Iron Carbon Diagram MCQ Quiz in বাংলা - Objective Question with Answer for Iron Carbon Diagram - বিনামূল্যে ডাউনলোড করুন [PDF]

Explanation:

Cast iron 

• It is an alloy of iron, carbon, and silicon and it is hard and brittle.
• Carbon content in CI may be within 1.7% to 3% and carbon may be present as free carbon or iron carbide Fe3C.
• In general, the types of cast iron are

1. Grey cast iron and
2. White cast iron
3. Malleable cast iron etc.

Wrought iron

• Wrought iron is a very pure iron where the iron content is of the order of 99.5%. 
• It is produced by re-melting pig iron and some small amount of silicon, sulfur, or phosphorus may be present. It is tough, malleable, and ductile and can easily be forged or welded.
• It cannot however take the sudden shock. Chains, crane hooks, railway couplings, and such other components may be made of this iron.

Pig iron

• Pig iron is an intermediate product of the iron industry. 
• Pig iron has a very high carbon content, typically 3.8–4.7%, along with silica and other constituents of dross, which makes it very brittle, and not useful directly as a material except for limited applications.

Steel

• Steel is basically an alloy of iron and carbon in which the carbon content can be less than 1.7% and carbon is present in the form of iron carbide to impart hardness and strength.
• Two main categories of steel are (a) Plain carbon steel and (b) alloy steel.
• Thus, wrought iron is an iron alloy with a very low carbon (less than 0.08%) content in contrast to cast iron.

Note: The best available option here will be 1.7 to 4.5% for Cast Iron.

Explanation:

Eutectoid/Pearlite steel:

• A 0.8% carbon steel or eutectoid steel is known as PEARLITE steel.
• This is much stronger than ferrite or cementite.
• It is a phase mixture of  ferrite and cementite.
• It contains ferrite 87% and cementite 13%.

Hypo-eutectoid Steel:

• Plain carbon steels in which carbon percentage is less than 0.8% are called hypo-eutectoid steel.

Hyper-eutectoid Steel:

• Plain carbon steels in which carbon percentage is more than 0.8% are called hyper-eutectoid steel.
• High-Speed Steel is a high carbon tool steel, containing a large dose of tungsten. A typical HSS composition is: 18% tungsten, 4% Chromium, 1% Vanadium, 0.7% carbon and the rest, Iron.
• Cast irons have a chemical composition of 2.15–6.7% carbon.

Concept:

Austenite is an interstitial solution of Carbon in γ iron (Carbon atoms are accommodated at interstitial spaces). The maximum solubility of carbon in γ iron is 2.11%. In pure form, Austenite has an FCC structure. It has larger interatomic spacings (1.02 A° ) than ferrite. Austenite is soft, tough, and highly ductile. It has high tensile strength also. Austenite is non-magnetic at any temperature.

The liquid molten metal of iron on cooling will become solid at 1539°C (δ-iron) having BCC, on further cooling at 1410°C δ-iron is converted to γ – iron called as austenite having FCC and on further cooling, it counts to α – iron called ferrite at 910°C.

Explanation:

Eutectic System: 

• Two metals are completely soluble in the liquid state and completely insoluble in the solid-state. A eutectic system is a homogeneous mixture of substances that melts or solidifies at a single temperature lower than any of the constituents' melting points. This temperature is known as the eutectic temperature.

Eutectic reaction:

Liquid (L) ⇔  Solid 1 (α) + Solid 2 (β)

Peritectic System: 

• A peritectic reaction occurs at a specific temperature and composition, where a solid phase and a liquid phase combine to form a second solid phase.

Peritectic reaction: Liquid (L) + Solid 1 (α) ⇔ Solid 2 (β)

Monotectic System: 

• In the solidification of metallic alloys, peritectic solidification is a very common type of system. A liquid phase transforms into a solid phase and another liquid phase.

Monotectic reaction: Liquid 1 (L1) ⇔ Liquid 2 (L2) + Solid (α)​

Eutectoid Reaction

• At temperature 723° C and carbon composition 0.8%
• Austenite (γ ) (S) ⇄ Ferrite (α-iron) (S) + Carbide (Fe3C) (S)
• One solid converted into two solids.

Explanation:

The iron-carbon Phase diagram is shown below

Iron can exist in three different crystalline forms each having limited solubility of carbon.  

The stability of these depends on temperature and composition. The two high-temperature forms of iron are δ ferrite which is BCC (stable above 1394°C) and austenite (γ: stable above 910°C) which is FCC. 

