Eutectoid, Hypoeutectoid and Hypereutectoid steels

Introduction to Materials Science and Engineering
11 Mar 201812:10
EducationalLearning
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TLDRThis script delves into the iron carbon system, the foundation of steel production. It explains the phase diagram, highlighting the eutectoid reaction at 725ยฐC, where austenite transforms into ferrite and cementite. The eutectoid, hypo-eutectoid, and hyper-eutectoid steels are defined by their carbon content. The eutectoid steel, with 0.8% carbon, forms a distinctive microstructure called pearlite, a layered mixture of alpha and Fe3C, upon cooling. Pearlite, a key micro-constituent, is not a phase but a visually distinct mixture in the steel's microstructure.

Takeaways
  • ๐Ÿ“š The script discusses the iron carbon system, which is crucial for steel production, the most important engineering alloy.
  • ๐Ÿ” It reviews the phase diagram of the iron carbon alloy system, highlighting three invariant reactions: peritectic, eutectic, and eutectoid reactions.
  • ๐Ÿ”จ The eutectoid reaction at 725 degrees Celsius is identified as particularly significant for controlling the microstructure of iron carbon alloys.
  • ๐Ÿง  The terms 'hypoeutectoid' and 'hypereutectoid' steels are introduced, based on their carbon content in relation to the eutectoid composition of 0.8% carbon.
  • ๐ŸŒก The eutectoid steel, with 0.8% carbon, undergoes a phase change at 725 degrees Celsius, transforming from austenite (gamma phase) to a mixture of alpha (ferrite) and Fe3C (cementite).
  • ๐Ÿ“‰ Hypoeutectoid steels, with less than 0.8% carbon, and hypereutectoid steels, with more than 0.8% carbon, are differentiated based on their carbon content.
  • ๐Ÿ”ฌ The script explains the phase regions in the diagram, indicating where alpha, gamma, and Fe3C phases exist and interact.
  • ๐Ÿ›  The eutectoid reaction is described in detail, where cooling austenite with 0.8% carbon results in the formation of alpha and Fe3C.
  • ๐Ÿ“ The alpha phase has a carbon composition of 0.02%, while Fe3C, a compound, has a fixed composition of 6.67% carbon, with all compositions measured in weight percent.
  • ๐ŸŒŒ The microstructure of eutectoid steel below 725 degrees Celsius is characterized by alternating plates of alpha and Fe3C, known as pearlite.
  • ๐Ÿ”ฎ Pearlite is not a phase but a microconstituent, a specific appearance in the microstructure resulting from the mixture of alpha and Fe3C phases.
Q & A

  • What is the significance of the iron carbon system in metallurgy?

    -The iron carbon system is significant because it is the basis for steel production, which is the most important engineering alloy used in various applications.

  • What are the three different types of reactions in the iron carbon phase diagram?

    -The three types of reactions in the iron carbon phase diagram are a peritectic reaction at high temperature, a eutectic reaction at an intermediate temperature, and a eutectoid reaction at a lower temperature.

  • Why is the eutectoid reaction considered the most important in the iron carbon system?

    -The eutectoid reaction is considered the most important because it plays a crucial role in controlling the microstructure of steel, which in turn affects its mechanical properties.

  • What are the different types of steel based on carbon content relative to the eutectoid composition?

    -Steels are classified as eutectoid steel (0.8% carbon), hypo eutectoid steel (less than 0.8% carbon), and hyper eutectoid steel (more than 0.8% carbon).

  • What is the temperature at which the eutectoid reaction occurs in the iron carbon system?

    -The eutectoid reaction occurs at 725 degrees Celsius in the iron carbon system.

  • What happens to austenite when it cools below 725 degrees Celsius in the iron carbon system?

    -When austenite cools below 725 degrees Celsius, it undergoes the eutectoid reaction and transforms into a mixture of alpha (ferrite) and Fe3C (cementite).

  • What is the composition of alpha and Fe3C in the eutectoid reaction?

    -In the eutectoid reaction, the composition of alpha is approximately 0.02% carbon, and Fe3C has a composition of 6.67% carbon.

