5.4.3
Ek metal ki properties (hardness, strength, ductility, toughness) uski
chemical formula se fix nahi hoti — yeh uski microstructure pe depend karti hain: kaun se crystal phases present hain,
grains kitne bade hain, aur atoms kaise arrange hain. Heat treatment ek art hai jisme metal ko heat aur cool karte hain
deliberately, taaki us microstructure ko control kar sakein.
Sabse important idea: cooling rate decide karta hai ki kaun sa phase trap hoga
Aerospace parts (turbine blades, landing gear, airframe) mein contradictory cheezein chahiye hoti hain: high
strength aur itni toughness ki cracks na chalein. Ek single cooling path dono nahi de sakta,
isliye hum treatments ki sequences use karte hain. Puri field ek fact pe bani hai:
Definition Diffusion-controlled vs diffusionless
Diffusion-controlled transformations mein atoms ko move karna padta hai (lattice sites ke beech hop karna).
Iske liye time aur temperature chahiye. Slow cooling → equilibrium phases.Diffusionless (martensitic) transformation ek sudden lattice shear se hota hai
jab diffuse karne ka time hi nahi hota . Fast cooling → trapped, hard phase.
Plain-carbon steel ke liye key equilibrium phases hain:
Austenite (γ) — FCC iron, bahut carbon dissolve karta hai, hot hone pe stable (>~723 °C).Ferrite (α) — BCC iron, almost koi carbon dissolve nahi karta, soft.Cementite (Fe₃C) — hard, brittle iron carbide.Pearlite — ferrite + cementite ki alternating layers (austenite ke slow cooling se).Martensite — distorted BCT iron mein trapped carbon; bahut hard, brittle (quench se).
Transformation temperature se upar heat karo taaki austenite bane, hold (soak) karo, phir bahut slowly cool karo
(usually furnace mein).
YEH KYA KARTA HAI: coarse pearlite + large grains deta hai → maximum softness aur ductility ,
internal stresses hatata hai, machinability improve karta hai.
SLOW COOLING KYUN: atoms ke paas diffuse hokar equilibrium tak pahunchne ka poora time hota hai, isliye sabse
soft possible structure milti hai.
Annealing jaisi hi heating, lekin still air mein cool karo (furnace se faster, quench se slower).
YEH KYA KARTA HAI: finer pearlite aur chhote, zyada uniform grains → annealed steel se
tougher aur thoda stronger.
AIR-COOL KYUN: moderately faster rate se grains coarse nahi ho paate, structure refine hoti hai.
(Hall–Petch: chhote grains ⇒ harder, neeche formula dekho.)
Austenite tak heat karo, phir bahut fast cool karo (water/oil/brine) taaki carbon ke paas
FCC lattice se bahar diffuse karne ka time na ho . Carbon trap hota hai → lattice BCT martensite mein distort ho jaata hai.
YEH KYA KARTA HAI: maximum hardness aur strength, lekin residual stress ke saath bahut brittle .
HARD KYUN: trapped carbon atoms lattice ko strain karte hain aur dislocation motion rok dete hain.
Quenched (martensitic) steel ko moderate temperature (150–650 °C) par reheat karo, hold karo,
phir cool karo. Kuch carbon fine carbides ke roop mein precipitate hota hai, strain relieve hoti hai.
YEH KYA KARTA HAI: thodi hardness trade karke toughness/ductility mein bada gain milta hai, quench stresses
remove hote hain. Almost sab quenched steel ko temper kiya jaata hai → "quench & temper" ek hi process hai.
Aluminium (jaise Al–Cu , Duralumin family), Ni-superalloys aur Ti alloys ko steel ki tarah
quench-harden nahi kar sakte. Iske bajaye hum precipitation hardening use karte hain, ek aise
solubility ka fayda uthate hue jo temperature girne pe kum ho jaati hai.
Solution treatment: itna heat karo ki saara solute (jaise Cu) ek single solid
solution (α) mein dissolve ho jaaye.Quench: fast cool karo → ek supersaturated solid solution (SSSS); solute trap ho jaata hai,
bahar aane ka time nahi milta.Ageing: low/moderate temperature par hold karo (ya room temp = natural ageing ). Solute ab
dheere dheere tiny coherent precipitates (jaise GP zones, θ″) banata hai jo matrix mein disperse hote hain.
Intuition WHY tiny precipitates harden karte hainStrength dislocations ko rokne se aati hai. Fine, closely spaced coherent precipitates surrounding
lattice ko strain karte hain aur dislocations ko bend around ya cut through karne par majboor karte hain — dono mein
energy lagti hai, isliye metal deformation resist karta hai. Over-ageing (bahut lamba/hot) se precipitates
coarse aur door ho jaate hain → dislocations unke beech se aasaani se slip kar jaate hain → strength gir jaati hai . Isliye
ek optimum ageing time par peak hardness hoti hai.
Worked example Example 2 — Hall–Petch numbersEk steel mein σ 0 = 150 k = 0.7 MPa⋅m 1/2 d = 100 μ m d = 25 μ m
Annealed: σ y = 150 + 0.7/ 100 × 1 0 − 6 = 150 + 70 = 220 1 0 − 6 1 μ m = 1 0 − 6 k 1/2
Normalised: σ y = 150 + 0.7/ 25 × 1 0 − 6 = 150 + 140 = 290 kyun normalising strengthen karta hai .
Worked example Example 3 — Al–Cu age hardening pathDuralumin (Al–4%Cu).
Solution treat ~500 °C: saara Cu dissolve ho jaata hai. Kyun? Trap karne ke liye single phase chahiye.
