Material Science, Heat Treatment, Foundry, Welding and Forging

What to remember

• The iron-carbon diagram governs steel and cast iron: eutectoid at 0.8% C and 727 °C (pearlite), eutectic at 4.3% C and 1147 °C (ledeburite), maximum carbon in austenite 2.14%. Steel has below 2.14% carbon; cast iron has above it.
• Heat treatment changes properties without changing shape: annealing softens, normalising refines grain, hardening makes martensite, tempering restores toughness. Case hardening hardens only the surface.
• Manufacturing processes are chosen by property and shape: casting for complex shapes, welding for joining, forging and rolling for strong wrought parts (hot working above the recrystallisation temperature).

1. Crystal structure and defects

• FCC metals are very ductile because they have many slip systems (12). HCP metals have few slip systems and are less ductile.
• Miller indices describe planes: take intercepts, take reciprocals and clear fractions.
• Defects: point defects (vacancy, interstitial, substitutional impurity), line defects (edge and screw dislocations), surface defects (grain boundaries) and volume defects (voids).
• Plastic deformation occurs by slip of dislocations. Grain boundaries block dislocations, so finer grains give higher strength (Hall-Petch relation). Cold working multiplies dislocations and causes strain hardening.
• Gibbs phase rule: F = C - P + 2. At constant pressure: F = C - P + 1.
• Lever rule gives the fraction of phases in a two-phase region.

2. Mechanical properties and tests

• Strength: yield, ultimate tensile strength (UTS). Ductility: percentage elongation. Toughness: energy absorbed before fracture. Hardness: resistance to indentation.
• Hardness tests: Brinell (steel ball, soft metals), Rockwell (depth of indentation, direct reading, scales A, B, C), Vickers (diamond pyramid, any hardness).
• Impact tests: Charpy (simply supported beam) and Izod (cantilever).
• Creep: slow plastic deformation under constant stress at high temperature; important for boiler and turbine parts. Fatigue: failure under repeated stress below the yield strength; endurance limit is the stress below which steel does not fail.
• Alloys: stainless steel needs at least about 10.5% chromium; the common 18-8 grade (18% Cr, 8% Ni) is austenitic. Brass is copper and zinc; bronze is copper and tin; duralumin is aluminium with about 4% copper; high speed steel carries tungsten, chromium and vanadium.

3. Iron-carbon diagram

Phases: ferrite (α, BCC, very low carbon, soft and ductile), austenite (γ, FCC, non-magnetic, can dissolve up to 2.14% C), cementite (Fe₃C, 6.67% C, hard and brittle) and mixtures pearlite (ferrite plus cementite in layers, eutectoid mixture) and ledeburite (austenite plus cementite, eutectic).

• Hypoeutectoid steel: below 0.8% C (ferrite plus pearlite). Hypereutectoid steel: 0.8% to 2.14% C (pearlite plus cementite).
• Cast irons: grey (graphite flakes, good damping and machinability), white (cementite, hard and brittle), malleable (white iron heat-treated to give nodules of temper carbon), SG or nodular (magnesium or cerium added to give spheroidal graphite; high strength and ductility).

4. Heat treatment

• Martensite is a supersaturated solid solution of carbon in iron with a body centred tetragonal (BCT) lattice, formed by diffusionless shear transformation. Bainite forms at intermediate cooling rates (austempering gives tough bainite). Retained austenite remains if quenching is incomplete.
• Quenching severity: brine > water > oil > air.
• TTT (time-temperature-transformation) diagram shows isothermal transformation; the critical cooling rate just misses the nose of the curve and gives martensite. Hardenability (depth of hardening) is measured by the Jominy end-quench test; alloying elements increase hardenability.
• Case hardening gives hard surface with tough core: carburising (adds carbon to low-carbon steel), nitriding (nitrogen, no quench needed), cyaniding and carbonitriding, flame hardening and induction hardening (medium carbon steel).
• Martempering and austempering reduce distortion and cracking.

5. Foundry (casting)

Steps: pattern making, mould making, melting and pouring, solidification, cleaning.

