Steel in humanoid robots is usually there for one of five reasons:
- Wear and rolling contact (bearings, races, gear teeth)
- High-cycle fatigue (shafts, pins, gear hubs, clevises)
- Stiffness and compact strength (thin sections under high load)
- Threads and clamp load reliability (fasteners, inserts, studs)
- Corrosion control (sweat/humidity, outdoor use, galvanic couples)
A good selection flow is:
Function → failure mode → required heat treat / surface → corrosion environment → manufacturing route.
Where steel shows up in humanoids
Joint/actuator internals
- Output shafts, input shafts, pins, eccentric cams
- Gear stages, splines, couplers
- Bearing seats, hardened races, wear sleeves
Structure & interfaces
- High-load brackets and clevises near joints
- Mounting plates that see repeated impact (falls)
- Threaded hardware, studs, dowel pins
Sensors & precision
- Encoder shafts, reference pins, datum tooling (often for stability and wear)
The “short list” of steels that cover most humanoid robot needs
4140 / 4142 (chromoly) — the default for shafts, pins, and general high-strength parts
Best for: actuator shafts, clevis pins, hubs, mounts, load-bearing brackets
Why: strong, tough, widely available, predictable heat treat, machines well in pre-hard states
Common condition: pre-hard (e.g., ~28–32 HRC) for good machinability; through-harden if needed for fatigue/wear
Surface upgrades: nitriding or QPQ for wear + some corrosion help
References (properties / overview):
Use it when: you need “strong and tough” without exotic cost.
4340 — when impact and fatigue are more severe
Best for: high-load output shafts, compact high-stress links, parts that must survive drops and shock
Why: higher hardenability and toughness than many general steels; strong fatigue performance when heat treated correctly
References:
Use it when: your FEA says 4140 is marginal, or field testing shows bending/fatigue cracking.
17-4 PH stainless — the workhorse “strong stainless” for robotic mechanisms
Best for: shafts, hubs, brackets, housings that need strength and corrosion resistance
Why: good strength with precipitation hardening, relatively stable, common in precision parts
Common condition: H900/H1025/etc. depending on strength vs toughness tradeoff
Watchouts: some environments and tempers can be less ideal for stress-corrosion than other stainless options; also not a “bearing steel.”
References:
Use it when: you want to reduce corrosion headaches (sweat/humidity) without giving up too much strength.
316 / 304 stainless — for corrosion-first components (not wear-first)
Best for: exposed hardware, covers, brackets in sweat / outdoor environments, fasteners when corrosion dominates
Why: excellent general corrosion resistance (316 better than 304 in chlorides)
Watchouts: lower strength than PH stainless or alloy steels; not ideal for wear or high clamp-load fasteners unless sized appropriately.
References:
Use it when: rust risk is your main enemy and loads are moderate.
52100 — for bearing races, rolling contact, and wear-critical precision parts
Best for: custom bearing races, rolling contact elements, hardened wear components
Why: classic high-carbon chromium bearing steel designed for rolling contact fatigue
Watchouts: corrosion (it will rust), heat treat control is critical, machining is harder in hardened state.
References:
Use it when: you need real bearing-grade performance, not just “hard steel.”
Tool steels (A2 / D2) — for dies, precision wear plates, and “it must not dent”
Best for: wear plates, detents, hard stops, forming tools, long-life jigs/fixtures, some gear-like wear applications
Why: excellent wear resistance and dimensional stability (varies by grade)
Watchouts: can be brittle if misapplied; corrosion still an issue.
References:
Use it when: you’re fighting indentation, fretting, or surface damage, not bending loads.
Spring steels (music wire / 1075–1095 / 17-7 PH / 301) — for compliant elements
Best for: springs, compliant flexures, latches, retaining clips
Why: high yield strength and fatigue performance when formed and heat treated properly
Common picks:
- Music wire (ASTM A228) for small, high-performance springs
- 17-7 PH when you need corrosion-resistant spring behavior
References:
- Music wire overview: https://www.asminternational.org/documents/10192/1849770/06701G_Chapter_03.pdf
A “which steel should I use?” map for humanoid parts
1) Shafts, pins, and joint axles
- Default: 4140/4142 (pre-hard)
- Higher fatigue/impact: 4340
- Need corrosion resistance: 17-4 PH (or 316 if loads are modest)
- Wear surface needed: add nitriding/QPQ or a hardened sleeve
2) Gears, splines, and torque-carrying interfaces
- For compact, high torque: consider steels intended for carburizing / case hardening (common industrial practice).
