Is 304 or 316 the better grade for a part?

Neither is better in the abstract; the environment decides. 304 handles atmosphere, fresh water, most food contact and general fabrication at a lower cost per kilogram. Once chlorides are present in any real concentration — salt spray, brine, cleaning chemicals left on a surface, coastal air — the molybdenum in 316 is what keeps pits from starting, and the upgrade pays for itself the first time a part has to be replaced. If the part is also welded and will not be annealed afterwards, specify the low-carbon L variant so the heat-affected zone does not sensitise.

What does the L suffix change?

It caps carbon at 0.03% instead of 0.08%. The lower carbon leaves less chromium carbide able to form at grain boundaries during welding, which is the mechanism behind sensitisation and intergranular corrosion. Mechanical strength is slightly lower than the standard grade, so a design that was sized on the higher yield figure needs rechecking rather than a straight swap.

Can a 400-series grade replace 304 to save money?

Sometimes, and only when the duty allows it. 430 and 434 are ferritic: cheaper because they carry little or no nickel, but they are less ductile, do not weld as forgivingly, and lose corrosion resistance in the same chloride conditions that trouble 304. 410 and 420 are martensitic and can be hardened, which makes them right for wear surfaces and wrong for anything needing deep forming or a corrosion-critical weld.

How do I know the grade I received is the grade I ordered?

Ask for the mill certificate with the heat number and check it against the marking on the material, then have the lot tested rather than trusting the paperwork alone. Positive material identification on a sample confirms the alloy family, and a chemical analysis confirms the balance. For thin sheet and strip, a hardness or tensile check on the same lot catches a temper that was rolled to the wrong condition.

Does surface finish change the grade choice?

Finish changes performance inside the same grade, so it belongs in the decision. A polished or bright-annealed surface has fewer places for chlorides to sit and resists pitting longer than a rough rolled finish. Pickled and passivated stock removes the heat tint and free iron left by welding or handling; without that step, a 316 part can corrode as if it were a lower grade.

Which grades can be supplied as tube, sheet, bar and strip?

The austenitic range is the broadest: 304, 304L, 316, 316L and 321 run as welded and seamless tube, sheet, plate, round bar and strip. 317L and 904L are narrower and usually quoted per order. On the ferritic and martensitic side, 409 and 430 are common in sheet and coil, 410 and 420 in bar and strip. Availability moves with mill schedules, so the form and size belong on the enquiry.

Stainless Steel Grades: Families and Selection

Stainless steel strip coiled in a warehouse

Grade is only half the specification: the form, the finish and the certification travel with it.

Why One Grade Carries Three Names

Ask three suppliers for the same material and the answers can come back as 304, 1.4301 and S30400. None of them is wrong: the first is the American three-digit designation, the second the European number, the third the unified numbering system entry. Behind them sits one chemistry range, and that is the number a quote should actually be checked against.

The suffix matters as much as the digits. An L marks the low-carbon version, an H the high-carbon version, and a Ti or Nb marks a stabilised grade. Two grades that look almost identical on a drawing can therefore behave quite differently after welding, which is why a purchase order that names only “stainless” is an invitation to a dispute later. The grade, the standard it is ordered to, and the condition it arrives in are three separate lines on a good enquiry.

The families underneath the numbers are only four, plus the super-austenitic group that sits above them. Everything else is a variation on those.

The Four Families at a Glance

What separates the families is the crystal structure, and the structure is set by the balance of chromium, nickel and the other alloying elements. That balance decides whether a grade can be hardened by heat treatment, how far it can be formed, and how it behaves when welded.

Family	Typical chemistry	Hardenable by heat	Where it earns its place
Austenitic (300 series)	16–26% Cr, 6–22% Ni	No	General fabrication, process equipment, food and sanitary service, deep-formed parts
Ferritic (409, 430, 434)	10.5–18% Cr, little or no Ni	No	Automotive exhaust and trim, appliances, decorative sheet where cost rules
Martensitic (410, 420)	11.5–14% Cr, 0.1–0.4% C	Yes	Wear surfaces, cutlery, shafts, parts needing hardness rather than corrosion depth
Duplex (2205, 2507)	21–26% Cr, 4–7% Ni, Mo, N	No	Chloride service needing roughly twice the yield of 304 at moderate cost

Stainless steel coil end faces stacked in a warehouse

The same chemistry can arrive as coil, sheet, bar or tube; the form and the finish often decide the price as much as the grade does.

Austenitic: the 300 Series

This is the family most drawings mean when they say stainless. It is ductile, welds cleanly, and the passive chromium oxide film reforms within hours of being scratched as long as oxygen can reach the surface. The trade is price: nickel is the expensive ingredient, and the higher the nickel, the more chloride and acid the grade will tolerate.

