Heat Treatment Basics (Quenching, Tempering, Annealing, Normalizing)

Heat treatment heats and cools metal to adjust hardness and toughness. Drawings often specify "HRC〇〇," and it affects the machining sequence. This article compiles the methods, hardness, materials, and cautions.

Methods and their purpose

Method	Purpose	Rough flow
Quenching	Harden	Heat → rapid cool
Tempering	Remove brittleness, restore toughness	After quench, reheat → cool
Annealing	Soften, relieve stress	Heat → slow cool
Normalizing	Make structure uniform	Heat → air cool

Quenching and tempering are always a pair (quenching alone is brittle).

Case hardening (carburizing / nitriding)

• Carburizing: adds carbon to the surface to harden it, keeping the core tough (gears, shafts)
• Nitriding: adds nitrogen to the surface; low distortion, high hardness
• For parts needing a wear-resistant surface with a tough core

Hardness (HRC)

Hardness is often specified on the Rockwell HRC scale. Higher is harder and more wear-resistant but more brittle — so tempering balances toughness.

Treatable materials

• Carbon steel (S45C, etc.): hardens by quenching
• Tool steel (SK, SKD, etc.): high hardness, wear resistance
• Alloy steel (SCM, etc.): strength, toughness
• General structural steel (SS400) and aluminum (A5052) don't harden by quenching

Impact on sequence & distortion

• After quenching, the material is hard and normal cutting is difficult → usually finished by grinding
• A common flow is "rough machining → heat treatment → grind to finish"
• Heat treatment causes distortion (warping), so leave allowance

FAQ

Q. Is quenching alone enough? A. Right after quenching it's hard but brittle. Temper to restore toughness and resist cracking.

Q. Can any material be hardened? A. No. Low-carbon SS400 and aluminum don't harden by quenching. Targets are carbon/tool steels like S45C.

Q. Does heat treatment change dimensions? A. It can cause distortion. That's why you leave allowance before treatment and grind to finish after.

Conclusion

Heat treatment is "balancing hardness and toughness." Quenching and tempering are a pair, finishing is by grinding — grasp this flow and drawing intent becomes easier to read.

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