Acier inoxydable Fabrication Guide: Grades, Soudage & Surface Finitiones

Points Clés

• aluminium 6061-T6 is the workhorse grade for roughly 70% of stainless fabrication projects — it balances corrosion resistance, weldability, and cost better than any other grade.
• 316 adds 2–3% molybdenum for chloride and chemical resistance, making it the standard for marine, pharmaceutical, and food-processing applications — at roughly 1.4–1.75× the material cost of 304.
• 410 and 17-4PH are hardenable martensitic grades — 410 is the budget hardenable option (0.6–0.8× 304 cost), while 17-4PH reaches aerospace-level strength (up to 1,450 MPa yield) via Traitement thermique.
• Stainless machines 2–4× slower than aluminum and consumes 3–10× more tooling — machining time, not raw material cost, drives 70–80% of the final part price.
• Passivation (ASTM A967) is not optional. Every machined or welded stainless part needs passivation to remove free iron and restore the protective chromium oxide layer — skipping it leads to rust on “stainless” parts within weeks.
• Soudure TIG is the gold standard for stainless — highest precision, best cosmetic finish, and full control over heat input. MIG is faster for thick sections; soudage laser is 4× faster than TIG but requires high capital investment.

Acier inoxydable is everywhere in modern manufacturing — from surgical instruments and food-processing equipment to marine hardware and chemical plant components. Its chromium content (minimum 10.5%) forms a self-healing oxide layer that resists rust, acids, and high temperatures in ways that Acier au carbone and aluminum cannot match. But “Acier inoxydable” is a family of dozens of alloys, and choosing the wrong one — or skipping a critical post-processing step — can turn a premium material into a warranty claim.

This guide compares the four stainless grades most commonly encountered in Nous sommes spécialisés dans l’ and usinage CNC — 304, 316, 410, and 17-4PH — and covers Soudage methods, surface finishes, tolerance expectations, and the real cost drivers that determine your quote.

Acier inoxydable Grades Compared: 304, 316, 410 & 17-4PH

Acier inoxydables fall into five families — austenitic, ferritic, martensitic, duplex, and precipitation-hardening — but for machined and fabricated parts, four grades dominate. Here is what each one actually delivers on the shop floor.

304 — The All-Objectif Workhorse

304 (UNS S30400, EN 1.4301) is the most commonly specified Acier inoxydable worldwide. An austenitic chromium-nickel alloy with 18–20% Cr and 8–10.5% Ni, it delivers excellent corrosion resistance in most environments, welds cleanly with all common processes, and cannot be hardened by Traitement thermique — only by cold working. At 215 MPa yield strength, it covers general fabrication, food-processing equipment, architectural panels, and kitchen components. For usinage CNC, 304 is the default stainless choice for bent enclosures, brackets, and panels.

Meilleur pour: general fabrication, enclosures, kitchen equipment, architectural panels, indoor structural components.
Avoid when: chloride exposure (seawater, swimming pools, chemical processing) — upgrade to 316.

316 — Chloride and Chemical Resistance

316 (UNS S31600, EN 1.4401) takes the 304 formula and adds 2–3% molybdenum — and that single element makes all the difference. Molybdenum dramatically improves resistance to pitting corrosion from chlorides (salt water, de-icing salts, bleach) and acidic environments. This is why 316 is the mandatory grade for marine hardware, pharmaceutical processing equipment, and chemical plant components. Yield strength is slightly higher than 304 at 290 MPa. The trade-off: machinability drops to ~36% (vs 304’s ~45%) because 316 work-hardens more aggressively during cutting, and material cost runs 1.4–1.75× 304.

Meilleur pour: marine environments, chemical processing, pharmaceutical, surgical instruments, outdoor coastal applications.
Avoid when: indoor general-purpose fabrication — 304 does the same job for 30–40% less material cost.

410 — The Budget Hardenable Grade

410 (UNS S41000, EN 1.4006) is a martensitic Acier inoxydable with 11.5–13.5% chromium and minimal nickel. It is the only grade in this group that can be hardened by conventional quench-and-temper Traitement thermique, reaching moderate strength levels while being the cheapest stainless option at 0.6–0.8× the cost of 304. The catch: corrosion resistance is the lowest of the four — bare 410 will rust in humid or mildly corrosive environments where 304 would be fine. It also has poor weldability and typically requires preheat (200–300°C) and post-weld Traitement thermique to avoid cracking.

