304 and 316 stainless steel — 1.4301 / 1.4401

304 (1.4301) and 316 (1.4401) are austenitic chromium-nickel stainless steels, and ISCAR calls them the most popular stainless grades. 316 adds 2.0–2.5 % molybdenum, which raises resistance to pitting in chlorides. Both are essentially non-magnetic when annealed, cannot be hardened by heat treatment, and harden quickly when cold worked; that last property is also what makes them awkward to machine.

Designations

Deutsche Edelstahlwerke gives the JIS names SUS304, SUS303, SUS316, SUS316L and SUS316Ti. Bars and rods are covered by EN 10088-3, whose 2023 edition replaced the 2014 edition in December 2023. The limits and properties below are as printed in producer data sheets issued between 2007 and 2019, under the earlier editions; the changes in the 2023 text could not be checked.

Chemical composition

EN 10088-3 limits for bars, % by mass. For all six grades Si ≤ 1.0, Mn ≤ 2.0 and P ≤ 0.045.

The nitrogen limit is printed as ≤ 0.1 by Deutsche Edelstahlwerke but as ≤ 0.11 by Ugitech (1.4301, 1.4305, 1.4404), the stockist MARKS (1.4301, 1.4404) and Matthey’s strip sheet (1.4301). The sheets for flat products (MARKS, Matthey) print a lower sulphur maximum, 0.015 %, consistent with EN 10088-2; 0.030 % is the value for bars.

1.4301 (304)1.4401 (316)

ASTM limits differ from EN. ASTM A276 sets 304 at Cr 18.0–20.0 % and Ni 8.0–11.0 % (A240 uses the EN windows for 304); ASTM A276 and A240 allow 316 Ni 10.0–14.0 % and Mo 2.0–3.0 %, but Cr only 16.0–18.0 %. A certificate to one standard therefore does not prove the other: a 316 with 2.7 % Mo is inside the ASTM limits and outside EN 1.4401.

Mechanical properties

Bars, solution annealed (+AT), d ≤ 160 mm, EN 10088-3. Minimums, except Rm (range) and HB (maximum); A and KV in the longitudinal direction.

For 1.4301, 1.4307 and the Mo grades, from 160 to 250 mm the strength values stay the same and only transverse values are given: A 35 % (1.4301, 1.4307) or 30 % (Mo grades), KV 60 J. Larger sizes are by agreement. Rp1.0 is from Deutsche Edelstahlwerke only, and one stockist prints Rp0.2 ≥ 145 MPa for 1.4307. Deutsche Edelstahlwerke’s typical values for 1.4301 bar are well above these minimums: Rp0.2 360 MPa, Rm 660 MPa and A 50 %.

ASTM A240’s minimums for annealed plate, sheet and strip are Rp0.2 205 MPa, Rm 515 MPa and elongation 40 % for 304, 316 and 316Ti, and 170 MPa, 485 MPa and 40 % for the L grades; for 304L bar to A276, Deutsche Edelstahlwerke prints 205 / 515 MPa. For sheet and plate, EN 10088-2 asks for Rp0.2 ≥ 230 MPa (cold rolled) and ≥ 210 MPa (plate) for 1.4301, and 240 and 220 MPa for 1.4401 and 1.4404 (Outokumpu).

Cold work

These steels cannot be hardened by heat treatment, but cold work raises their strength sharply. Allegheny Ludlum’s figures for 316 sheet:

Megapascal values as Allegheny Ludlum prints them. Ten percent cold reduction nearly doubles the yield strength; sixty percent raises it almost four times. Cold work also makes 304 and 316 slightly magnetic, because part of the austenite transforms to martensite (ASSDA); annealed 1.4301 has a relative permeability µr ≤ 1.3 (Deutsche Edelstahlwerke).

Physical properties

The stainless values are from Deutsche Edelstahlwerke and Outokumpu, which agree; C45E is from Deutsche Edelstahlwerke’s C45E sheet. Atlas Steels and the stockist MARKS give 193 GPa for the elastic modulus.

Two consequences for the shop follow from the table. Less of the cutting heat flows away into the workpiece, so the edge and the chip run hotter than in carbon steel. And the part grows more as it warms: from the expansion coefficients, a 100 mm length grows 0.016 mm for a 10 K rise, against 0.011 mm for C45E. Measure tight dimensions after the part has cooled.

Heat treatment

These are Deutsche Edelstahlwerke’s ranges; Outokumpu gives 1000–1100 °C for its whole range and Atlas Steels 1010–1120 °C. Cooling must be fast enough to keep the carbon in solution. Outokumpu warns that annealing titanium-stabilized grades above 1070 °C can impair their resistance to intergranular corrosion.

