Austenitic stainless steel grades and how they differ

Start from 304 and think of every other grade as 304 with one setting changed.

Carbon is the first lever. Straight 304 allows up to 0.07% carbon in plate and sheet (0.08% in bar and pipe). During welding, carbon can combine with chromium at the grain boundaries next to the weld, which leaves those zones short of chromium and open to intergranular attack. The L grades cap carbon at 0.030%, so the heat-affected zone stays sound without post-weld annealing.

Molybdenum is the second. 316 and 316L carry 2.00–3.00% Mo and more nickel than 304. That addition is what holds back pitting and crevice corrosion in brine, seawater spray, bleach and many process chemicals. The 304 vs 316 comparison goes into the trade-offs in detail.

Titanium is the third. Grade 321 ties up its carbon as titanium carbide, at least five times the carbon plus nitrogen content. That protects it in long service between roughly 425 and 815 °C, where ordinary 304 would slowly sensitise. Exhaust systems, expansion bellows and furnace internals use it for that reason.

Sulphur is the fourth. Grade 303 carries 0.15% minimum sulphur, which forms manganese sulphide stringers that break chips cleanly on a lathe. The price is lower corrosion resistance and poor weldability, so 303 belongs on turned parts that will not be welded.

In the annealed condition, yes. The face-centred cubic austenite structure does not respond to a magnet. Cold work changes that. Bending, drawing, cold rolling and heavy machining convert some austenite near the surface into martensite, and that martensite is magnetic. Cold-drawn 304 wire or a bent 304 elbow can pick up a noticeable pull. 316, with its higher nickel, resists this change better, so it usually stays closer to non-magnetic after forming.

Weld beads can also attract a magnet slightly. Filler metals are balanced to leave a little delta ferrite in the weld, which helps prevent hot cracking. None of this means the grade is wrong; only a chemical analysis can tell you that.

• Dairy tanks, water lines, general fabrication: SS 304.
• Pharma vessels, coastal structures, chloride process lines, especially where welds are not annealed afterwards: SS 316L.
• Parts that run hot for long periods, such as flue ducts and heat exchanger internals: SS 321.
• Shafts, bushes, fittings and fasteners turned in volume on auto lathes: 303.
• Welded 304 assemblies too thick or too large to solution anneal: 304L.

Much of the plate, pipe and bar now made is certified to both the straight and the L grade at once. The heat meets the 0.030% carbon cap of the L grade and also the higher strength minimums of the straight grade. If your specification says 316L and the certificate reads 316/316L, the material complies. Ask for the dual certificate when one order has to cover both.