Material Science and Heat Treatment for SSC JE Mechanical: Crystal Structures, Iron Carbon Diagram, Hardness Tests and Alloys
Rahul Kumar
SSC Exam Expert & Content Editor
In this note
- 1.Crystal Structures
- 2.Iron–Carbon Diagram
- 3.Phases and Microconstituents
- 4.Heat Treatment Processes
- 5.Mechanical Property Tests
- 6.Alloy Steels and Their Elements
- 7.Cast Irons
- 8.Non-Ferrous Alloys
- 9.Solved Examples
- 10.Quick Tips
- 11.Quick Revision Points
- 12.Practice Questions With Answers
- 13.How to Practise This Chapter
- 14.Frequently Asked Questions
PDF Notes: Material Science and Heat Treatment for SSC JE Mechanical: Crystal Structures, Iron Carbon Diagram, Hardness Tests and Alloys
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Material science explains the structure and properties of engineering materials, and heat treatment changes these properties. SSC JE Mechanical asks questions on the crystal structures, the iron–carbon diagram, the heat treatment processes, the hardness tests and the alloys. This guide on material science and heat treatment for SSC JE covers all of them, with tables, solved examples, practice questions and FAQs.
Crystal Structures
| Structure | Examples | Atoms per Unit Cell | Coordination Number | Atomic Packing Factor |
|---|---|---|---|---|
| Simple cubic | Polonium | 1 | 6 | 0.52 |
| Body-centred cubic (BCC) | Alpha-iron (ferrite), chromium, tungsten, molybdenum, vanadium | 2 | 8 | 0.68 |
| Face-centred cubic (FCC) | Gamma-iron (austenite), copper, aluminium, gold, silver, nickel, lead | 4 | 12 | 0.74 |
| Hexagonal close-packed (HCP) | Zinc, magnesium, titanium, cadmium | 6 | 12 | 0.74 |
- FCC metals are the most ductile because of the many slip systems (12); HCP metals are the least ductile.
- Allotropy is the existence of an element in more than one crystal form; iron is BCC below 912°C (alpha), FCC between 912°C and 1394°C (gamma) and BCC again above 1394°C (delta).
- Imperfections: point defects (vacancies, interstitials), line defects (dislocations), surface defects (grain boundaries) and volume defects (voids).
- A smaller grain size gives a higher strength and hardness (Hall–Petch relation).
- Cold working increases the strength and hardness (strain hardening) but reduces the ductility; recrystallisation annealing restores it.
Iron–Carbon Diagram
| Item | Value |
|---|---|
| Pure iron melting point | 1539°C |
| Eutectoid point | 0.8% carbon at 727°C (austenite → pearlite) |
| Eutectic point | 4.3% carbon at 1147°C (liquid → austenite + cementite = ledeburite) |
| Maximum solubility of carbon in austenite | 2.14% at 1147°C |
| Maximum solubility of carbon in ferrite | 0.022% at 727°C |
| Cementite (Fe₃C) | 6.67% carbon |
| Steel | Carbon up to 2.14% |
| Cast iron | Carbon above 2.14% (up to about 6.67%) |
| Hypo-eutectoid steel | Less than 0.8% C (ferrite + pearlite) |
| Hyper-eutectoid steel | 0.8% to 2.14% C (cementite + pearlite) |
Phases and Microconstituents
| Constituent | Details |
|---|---|
| Ferrite (alpha) | Interstitial solid solution of carbon in BCC iron; soft, ductile and magnetic |
| Austenite (gamma) | Solid solution of carbon in FCC iron; non-magnetic, soft and ductile; stable above 727°C |
| Cementite | Iron carbide; very hard and brittle |
| Pearlite | Lamellar mixture of ferrite and cementite; formed at the eutectoid composition; good strength |
| Martensite | Supersaturated solid solution of carbon in iron with a body-centred tetragonal (BCT) structure; formed by quenching; the hardest and most brittle structure of steel |
| Bainite | Formed by cooling at an intermediate rate; tougher than pearlite |
| Ledeburite | Eutectic mixture of austenite and cementite in cast iron |
Heat Treatment Processes
| Process | Method | Purpose |
|---|---|---|
| Annealing (full) | Heat to 30–50°C above the upper critical temperature, hold and cool very slowly in the furnace | Softens the steel, improves the machinability, relieves the internal stresses, refines the grain |
