Heat Treatment

Heat treatment changes a metal’s microstructure through controlled heating and cooling to target hardness, strength, toughness, and wear or fatigue performance.

Overview

Heat treatment is a set of controlled thermal cycles that tune a metal’s microstructure to hit specific mechanical properties. Common sub-processes include annealing and normalizing (soften, relieve stress, improve machinability), quenching and tempering (raise strength/hardness while restoring toughness), and case hardening like carburizing or nitriding (hard wear-resistant surface with a tougher core).

Choose heat treatment when material properties—not appearance—are the limiting factor: wear, fatigue life, impact resistance, springback control, or a hardness spec (e.g., HRC) must be met. It fits prototypes through production as long as the part can be fixtured and the shop can certify the cycle.

Tradeoffs: heat treat can cause distortion, size change, and surface oxidation/decarb (unless vacuum/inert). Thin sections, asymmetric geometry, and tight flatness/straightness requirements may need post-heat-treat grinding or finish machining and clearly defined inspection conditions.

Common Materials

  • 4140 steel
  • D2 tool steel
  • 17-4 PH stainless
  • 8620 steel
  • A2 tool steel
  • Ti-6Al-4V

Tolerances

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Applications

  • Gears and sprockets
  • Injection mold cores and inserts
  • Power transmission shafts
  • Wear plates and guide rails
  • Cutting dies and punches
  • High-strength fasteners

When to Choose Heat Treatment

Choose heat treatment when your drawing calls out hardness, tensile/yield strength, toughness, or case depth, and those properties drive performance or life. It’s a good fit for steels and precipitation-hardening alloys where process certification and lot traceability matter. Plan for it early if distortion risk could force secondary finishing operations.

vs Machined Surface Finishing

Choose heat treatment when the requirement is mechanical performance (HRC, strength, fatigue, wear), not just surface roughness. Machined surface finishing improves texture and can reduce friction, but it won’t deliver through-thickness property changes or a specified case depth.

vs Polishing

Choose heat treatment when you need a hardened or strengthened substrate before any cosmetic finishing. Polishing only modifies surface topography; it can’t prevent brinelling, galling, or tooth wear if the base material hardness is too low.

vs Coatings

Choose heat treatment when the surface must resist wear under load without relying on a thin applied layer, or when adhesion risk is unacceptable. Coatings can add corrosion or wear resistance, but they don’t fix an under-strength core and may crack or spall if the substrate is too soft or flexes.

vs Passivation

Choose heat treatment when you need higher strength or hardness; passivation is for corrosion performance of stainless surfaces and doesn’t increase wear or fatigue capability. Use heat treatment to hit mechanical specs, then apply passivation if corrosion requirements remain.

vs Hard Coatings

Choose heat treatment when you need bulk strength/toughness or a robust hardened case that won’t be compromised by local coating damage. Hard coatings add a very hard surface, but they depend on substrate support; inadequate base hardness can lead to premature cracking or delamination.

Design Considerations

  • Call out the exact heat treat condition and standard (e.g., material spec + HRC range + case depth + effective depth definition) instead of vague notes like “heat treat as needed.”
  • Indicate whether critical dimensions are to be held before or after heat treat and specify any post-heat-treat grinding/finish machining allowances.
  • Avoid thin, asymmetric sections and long slender features if straightness/flatness matters; add stock and plan secondary ops where distortion risk is high.
  • Specify atmosphere requirements (vacuum, inert, endothermic) when scale, decarb, or surface chemistry affects fit, fatigue, or subsequent coating/plating.
  • Define allowable hardness test locations and methods (Rockwell vs microhardness) so the shop can fixture parts and certify results without ambiguity.
  • Bundle parts by material/section thickness and consistent requirements to reduce setup variation, improve uniformity, and lower per-part cost.