• Pattern allowances: shrinkage (pattern made larger), machining (finish allowance), draft (taper for withdrawal), shake or rapping (pattern made slightly smaller), distortion allowance.
• Moulding sand properties: permeability (escape of gases), refractoriness, cohesiveness, green strength, collapsibility, flowability.
• Gating system: pouring basin, sprue, runner, ingate. The riser feeds liquid metal to compensate for solidification shrinkage and must solidify last. Chills help directional solidification; chaplets support cores; core prints hold cores.
• Chvorinov's rule: solidification time t = B (V / A)², where V is volume and A is cooling surface area. A riser needs a larger V/A than the casting (commonly about 1.2 times).
• Cupola melts cast iron. Crucible and electric arc or induction furnaces melt steel and non-ferrous metals.
• Special casting: die casting (hot chamber for low melting alloys like zinc, cold chamber for aluminium), investment or lost-wax casting (excellent accuracy), centrifugal casting (pipes), shell moulding (thin resin-bonded shells), permanent mould casting.
• Defects: blow holes and porosity (trapped gas), shrinkage cavity, misrun (incomplete filling), cold shut (two streams not fusing), hot tear (cracking during cooling), sand inclusion, mould shift.

6. Welding

• Arc welding: an electric arc melts the joint. Shielded metal arc welding uses a flux-coated consumable electrode. TIG (GTAW) uses a non-consumable tungsten electrode with argon or helium shield. MIG/MAG (GMAW) uses a continuously fed consumable wire. Submerged arc welding uses a granular flux blanket and gives deep penetration.
• Polarity: DC electrode negative (straight polarity) puts more heat on the workpiece; DC electrode positive (reverse polarity) puts more heat on the electrode. Alternating current is used for general work with transformers.
• Heat input per unit length H = η V I / S (S = travel speed).
• Gas welding: oxy-acetylene flame. Neutral flame (equal gas) for steel; carburising flame (excess acetylene) for hardfacing; oxidising flame (excess oxygen) for brass and bronze.
• Resistance welding: spot, seam, projection and butt. Heat = I² R t.
• Other processes: thermit welding (aluminium and iron oxide, used for rails), friction welding, electron beam, laser beam, ultrasonic welding.
• Brazing uses filler above 450 °C below the base metal melting point; soldering uses filler below 450 °C.
• Heat affected zone (HAZ) is the base metal region whose structure changes. Preheating and slow cooling prevent cracks in high-carbon steels.
• Defects: porosity, slag inclusion, undercut, lack of fusion, incomplete penetration, cracks, spatter.
• Welded joints between boiler pressure parts and pipes in power plants need qualified procedures and radiography or ultrasonic testing.

7. Forging, rolling and other metal forming

• Hot working is done above the recrystallisation temperature (about 0.3 to 0.5 of the absolute melting temperature). It needs lower force, gives no strain hardening and refines grain, but surface finish is poorer and oxidation occurs. Cold working is done below it: better finish and accuracy, strain hardening, higher strength, but higher forces.
• Forging: open die (smith forging), closed die or impression die (flash is excess metal), drop forging, press forging, upset forging (heads of bolts). Volume constancy: A0 L0 = A1 L1.
• Rolling: reduction in thickness is draft = h0 - hf. The maximum possible draft is μ² R, where μ is the coefficient of friction and R is roll radius. The bite angle satisfies tan α = μ.
• Extrusion: direct and indirect. Drawing: wires and tubes pulled through a die. Deep drawing: cups from sheet.
• Sheet metal: blanking (the cut piece is the product, so die size equals blank size and the punch is smaller), piercing (the punched hole is the product, so punch size equals hole size and the die is larger). Cutting force = shear strength × perimeter × thickness. Springback occurs after bending.
• Powder metallurgy: blending, compacting and sintering; used for porous bearings and tungsten carbide tools.

Exam traps

• Ferrite is BCC; austenite is FCC; cementite is a compound, not a solid solution.
• Eutectoid (0.8% C, solid to solid) is different from eutectic (4.3% C, liquid to solid).
• Annealing cools in the furnace; normalising cools in air.
• Martensite is hard because of trapped carbon in BCT lattice, not because it contains cementite.
• Nitriding does not need quenching; carburising does.
• Shrinkage allowance makes the pattern larger; shake allowance makes it smaller.
• In blanking, die size equals blank size; in piercing, punch size equals hole size.
• Brazing is above 450 °C; soldering is below.

