Stainless Steel 304L CNC Machining
304L is the low-carbon version of the general-purpose austenitic grade, and on a welded part it is the correct call-out every time. What makes it interesting commercially is that a great deal of the 304 bar and plate in circulation is already dual-certified as 304/304L — so the L often costs nothing, and the only real question on a machined part is whether the slightly lower specified minimums matter. They usually do not.
Specification at a glance
- Alloy family
- Austenitic stainless steel, non-hardenable, low carbon
- Composition
- ~18–20% chromium, ~8–12% nickel; carbon ≤0.03%
- Yield strength (annealed)
- ~170 MPa minimum — below standard 304
- Hardened by heat treatment
- No — only by cold work
- Machinability
- Moderate — marginally gummier than 304; 303 is the free-machining alternative
- Corrosion resistance
- Same as 304 in service, and better than 304 beside a weld
- Weldability
- Excellent — this is the reason the grade exists
- Magnetic
- Essentially non-magnetic annealed; slightly magnetic after cold work
- Typical finishes
- Passivation, electropolishing, black oxide, blasting, polishing, brushing
- Finishing route
- Through our vetted partner network, managed under one quality process
- Precision tolerance
- ±0.01 mm on precision features
- General tolerance
- ISO 2768-m unless stated on the drawing
- MOQ
- From 50 pieces per style and material
The L grade on the general-purpose alloy
304 is the stainless steel most buyers picture when they picture stainless: the austenitic 18/8 grade, stocked everywhere, welded and formed everywhere. 304L is the same alloy with carbon held below about 0.03% instead of 0.08%.
The two grades are otherwise interchangeable:
| 304 | 304L | |
|---|---|---|
| Carbon maximum | ~0.08% | ~0.03% |
| Minimum yield (annealed) | ~205 MPa | ~170 MPa |
| Welded, then in service | Sensitises | Standard choice |
| Machined only, no weld | Fine, marginally stronger | No advantage |
| Commonly dual-certified | — | Yes, widely available as 304/304L |
That is the entire difference. Everything that follows is about which of those rows actually applies to your part.
Why the L grade exists
Stainless resists corrosion because chromium forms a self-repairing oxide film. Take chromium out of the metal and you take the protection with it.
Welding does that indirectly. Beside a weld, the heat-affected zone passes through roughly 425–815 °C as the joint cools. In that window, chromium combines with carbon and forms chromium carbides, which precipitate at the grain boundaries. The carbides are not the problem; what they leave behind is. Chromium has been pulled out of the metal immediately around them, and that metal now sits below the level needed to maintain a passive film.
The consequence is a narrow band running beside the weld that is chemically the weakest part of the whole assembly. It corrodes first, it corrodes in a line that follows the joint, and the rest of the part being perfectly good material does not help. That is sensitisation.
Hold carbon to 0.03% and there is not enough of it present to form the carbides. The grain boundaries stay clean and the heat-affected zone keeps its corrosion resistance. The mechanism is set out in more detail on the 316L page, where the same metallurgy applies.
Dual certification: why the L often costs nothing
The commercially useful fact about 304L is that a large share of the 304 bar, plate and tube in circulation is dual-certified 304/304L. The heat is made to satisfy both specifications at once — its chemistry and mechanical results meet the requirements of 304 and of 304L — and the material test report states both designations.
This exists because mills and stockholders would rather hold one line of stock than two. The practical effects for a buyer are worth knowing:
- Where stock is dual-certified, the L designation costs nothing. The same bar serves either call-out and the price is the same.
- It also removes the paper-strength objection. If the certification satisfies 304 as well as 304L, the higher 304 minimums are met, and there is no trade to weigh at all.
- It does not remove the specification question. A drawing that says 304 is still satisfied by dual-certified stock; a drawing that says 304L is not satisfied by material certified only to 304. If welding is in the route, the safer call-out is the L.
So on a welded part the decision is easy. On a machined part there is nothing to gain from the L — but if that is the material already on the shelf and it is dual-certified, there is nothing to lose either.