301 — the work-hardening grade

Lower nickel and higher carbon than 304, so it work-hardens steeply and holds strength after forming. Springs, clips, conveyor parts and structural sections that are formed rather than machined belong here. It is not the grade for a part that has to stay soft through a deep draw.

304 and 304L — the default

Roughly 18% chromium with 8% nickel, and the grade that covers most of what industry needs at a price that survives a budget review. Tanks, frames, covers, guards, pipework and general machined parts are all ordinary work for it. Two limits are worth remembering: chlorides cause pitting, and the standard grade sensitises at the weld if it is not annealed afterwards. Specify 304L when a weld will not see a solution anneal.

316 and 316L — the chloride answer

Two to three percent molybdenum is the difference. It raises resistance to pitting and crevice attack in salt water, brines, cleaning agents and coastal air, which is why sanitary, marine and chemical service default to it. It is not a universal upgrade: 316 still pits in warm concentrated chloride, and paying for it where 304 would do is money spent for nothing.

317L — more molybdenum again

A step beyond 316 for aggressive chloride and mild acid duty, with higher molybdenum and a low carbon content for welded assemblies. Availability is narrower than 316, so size and form belong on the enquiry rather than in an assumption.

Ferritic and Martensitic: the 400 Series

The 400 family drops most of the nickel and saves cost, at the price of ductility and weldability. It is split by what happens on cooling: the ferritic grades stay soft, the martensitic grades harden.

409 — exhaust and heat

Stabilised with titanium and cheap enough for high-volume exhaust systems, where thermal cycling matters more than a polished surface. It is a thin-gauge grade, rarely the right answer for a machined component.

410 and 420 — hardenable wear grades

410 is the general-purpose martensitic grade and 420 carries more carbon, so it reaches a higher hardness after quench and temper. Both are used for blades, wear plates, shafts and valve parts. Neither likes deep forming, and both need a corrosion allowance in the design because their chromium is spent on hardness rather than on a thick passive film.

430 and 434 — sheet and trim

430 is the everyday ferritic for appliances, kitchen equipment and decorative panels. 434 adds molybdenum for automotive trim and other applications that see road salt, where plain 430 would stain. Both are weldable with care but are not substitutes for 304 in a pressure or sanitary duty.

904L and the Super-Austenitic Grades

Above the 300 series sit the high-alloy austenitics: around 20% chromium, 25% nickel, 4.5% molybdenum and a copper addition in the case of 904L. That combination is what stands up to sulphuric and phosphoric acid service, and it is why the grade appears in chemical plant, pulp processing and scrubber systems rather than in general fabrication. The 6% molybdenum grades and their relatives take the same idea further for seawater and chloride-heavy processes.

These are specification-led purchases. Availability, lead time and price move with the mill, and a supplier who quotes a date before checking is guessing. The honest sequence is to send the specification, the form, the size and the certification requirement, and to hear back what can actually be supplied.

Choosing Between Them

Work through the environment first, then the mechanical duty, then the fabrication route. The grade that survives all three is the one to quote; a grade chosen on price alone tends to come back as a corrosion claim.

What is on the surface? Fresh water and atmosphere point to 304. Chlorides in any concentration point to 316 or above. Acids point to 904L or a nickel alloy.
Will it be welded, and will it be annealed? If not, use the L grade or a stabilised grade so the heat-affected zone does not sensitise.
Does it need to be hard? Wear surfaces belong to 410 or 420; structural parts that must stay ductile do not.
How much forming? Deep draws and tight bends favour the austenitics. Ferritics crack where austenitics stretch.
What finish is specified? 2B, BA, No.4, hairline or mirror changes both the look and the corrosion behaviour, and it changes the price.

Where two grades both pass, take the cheaper one and write the reason into the file. A documented decision is what makes the next order repeatable.

What Belongs on the RFQ

Most grade disputes trace back to an enquiry that named a family but not a condition. Six lines close that gap:

The grade and the standard it is ordered to — for example 316L to ASTM A240 for sheet, or A312 for welded tube.
The form and the exact size range, including wall or thickness tolerance class.
The condition: annealed, pickled, bright annealed, polished, or as-rolled.
The quantity, and whether cut lengths or offcuts are acceptable.
The certification: mill certificate to EN 10204 3.1, plus any test reports the application needs.
Any extra testing — positive material identification, intergranular corrosion, hardness, or a dimensional check on the lot that ships.

Send those and the answer is a quote that can be compared. Leave them out and the answer is a price against an assumption. Our stainless tube, sheet and material pages carry the forms and grades we hold, and the machining side covers parts made from them. If a grade is missing from those pages, send the specification and we will tell you what the mills can supply rather than guess.

Stainless Steel Grades: Families and Selection

Why One Grade Carries Three Names

The Four Families at a Glance