One-liners

• 1. BCC packing factor is 0.68; FCC and HCP is 0.74.
• 2. Eutectoid steel has 0.8% carbon.
• 3. Pearlite is a mixture of ferrite and cementite.
• 4. Cementite contains 6.67% carbon.
• 5. Jominy test measures hardenability.
• 6. Nodular cast iron contains spheroidal graphite from magnesium treatment.
• 7. A riser must solidify after the casting.
• 8. TIG welding uses a non-consumable tungsten electrode.
• 9. Neutral flame has equal oxygen and acetylene.
• 10. Hot working is done above recrystallisation temperature.
• 11. Chvorinov's rule: t = B (V/A)².
• 12. Tempering follows hardening to reduce brittleness.

Practice questions

1. The number of atoms per unit cell in a BCC structure is

   1. 2
   2. 1
   3. 4
   4. 6
   Answer

   A. 2

   One at the centre plus eight corners shared by eight cells: 1 + 8 × 1/8 = 2.

2. The atomic packing factor of an FCC structure is

   1. 0.68
   2. 0.34
   3. 0.52
   4. 0.74
   Answer

   D. 0.74

   FCC and HCP are close-packed; BCC is 0.68 and simple cubic 0.52.

3. The eutectoid composition of the iron-carbon system is

   1. 4.3% carbon at 1147 °C
   2. 0.8% carbon at 727 °C
   3. 6.67% carbon at 1493 °C
   4. 2.14% carbon at 1147 °C
   Answer

   B. 0.8% carbon at 727 °C

   Austenite changes to pearlite at 0.8% C and 727 °C.

4. Cementite contains carbon of

   1. 0.8%
   2. 4.3%
   3. 2.14%
   4. 6.67%
   Answer

   D. 6.67%

   Fe₃C has 6.67% carbon by weight.

5. Pearlite is a mixture of

   1. Austenite and martensite
   2. Ferrite and graphite
   3. Ferrite and cementite
   4. Cementite and ledeburite
   Answer

   C. Ferrite and cementite

   It is the lamellar eutectoid product of austenite.

6. Iron-carbon alloys with carbon above 2.14% are classified as

   1. Mild steel
   2. Cast iron
   3. Wrought iron
   4. High speed steel
   Answer

   B. Cast iron

   2.14% is the maximum solubility of carbon in austenite and the steel/cast iron boundary.

7. Martensite has a

   1. Simple cubic structure
   2. Body centred tetragonal structure
   3. Hexagonal close packed structure
   4. Face centred cubic structure
   Answer

   B. Body centred tetragonal structure

   Trapped carbon distorts the BCC lattice into BCT.

8. A steel with 0.4% carbon is cooled slowly. Taking the carbon in ferrite as zero, the fraction of pearlite is about

   1. 40%
   2. 25%
   3. 50%
   4. 80%
   Answer

   C. 50%

   Lever rule: 0.4/0.8 = 0.5.

9. At constant pressure, the number of degrees of freedom in a two-phase region of a binary alloy is

   1. 0
   2. 2
   3. 1
   4. 3
   Answer

   C. 1

   F = C - P + 1 = 2 - 2 + 1 = 1.

10. Slow cooling in the furnace after heating above the critical temperature is called

    1. Full annealing
    2. Quenching
    3. Austempering
    4. Normalising
    Answer

    A. Full annealing

    Normalising cools in air; annealing cools in the furnace.

11. Which treatment gives a finer pearlite and higher strength than annealing?

    1. Spheroidising
    2. Stress relief
    3. Tempering
    4. Normalising
    Answer

    D. Normalising

    Faster air cooling gives finer grain.

12. The Jominy end quench test measures

    1. Impact toughness
    2. Hardenability
    3. Fatigue limit
    4. Creep rate
    Answer

    B. Hardenability

    It shows how deep hardness penetrates in a quenched steel.

13. Tempering is done after hardening mainly to

    1. Reduce brittleness and internal stress
    2. Add carbon
    3. Increase hardness
    4. Change the grain to austenite
    Answer

    A. Reduce brittleness and internal stress

    Heating below A1 lets martensite transform to tougher structures.

