316 vs 316L Stainless Steel Filter Housing: When L Matters
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For a welded filter housing, 316L matters in three situations: when the shell or flanges are thick, roughly 5 mm and up; when the welds will not be annealed afterwards; and when the housing runs hot, in the 425 to 860 °C band where carbides form. A 316L stainless steel filter housing in those conditions keeps its corrosion resistance next to the welds, where plain 316 can suffer weld decay. For a thin-wall housing in normal liquid service, 316 and 316L corrode the same, and most sheet made today is dual-certified to both grades anyway.
- Same alloy, less carbon: 316 allows up to 0.08% carbon, 316L up to 0.030%. Chromium, nickel and molybdenum are the same.
- What the L buys you: resistance to intergranular corrosion in the heat-affected zone of welds, especially in thick sections.
- What it costs you: slightly lower minimum strength on paper, which pressure codes turn into lower allowable stress.
- What usually happens: the sheet is dual-certified 316/316L and meets both.
This article is the welding footnote to 304 vs 316 for filter housings, which covers when you need molybdenum at all. The wider selection is in our stainless steel filter housing buyer's guide.
What is the difference between 316 and 316L?
Only the carbon limit. Both grades contain 16 to 18% chromium, 10 to 14% nickel and 2 to 3% molybdenum, which is the element that resists chloride pitting. Standard 316 (UNS S31600, EN 1.4401) is limited to 0.08% carbon; 316L (UNS S31603, EN 1.4404) is limited to 0.030% (Atlas Steels, 316/316L/316H data sheet). The British Stainless Steel Association puts it the same way: apart from carbon, "the element ranges are essentially the same" (BSSA, 316 vs 316L compositions).
| 316 | 316L | |
|---|---|---|
| UNS / EN number | S31600 / 1.4401 | S31603 / 1.4404 |
| Carbon | 0.08% max | 0.030% max |
| Chromium, nickel, molybdenum | 16–18, 10–14, 2–3% | Same |
| Minimum tensile / yield (flat product, ASTM A240) | 515 / 205 MPa | 485 / 170 MPa |
| Resistance to weld decay | Needs annealing after heavy welding | Highly resistant without annealing |
| Typical supply | Dual-certified 316/316L sheet that meets both | |
Why does carbon matter in a welded filter housing?
A filter housing is welded at the shell seam, every nozzle, the element seat and the legs. Each weld heats a band of metal on either side, the heat-affected zone, through 450 to 850 °C for a short time. In that band, carbon combines with chromium to form chromium carbides at the grain boundaries, which strips chromium from the metal next to them. The steel is then "sensitized": in an aggressive liquid it corrodes along the grain boundaries beside the weld, which is what weld decay means. BSSA states that if the carbon is below 0.030% this intercrystalline corrosion does not take place under those exposure conditions (BSSA). Thick sections stay in that temperature band longer, so the risk grows with wall thickness and with the number of weld passes.
When does the L matter in a 316L stainless steel filter housing?
- Thick shells and flanges. Atlas Steels describes 316L as the grade for heavy gauge welded components of about 5 mm and over, and notes that heavy welded sections in 316 need post-weld annealing for full corrosion resistance, which 316L does not. Large bag and multi-cartridge vessels with flanged nozzles and welded legs fall in this range.
- No annealing after welding. Annealing a finished vessel is rarely practical, so a thick 316 vessel usually goes into service as welded. 316L removes the need.
- Hot service. 316L is preferred for continuous use between 425 and 860 °C where aqueous corrosion resistance matters afterwards. Filter housings seldom run that hot, but a housing that sees a hot cleaning or steam cycle through the sensitizing band, on aggressive liquid, is the exception.
- Aggressive liquids at the weld. Weld decay needs both sensitized metal and a liquid that attacks it: chlorides, acids, hot brine. On town water, a sensitized weld often shows nothing for years.
- A specification that says 316L. Many sanitary and pharmaceutical specifications call for 316L by convention. If yours does, write it on the request and the quotation should confirm it for the housing body.
When does the L not matter?
For a thin-wall housing, such as a cartridge housing formed from sheet of a couple of millimeters, the heat-affected zone cools fast and the carbide problem is small. The Australian Stainless Steel Development Association puts it directly: without heavy-section welding or high-temperature exposure, "the corrosion resistances of the standard and L grades are usually identical" (ASSDA, L, H and standard grades). Pitting resistance in chlorides comes from molybdenum, not carbon, so 316L does not out-perform 316 in seawater or brine away from the welds. If the housing is thin and the liquid is mild, specifying 316L buys little; if it comes dual-certified anyway, you have it regardless.