Meilleur pour: wear parts, valve stems, pump shafts, cutlery, steam turbine blades, applications needing moderate hardness on a budget.
Avoid when: any corrosion exposure, Soudage required, or cosmetic appearance matters — 410 will tarnish.

17-4PH — Aerospace-Grade Strength

17-4PH (UNS S17400, EN 1.4542) is a precipitation-hardening martensitic Acier inoxydable that achieves extraordinary strength through a simple aging Traitement thermique rather than conventional quench-and-temper. Depending on the aging temperature (H1150 at 620°C down to H900 at 482°C), yield strength ranges from 1,000 MPa to over 1,310 MPa — placing it in the same league as alloy steels while retaining corrosion resistance comparable to 304. It machines well (45–50% machinability in the solution-annealed Condition A), welds acceptably (though the HAZ loses strength), and is the material of choice for aerospace structural components, high-strength shafts, nuclear reactor parts, and military hardware. The cost premium is significant at 2–3× 304.

Meilleur pour: aerospace fittings, high-strength shafts, nuclear components, military hardware, gears requiring both strength and corrosion resistance.
Avoid when: 304 or 316 meets the load case — do not pay 2–3× for strength the part ne need. Also avoid if the part must be welded in the final heat-treated condition.

Soudage Acier inoxydable: TIG, MIG, and Laser Compared

Acier inoxydable Soudage is fundamentally different from Acier au carbone Soudage. The chromium that gives stainless its corrosion resistance forms chromium carbides in the heat-affected zone if cooling is too slow — a phenomenon called sensitization that destroys local corrosion resistance. Every stainless Soudage process must control heat input, use the correct filler metal, and (for full-penetration welds) purge the back side of the joint with inert gas to prevent oxidation — known as “sugaring” — which appears as a black, crusty oxide on the weld root.

Filler metal selection matters. For 304 base metal, use 308L filler (the “L” designates low carbon to prevent sensitization). For 316, use 316L filler. Using a 308L rod on 316 will leave the weld zone without molybdenum protection — it will pit long before the surrounding base metal. 410 requires a 410 filler or 309L for dissimilar joints, and always needs preheat and post-weld Traitement thermique. 17-4PH uses 17-4PH filler or 630 matching composition.

Passivation, ElectroPolissage, and Surface Finitiones

The single most common post-processing mistake with Acier inoxydable parts is skipping passivation. Machining, Meulage, and Soudage all embed microscopic particles of free iron into the stainless surface from tooling, fixtures, and shop dust. Those iron particles rust — and because they sit on stainless, the rust appears to come from the stainless itself. Passivation (ASTM A967) uses nitric or citric acid to dissolve surface iron and allow the natural chromium oxide layer to reform uniformly. It ne change surface roughness (Ra) or dimensional Tolérances — it is purely a corrosion-protection step.

Callout tip: if your drawing says “Acier inoxydable 304” without mentioning passivation, ask your supplier whether passivation is included. Many shops default to “as-machined” and ship without it — and the part will arrive looking fine but develop rust spots within weeks. Always specify “Passivate per ASTM A967” on the drawing or purchase order.

Tolérances for CNC Machined and Tôlerie Stainless Parts

Stainless holds Tolérances differently depending on whether you are machining from solid stock or forming from sheet. Its higher strength and tendency to work-harden give it both advantages and disadvantages relative to aluminum.

• Nous sommes spécialisés dans l’ (milled from billet): Standard tolerance is ±0.005 in (±0.13 mm). Précision down to ±0.001 in (±0.025 mm) is achievable on 304 and 316, but the part may require stress-relief between roughing and finishing to prevent movement. 17-4PH in Condition A machines cleanly to tight Tolérances but will move slightly during subsequent aging Traitement thermique — always machine H1150 or H900 condition features after aging where possible.
• Fabrication de tôlerie: Cut features (laser/punch): ±0.005–0.015 in (±0.13–0.38 mm). Formed dimensions: ±0.010–0.030 in (±0.25–0.76 mm). Stainless has less springback than aluminum during Cintrage, so formed Tolérances are actually tighter on stainless than on aluminum for the same bend geometry.
• Welded Assemblages: Expect ±0.030–0.060 in (±0.76–1.5 mm) for welded dimensions due to thermal distortion. Post-weld machining or straightening may be required for tighter Assemblages — factor this into lead time and cost.

