Stainless steel 304 or 316L: which grade should you choose for your parts?

Austenitic stainless steels 304 and 316L are the most widespread for machined parts, sheet and assemblies. Their name changes according to the standard: the American AISI designation (304, 316L) and the European designation of EN 10088, by number (1.4301, etc.) or by chemical composition. For manufacturing in Europe, the EN designation is generally the one on the material certificate: show it on your drawing, or state “equivalent accepted”.

The “L” stands for low carbon. Density is about 7.9 g/cm³ for 304 and 8.0 g/cm³ for 316L: for the same volume, the mass is almost identical, and the mass calculation tool gives you the weight of the raw stock. Both grades are non-magnetic in the annealed state, but can become slightly magnetic after cold working: a magnet therefore cannot tell them apart.

In the annealed state, the two grades have similar mechanical properties (a minimum yield strength of around 200 MPa, with high elongation): they are therefore rarely told apart on strength, but on corrosion resistance. Neither hardens by quenching; only work hardening increases their hardness. For high hardness, you need to turn to other stainless families, martensitic in particular.

Stainless steel resists corrosion thanks to an extremely thin layer of chromium oxide, which re-forms in air. Chloride ions (sea salt, de-icing salts, bleach, many cleaners and disinfectants) attack this layer locally and cause pitting corrosion, or crevice corrosion in gaps and under deposits. The molybdenum in 316L strengthens the layer against these attacks.

An index makes it possible to compare grades: the PREN (pitting resistance equivalent number), calculated from chromium, molybdenum and nitrogen. Its typical values are in the range of 18 to 20 for 1.4301 and 23 to 26 for 1.4404. The higher it is, the better the grade resists pitting, all else being equal.

What this changes depending on the use

• 304 is enough indoors, in a chlorine-free atmosphere: professional kitchens, furniture, guards, many mechanical parts.
• 316L is preferred at the seaside, in marine environments, in chemical and pharmaceutical processes, and whenever chlorinated cleaning or disinfection is regular.
• Beyond 316L: it too has its limits (prolonged immersion in seawater, hot chlorides, tight clearances). Duplex steels, such as 1.4462, or super-austenitic grades should then be considered.

Surface finish counts as much as the grade: a smooth, clean surface resists better than a rough or contaminated one.

Austenitic stainless steels are harder to machine than steel or aluminium. They work harden: the material hardens under the tool if it rubs instead of cutting. Their low thermal conductivity concentrates heat on the cutting edge, and their long, stringy chips break poorly. 316L is generally a little trickier than 304: tougher, it wears tools more.

CNC machining workshops compensate with sharp, suitable tools, lower cutting speeds than on steel, a sufficient feed to cut beneath the work-hardened zone, abundant coolant and chip-breaking strategies. The result: longer cycle times and tools replaced sooner, hence a higher machining cost. The cutting speed tool gives a reference for calculating speeds.

303 (1.4305), when corrosion is not critical

For turning and bar machining in production, a sulphur grade, 303 (1.4305), breaks chips more easily and machines much faster. In return, its corrosion resistance is lower (sulphides promote pitting) and welding it is not recommended. It does not replace 304 or 316L for a welded part or one exposed to corrosion, but, if the use allows, it can reduce the price: ask the workshop.

304 and 316L weld very well, in TIG as in MAG (see the guide TIG, MIG, MAG or stick welding). Two points need attention.

• Sensitisation. Between about 450 °C and 850 °C, carbon can combine with chromium and form carbides at the grain boundaries; the metal becomes depleted in chromium around the weld and susceptible to intergranular corrosion. The “L” versions (carbon of 0.03% or less) limit this risk: they are the natural choice for welded parts, especially thick ones. Titanium-stabilised grades, such as 316Ti (1.4571), are another answer.
• Distortion. Austenitic steels expand by about 16 µm per metre per kelvin, against 12 for structural steel, and conduct heat three times less well: welding distortion is greater, which calls for clamping, a welding sequence and sometimes wider tolerances.

After welding, the heated zone carries coloured oxides, less resistant to corrosion than the sound metal. Pickling, then passivation, restore the protective layer. Tools that have been used on carbon steel (brushes, grinding wheels) leave free iron and cause rust: ask for tools reserved for stainless steel.

316L costs more than 304 to buy as material: molybdenum and nickel are expensive alloying elements. Steel producers charge alloy surcharges that change every month: the gap between the two grades is therefore not constant, and we do not put a figure on it. Ask for the price of both grades rather than assuming it.

The price of the finished part depends on far more than the kilo of material: longer machining time than in steel, tools, welding distortion, finishes, inspection. For a small part, the material surcharge for 316L can remain modest compared with the machining cost; for a large sheet or a large blank, it weighs more. As for availability, 304 (or 304L) and 316L exist as sheet, bar, tube and sections at most distributors; unusual sizes and special grades lengthen lead times. Sheet finish also matters: a brushed or polished finish or a protective film add a cost that the price per kilo does not show.

The table summarises the differences. The decision is made on the part’s environment, not on habit.

A simple rule: start from 304 (or 304L if the part is welded), then move to 316L if it meets chlorides, chlorinated cleaning, a marine environment or a more severe corrosion requirement. This is common in the marine and boating industry as in the food industry, where disinfectants are often chlorinated.

Two cases call for caution. For food contact, say so: the grade, surface finish and finishing must meet the applicable requirements (Regulation (EC) No 1935/2004 on materials in contact with food). For medical use, the grade is most often fixed by a precise specification: do not replace it with an “equivalent” without validation.

• the exact designation, preferably in EN (1.4301, 1.4307, 1.4404, etc.), or “304L or 316L, equivalent accepted”;
• the form of the raw stock: round bar, flat, sheet (thickness, surface finish such as 2B or brushed), tube;
• the material certificate wanted (for example 3.1 according to EN 10204), the only reliable way to verify the grade and the heat: neither a magnet nor appearance distinguishes a 304 from a 316L;
• the environment of the part: outdoors, seaside, cleaning products, food contact;
• any welding, so the workshop selects the low-carbon version;
• the finishes: pickling, passivation, electropolishing, surface finish (Ra);
• the tolerances, quantities and lead time.

If the use does not justify 316L, say so: the workshop can then propose 304. To compare stainless steel with other materials, see the guide materials for machined parts. On Meaflow, posting a request is free and commission-free: you compare quotes from several workshops and talk by messaging before choosing.