- Many teams use alloy steels with case hardening for wear + strong core toughness (process choice matters as much as alloy).
Good background on case hardening concepts:
3) Bearings and races
- Use off-the-shelf bearings when possible (they’re optimized systems).
- If you must make races: 52100 (with correct heat treat) is the typical direction.
4) Fasteners and threaded reliability
- Default clamp-load fasteners: high-strength alloy steel fasteners (properly specified grades)
- In corrosion environments: stainless fasteners can gall and may not deliver the same clamp load without careful selection and lubrication.
Useful fastener background:
- NASA fastener design manual (great general reference): https://ntrs.nasa.gov/citations/19900009424
5) Structural brackets near joints (where falls happen)
- If weight is less critical than durability: 4140 or 17-4 PH
- If you see denting/fretting at hard stops: add tool steel wear plates (A2/D2) or hardened inserts
The failure modes that should drive your steel choice
Fatigue cracking at fillets, keyways, and cross-holes
- If you see cracks after cycles: prefer tough alloy steels (4140 → 4340), improve fillets, and manage surface finish.
- Heat treat and shot peening can matter as much as alloy choice for fatigue.
Fretting wear at splines, clamp interfaces, dowel fits
- Consider surface hardening (nitriding/QPQ), wear sleeves, or tool-steel inserts.
Rolling contact failure (bearings)
- Don’t “fake” a bearing race with random hardened steel unless you’re prepared to validate it—rolling contact is specialized.
Corrosion-driven seizure, encoder drift, and service pain
- Sweat/humidity + dissimilar metals = galvanic mess.
- Use stainless strategically (17-4, 316), coatings, and isolation.
Heat treat and surface treatments that work well in humanoids
Nitriding (and QPQ / ferritic nitrocarburizing)
- Great for shafts/pins: improved wear, can help fretting, minimal distortion compared to some through-hardening
Background: https://www.bodycote.com/services/heat-treatment/thermochemical-diffusion/ferritic-nitrocarburising/
Black oxide / phosphate
- Basic corrosion mitigation and cosmetics; not a “real” corrosion solution without oil/wax/paint
Hard chrome / HVOF / DLC (selectively)
- Good for wear, but cost and process control go up; validate adhesion and thickness impact on fits
Passivation (stainless)
- Important after machining to improve corrosion performance
Background: https://www.astm.org/a0967-22.html
Practical rules of thumb for humanoid robots
- If it rotates on a bearing and you care about life: treat rolling contact as a system, not just a material.
- If it’s a shaft/pin and you’re iterating: 4140 pre-hard is the fastest “serious” choice.
- If it’s failing from impact/fatigue: 4340 + correct heat treat is a common upgrade.
- If corrosion is dominating your maintenance cycle: move exposed parts to 17-4 or 316, plus good finishing and isolation.
- If wear is dominating: surface engineering (nitriding/QPQ, sleeves, inserts) often beats switching alloys.
RFQ checklist for steel robot parts
When you request quotes, include:
- Part function: shaft / pin / gear / bracket / wear plate / fastener
- Alloy + spec: e.g., “4140 pre-hard” or “17-4 PH H1025”
- Heat treat target: hardness range (HRC), case depth (if case hardened), or “nitrided/QPQ”
- Critical fits: bearing seats, spline fits, threads (class), dowel bores
- Surface finish: Ra on sealing/wear faces, coating requirement
- Environment: indoor, humidity/sweat, outdoor, coastal
- Validation needs: cycle life target, proof torque, impact/drop expectations
Outbound references used in this guide
MatWeb (general material datasheets): https://www.matweb.com/
AZoM (alloy overviews): https://www.azom.com/
NASA Fastener Design Manual (NTRS): https://ntrs.nasa.gov/citations/19900009424
Ferritic nitrocarburizing overview (Bodycote): https://www.bodycote.com/services/heat-treatment/thermochemical-diffusion/ferritic-nitrocarburising/
ASTM A967 stainless passivation standard landing page: https://www.astm.org/a0967-22.html
Case hardening and gears overview: https://www.geartechnology.com/articles/0804/case-hardening-and-heat-treating-gears
Takeaways
- Default most shafts/pins to 4140/4142 pre-hard, then add nitriding/QPQ if wear shows up.
- Upgrade to 4340 when shock and fatigue drive failures.
- Use 17-4 PH when you need strength with much better corrosion behavior.
- Use 52100 (or commercial bearings) for real rolling-contact performance.
- For wear and denting, tool steel inserts / wear plates plus good surface engineering often beat “stronger steel.”