14. Which process hardens the surface of low-carbon steel by adding carbon?

    1. Normalising
    2. Nitriding
    3. Annealing
    4. Carburising
    Answer

    D. Carburising

    Carburising adds carbon in the surface layer, then the part is quenched.

15. Which case hardening process does not need quenching after treatment?

    1. Nitriding
    2. Induction hardening
    3. Carburising
    4. Flame hardening
    Answer

    A. Nitriding

    Nitrogen forms hard nitrides at low temperature; no phase change is involved.

16. In nodular (SG) cast iron, the graphite is spheroidal because of the addition of

    1. Tin
    2. Magnesium
    3. Sulphur
    4. Silicon carbide
    Answer

    B. Magnesium

    Mg or Ce treatment produces spheroids and gives ductility.

17. White cast iron is hard and brittle because carbon is present as

    1. Flakes of graphite
    2. Spheroidal graphite
    3. Temper carbon
    4. Cementite
    Answer

    D. Cementite

    Rapid cooling gives iron carbide instead of graphite.

18. Consider: 1. Annealing involves furnace cooling. 2. Normalising gives finer grain than annealing. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct statements.

19. The pattern allowance that makes the pattern larger than the casting is the

    1. Distortion allowance
    2. Draft allowance
    3. Shake allowance
    4. Shrinkage allowance
    Answer

    D. Shrinkage allowance

    Shrinkage allowance compensates for contraction on cooling. Shake makes the pattern slightly smaller.

20. The ability of moulding sand to let gases escape is called

    1. Refractoriness
    2. Permeability
    3. Collapsibility
    4. Cohesiveness
    Answer

    B. Permeability

    Low permeability causes blow holes.

21. According to Chvorinov's rule, solidification time is proportional to

    1. (V / A)²
    2. V / A
    3. V × A
    4. A / V
    Answer

    A. (V / A)²

    t = B (V/A)².

22. A casting has B = 2 min/cm² and V/A = 3 cm. The solidification time is

    1. 6 min
    2. 12 min
    3. 18 min
    4. 36 min
    Answer

    C. 18 min

    t = 2 × 3² = 18 min.

23. If V/A of a casting is doubled, the solidification time becomes

    1. Same
    2. Four times
    3. Twice
    4. Half
    Answer

    B. Four times

    t is proportional to (V/A)².

24. A riser should

    1. Solidify after the casting
    2. Solidify before the casting
    3. Have a high surface area to volume ratio
    4. Be smaller than the casting
    Answer

    A. Solidify after the casting

    It must feed liquid metal until the casting is solid.

25. Hot chamber die casting is best suited for

    1. Aluminium alloys at high melting point
    2. Copper alloys
    3. Steel
    4. Zinc alloys
    Answer

    D. Zinc alloys

    Low-melting alloys do not attack the submerged pump parts as strongly.

26. Investment casting is also called

    1. Centrifugal casting
    2. Shell moulding
    3. Lost wax process
    4. Sand casting
    Answer

    C. Lost wax process

    The wax pattern is melted out of the ceramic mould.

27. Blow holes in castings are mainly caused by

    1. Trapped gases and poor venting
    2. Too much pouring speed only
    3. Low melting point
    4. Excess draft
    Answer

    A. Trapped gases and poor venting

    Gas that cannot escape stays as cavities.

28. A neutral oxy-acetylene flame is used for

    1. Welding brass
    2. Hardfacing
    3. Cutting only
    4. Welding mild steel
    Answer

    D. Welding mild steel

    Equal gas ratio gives a flame that neither adds carbon nor oxidises steel.

29. An oxidising flame is preferred for welding

    1. Mild steel
    2. Brass
    3. Aluminium
    4. Cast iron
    Answer

    B. Brass

    Oxidising flame forms an oxide layer that restricts zinc evaporation.

30. In TIG welding the electrode is

    1. Consumable wire
    2. Coated stick
    3. Non-consumable tungsten
    4. Carbon block only
    Answer

    C. Non-consumable tungsten

    Filler rod is added separately; argon or helium shields the weld.

31. In DC arc welding, straight polarity (electrode negative) puts more heat on the

    1. Nozzle
    2. Flux
    3. Electrode
    4. Workpiece
    Answer

    D. Workpiece

    With electrode negative, electrons strike the workpiece, giving more heat there.

