SS 316L for welded hygienic and chemical systems

Only carbon. ASTM A240 allows 316 up to 0.08% carbon and limits 316L to 0.030%. Chromium, nickel and molybdenum ranges are the same.

That carbon difference controls what happens beside a weld. With more carbon, chromium carbides form at grain boundaries in the heat-affected zone and leave chromium-depleted paths that corrode first. With 316L there is not enough carbon to do real damage during normal welding. You give up a little strength for it: 170 MPa minimum yield instead of 205 MPa.

Process lines in pharma, biotech, dairy and beverage plants are orbital-welded, so there may be hundreds of welds in one skid. Every one of them has a heat-affected zone, and every one will see the same cleaning chemicals as the parent tube. 316L means the weld zones keep corrosion resistance close to the rest of the line.

Low carbon also helps electropolishing, which these systems often use for very smooth internal surfaces. Our pharmaceutical industry page goes into tube finish and documentation in more detail.

• WFI, purified water and clean steam lines
• Product transfer piping and CIP return lines
• Mixing, holding and fermentation vessels
• Dairy pasteurisers and cheese brine handling
• Instrument impulse lines and capillary tubing

In everyday language, yes. Watches, jewellery and some instruments marketed as surgical steel are usually 316L. In medical engineering, though, implant-grade 316L is covered by its own standard, ASTM F138, with tighter limits on inclusions and chemistry, and it is usually vacuum remelted. Industrial 316L to ASTM A240, A312 or A276 is not implant-grade material, and we do not supply it for implants.

In hygienic service, the internal surface of the tube decides how easily product residues and bacteria can be cleaned away. Pharma and biotech specifications usually call for a maximum surface roughness, expressed as Ra in micrometres, and often for electropolishing on top of mechanical polishing. Dairy and beverage lines commonly accept a less demanding finish.

The same 316L chemistry can be supplied as mill-finish pipe for chemical service, bright annealed tube for instrumentation and polished tube for hygienic lines. Price and availability differ a lot between them. When you enquire, give the finish and Ra requirement along with size and wall thickness, so we quote the right material the first time.

316L is not only a clean-room material. Effluent treatment plants, distillery spent-wash lines and sugar mill juice handling all involve acidic, chloride-bearing liquids where welded 304L can pit within a few years. 316L welded fabrication holds up noticeably better there. On coastal sites, welded 316L railings, frames and enclosures keep their appearance longer than 304L. Our sugar and distillery page covers where 316L sits against brass and other materials in that industry.

316L resists oxidation in air to roughly the same temperatures as 316. Its weak spot is strength under sustained load at high temperature, where the low carbon reduces creep strength. For pressure parts running hot for long periods, 316 or 316H is normally specified. 316L is a corrosion grade first.

For moderate temperatures such as hot water, steam condensate and CIP solutions, 316L is entirely suitable.

Most 316L sold today is dual-certified 316/316L, meeting the low-carbon limit and the higher 316 strength minimums in a single heat. When you order, state the product standard you need. That could be ASTM A312 for pipe, A269 or A213 for tube, A240 for sheet and plate, or A276/A479 for bar. Also state the finish, such as mill finish, bright annealed or polished.

We supply 316L as seamless pipe, ERW pipe and tube, capillary tube, sheet, strip, bar, wire, flanges and fittings. Sizes are confirmed at enquiry, with a test certificate for each heat.