Heat Treatment and Microstructure of Steel

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Questions and Answers

What is the temperature at which δ-iron transforms into γ-iron?

  • 1536°C
  • 1392°C (correct)
  • 727°C
  • 1000°C

At what temperature does iron crystallize into a body-centered cubic lattice structure (bcc)?

  • 1536°C (correct)
  • 1392°C
  • 727°C
  • 1000°C

What is the name of the process that transforms steel by altering its properties and structure using heat?

  • Thermal arrest
  • Heat treatment (correct)
  • Solidification
  • Microstructure formation

What is the term used for a change in the chemical structure of steel during heat treatment?

<p>Thermochemical (B)</p> Signup and view all the answers

Which of these is NOT a type of iron mentioned in the text?

<p>β-iron (A)</p> Signup and view all the answers

What is the significance of the "thermal arrest" mentioned in the text?

<p>It marks the point where the steel undergoes a phase change. (D)</p> Signup and view all the answers

What is the primary factor that determines the structure of steel at different temperatures?

<p>The atomic structure and binding energies (A)</p> Signup and view all the answers

What is the key difference between thermochemical and thermomechanical processes?

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What is the eutectoid composition of carbon in retained austenite at 723 °C?

<p>0.8% carbon (D)</p> Signup and view all the answers

What happens to the carbon when the austenite converts to ferrite at 723 °C?

<p>Carbon precipitates out as cementite (D)</p> Signup and view all the answers

Which structure does the austenite transform into after reaching the eutectoid composition?

<p>Pearlite (D)</p> Signup and view all the answers

What is the relationship between hypoeutectoid and hypereutectoid steels based on carbon content?

<p>Hypoeutectoid steels have less than 0.8% carbon (D)</p> Signup and view all the answers

What type of phase mixture results from the transformation of retained austenite in the process described?

<p>Pearlite (A)</p> Signup and view all the answers

At what temperature does iron transform from face-centered cubic to body-centered cubic structure?

<p>911 °C (C)</p> Signup and view all the answers

What magnetic state is iron in above its Curie temperature?

<p>Paramagnetic (D)</p> Signup and view all the answers

What is the lattice structure of iron below the Curie temperature?

<p>Body-centered cubic (A)</p> Signup and view all the answers

What happens to carbon when the temperature falls below the γ-α transformation line?

<p>Precipitates at grain boundaries (B)</p> Signup and view all the answers

What is the term for the magnetic state of iron below the Curie temperature?

<p>α-iron (D)</p> Signup and view all the answers

What is formed from the austenite lattice when the temperature decreases?

<p>Ferrite (C)</p> Signup and view all the answers

What is the relationship between austenite and ferrite in the context of phase transformation?

<p>Ferrite precipitates from austenite during cooling (B)</p> Signup and view all the answers

What effect does the body-centered cubic lattice structure have on iron’s magnetic properties?

<p>It causes the iron to become non-magnetic (A)</p> Signup and view all the answers

What is the primary component of the microstructure in hypereutectoid steel at room temperature?

<p>Grain boundary cementite (D)</p> Signup and view all the answers

Which statement accurately describes the microstructure of hypoeutectoid steel at room temperature?

<p>It contains ferrite grains and pearlite. (B)</p> Signup and view all the answers

What is the microstructure composition of eutectoid steel at room temperature?

<p>Only pearlite grains (A)</p> Signup and view all the answers

What happens to carbon during the quenching of austenitized steel?

<p>It remains dissolved in the austenite lattice. (D)</p> Signup and view all the answers

In the context of steel microstructure, what does the term 'lattice transformation' refer to?

<p>Structural change from austenite to ferrite due to quenching (D)</p> Signup and view all the answers

What characterizes the structure of ferrite when carbon is dissolved during quenching?

<p>Ferrite structure becomes expanded tetragonally. (B)</p> Signup and view all the answers

Which of the following statements about the microstructure formation in steel is true?

<p>Eutectoid steel lacks ferrite completely. (B)</p> Signup and view all the answers

What does the term 'grain boundary cementite' refer to in hypereutectoid steel?

<p>Precipitated cementite at the boundaries of grains (A)</p> Signup and view all the answers

What defines the martensite microstructure in steel?

