IDB-HTS-040
Heat treatment · case hardening · plating · coatings
Heat & surface treatment
Changing a part after machining — steel heat treatment, surface hardening, and the plating/coating choices for corrosion, wear and appearance, plus the gotchas.
Abstract
After a part is made, treatments tune what the material alone can't: heat treatment changes bulk strength and hardness, surface hardening adds a wear-resistant skin over a tough core, and plating/coating add corrosion resistance, wear resistance or appearance. Each has dimensional and process consequences to design for.
Section 1 frames it. Section 2 is steel heat treatment. Section 3 is surface hardening. Section 4 is plating and coatings. Section 5 covers aluminium and stainless specifics. Section 6 is selection and gotchas.
1.What treatments do
Two questions decide the route: do you need to change the bulk (strength/hardness/ductility) or just the surface (wear, corrosion, looks)?
2.Steel heat treatment
| Process | Purpose | Result |
|---|---|---|
| Annealing | soften, relieve stress, ease machining | soft, ductile, coarse grain |
| Normalizing | refine grain, uniform structure | moderate strength, consistent |
| Quench & temper (harden) | high strength/hardness | hard then tempered to set toughness |
| Stress relief | remove residual stress (post-weld/machining) | little property change, stabilises dimensions |
The temper after quenching is the design lever: low temper = harder/more brittle, high temper = tougher/softer. Specify the target hardness (e.g. "harden & temper to 40–45 HRC"), not just "harden". See the Hardness reference for HRC↔strength.
3.Surface hardening
A hard skin over a tough core resists wear and contact fatigue (gears, shafts, cams) without making the whole part brittle:
| Method | Case depth | Notes |
|---|---|---|
| Carburizing | 0.5–2 mm | low-carbon steel; deep, hard case; some distortion |
| Nitriding | 0.1–0.5 mm | low distortion (low temp), very hard, corrosion resistant |
| Induction / flame | selective | medium-carbon steel; harden only where needed |
Specify case depth and surface hardness; the hard case raises bending and pitting fatigue strength (pairs with the Hertz and Gear references).
4.Plating and coatings
| Finish | Function | Notes |
|---|---|---|
| Zinc plate / galvanize | sacrificial corrosion | cheap, common; H-embrittlement risk on high-strength steel |
| Zinc-nickel | better corrosion | automotive, harsh environments |
| Electroless nickel (EN) | uniform hard + corrosion | even on complex shapes (no current path) |
| Hard chrome | wear, hardness | shafts, cylinders; decorative chrome is thin |
| Anodize (aluminium) | corrosion + wear + colour | type II decorative/dyeable, type III hardcoat (wear) |
| Passivation (stainless) | restore corrosion resistance | removes free iron; no dimensional change |
| Phosphate | paint base, mild corrosion, break-in | manganese/zinc phosphate |
| PVD / DLC | very hard, low friction, thin | tools, decorative, dry-running surfaces |
| Powder coat / e-coat | durable paint | thick build-up — account for it on fits |
5.Aluminium and stainless specifics
- Aluminium tempers (e.g. 6061-T6) come from solution + age heat treatmentdon't anneal a structural temper away with welding heat. Anodizing grows an oxide that adds ~half its thickness per side to dimensions; type III hardcoat is thicker — mask threads and account for build-up on fits.
- Stainless relies on its chromium-oxide film; passivation restores it after machining (which smears free iron and causes rust spots). It doesn't change dimensions or hardness.
- Galvanic pairing: plating and dissimilar coatings change the galvanic couplesee the Galvanic compatibility chart.
6.Selection and gotchas
- Hydrogen embrittlement: electroplating high-strength steel (≥ ~10.9 / 40 HRC) can cause delayed crackingbake within hours of plating, or use mechanical zinc / non-electrolytic finishes.
- Dimensional change: plating and anodizing add thickness; carburizing/quench can distort. Treat after rough machining and finish-grind critical features after treatment, or allow for growth.
- Masking: threads, bearing seats and electrical contacts usually need masking from coating build-up.
- Distortion: thin/asymmetric parts warp in quenchdesign symmetric sections, use press-quench or low-distortion nitriding where flatness matters.
- Checklist: decide bulk vs surface → pick heat-treat (with target hardness) and/or case hardening (depth + hardness) → choose corrosion/wear/appearance finish → check H-embrittlement, dimensional growth, masking and distortion → sequence treatment vs finish-machining on the drawing.