How 304L machines
It machines like 304, which means like an austenitic stainless: it rewards method and punishes improvisation.
It work-hardens under the cutter. Deformation transforms some austenite to martensite, and martensite is considerably harder. A light pass rubs the surface instead of shearing it, the rubbed skin comes back harder, and the next pass has to cut hardened metal. A decisive depth of cut gets underneath it in one go.
It galls the tool edge. Austenitic stainless tends to weld to the cutting edge, and the low carbon in 304L makes it marginally gummier than standard 304. That shows up chiefly on light finishing passes, where built-up edge degrades the surface finish. Sharp, correctly ground edges and a suitable lubricant are the answer.
It conducts heat poorly, so heat concentrates at the cutting edge, tool life suffers and cutting speeds are limited.
What the material rewards:
- A decisive depth of cut — never a light spring pass to creep up on a dimension
- Sharp tooling, replaced on condition rather than on a schedule
- Flood coolant, to carry heat out of the cutting zone
- Rigid workholding, because thin sections deflect and a deflected stainless surface hardens where it moved
One practical alternative is worth naming: if the part is a high-volume turned item and corrosion resistance, not weldability, is the requirement, then 303 is the free-machining grade of the same family. The sulphur addition that makes 303 machine so much faster is also why it is a poor choice for welding, so the two grades solve opposite problems. The comparison is worked through in our 303 vs 304 article.
Threading needs attention on 304L as on any austenitic grade: taps rub, the material hardens against them, and torque climbs until something fails. A sharp tap used once, a suitable lubricant and a marginally larger tap drill are the standard route; thread milling avoids the tap altogether where the thread is critical. See thread standards.
Precision features are held to ±0.01 mm with ISO 2768-m as the general tolerance block — see tolerances we hold.
Where 304L is the right call
- Welded food and beverage equipment, where the surface is passivated
- Tanks, vessels, hoppers and pipework, welded and put into service
- Welded frames, brackets, guards and machine bases
- Architectural and structural fabrications in benign atmospheres
- Parts hot formed or welded part-way through the production route
- Any 304 part where the drawing already says L, which is common on food and process work
Where 304L is not the answer
The duty needs hardness or wear resistance. 304L cannot be hardened by heat treatment. Where the drawing calls for a hard surface or a high strength class, the answer is 410 or 416 for a turned wear part, or 17-4PH where strength and corrosion resistance are both needed.
The duty contains chlorides. The L grade does not extend the alloy’s corrosion limits. Salt water, coastal air, bleach and chloride process fluids will pit 304L exactly as they pit 304 — welded or not. The specification for those duties is 316 or 316L. The mechanism is in our 304 vs 316 article.
The part is a high-volume turned item with no welding. Then 303 machines several times faster and the L designation buys nothing.
The requirement is non-magnetic. Cold work makes 304L slightly magnetic, and heavy cold work makes it noticeably so. If non-magnetic is a hard requirement, state it on the drawing rather than assuming.
Finishing 304L
Passivation is the standard step after machining, and on a welded part the sequence matters.
Machining smears free iron into the surface, and free iron rusts while the stainless around it does not — which reads as a material failure when it is a cleaning problem. Passivation removes that contamination and restores the chromium oxide film.
Welding leaves oxide scale and heat tint beside the joint. Heat tint is not cosmetic: it is a chromium-depleted oxide layer sitting exactly where the sensitisation risk already is, and it has to be removed before passivation rather than after. Mechanical cleaning, pickling or electropolishing all serve; the order is clean first, passivate second.
Other finishes that pair with 304L:
- Electropolishing — smoother, brighter surface for hygienic and clean service
- Black oxide — a uniform matte black for cosmetic and low-reflectance parts
- Sand blasting and polishing and brushing — matte textures and directional finishes
All are arranged through our vetted partner network, managed under one quality process. The grades we run across every process are listed on the materials page.