<p>It appears needle-shaped or plate-shaped under a microscope. (A)</p> Signup and view all the answers

Why can't the martensite formation be explained by the iron-carbon phase diagram?

<p>They are based on thermodynamic equilibrium that is not achieved during quenching. (D)</p> Signup and view all the answers

Which condition is necessary for effective quenching and tempering of steel?

<p>There must be sufficient carbon for significant increases in hardness. (B)</p> Signup and view all the answers

What happens to carbon atoms during the formation of martensite?

<p>They are forcibly dissolved and distort the lattice structure. (D)</p> Signup and view all the answers

Which of the following statements best describes quenching?

<p>It prevents the formation of stable phases by cooling rapidly. (A)</p> Signup and view all the answers

What is required for the γ-α lattice transformation in steel?

<p>The transformation should not be suppressed by alloying elements. (B)</p> Signup and view all the answers

What can occur if the carbon content in steel is too low during quenching?

<p>It may hinder the formation of martensitic structures. (A)</p> Signup and view all the answers

What effect does the quenching process have on the microstructure of steel?

<p>It creates a hardened martensitic structure. (B)</p> Signup and view all the answers

What is the primary purpose of heat treatment in metals?

<p>To change the properties and/or structure of the metal (A)</p> Signup and view all the answers

What lattice structure does iron crystallize into at 1536°C?

<p>Body-centered cubic (bcc) (A)</p> Signup and view all the answers

During the phase transformation at 1392°C, which structure does δ-iron transform into?

<p>Face-centered cubic (fcc) (D)</p> Signup and view all the answers

What is a thermal arrest in the context of heat treatment?

<p>A constant temperature during a lattice transformation (D)</p> Signup and view all the answers

What type of process is thermochemical in heat treatment?

<p>Involves a change in chemical structure (B)</p> Signup and view all the answers

Which metals are primarily discussed in the heat treatment content?

<p>Steel and Iron (B)</p> Signup and view all the answers

At what point does solidification of the iron complete its microstructure?

<p>At 1536°C (C)</p> Signup and view all the answers

Which process is NOT a characteristic of heat treatment methods?

<p>Maintaining uniform weight (C)</p> Signup and view all the answers

Flashcards

Heat Treatment

A process involving controlled heating and cooling to alter metal properties and structure.

Thermochemical Treatment

Heat treatment that involves changes in the chemical structure of metals.

Thermomechanical Treatment

Heat treatment focused on altering the mechanical properties of metals through forming.

Microstructure

The internal structure of a material that affects its properties, observable under a microscope.

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Body-Centered Cubic (bcc)

A lattice structure where atoms are located at the corners and one at the center of the cube.

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Face-Centered Cubic (fcc)

A lattice structure with atoms at each corner and in the center of each face of the cube.

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Austenite

A phase of iron with a face-centered cubic structure formed above 1392°C.

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Thermal Arrest

A phase transformation in metals that occurs at a constant temperature during heat treatment.

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Curie Temperature

The temperature at which iron becomes non-magnetic, transitioning to a magnetic state below 769 °C.

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α-iron

The magnetic state of iron in body-centered cubic lattice structure, stable below the Curie temperature.

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β-iron

The state of iron in face-centered cubic structure, stable above 911 °C.

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Ferrite

The form of iron that precipitates from austenite as the temperature drops; characterized by a body-centered cubic structure.

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Cementite

A compound of iron and carbon (Fe3C) that forms at grain boundaries in steel, depleting carbon from surrounding areas.

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Phase Transformation

The process through which steel changes from one structural form to another, like from austenite to ferrite.

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Solubility Limit

The maximum concentration of carbon that can dissolve in austenite before cementite begins to form.

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Eutectoid Composition

The composition where 0.8% carbon at 723°C allows full transformation to pearlite.

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Retained Austenite

Austenite that remains after cooling and can still dissolve carbon before reaching eutectoid.

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Cementite Lamellae

Carbides that precipitate from ferrite as carbon can't be dissolved in it during cooling.

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Pearlite

A microstructure formed from the combination of ferrite grains and cementite lamellae.

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Hypoeutectoid vs Hypereutectoid

Hypoeutectoid has <0.8%C; hypereutectoid has >0.8%C carbon content.

