Heat treatment — annealing, normalising, quenching, tempering; precipitation hardening

WHY heat treatment exists

Aerospace parts (turbine blades, landing gear, airframe) demand contradictory things: high strength and enough toughness so cracks don't run. A single cooling path cannot give both, so we use sequences of treatments. The whole field is built on one fact:

For plain-carbon steel the key equilibrium phases are:

Austenite (γ) — FCC iron, dissolves lots of carbon, stable when hot (>~723 °C).
Ferrite (α) — BCC iron, dissolves almost no carbon, soft.
Cementite (Fe₃C) — hard, brittle iron carbide.
Pearlite — alternating layers of ferrite + cementite (from slow cooling of austenite).
Martensite — carbon trapped in distorted BCT iron; very hard, brittle (from quench).

The four classical steel treatments

1. Annealing — soften & relieve stress

2. Normalising — refine the grain

3. Quenching (hardening) — trap martensite

4. Tempering — restore toughness

Figure — Heat treatment — annealing, normalising, quenching, tempering; precipitation hardening

Precipitation (age) hardening — for alloys that can't form martensite

Aluminium (e.g. Al–Cu, the Duralumin family), Ni-superalloys and Ti alloys can't be quench-hardened like steel. Instead we use precipitation hardening, exploiting a solubility that falls as temperature drops.

Worked examples

Common mistakes (Steel-man → fix)

Flashcards

What microstructural variable does cooling rate control?

Which phase is trapped — slow cooling → equilibrium (pearlite/ferrite), fast cooling → martensite.

Define austenite vs ferrite.

Austenite = FCC iron, high-T, dissolves much carbon; ferrite = BCC iron, low-T, dissolves almost no carbon.

What is martensite and why is it hard?

Carbon trapped in a distorted BCT iron lattice (diffusionless shear); the strain blocks dislocation motion → very hard, brittle.

Annealing: cooling medium and goal?

Furnace (very slow) cooling; max softness/ductility, stress relief, coarse pearlite.

Normalising: cooling medium and benefit over annealing?

Still air (faster); finer grain → tougher & slightly stronger (Hall–Petch).

Why must quenched steel be tempered?

Pure martensite is glass-brittle with residual stress; tempering forms fine carbides to restore toughness.

State the Hall–Petch equation and meaning.

\sigma_y=\sigma_0+k/\sqrt{d}

; smaller grain

d

→ higher yield strength.

Three steps of precipitation hardening?

Solution treat (dissolve solute) → quench (supersaturate) → age (form fine precipitates).

Why does over-ageing reduce strength?

Precipitates coarsen, spacing

L

increases, Orowan stress

Gb/L

falls → easier dislocation bowing.

Why can't aluminium be hardened by quenching alone like steel?

It has no martensitic transformation; quench just gives a soft supersaturated solution that needs ageing.

State the Orowan bowing stress.

\Delta\tau = Gb/L

, extra shear to bow a dislocation through precipitates spaced

L

Recall Feynman: explain to a 12-year-old

Imagine hot toffee. If you cool it slowly, the sugar arranges neatly and the toffee is soft and easy to bite (annealing). If you dunk it in ice water super fast, the sugar gets frozen in a messy, locked-up way and it goes rock hard but cracks like glass (quenching). Then if you warm it gently for a bit, it relaxes just enough to stop shattering while staying hard (tempering). Some metals (like aeroplane aluminium) instead get sprinkled with tiny invisible lumps inside that trip up anything trying to bend the metal — that's precipitation hardening. Too many big lumps spaced far apart, though, and the trips become easy to dodge (over-ageing).

Connections

Iron-Carbon Phase Diagram — source of austenite/ferrite/cementite regions.
Dislocations and Plastic Deformation — why blocking dislocations = strength.
Hall-Petch Strengthening — grain-size effect used in normalising.
Diffusion in Solids — controls every slow-cooling transformation and ageing.
Aluminium Alloys (Duralumin) — main precipitation-hardening aerospace material.
Nickel Superalloys — γ′ precipitate hardening in turbine blades.
TTT and CCT Diagrams — cooling-rate vs phase maps that quantify quenching.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, heat treatment ka core funda ek hi hai: metal ki property uski microstructure se decide hoti hai, sirf composition se nahi. Aur microstructure ko hum heating-cooling se control karte hain. Sabse important baat — cooling rate se decide hota hai ki kaunsa phase trap hoga. Slow cooling = atoms ko diffuse karne ka time milta hai → soft equilibrium structure (pearlite/ferrite). Fast cooling = atoms ko time nahi milta, lattice achanak shear ho jaata hai aur carbon andar trap ho jaata hai → martensite, jo bahut hard par brittle hota hai.

Steel ke chaar classic treatments yaad rakho: Annealing (furnace mein slow cool, sabse soft), Normalising (air mein cool, grain fine ho jaata hai, isliye Hall–Petch se thoda strong aur tough), Quenching (paani/oil mein fast cool, sabse hard par bhangur), aur Tempering (quench ke baad halka reheat karke thodi hardness chhod ke toughness wapas laana). Aerospace gear ke liye "quench + temper" combo use hota hai — surface hard, core shatter na ho.

Aluminium aur Ni-superalloys ko quench se harden nahi kar sakte, kyunki unmein martensite nahi banta. Inke liye precipitation (age) hardening: pehle solution treat (solute ghol do), phir quench (supersaturated solution bana ke solute trap karo), phir ageing (low temp pe chhote-chhote precipitates banao jo dislocations ko rok dein). Yaad rakho — ageing zyada karoge to precipitates coarse ho jaate hain, spacing $L$ badh jaati hai, aur Orowan stress $Gb/L$ gir jaata hai → over-ageing se metal soft ho jaata hai. Isliye ek peak hardness time hota hai, usse zyada mat karo. Yeh sab aeroplane ke turbine blade, landing gear, airframe ke liye critical hai — strong bhi chahiye aur crack-resistant bhi.