  • What is the microstructure of eutectoid steel above 725 degrees Celsius?

    -Above 725 degrees Celsius, the microstructure of eutectoid steel is a polycrystalline single phase of austenite (gamma).

  • What is the special name given to the microstructure formed when eutectoid steel is cooled below 725 degrees Celsius?

    -The microstructure formed in eutectoid steel when cooled below 725 degrees Celsius is called pearlite.

  • What is the difference between pearlite and a single phase in the context of steel microstructure?

    -Pearlite is not a single phase but a micro constituent, which is a mixture of two phases, alpha and Fe3C, with a specific appearance in the microstructure, whereas a single phase refers to a uniform composition throughout the material.

  • What does the term 'hypoeutectoid steel' refer to and what is its carbon content range?

    -Hypoeutectoid steel refers to steels with a carbon content less than 0.8%, which means their composition is below the eutectoid composition.

  • What does the term 'hypereutectoid steel' refer to and what is its carbon content range?

    -Hypereutectoid steel refers to steels with a carbon content more than 0.8%, indicating that their composition is above the eutectoid composition.

Outlines
00:00
๐Ÿ” Introduction to the Iron-Carbon System and Its Invariant Reactions

This paragraph introduces the iron-carbon system and its significance in producing steel, the most important engineering alloy. It reviews the phase diagram of the iron-carbon alloy system and highlights three key invariant reactions: peritectic, eutectic, and eutectoid. The eutectoid reaction, occurring at 725ยฐC, is emphasized as crucial for microstructure control in the iron-carbon system. The paragraph outlines the phases above and below the eutectoid horizontal, detailing the transformation of austenite (gamma) to alpha and Fe3C (cementite).

05:06
๐Ÿ› ๏ธ Nomenclature and Types of Steels Based on Carbon Content

This paragraph explains the classification of steels based on carbon content, focusing on eutectoid steel (0.8% carbon). It describes the nomenclature for hypo-eutectoid (less than 0.8% carbon) and hyper-eutectoid (more than 0.8% carbon) steels. The microstructure of eutectoid steel is discussed, noting that above 725ยฐC, it consists of polycrystalline austenite (gamma). Upon cooling below 725ยฐC, it transforms into a mixture of alpha (ferrite) and Fe3C (cementite), forming alternate plates in the microstructure.

10:16
๐Ÿ”ฌ Microstructure of Eutectoid Steel and the Formation of Pearlite

This paragraph delves into the microstructure of eutectoid steel cooled slowly through 725ยฐC, resulting in a structure known as pearlite. It explains that pearlite consists of alternating plates of alpha and Fe3C, which form distinct colonies in the microstructure. The paragraph emphasizes that pearlite is not a single phase but a micro constituent, meaning it is a mixture of two phases (alpha and Fe3C) that exhibit a specific appearance in the microstructure. The entire microstructure of eutectoid steel cooled under these conditions will be 100% pearlite.