| Normalising | Heat above the upper critical temperature and cool in still air | Refines the grain, removes the stress and gives a better strength than annealing |
| Hardening | Heat to the austenite range and quench rapidly in water, oil or brine | Forms martensite; high hardness; brittle |
| Tempering | Reheat a hardened steel below the lower critical temperature (727°C) and cool | Reduces the brittleness and the internal stress, increases the toughness |
| Spheroidising | Prolonged heating just below 727°C | Makes the cementite spherical, improves the machinability of high-carbon steel |
| Austempering | Quench to a temperature above the martensite start and hold to form bainite | Hard and tough without distortion |
| Martempering | Quench to just above the martensite start, hold, then air cool | Reduces distortion and cracks |
- The critical cooling rate is the minimum rate of cooling that gives a fully martensitic structure.
- Hardenability is the ability of steel to be hardened to a depth; tested by the Jominy end-quench test; alloying elements increase the hardenability.
- Case hardening gives a hard surface and a tough core: carburising (adding carbon), nitriding (adding nitrogen), cyaniding (carbon and nitrogen), flame hardening and induction hardening.
Mechanical Property Tests
| Test | Details |
|---|---|
| Brinell hardness (HB) | A hardened steel or tungsten carbide ball indenter; used for soft metals and castings |
| Rockwell hardness (HRB, HRC) | Depth of indentation; scale B uses a 1/16 inch steel ball and 100 kgf; scale C uses a diamond cone and 150 kgf; fast and direct reading |
| Vickers hardness (HV) | A diamond pyramid indenter with an angle of 136°; suitable for all materials and thin sheets |
| Mohs scale | Scratch hardness for minerals: talc 1 and diamond 10 |
| Charpy and Izod impact tests | Measure the toughness (energy absorbed in fracture) of a notched specimen; Izod is a cantilever type and Charpy is a simple beam |
| Tensile test | Yield strength, ultimate tensile strength, percentage elongation and reduction in area |
| Creep test | Slow deformation under a constant load at a high temperature |
| Fatigue test | Failure under cyclic stresses; endurance limit |
| Jominy test | Hardenability |
Alloy Steels and Their Elements
| Element | Effect |
|---|---|
| Chromium (Cr) | Increases hardness, corrosion and wear resistance; stainless steel has more than 11% Cr |
| Nickel (Ni) | Increases the toughness and strength; stainless steel (18-8) has 18% Cr and 8% Ni |
| Manganese (Mn) | Increases hardness and toughness; Hadfield steel has 12–14% Mn, wear resistant |
| Molybdenum (Mo) | Increases the strength at a high temperature |
| Tungsten (W) | Retains hardness at a high temperature; high-speed steel (18-4-1) has 18% W, 4% Cr and 1% V |
| Vanadium (V) | Grain refinement and strength |
| Silicon (Si) | Increases the elasticity; used in spring steel and electrical steel |
| Sulphur and phosphorus | Impurities; sulphur causes hot shortness and phosphorus causes cold shortness |
Cast Irons
| Type | Features |
|---|---|
| Grey cast iron | Graphite in flakes; good machinability and damping; brittle in tension; used in machine beds and engine blocks |
| White cast iron | Carbon as cementite; very hard and brittle; used for wear-resistant parts |
| Malleable cast iron | Made by heat-treating white cast iron; ductile |
| Spheroidal graphite (SG or ductile) iron | Graphite as nodules; high strength and ductility; used in crankshafts and pipes |
| Chilled cast iron | White iron surface with a grey core, made by rapid cooling |
Non-Ferrous Alloys
| Alloy | Composition |
|---|---|
| Brass | Copper and zinc |
| Bronze | Copper and tin |
| Gunmetal | Copper, tin and zinc |
| Duralumin | Aluminium with copper, magnesium and manganese |
| Babbitt metal | Tin, antimony and copper (bearing material) |
| Monel | Nickel and copper |
Solved Examples
Example 1. What is the carbon percentage at the eutectoid point?
