Heat Treatment of Steel: Hardening, Tempering, Annealing, and More

Why Heat Treat?

Raw steel from the mill has whatever properties the manufacturing process gave it. Heat treatment changes the internal crystal structure to make it harder, tougher, softer, or more uniform — depending on what you need.

The Big Four

Hardening (Quenching)

Heat steel above its critical temperature (~1500°F for most carbon/alloy steels), then rapidly cool (quench) in oil, water, or air. The fast cooling traps carbon in the crystal structure, creating martensite — extremely hard but brittle.

• Result: Maximum hardness (up to 65 HRC depending on grade)
• Requires: Sufficient carbon content (≥0.30% C). Low carbon steels like 1018 don’t harden significantly.
• Problem: As-quenched parts are brittle — almost always followed by tempering.

Tempering

After hardening, reheat to a lower temperature (300–1100°F) and hold, then cool. This trades some hardness for toughness and ductility. Higher tempering temperature = softer but tougher.

Annealing

Heat above critical temperature, then cool SLOWLY (usually in the furnace). Produces the softest, most ductile condition. Used to relieve stress, improve machinability, or restore formability after cold working.

• Full anneal: Slow furnace cool. Maximum softness.
• Process anneal (stress relief): Lower temperature (1000–1200°F). Doesn’t fully soften but relieves internal stress from machining, welding, or forming.

Normalizing

Heat above critical temperature, then cool in still air (not quenched, not furnace-cooled). Produces a uniform, fine-grained structure with moderate hardness. Used to “reset” steel that’s been hot-worked or has inconsistent properties.

Case Hardening

Sometimes you want a hard surface (wear resistance) with a tough core (impact resistance). Case hardening achieves this:

Carburizing

Expose low-carbon steel (1018, 8620) to carbon-rich atmosphere at ~1700°F. Carbon diffuses into the surface (0.020–0.080″ deep). Then quench — the carbon-rich case hardens while the low-carbon core stays tough.

Nitriding

Expose steel to nitrogen atmosphere at ~975°F. Nitrogen diffuses into surface, creating extremely hard nitride layer. No quenching needed — minimal distortion. Used on 4140, nitralloy, and stainless steels.

Induction Hardening

Electromagnetic coil heats only the surface, which is then quenched. Hardens a specific area (gear teeth, bearing journals, wear surfaces) while the bulk remains tough. Fast, precise, local.

When to Specify What

• “Machine, then harden”: Machine in soft condition for ease, then heat treat. Expect 0.001–0.003″ distortion — plan for grinding after.
• “Harden, then grind”: For precision surfaces. Heat treat first, then grind to final dimension.
• “Stress relieve after welding”: Weldments in alloy steel should be stress relieved to prevent cracking. Spec: hold at 1100°F for 1hr per inch of thickness.

Need heat-treated parts? Tell us the material and required hardness — we’ll manage the heat treatment process.