Sensitization. Held in roughly 400 to 860 °C, these steels precipitate chromium carbides at the grain boundaries, and the chromium-depleted zones next to them corrode. The exact window differs by source. A low carbon content (the L grades, C ≤ 0.03 %) lengthens the time needed to sensitize, and titanium in 1.4571 ties up carbon as titanium carbide. That is why the producers rate 1.4307, 1.4404 and 1.4571 resistant to intergranular corrosion after welding, while 1.4301 and 1.4401 can lose that resistance in welded sections that are not annealed afterwards.

Corrosion resistance

The usual index for pitting resistance in chlorides is the pitting resistance equivalent:

$$\mathrm{PRE} = \%\mathrm{Cr} + 3.3 \cdot \%\mathrm{Mo} + 16 \cdot \%\mathrm{N}$$

Molybdenum is weighted by 3.3, so the 2 % Mo of 316 is worth more than six points. Outokumpu gives typical values of 18 for 304 and 24 for 316; Allegheny Ludlum gives 19.0 and 24.2.

For water, Allegheny Ludlum puts the limit at about 100 ppm chloride for 304 and about 2000 ppm for 316; seawater, at about 19,000 ppm, is beyond both. Acerinox gives below 200 ppm for 304; the criteria behind the two figures differ. 316 is usually called the standard marine grade, but it is not fully resistant to seawater, and Atlas Steels notes that both grades can suffer stress corrosion cracking in warm chloride environments above about 60 °C. Allegheny Ludlum gives a critical pitting temperature of 15 °C for 316 in the ASTM G48 test.

303 pays for its machinability with corrosion resistance: Atlas Steels rates it significantly below 304 because of the sulphur, and Atlas Steels and Ugitech advise against marine use. Deutsche Edelstahlwerke notes that the PRE formula ignores sulphur, so a PRE calculated for 303 should be read with caution.

Welding

• 304 and 304L. 1.4301 welds with or without filler (filler 1.4316, type 308L). Deutsche Edelstahlwerke advises solution annealing after welding if the carbon content is at the top of the range; where no heat treatment is planned, and for thick welded sections, it recommends 1.4307.
• 316 and 316L. 1.4401 needs a heat treatment after welding to keep its intergranular corrosion resistance; where that cannot be done, use 1.4404, which needs none. Filler 1.4430.
• 316Ti. 1.4571 is resistant to intergranular corrosion after welding without heat treatment.
• 303. Not recommended for welding: the sulphur causes hot cracking. Friction welding is the exception Deutsche Edelstahlwerke names.

Machining

Austenitic stainless steel is the core of ISO 513 group M. ISCAR’s group 14 (austenitic, 180 HB) uses kc1.1 = 2150 N/mm² and mc = 0.20; Seco’s group M2, with X5CrNi18-10 as its reference, uses kc1.1 = 1920 N/mm² and mc = 0.18. The two give 2966 and 2565 N/mm² at h = 0.2 mm; read cutting forces before mixing them. Seco puts free-machining 303 in M1, 304 in M2 and its 316L-type reference in M3.

Four properties make these steels hard on tools:

• Work hardening. The surface left by the previous pass is harder than the material under it, and a worn edge hardens it further. Alleima advises changing inserts regularly for this reason.
• Low thermal conductivity. The heat stays in the cutting zone.
• Built-up edge. Outokumpu describes a high tendency to built-up edge; raising the cutting speed or changing the coating helps (Marcegaglia).
• Notch wear at the depth-of-cut line, which Sandvik Coromant (through AMTIL) puts down to hard chips and the hardened surface. A smaller entering angle (60–80°), a varying depth of cut or a softer insert grade helps (Marcegaglia).

Deutsche Edelstahlwerke’s rule for 1.4301 is to choose the depth of cut large enough to cut under the hardened zone. Rolled Alloys adds that the tool must not ride on the work, and that the feed must be high enough to get under the previous cut. Sharp edges, a rigid set-up and plenty of coolant complete the list (Sandvik Coromant through AMTIL; Outokumpu; Alleima).

For drilling 1.4301, Deutsche Edelstahlwerke gives 30–70 m/min at 0.04–0.14 mm/rev with a 140° point, and for milling 105–220 m/min.

Machinability of the variants. 303 breaks chips well and suits automatic lathes; Atlas Steels rates it at about 78 % of the free-cutting carbon steel B1212. Deutsche Edelstahlwerke finds 1.4401 somewhat harder to machine than 1.4404 because of its higher carbon. Producers also sell 304 and 316 in versions made to machine better: Ugitech claims 10 to 50 % more productivity at the same tool life for its UGIMA grades, and Outokumpu describes faster machining and longer tool life for Prodec with similar corrosion resistance.

Choosing a grade

Mild environmentindoors, fresh water, food equipmentChloridescoast, road salt, chloride-bearing process water

Thick sections weldedno solution anneal after weldingMild environment304L1.4307Chlorides316L1.4404or316Ti1.4571

Not welded, or thin sectionsgeneral fabricationMild environment3041.4301Chlorides3161.4401

Heavily machinednot weldedMild environment3031.4305or304 / 304Limproved machinabilityChlorides316L1.4404improved machinability