It has 3 invariant reactions (transformation):

• The one at 727°C is known as Eutectoid transformation where austenite decomposes into a mixture of ferrite and cementite. This is known as Pearlite. Eutectoid reaction at 723° C and 0.8% C
  γ – iron ⟷ α – ferrite + Fe3C (cementite) (pearlite)

• The one occurring at 1495°C is the peritectic reaction. The delta ferrite reacts with liquid to form austenite.
  Peritectic reaction at 1495° and 0.16% C
  δ-ferrite + L ⟷ γ-iron (austenite)
• The one at 1148°C is known as the eutectic reaction where the liquid transforms into a mixture of austenite and cementite. The eutectic is known as Ledeburite.
  Eutectic reaction at 1147° C and 4.3% C
  L ⟷ γ-iron + Fe3C (cementite)(Ledeburite)

Concept:

Steel is an alloy of iron and carbon. Composition up to 2.1%C is steel and beyond this, it is considered as cast iron.

Eutectoid/Pearlite steel:

A 0.84% carbon steel or eutectoid steel is known as PEARLITE steel. This is much stronger than ferrite or cementite.

• Hypo-eutectoid Steel
  • Plain carbon steels in which carbon percentage is less than 0.8% are called hypo-eutectoid steel.
• Hypereutectoid Steel
  • Plain carbon steels in which carbon percentage is more than 0.8% are called hyper-eutectoid steel.

Concept:

Wrought iron is a mixture of very pure iron and slag. Wrought iron is a soft, ductile, fibrous variety that is produced from relatively pure iron globules partially surrounded by slag.

• It usually contains less than 0.1% carbon and 1 or 2% slag.
• Wrought iron never cast.
• The composition of Wrought iron is,
  • Iron - 99-99.8%
  • Carbon - 0.05-0.25%
  • Manganese - 0.01-0.1%
  • Sulfur - 0.02-0.1%
  • Phosphorus - 0.05-0.2%
  • Silicon - 0.02-0.2%

Explanation:

Mild steel means the presence of carbon is very low that is less than 0.3%.

Explanation:

• Cast irons typically contain 2 - 4 wt % of carbon with a high silicon concentration and a greater concentration of impurities than steel.
• Carbon in cast iron can be present in two forms:
  • Free carbon or graphite form
  • Chemically combined form
• Carbon is present in the form of graphite in grey cast iron and in the form of combined cementite in white cast iron.
• A high cooling rate and a low carbon equivalent favor the formation of white cast iron whereas a low cooling rate or a high carbon equivalent promotes grey cast iron.
• The higher the percentage of carbon more will be cementite and the harder will be the iron.​

​Additional Information

​Ferrite:

• ​Ferrite, a ceramic-like material with magnetic properties that is useful in many types of electronic devices.
• Ferrites are hard, brittle, iron-containing, and generally grey or black and are polycrystalline i.e., made up of a large number of small crystals
• They are composed of iron oxide and one or more other metals in chemical combination.
• A ferrite is formed by the reaction of ferric oxide (Fe2O3, iron oxide, or rust) with any of a number of other metals, including magnesium, aluminum, barium, manganese, copper, nickel, cobalt, or even iron itself.

Cementite:

• Cementite (or iron carbide) is an interstitial compound of iron and carbon, with the formula Fe3C.
• By weight, it is 6.67% carbon and 93.3% iron.
• It has an orthorhombic crystal structure.
• It is a hard, brittle material, normally classified as a ceramic in its pure form, and is a frequently found and important constituent in ferrous metallurgy.

Pearlite:

• It is formed by the decomposition of austenite at 723°C.
• The decomposition of austenite leads to the formation of a eutectoid mixture of 87% ferrite and 13% cementite called pearlite.
• The pearlite constituent consists of alternate lamellae of ferrite and cementite and contains 0.8% carbon.

Concept:

According to the Iron-Carbon phase diagram, we call the composition of steel that contains 0.8 percent of carbon as eutectoid composition.

It is a certain composition where, upon cooling austenite to lower than 723°C (eutectoid reaction), the austenite turns into pearlite (alternate layers of ferrite and cementite).

Eutectoid reaction is given by

\(\gamma \underset{{0.8}}{\overset{{{{723}^o}}}{\longleftrightarrow}}\alpha + F{e_3}C \)

The eutectoid reaction can be observed in the Iron-carbon phase diagram.

Additional Information

Hypo-eutectoid Steel

Plain carbon steels in which carbon percentage is less than 0.8% are called hypo-eutectoid steel.

Hypereutectoid Steel

Plain carbon steels in which carbon percentage is more than 0.8% are called hypereutectoid steel.