What to put on the drawing
| Instead of | Write |
|---|---|
| Stainless steel | 304L if welded; 304 if machined only |
| 304, will be welded | 304L, or specify a post-weld solution anneal |
| 304L, machined only | Either grade — check whether the stock is dual-certified |
| 304L, needs hardness | Wrong grade — 410, 416 or 17-4PH |
| 304L, marine duty | Wrong grade — the L does not make it a chloride grade |
| 304L, high-volume turned part | 303 machines faster if weldability is not needed |
| 304L, welded and visible | State the finish — heat tint comes off before passivation |
Two lines decide most 304L specifications: is the part welded, and does the duty contain chlorides. The first is what the L grade is for. The second is what it cannot fix.
Material is bought against your drawing for your job, and every batch ships with a material certificate (MTR) tracing the grade to its mill heat plus pre-shipment photos of the actual parts. Orders start at 50 pieces per style and material, samples from a single piece in 5–10 days, and production runs in 25–35 days after sample approval. Send the drawing through the request a quote form and the reply comes back within 24 hours with DFM feedback — including a note where the grade and the duty are working against each other.
Frequently asked
- What is the difference between 304 and 304L?
- Carbon content. 304 permits up to about 0.08% carbon; 304L holds it to about 0.03% maximum. The chromium and nickel ranges are the same and the material behaves identically in service. The lower carbon limit changes what happens beside a weld, and it lowers the specified minimum strength values slightly.
- Should I specify 304 or 304L?
- If the part is welded and then used without a post-weld anneal, specify 304L — standard 304 sensitises beside the weld and that band corrodes first. If the part is only machined, standard 304 is marginally stronger and does the same job. If the material is dual-certified 304/304L, the question does not arise, because the same stock satisfies either call-out.
- What does dual-certified 304/304L mean?
- It means the mill heat satisfies both specifications at once — the chemistry and mechanical properties meet the requirements of 304 and of 304L simultaneously, and the material test report states both. A great deal of 304 bar, plate and tube in circulation is supplied this way, because it lets mills and stockholders hold one line of stock instead of two. Where it applies, the L designation costs nothing.
- Is 304L weaker than 304?
- On paper, yes — roughly 170 MPa minimum yield against about 205 MPa, with a similar gap in tensile. In practice the difference rarely decides anything. If a machined part genuinely needs more strength than 304L provides, standard 304 does not fix it either, and the real answer is 17-4PH, a 400-series martensitic grade, or more section.
- Why does 304L resist corrosion better beside a weld?
- Because sensitisation cannot happen. Welding heats a band of metal beside the joint to roughly 425–815 °C, and in that range chromium combines with carbon to form carbides at the grain boundaries. Those carbides draw chromium out of the metal around them, leaving a narrow band short of the element that forms the passive film. At 0.03% carbon there is too little carbon present for the carbides to form, so the grain boundaries and the chromium stay intact.
- Can 304L be used in a marine environment?
- No, and the L grade does not change this. 304 resists fresh water, food contact and most mild chemicals well, but it is not a chloride grade — salt water, coastal air, bleach and chloride process fluids will pit it, whether it is welded or not. For marine duty the specification is 316 or 316L.
- How does 304L machine?
- Like 304: it work-hardens under the cutter, conducts heat poorly and tends to gall the tool edge, so it needs sharp tooling, a decisive depth of cut and flood coolant. The low carbon makes it marginally gummier than standard 304, which shows up as built-up edge on lighter finishing passes. If the part is a high-volume turned item, 303 is the family's free-machining grade and machines considerably faster.
- What is 304L used for?
- Welded food and beverage equipment, welded frames and brackets, tanks and vessels, pipework and fittings, architectural fabrications, and any 304 part that is welded and then used without a post-weld solution anneal. It is also the grade specified for parts that are hot formed or welded in the middle of the production route.
Related pages
Other Stainless Steel materials
Have a part in Stainless Steel 304L?
Send the drawing, the grade and the quantity. You get DFM feedback and a price within 24 hours, and orders start at 50 pieces per style and material.