Mindmap
Keywords
๐Ÿ’กIron Carbon System
The Iron Carbon System refers to the alloy system of iron and carbon, which is fundamental in the creation of steel, the most important engineering alloy. In the video's context, it is the focus of the discussion, particularly the phase diagram that illustrates the various reactions and transformations within this system.
๐Ÿ’กPhase Diagram
A phase diagram is a graphical representation of the phase behavior of a material as a function of temperature and composition. In the video, the phase diagram of the iron carbon system is discussed, showing the different phases and reactions, such as peritectic, eutectic, and eutectoid reactions, which are crucial for understanding steel's properties.
๐Ÿ’กEutectoid Reaction
The eutectoid reaction is a specific type of phase transformation that occurs at a lower temperature in the iron carbon system. It is highlighted in the script as the most important reaction for controlling the microstructure of steel. The eutectoid reaction involves the transformation of austenite into a mixture of ferrite and cementite at 725 degrees Celsius.
๐Ÿ’กHypo Eutectoid and Hyper Eutectoid Steels
These terms describe steels based on their carbon content in relation to the eutectoid composition. Hypo eutectoid steels have less than 0.8% carbon and transform into a mixture of ferrite and pearlite upon cooling. Hyper eutectoid steels contain more than 0.8% carbon and form a mixture of ferrite and cementite. The distinction is important for the resulting microstructure and properties of the steel.
๐Ÿ’กAustenite
Austenite, denoted as gamma in the script, is a phase of iron that is stable at high temperatures and contains carbon. It is a key phase in the iron carbon system, and its transformation upon cooling is central to the formation of steel's microstructure, as discussed in the video.
๐Ÿ’กFerrite
Ferrite is a phase of iron that is stable at lower temperatures and contains a small amount of carbon. In the context of the video, ferrite forms as a result of the eutectoid reaction when austenite cools below 725 degrees Celsius, contributing to the steel's microstructure.
๐Ÿ’กCementite
Cementite is a compound with the formula Fe3C and is a phase in the iron carbon system. It forms alongside ferrite during the eutectoid reaction in steels with exactly 0.8% carbon content. Cementite is an important constituent in the microstructure of steel, affecting its hardness and strength.
๐Ÿ’กEutectoid Steel
Eutectoid steel is a specific type of steel with a carbon content of exactly 0.8%. As discussed in the script, this steel undergoes the eutectoid reaction at 725 degrees Celsius, resulting in a microstructure of alternating plates of ferrite and cementite, known as pearlite.
๐Ÿ’กPearlite
Pearlite is a microconstituent of steel, not a phase, but a mixture of two phases: ferrite and cementite. It forms as a result of the eutectoid reaction in eutectoid steels and is characterized by its layered, or 'lamellar,' structure, which gives steel certain mechanical properties.
๐Ÿ’กMicrostructure
Microstructure refers to the small-scale structure of a material, which can be observed with tools like microscopes. In the video, the microstructure of steel is discussed in relation to the phases present (austenite, ferrite, cementite) and how they arrange themselves, such as in pearlite, which significantly influences the material's properties.
Highlights

The iron carbon system is a crucial alloy system for producing steel, the most important engineering alloy.

The phase diagram of the iron carbon system was discussed, featuring three invariant reactions: peritectic, eutectic, and eutectoid.

The eutectoid reaction at 725 degrees Celsius is pivotal for controlling the microstructure of the iron carbon system.

Eutectoid, hypo eutectoid, and hyper eutectoid steels are differentiated based on their carbon content relative to the eutectoid point.

At 725 degrees Celsius, a eutectoid steel with 0.8% carbon transforms from austenite to a mixture of alpha and Fe3C.

The eutectoid reaction is characterized by the formation of alpha ferrite and cementite from austenite.

The alpha phase composition is 0.02% carbon, while Fe3C, a compound, has a composition of 6.67% carbon.

Hypo eutectoid steels have less than 0.8% carbon, hyper eutectoid steels have more, and eutectoid steels are exactly at this percentage.

The microstructure of eutectoid steel above 725 degrees Celsius is polycrystalline single-phase austenite.

Upon cooling through 725 degrees Celsius, eutectoid steel forms a microstructure of alternating alpha and Fe3C plates.

The term 'pearlite' refers to the microstructure of eutectoid steel, which is a mixture of alpha and Fe3C with a specific appearance.

Pearlite is not a phase but a microconstituent, indicating a certain appearance in the microstructure of a two-phase mixture.

The phase diagram does not dictate the morphology of the microstructure but informs the presence of phase mixtures.

Slow cooling of eutectoid steel results in a microstructure that is 100% pearlite.

Pearlite's appearance in the microstructure consists of alternating dark Fe3C and light alpha regions, forming colonies.

The eutectoid reaction is the most important invariant reaction in the iron carbon system for microstructure control.

Understanding the eutectoid reaction is key to manipulating the properties of steel through microstructure control.

Transcripts

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Related Tags
Iron-CarbonPhase DiagramSteel TypesEutectoid ReactionMicrostructureEngineering AlloyAustenitePearliteFerriteCementite