0.8%.
Example 2. What is the number of atoms in an FCC unit cell?
4.
Example 3. Which heat treatment gives the maximum hardness in steel?
Hardening (quenching), which forms martensite.
Example 4. Which test uses a diamond pyramid indenter?
The Vickers hardness test.
Quick Tips
- Memorise the key points of the iron–carbon diagram: 0.8%, 2.14%, 4.3% and 6.67%.
- Remember the atoms per cell: 2, 4 and 6.
- Remember that martensite is the hardest constituent.
- Remember that tempering follows hardening.
Quick Revision Points
- Eutectoid: 0.8% C at 727°C; eutectic: 4.3% C at 1147°C.
- FCC has 4 atoms per cell and an APF of 0.74; BCC has 2 and 0.68.
- Martensite is BCT and the hardest.
- Annealing gives the softest steel; hardening gives the hardest.
- Vickers uses a diamond pyramid.
Practice Questions With Answers
- What is the carbon content of the eutectoid steel?
Answer: 0.8% - What is the structure of martensite?
Answer: Body-centred tetragonal (BCT) - What is the atomic packing factor of FCC?
Answer: 0.74 - What is the maximum solubility of carbon in austenite?
Answer: 2.14% - Which heat treatment process follows quenching to reduce brittleness?
Answer: Tempering - Which constituent is a lamellar mixture of ferrite and cementite?
Answer: Pearlite - Which hardness test uses a 136° diamond pyramid?
Answer: Vickers - Which cast iron has graphite flakes?
Answer: Grey cast iron - What is brass made of?
Answer: Copper and zinc - What does high-speed steel 18-4-1 contain?
Answer: 18% tungsten, 4% chromium and 1% vanadium
How to Practise This Chapter
Practise 10 questions every day and write the steps on paper. Then attempt timed sets from our practice papers and the previous year papers page. See all topics on the Mechanical Engineering notes page, read the previous topic Steam Boilers, Turbines and Power Plants and the next topic Industrial Engineering and Operations Management.
Frequently Asked Questions
What is allotropy?
The existence of an element in more than one crystal structure; iron shows it.
What is the iron–carbon diagram?
A phase diagram showing the phases of iron–carbon alloys at different temperatures and carbon contents.
What is the eutectoid reaction?
The transformation of austenite into pearlite at 727°C and 0.8% carbon.
What is martensite?
A very hard phase of steel formed by rapid quenching.
What is annealing?
Heating and slow cooling to soften steel and relieve the stress.
What is normalising?
Heating and air cooling to refine the grain.
What is tempering?
Reheating hardened steel to reduce the brittleness.
What is case hardening?
Hardening only the surface of a part while the core remains tough.
What is the Brinell test?
A hardness test that uses a steel ball indenter.
What is stainless steel?
Steel with more than 11% chromium, which resists corrosion.
Is this topic asked in SSC JE?
Yes. Material science and heat treatment are asked in the mechanical paper.
What are grey and white cast irons?
Cast irons with graphite flakes (grey) and with cementite (white).
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Rahul Kumar
SSC Exam Expert & Content Editor
Rahul is a senior SSC exam strategist with 8+ years of experience helping aspirants crack CGL, CHSL and MTS exams. He writes in-depth notification breakdowns, exam pattern guides and preparation strategies.