304 vs 316 Stainless Steel: Which Grade for Machined Parts?
Composition, corrosion resistance, machinability, and the 2026 price gap — plus the selection rules our own engineers apply when a drawing just says "stainless."
IsoCNC Engineering Team
CNC Machining & DFM Engineers · Updated · 8 min read
On this page
The 30-Second Answer
Default to 304. It machines the same, costs roughly half as much per kilogram, and covers the majority of indoor, food-equipment, and general industrial service. Pay for 316 when the part lives near chlorides — saltwater, coastal air, bleach washdown, chemical splash — because its molybdenum content is what stands between the part and pitting corrosion. Specify 316L when the part gets welded and the corrosion resistance has to survive the weld zone. Everything else is detail, and the details follow.
The One Difference That Matters: Molybdenum
304 (the default)
Chromium 18–20%, nickel 8–10.5%, molybdenum 0%. The general-purpose austenitic grade: good corrosion resistance in mild environments, excellent formability, the lowest price of the family — and, like all austenitic steels, essentially non-magnetic in the annealed state (see which metals are magnetic).
316 (chloride service)
Chromium 16–18%, nickel 10–14%, molybdenum 2–3%. The Mo is the whole story: it stabilizes the passive film against chloride attack, which is what pitting and crevice corrosion need to get started.
316L (welded assemblies)
Same alloy family as 316 with carbon cut from 0.08% to 0.03% max. Less carbon means less carbide precipitation at weld heat-affected zones — corrosion resistance survives the weld.
On paper the chemistries look like small variations. In service, the 2–3% molybdenum is the difference between a fitting that lasts a decade on a boat deck and one that tea-stains in a season.
Corrosion Resistance in Practice
Engineers compare pitting resistance with PREN — Pitting Resistance Equivalent Number, calculated as %Cr + 3.3 × %Mo + 16 × %N. The worked numbers: 304 lands around 18–20; 316 lands around 23–28. That roughly 40% jump in PREN is bought entirely with molybdenum, and it shows up exactly where chlorides concentrate: marine hardware, coastal enclosures, food plants that sanitize with hypochlorite, pharmaceutical and chemical wetted parts.
The industries that write 316 or 316L into their standards — marine, chemical processing, pharma — do it because pitting is a hole-through-the-wall failure mode, not a cosmetic one. If your part sits indoors away from chlorides, 304 is not a compromise; it is the correct engineering choice at half the material cost.
Machinability: What Changes on the Machine
Both grades are austenitic, gummy, and work-hardening — machinability around 45% against the 1212 free-machining-steel baseline. Neither is a joy to cut, and the technique is identical for both: sharp tools, positive rake, continuous feed that stays under the work-hardened layer, no rubbing, no dwell. 316 runs slightly harder on tooling because the molybdenum-bearing structure work-hardens a bit more aggressively, so expect cycle times a few percent longer and tools changed a bit sooner.
The quoting consequence: machine time is nearly the same for both grades, so the price difference between a 304 part and its 316 twin is mostly the material line. When a buyer asks us to shave cost on a stainless job, switching 316 → 304 where the environment allows is the cleanest saving available — bigger than any cycle-time optimization. More on the process side in our stainless steel CNC machining services page.
Strength, and the Magnet Question
Mechanically the grades are near-identical. Annealed 304 runs about 515 MPa tensile / 205 MPa yield minimum; 316 is in the same band (some specs list 485 MPa tensile / 170 MPa yield); 316L sits slightly lower again. For machined parts, strength essentially never decides between them — corrosion does.
On magnetism: in the annealed condition both are effectively non-magnetic. After cold work — heavy machining, bending, thread rolling — both can pick up a weak magnetic response. If your assembly is magnetic-sensitive (instruments, sensor housings), call out the annealed condition on the drawing rather than hoping the grade saves you; neither grade is magnetically bulletproof after fabrication.
The Price Gap in 2026
Current mill pricing tells the story plainly: 304 sheet trades around $2.0–2.2/kg while 316/316L sheet runs $4.0–4.2/kg — 316 is roughly double by weight, a gap of about $2–4/kg depending on form and market. In a year when the World Bank projects record aluminum prices and industrial metals stay elevated, that spread is not narrowing.
