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The specification of bar, (to BS970), and coil / plate, (to BS1449), before 1983 covered two type 316 grades: a ‘low’ carbon with 0.03% max, (316S12), and a ‘standard’ carbon with 0.07% max 316S16. Both had a molybdenum content in the range of 2.25-3.0 %.
The 1983 and subsequent versions of the standards introduced new molybdenum content ranges of 2.0-2.5% and 2.5-3.0% for both the low and ‘standard’ carbon grades, thereby increasing the number of these grades from two to four:
316S11 low carbon, lower molybdenum range
316S13 low carbon, higher molybdenum range
316S31 standard carbon, lower molybdenum range
316S33 standard carbon, higher molybdenum range
The chromium range was maintained across all of the grades at 16.5-18.5%, but the nickel range was adjusted to ‘balance’ the structure of these austenitic steels. Nickel is an ‘austenite stabiliser’ and so is used to balance the combined effects of carbon, (also an austenite stabiliser), and molybdenum which has the opposite effect as a ‘ferrite stabiliser’.
Strictly, the nearest equivalent for the old 316S12 is the 316S13, so that the minimum molybdenum of 2.25 is met. Similarly, the nearest equivalent for the old 316S16 is the 316S33. For atmospheric exposure service conditions, except in extreme marine environments, the substitution of either
316S11 for 316S12 or
316S31 for 316S16
should give satisfactory corrosion resistance, depending on design and finishing techniques.
|% min||% max.||% min||% max|
|% min||% max||% min||% max||% min||% max||% min||% max|
In 1995 BS EN 10088-2 replaced BS1449 and BS EN 10088-3 replaced BS 970-1. The four 316 sub-grades have numerical designations as follows:
|Former BS designation||316S11||316S13||316S31||316S33|
|Current BS EN numbers||1.4404||1.4432||1.4401||1.4436|