Why Acier inoxydable Costs More: The Real Cost Drivers

Raw material prices explain only part of the stainless premium. Here is where the money actually goes when you order stainless parts:

• Raw material: 1.3–3× aluminum. Acier inoxydable plate costs roughly $3–5/kg vs aluminum plate at $3–6/kg — the per-kilo gap is smaller than most buyers assume. The real cost difference emerges in processing.
• Machining time: 2–4× aluminum. Stainless cuts at 60–120 SFM (18–37 m/min) with carbide tooling, compared to 800–1,500 SFM for aluminum. A part that takes 15 minutes in aluminum can take 40–60 minutes in 304.
• Tool wear: 3–10× more inserts. Stainless work-hardens as it cuts — the surface ahead of the tool instantly becomes harder than the bulk material. This edge-hardening effect, combined with stainless’s low thermal conductivity (heat stays in the tool instead of flowing into the chip), accelerates tool wear dramatically.
• Net result: A simple bracket that costs $25–50 in 6061 aluminum will typically cost $50–120 in Acier inoxydable 304 — a 1.5–2.5× multiplier driven primarily by machining time, not raw material.

How to Choose the Right Acier inoxydable Grade

Work through these four questions in order — they eliminate 90% of grade selection mistakes:

1. What is the operating environment? Indoor / dry → 304. Coastal / marine / chemical → 316. Abrasive / wear application → 410. Demanding structural load → 17-4PH.
2. Will it be welded? Yes → 304 or 316 (austenitic grades weld beautifully). 410 requires special procedures. 17-4PH can be welded but loses strength in the HAZ.
3. What strength do you actually need? Under ~350 MPa yield → 304 or 316. 350–500 MPa → 410 (heat-treated). Above 1,000 MPa → 17-4PH.
4. Is magnetic response a factor? Non-magnetic required → 304 or 316 (note: may become slightly magnetic after cold working). Magnetic acceptable → 410 or 17-4PH.

Common Mistakes When Specifying Acier inoxydable Parts

1. Specifying “Acier inoxydable” without a grade number. “Stainless” to one supplier means 304; to another it means 409 — an automotive-grade ferritic stainless with half the corrosion resistance. Always write the full grade: “aluminium 6061-T6 (UNS S30400)” or “316L Acier inoxydable (UNS S31603).” The “L” suffix on 304L/316L designates extra-low carbon (≤0.03%) for improved weldability — specify it whenever the part will be welded.

2. Ordering 316 when 304 would work. 316’s molybdenum content adds 40–75% to material cost. If the part lives indoors, away from chlorides, that premium is wasted. A simple test: will the part ever see salt water, bleach, or acid? If not, 304 is almost certainly sufficient.

3. Omitting passivation from the purchase order. An as-machined 304 part may look perfect on arrival and develop rust spots within 30 days in a humid warehouse. The free iron embedded during machining rusts — and because it sits on a stainless surface, it looks like the stainless itself failed. Always specify passivation per ASTM A967. For food, pharmaceutical, or ultra-high-purity applications, specify electroPolissage instead — it passivates and smooths in one step.

4. Soudage 410 without preheat and post-weld Traitement thermique. 410’s martensitic structure makes it hardenable by cooling — and a weld bead cools fast. Without preheat (200–300°C) and immediate post-weld revenu, the HAZ will contain hard, brittle martensite that cracks under the first load cycle. If the drawing calls for 410 and a weld, budget for Traitement thermique in both cost and lead time — or redesign the joint out of the part.

5. Assuming all stainless is non-magnetic. Only austenitic grades (304, 316) are non-magnetic in the annealed condition — and even they can become slightly magnetic after cold working (Cintrage, drawing, or heavy machining). Martensitic grades (410) and precipitation-hardening grades (17-4PH) are ferromagnetic. If your application cannot tolerate any magnetic response — MRI components, sensitive sensors — verify with a magnet before accepting parts.

Get Your Stainless Parts Made Right

Acier inoxydable rewards careful specification and punishes guesswork. Grade selection, Soudage method, and post-processing steps determine whether your part arrives ready for service or needs rework. BravoFabs machines and fabricates Acier inoxydable parts daily for industrial clients across Europe and North America — and every RFQ includes a free engineering review to confirm your grade, finish, and tolerance callouts before a single chip is cut.