Heat treatment and surface hardening
These processes change what a finished metal part can do, not its shape. Controlled heating and cooling, atoms soaked into the surface, or millions of tiny impacts make parts harder, tougher or longer-lasting under repeated loads. They usually come near the end of the route, after the part has been formed or machined.
The big idea
A metal's strength depends on how its atoms are arranged inside, and that arrangement can be changed without moving the outside shape. Think of a loaf of bread: the same dough becomes soft or crusty depending on how you bake it. Heat treatment changes the whole part; case hardening changes only a skin a fraction of a millimeter to a few millimeters deep; shot peening squeezes the surface so fatigue cracks struggle to grow. Three properties keep coming back: hardness (resistance to scratches and wear), toughness (resistance to cracking under shock) and fatigue life (how many load cycles a part survives).
The 3 processes in this chapter
Side by side
| Process | In one line | Best for | Batch size | Precision |
|---|---|---|---|---|
| 12.1Heat treatment (hardening, tempering, annealing) | Heat, quench and temper (or slow-cool) the whole part | Making steel hard and tough, or soft for machining | Single parts to continuous lines | Shape kept; slight distortion, often ground after |
| 12.2Case hardening and nitriding | Carbon or nitrogen diffused into the surface | Gears and shafts: a hard skin over a tough core | Furnace batches of hundreds to thousands | Nitriding: within hundredths of a mm; carburizing: slight distortion |
| 12.3Shot peening | Surface hammered by tiny balls to leave it compressed | Springs, gears, turbine parts that see repeated loads | Single parts to millions of springs | Shape essentially unchanged |
How to choose within this family
- The whole part must be hard or strong right through: heat treatment (quench and temper).
- The part must be soft to machine or bend first: annealing, then harden later if needed.
- The surface must resist wear while the core resists shock: case hardening.
- Distortion must be minimal: nitriding rather than carburizing.
- The part fails by fatigue (cracks after many cycles): add shot peening, often after hardening.
Key words
- Hardness
- resistance to scratching, denting and wear.
- Toughness
- ability to absorb a shock without cracking.
- Quench
- fast cooling in water, oil or polymer that locks in hardness.
- Temper
- gentle reheating after a quench that trades a little hardness for toughness.
- Anneal
- heating then very slow cooling that leaves metal soft.
- Case
- the hardened outer skin of a case-hardened part.
- Residual compressive stress
- a squeeze left inside the surface that keeps cracks closed.
- Fatigue
- slow cracking under many repeated loads, even small ones.
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