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Hypereutectoid Steel Microstructure

Consists of grain boundary cementite and pearlite at room temperature.

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Hypoeutectoid Steel Microstructure

Comprised of separate ferrite grains and pearlite at room temperature.

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Eutectoid Steel Microstructure

Contains only pearlite grains at room temperature.

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Quenching Process

Rapid cooling of austenitized steel that prevents carbon diffusion.

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Austenite Lattice

A face-centered cubic structure which can have dissolved carbon.

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Ferrite Lattice Transformation

The body-centered cubic structure expands tetragonally during quenching.

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Carbon Diffusion

Movement of carbon out of the austenite lattice during heat treatment.

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Martensite

A microstructure formed during rapid cooling known for its needle-like or plate-like structure.

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Quenching

A rapid cooling process used to form martensite, preventing equilibrium.

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Carbon Solubility

Carbon must be solvable in γ-iron and insoluble in α-iron for specific heat treatments.

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Lattice Transformation

The change from γ-iron to α-iron lattice structure during cooling.

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Tempering

A heat treatment process after quenching to reduce brittleness.

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Phase Diagram

A chart describing phases of a metal under various temperatures and compositions.

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Tetragonal Structure

A type of lattice structure that is elongated in one direction found in martensite.

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Hardness Increase

The significant enhancement of a material's hardness due to proper carbon content in heat treatment.

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Study Notes

Heat Treatment of Steel

  • Heat treatment is a series of steps using time and temperature procedures to change metal properties. This can involve chemical changes (thermochemical) or structural changes (thermomechanical).
  • The process is used on metals, focusing here on steel (steel)
  • Steps in the content of study:
    • Examination of microstructure in hardened metals.
    • Heat treatment procedures for metal (steel).
    • Testing of metals.

Microstructure Formation During Solidification

  • At 1536°C, the melt solidifies into a body-centered cubic (bcc) structure in
    • This is also called 8-iron
  • At 1392°C, the bcc structure transforms into a face-centered cubic (fcc) structure (γ-iron) at a constant temperature.
  • At 911°C, the fcc structure changes back to a bcc (β-iron) structure
    • This transformation isn't associated with a change in temperature
  • At 769°C there is no lattice transformation; however, iron becomes magnetic below this temperature. This temperature is called Curie temperature
    • The magnetic state of iron at this temperature is also known as a-iron; it also has a bcc structure.

Steel Phase Diagram

  • Phase diagrams show relationships between temperature and composition in a material system, and how phases (solid, liquid etc.) change during processes like cooling or heating.
  • This diagram maps phases like ferrite, cementite, and austenite alongside the changing percentage of carbon in the mixture.
  • Ferrite precipitates from the austenite lattice at temperatures below the γ–α transformation line, as the fcc austenite begins to change to the bcc ferrite.
  • Carbon atoms that can no longer be dissolved into the forming ferrite lattice migrate into the surrounding austenite.
  • At 723°C, and 0.8% carbon is the eutectoid composition. This results in the transformation of the remaining retained austenite fully into the body-centered cubic ferrite structure, with carbon precipitating out as cementite lamellae forming pearlite.

Microstructure Formation of Steel

  • Hypereutectoid steels have above 0.8% carbon content and therefore a microstructure consisting of the previously precipitated grain boundary cementite and the pearlite that formed.
  • Hypo-eutectoid steels (< 0.8% carbon) have a microstructure made up of the previously formed ferrite grains and the resulting pearlite.
  • Eutectoid steels (<0.8% carbon) consist only of pearlite grains at room temperature.

Lattice Transformation

  • Quenching austenitized steel makes it difficult for dissolved carbon to leave the austenite lattice.
  • This leads to a highly distorted lattice structure called martensite (a needle or plate-shape).

Requirements for Quenching and Tempering

  • Sufficient carbon is key for increasing hardness and strength in the steel
    • Too little carbon (<0.3%) means insufficient martensite formation.
  • Alloying elements can reduce critical cooling rates needed to form martensite.
  • Steels that contain alloying elements cannot undergo martensite transformation. Therefore they are not “hardenable.”

Hardness Testing

  • Various methods, like Brinell, Vickers, or Rockwell, are used for testing hardness in metals.
    • These tests quantify the material's resistance to indentation.

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