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Leaving Cert Engineering: Iron–carbon diagram

How often Iron–carbon diagram comes up on the Engineering papers, every year it was asked, and questions to try.

HL Asked on 9 of the last 9 Higher Level papers, most recently in 2025. banker

June 2028 is the first exam on the new Engineering course. These come from the 2016–2025 papers, so read them as a guide.

Iron–carbon diagram, Higher Level(7 marks)

Quick ones on Iron–carbon diagram.

(a)Eutectoid point on the iron–carbon diagram (as used on the papers)?
  1. 4.3% C at 1130 °C
  2. 0.83% C at 723 °C
  3. 0.02% C at 910 °C
(b)What is cementite?
  1. Carbon-free γ-iron
  2. A soft solid solution of carbon in α-iron
  3. Iron carbide (Fe3C), hard and brittle
(c)Eutectoid reaction in steel on slow cooling through 723 °C?
  1. Austenite changes to pearlite
  2. Ferrite changes to martensite
  3. Liquid changes to austenite
Show the answers

(a) 0.83% C at 723 °C

(b) Iron carbide (Fe3C), hard and brittle

(c) Austenite changes to pearlite

Your turn: Higher Level questions on Iron–carbon diagram.

Higher Level

Asked on 9 of the last 9 Higher Level papers, most recently in 2025. banker

Every paper, year by year

YearWhere it came up
2025Q4
2024Q4
2023Q4
2022Q4
2021Q4
2019Q3
2018Q3
2017Q3
2016Q3

Links open the State Examinations Commission’s paper for that year.

More Iron–carbon diagram questions

Iron–carbon diagram, 2 marks

Effect of raising the carbon content of a slowly cooled steel from 0.2% to 0.8%?

  1. Softer and more ductile but weaker
  2. No change, as long as it is cooled slowly
  3. Harder and stronger but less ductile
Show the answer

Harder and stronger but less ductile

More carbon means more pearlite (hard cementite plates) and less soft ferrite. Strength and hardness rise, while ductility and weldability fall.

Iron–carbon diagram, 2 marks

Ferrite is?

  1. FCC iron stable above 723 °C: tough
  2. Nearly pure alpha-iron: soft and ductile
  3. Iron carbide (Fe3C): very hard and brittle
Show the answer

Nearly pure alpha-iron: soft and ductile

Ferrite is BCC alpha-iron holding at most about 0.02% carbon. It is soft and ductile. Iron carbide is cementite; FCC iron is austenite.

Iron–carbon diagram, 3 marks

Why does martensite not appear on the iron–carbon equilibrium diagram?

  1. It forms only on rapid cooling, not at equilibrium
  2. It exists only as a liquid above the liquidus line
  3. It forms only above 1400 °C, beyond the diagram
Show the answer

It forms only on rapid cooling, not at equilibrium

The diagram shows slow-cooled equilibrium phases. Martensite forms when austenite is quenched so fast that carbon is trapped in a distorted lattice.

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