Water quench: SSSS, Cu frozen in. Fast kyun? Taaki Cu abhi precipitate na kare.
Age ~150 °C par ghanton tak: GP zones → θ″ form karte hain. Moderate temp kyun? Itni atom mobility ki
fine precipitates nucleate ho sakein, itna hot nahi ki coarsen ho jaayein.
✅ Peak hardness reach hoti hai; agar bahut lamba age kiya → over-aged aur soft (Orowan L
Common mistake "Quenching hamesha kisi bhi metal ko harder banata hai."Kyun sahi lagta hai: steel ke liye, fast cooling = hard martensite, isliye hum generalize kar dete hain.
Fix: martensite ke liye FCC→BCT shear aur dissolved carbon chahiye. Pure aluminium mein aisi
koi transformation nahi hoti — quenching sirf ek supersaturated solution deta hai jo tab bhi soft hota hai jab tak
tum use age nahi karte. Bilkul alag mechanism hai.
Common mistake "Lamba ageing = stronger, isliye jitna ho sake utna age karo."Kyun sahi lagta hai: ageing strengthening precipitates banata hai, isliye zyada achha lagta hai.
Fix: peak ke baad, precipitates coarsen ho jaate hain (Ostwald ripening), spacing L G b / L over-ageing softens . Strength vs time ek hump hai, ramp nahi.
Common mistake "Annealing aur normalising basically same hain."Kyun sahi lagta hai: dono austenite mein heat karte hain aur air/furnace cool karte hain.
Fix: cooling rate alag hoti hai — furnace (anneal, softest, coarse) vs still air
(normalise, finer grain, tougher). Same heating ≠ same product.
Common mistake "Tempered steel weaker hoti hai, isliye tempering ek downgrade hai."Kyun sahi lagta hai: tempering mein hardness girti hai.
Fix: untempered martensite uselessly brittle hoti hai. Tempering thodi hardness trade karke bahut
toughness leta hai — net engineering value badhti hai .
Cooling rate kaun sa microstructural variable control karta hai? Kaun sa phase trap hota hai — slow cooling → equilibrium (pearlite/ferrite), fast cooling → martensite.
Austenite vs ferrite define karo. Austenite = FCC iron, high-T, bahut carbon dissolve karta hai; ferrite = BCC iron, low-T, almost koi carbon dissolve nahi karta.
Martensite kya hai aur hard kyun hoti hai? Carbon ek distorted BCT iron lattice mein trapped hota hai (diffusionless shear); strain dislocation motion block karta hai → bahut hard, brittle.
Annealing: cooling medium aur goal? Furnace (bahut slow) cooling; max softness/ductility, stress relief, coarse pearlite.
Normalising: cooling medium aur annealing se fayda? Still air (faster); finer grain → tougher aur thoda stronger (Hall–Petch).
Quenched steel ko temper kyun karna padta hai? Pure martensite residual stress ke saath glass-brittle hoti hai; tempering fine carbides banata hai toughness restore karne ke liye.
Hall–Petch equation aur meaning batao. σ y = σ 0 + k / d ; chhota grain
d → zyada yield strength.
Precipitation hardening ke teen steps? Solution treat (solute dissolve karo) → quench (supersaturate karo) → age (fine precipitates banao).
Over-ageing strength kyun reduce karta hai? Precipitates coarsen ho jaate hain, spacing L G b / L
Aluminium ko steel ki tarah sirf quenching se harden kyun nahi kiya ja sakta? Isme koi martensitic transformation nahi hoti; quench sirf ek soft supersaturated solution deta hai jise ageing chahiye.
Orowan bowing stress batao. Δ τ = G b / L L
Recall Feynman: 12-saal ke bacche ko explain karoHot toffee imagine karo. Agar slowly cool karo, toh sugar neat arrange ho jaati hai aur toffee soft
aur kaatne mein aasaan hoti hai (annealing). Agar use ice water mein super fast daal do, toh sugar ek
messy, locked-up tarike mein freeze ho jaati hai aur bilkul hard ho jaati hai lekin glass ki tarah toot ti hai (quenching). Phir agar use
dheere se thoda warm karo, toh yeh relax ho jaati hai bas itna ki shattering band ho jaaye aur hard bhi rahe
(tempering). Kuch metals (jaise aeroplane aluminium) ki andar tiny invisible
lumps sprinkle ho jaate hain jo metal ko modne ki koshish karne wali cheez ko trip karate hain — yahi precipitation hardening hai.
Bahut zyada bade lumps jo door door spaced hain, aur trips dodge karna aasaan ho jaata hai (over-ageing).
Mnemonic Order aur effect"All Nice Quenchers Temper" → A nneal (softest) → N ormalise (finer/tougher) →
Q uench (hardest/brittle) → T emper (wapas toughen karo). Hardness A→N→Q badh ti hai, phir T use
thoda wapas kheench ta hai. Alloys ke liye: "SQA" = S olution, Q uench, A ge.
Iron-Carbon Phase Diagram — austenite/ferrite/cementite regions ka source.Dislocations and Plastic Deformation — kyun dislocations block karna = strength.Hall-Petch Strengthening — grain-size effect jo normalising mein use hota hai.Diffusion in Solids — har slow-cooling transformation aur ageing ko control karta hai.Aluminium Alloys (Duralumin) — main precipitation-hardening aerospace material.Nickel Superalloys — turbine blades mein γ′ precipitate hardening.TTT and CCT Diagrams — cooling-rate vs phase maps jo quenching quantify karte hain.
normalising refines grain
Pearlite ferrite plus cementite
Martensite BCT hard brittle
Microstructure sets properties
