond well to heat treatment and the commercial use of steels exceeds that of any other material.

    Reasons for Heat Treatment

    • Softening: to reduce strength or hardness, remove residual stresses, improve toughness, restore ductility, refine grain size, or change the electromagnetic properties of the steel.
    • Hardening: to increase the strength and wear properties.
    • Material Modification: to modify properties of materials in addition to hardening and softening, such as stress relieving, cryogenic treatment, or spring aging.

    Heat Treatment Processes

    • Annealing: heating the material above the critical temperature, holding long enough for transformation to occur, and slow cooling.
    • Normalizing: heating a steel above the critical temperature, holding for a period of time long enough for transformation to occur, and air cooling.
    • Quenching: heating the material to a suitable temperature and then quenching in water or oil to harden to full hardness.
    • Tempering: reheating the steel at a relatively low temperature to develop the required combination of hardness, strength, and toughness or to relieve the brittleness of fully hardened steels.

    Critical Temperatures

    • Decalescence point: the temperature at which steel continues to absorb heat without appreciably rising in temperature.
    • Critical point: the temperature at which the steel actually increases in temperature, although its surroundings may be colder.

    Hardening

    • The use of this treatment will result in an improvement of the mechanical properties, as well as an increase in the level of hardness, producing a tougher, more durable item.
    • Alloys may be air cooled, or cooled by quenching in oil, water, or another liquid, depending upon the amount of alloying elements in the material.

    Austempering and Martempering

    • Austempering: a technique used to form pure bainite, a transitional microstructure found between pearlite and martensite.
    • Martempering: similar to austempering, but the goal is to create martensite rather than bainite.

    Embrittlement

    • Occurs during tempering when, through a specific temperature range, the steel experiences an increase in hardness and a reduction in ductility.
    • Two types: tempered martensite embrittlement (TME) or one-step embrittlement, and temper embrittlement (TE) or two-step embrittlement.

    Recrystallization

    • A process by which deformed grains are replaced by a new set of defects-free grains that nucleate and grow until the original grains have been entirely consumed.
    • Usually accompanied by a reduction in the strength and hardness of a material and a simultaneous increase in the ductility.

    Time-Temperature Transformation (TTT) and Continuous Cooling Transformation (CCT) Diagrams

    • TTT diagrams: measure the rate of transformation at a constant temperature.
    • CCT diagrams: measure the extent of transformation as a function of time for a continuously decreasing temperature.
    • CCT diagrams are more practical than TTT diagrams as most of the processes employ continuous cooling rather than isothermal transformation.

    Hardenability

    • The ability of the material to be hardened by forming martensite.
    • Measured by the Jominy end-quench test.
    • Hardenability is then given as the dependence of hardness on distance from the quenched end.### Cooling and Thermal Gradients
    • Fast cooling of a metal piece can cause warping and crack formation due to large thermal gradients.
    • The shape and size of the piece, along with its heat capacity and heat conductivity, influence the cooling rate for different parts of the metal.

    Heat Capacity and Heat Conductivity

    • Heat capacity is the energy content of a heated mass that needs to be removed for cooling.
    • Heat conductivity measures how fast energy is transported to the colder regions of the piece.

    Precipitation Hardening

    • Precipitation hardening, also known as age hardening, enhances hardening by forming extremely small precipitates that hinder dislocation motion.
    • Precipitates form when the solubility limit is exceeded.

    Steps to Achieve Precipitation Hardening

    • Solution heat treatment: dissolve all solute atoms to form a single-phase solution.
    • Rapid cooling across the solvus line to exceed the solubility limit, resulting in a supersaturated solid solution.
    • Precipitation heat treatment: heat the supersaturated solution to an intermediate temperature to induce precipitation and keep it there for some time (aging).

    Overaging

    • If the precipitation hardening process is continued for a very long time, the hardness eventually decreases, a phenomenon known as overaging.

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    Description

    Heat treatment is a process to alter the physical and mechanical properties of metals without changing their shape. It's used to increase strength, improve machining, and achieve other manufacturability objectives.

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