What that means per part depends on how much of the part is material. A small fitting machined mostly into chips barely notices the difference. A large housing starting as a 12 kg billet feels every dollar of it. As a rule: the bigger the blank and the higher the buy-to-fly ratio, the more the grade choice moves your quote — and the more worthwhile it is to ask whether the service environment truly requires 316.
Selection Rules We Apply on RFQs
Indoor, dry, food-adjacent, general industrial? 304. Correct choice, lowest cost, machines identically.
Saltwater, coastal spray, bleach washdown, chemical splash? 316. Pitting is the failure mode and Mo is the defense.
Welded assembly that must stay corrosion-resistant at the welds? 316L (or 304L for mild service). The L grades exist for exactly this.
Drawing just says "stainless steel"? Expect a question from us. Grade, condition, and passivation spec change both price and performance — a two-minute answer at RFQ stage prevents a wrong material cert at delivery.
Whichever grade: specify passivation. Machining embeds free iron in the surface; passivation per ASTM A967 restores the passive film. It is cheap insurance on every stainless part, and we include it as an in-line option. Choosing between grades often pairs with choosing a finish — our materials guide and the 6061 vs 7075 aluminum comparison follow the same decide-by-environment logic.
At the 300-series entry level, the machinability-vs-corrosion trade is a different decision — see our 303 vs 304 comparison.
Related: Milling either grade? Our stainless steel milling guide covers carbide speeds and chip loads by grade, plus the work-hardening rules that protect tools.
FAQ
Is 316 worth it for indoor or dry-service parts?
Usually no. If the part never sees chlorides, salt spray, or chemical splash, 304 delivers the same mechanical service at roughly half the material price. Pay for 316 when chloride-driven pitting is a real failure mode — marine hardware, coastal installations, washdown with bleach-based cleaners, chemical process wetted parts.
Can you weld 304 and 316?
Both weld well, but standard grades can suffer carbide precipitation (sensitization) in the heat-affected zone, which attacks corrosion resistance right at the weld. For welded fabrications where corrosion must survive the weld zone, specify the L grades — 304L or 316L with carbon at 0.03% max. Machined-then-welded assemblies are exactly where 316L earns its premium.
Are 304 and 316 magnetic?
In the annealed condition both are essentially non-magnetic austenitic grades. Cold work — including heavy machining, bending, or thread rolling — can make them slightly magnetic. If a drawing requires low magnetic permeability after fabrication, call out the annealed condition and expect the supplier to minimize cold work.
Does 316 machine slower than 304?
Marginally. Both rate around 45% machinability against the 1212 free-machining-steel baseline, and both work-harden if the tool rubs or dwells. In practice 316 is slightly harder on tooling because of its molybdenum content and stronger work-hardening response, so cycle times run a few percent longer — but the material price gap, not machine time, is what separates a 304 quote from a 316 quote.
Is 18-8 stainless steel the same as 304?
Yes for practical purposes. "18-8" names the chromium (~18%) and nickel (~8%) content of the original austenitic family — 302, 304, and 305 all qualify, and fastener stock labeled 18-8 is typically 304. When a drawing says 18-8, quote 304 and note the equivalence; if the application is chloride-exposed, that is the moment to propose 316 instead.
Related Resources
6061 vs 5052 Aluminum: Which Alloy for Your Parts?
6061 vs 5052 aluminum compared: strength, machinability, bending, welding, corrosion, anodizing, and 2026 prices —…
6061 vs 6063 Aluminum: Machining vs Extrusion
6061 vs 6063 aluminum compared: strength, machinability, extrudability, anodizing, and selection rules — why…
6061 vs 7075 Aluminum: A Machinist's Decision Guide
6061 vs 7075 aluminum compared: strength, weldability, machinability, corrosion resistance, and cost — plus what…
How to Outsource CNC Machining to China, Step by Step
How to outsource CNC machining to China, step by step: vet suppliers, compare landed cost, control quality remotely,…
Not Sure Which Grade Your Part Needs?
Send the drawing and tell us the service environment. Our engineers will flag the right grade — and quote both options when the choice is close, typically within 1–2 business days.