32. Submerged arc welding uses

    1. Granular flux covering the arc
    2. Inert gas shield
    3. A tungsten electrode
    4. No filler wire
    Answer

    A. Granular flux covering the arc

    The flux blanket hides the arc and gives deep penetration.

33. Resistance spot welding with I = 1000 A, R = 0.001 Ω for 1 s generates heat of

    1. 1 J
    2. 100 J
    3. 1000 J
    4. 10 J
    Answer

    C. 1000 J

    H = I² R t = 10⁶ × 0.001 × 1 = 1000 J.

34. Arc voltage 25 V, current 200 A, travel speed 5 mm/s, efficiency 0.8. The heat input per mm is

    1. 4000 J/mm
    2. 800 J/mm
    3. 1000 J/mm
    4. 625 J/mm
    Answer

    B. 800 J/mm

    H = η V I / S = 0.8 × 25 × 200 / 5 = 800 J/mm.

35. Brazing differs from soldering in that the filler metal melts

    1. Below 450 °C
    2. Below 200 °C
    3. Above 450 °C
    4. Only below 100 °C
    Answer

    C. Above 450 °C

    Soldering uses fillers melting below 450 °C.

36. Hot working is carried out

    1. Below the recrystallisation temperature
    2. Above the recrystallisation temperature
    3. At room temperature only
    4. Only in vacuum
    Answer

    B. Above the recrystallisation temperature

    Strain hardening is removed as grains recrystallise during deformation.

37. A round bar of diameter 40 mm and length 100 mm is upset to a length of 25 mm without volume change. The new diameter is

    1. 160 mm
    2. 40 mm
    3. 57 mm
    4. 80 mm
    Answer

    D. 80 mm

    d² × 25 = 40² × 100, so d² = 6400 and d = 80 mm.

38. In rolling, the maximum draft possible is (μ = friction coefficient, R = roll radius)

    1. μ² R
    2. 2 μ R
    3. μ / R
    4. μ R
    Answer

    A. μ² R

    Bite condition tan α = μ gives h0 - hf = μ² R.

39. Rolls of radius 250 mm and μ = 0.1 can give a maximum draft of

    1. 25 mm
    2. 5 mm
    3. 2.5 mm
    4. 0.25 mm
    Answer

    C. 2.5 mm

    Draft = μ² R = 0.01 × 250 = 2.5 mm.

40. In blanking operation

    1. The die is smaller than the punch
    2. The die size equals the blank size
    3. The punch size equals the blank size
    4. The clearance is on the blank
    Answer

    B. The die size equals the blank size

    Clearance is taken on the punch. In piercing it is on the die.

41. A square blank 50 mm side is cut from 2 mm sheet with shear strength 300 N/mm². The cutting force is

    1. 120 kN
    2. 60 kN
    3. 30 kN
    4. 300 kN
    Answer

    A. 120 kN

    F = τ × perimeter × t = 300 × 200 × 2 = 120,000 N.

42. Consider: 1. Martempering reduces distortion and cracking. 2. Nitriding requires a quench to harden the case. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Nitriding needs no quench, so 2 is wrong.

43. Match the phase with its feature: 1 Ferrite, 2 Cementite, 3 Pearlite, 4 Martensite. P Hard, 6.67% C. Q Soft, BCC. R Lamellar eutectoid mixture. S Body centred tetragonal.

    1. 1-S, 2-R, 3-P, 4-Q
    2. 1-P, 2-Q, 3-S, 4-R
    3. 1-R, 2-S, 3-Q, 4-P
    4. 1-Q, 2-P, 3-R, 4-S
    Answer

    D. 1-Q, 2-P, 3-R, 4-S

    Ferrite is soft BCC, cementite hard with 6.67% C, pearlite layered mixture, martensite BCT.

44. Consider: 1. In blanking the punch is made smaller than the die. 2. In piercing the die is made larger than the punch. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both statements describe clearance applied to the opposite tool, so both are correct.

45. The Hall-Petch relation shows that

    1. Ductility is always highest in fine grains
    2. Yield strength increases as grain size increases
    3. Hardness is independent of grain size
    4. Yield strength increases as grain size decreases
    Answer

    D. Yield strength increases as grain size decreases

    Grain boundaries block dislocation motion.