Is 316L weaker than 316?
On paper, slightly. The minimum values in ASTM A240 are 515 MPa tensile and 205 MPa yield for 316, against 485 and 170 MPa for 316L. Pressure vessel codes turn that into a higher allowable stress for the standard grade, so a designer working to a code can use a thinner wall, or claim a higher rating, in 316 than in 316L for the same vessel. In practice mills usually produce low-carbon heats that meet the standard-grade strength too, which is why dual-certified product exists. What this means for a buyer: if your housing must be designed to a code, the grade affects the design calculation, so put both the code and the grade on the request. The design pressure and temperature of a housing are stated per model on the quotation and drawing.
What does dual-certified 316/316L mean?
Modern steelmaking produces low carbon as standard, so a sheet is often certified to both designations: carbon at or below 0.030% and strength at or above the 316 minimums. ASSDA notes that dual-certified product is "deliberately intended to fulfill requirements for both standard and L grades" and can be used wherever either is specified, but not where an H grade is called for (ASSDA). For a filter housing buyer that is good news: the sheet most fabricators buy already satisfies a 316L specification. The question is only whether the paperwork says so.
316 or 316L for your housing: a decision table
| Situation | Grade to specify | Why |
|---|---|---|
| Thin-wall cartridge housing, water or mild liquid | 316 (dual-certified is fine) | Heat-affected zone is small; corrosion resistance identical |
| Large bag or multi-cartridge vessel, welded flanges and legs | 316L or dual-certified | Thick sections, multi-pass welds, no post-weld anneal |
| Chlorides, acids or hot brine at the welds | 316L or dual-certified | Weld decay needs an aggressive liquid; remove the sensitized metal |
| Sanitary or pharmaceutical specification | 316L | Usually written into the specification |
| Housing designed to a pressure code, strength governs | Ask the designer | Standard grade has higher allowable stress; dual-certified satisfies both |
| Service above 425 °C | Neither L nor dual-certified | Codes exclude L grades at high temperature; a different grade is needed |
How do you verify which one you received?
A handheld XRF spectrometer, the usual positive material identification (PMI) tool, reads chromium, nickel and molybdenum in seconds and tells 316 from 304 by the molybdenum. It cannot read carbon, so it cannot tell 316 from 316L. For the L you need the mill test certificate of the sheet, which states the carbon, or an optical emission spectrometer (OES) test. Our 304 and 316 housings can be verified by spectrometer, and when your specification says 316L, ask for the carbon figure to be confirmed on the quotation. As always, the grade statement covers the housing body; valve, gasket and fastener materials are listed separately.
Our industrial bag and multi-cartridge housings are built in SUS304 or SUS316, and the sanitary multi-cartridge housing in the sanitary Tri-Clamp range is SUS316L. For a vessel built to your drawing, write the grade you need on the request; the custom bag and multi-cartridge housing page lists the rest of what to send, and the quotation and drawing confirm the grade of the housing body. Minimum order is one piece.
FAQ
Is 316L more corrosion resistant than 316?
Only at and around welds in thick sections, where 316 can be sensitized and 316L is not. Away from welds, and in thin sheet, their resistance to chlorides and acids is the same, because it comes from molybdenum and chromium, which both grades share.
Can a welded filter housing be made in plain 316?
Yes, and thin-wall housings commonly are. The risk appears with thick shells, flanges and legs welded in several passes and not annealed afterwards, on an aggressive liquid. That is where 316L, or dual-certified sheet, is the safer specification.
Can PMI testing tell 316 from 316L?
No. XRF reads molybdenum, so it confirms the 316 family, but it does not read carbon. To confirm the L, ask for the mill certificate or an OES test.
Do I need 316L for beer, beverage or dairy lines?
Many sanitary specifications call for 316L, and our sanitary multi-cartridge housing is built in it. Whether your line needs it depends on your specification, the product and the cleaning chemicals; send those three and we tell you what fits.
Does 316L cost more than 316?
Often not, because dual-certified sheet is the common supply. Where a true single-certified 316L is required, or a thicker wall is needed to meet a code in the lower-strength grade, the quotation shows the difference.
Need the grade confirmed before you order?
Tell us the liquid, its temperature and chloride level, the wall thickness or vessel size you have in mind, and whether your specification says 316 or 316L. We confirm the grade on the quotation and drawing, verify it by spectrometer, and deliver to the approved drawing. Start at custom bag and multi-cartridge filter housings or request a quote.