Grades of Stainless Steel Used in the Water Industry

Type 304L(1.4307) and 316L(1.4404) are the two grades of stainless steel most frequently used in the water industry. The 316L(1.4404) grade contains molybdenum, which provides a higher corrosion resistance and is therefore used for more exacting conditions. Both have an austenitic structure that in practical terms means they are ductile and strong and do not embrittle in freezing temperatures. A range of more highly alloyed “super”austenitic alloys is available for very critical applications. Their composition provides stronger alloys with very high corrosion resistance which means they can be used for handling higher chloride waters (even sea water) and higher levels of free chlorine in the waters than can be accommodated by the Type 304L(1.4307) and 316L(1.4404) grades.

The third most common grade used in the water industry is the duplex alloy 2205(1.4462). This has a higher corrosion resistance to waters than 316L(1.4404). The name duplex indicates that the alloy structure contains both austenite and ferrite in roughly similar amounts. This appreciably increases the alloy strength whilst retaining a reasonable ductility. It will embrittle at very low temperatures, but as this occurs at below -45^oC there is no practical significance. A range of super-duplex grades is available which are similar to the super-austenitic alloys in having very high corrosion resistance and are used for exceptionally severe conditions.

Table 2. Typical Chemical Composition of Some Commonly Used Stainless Steels

Popular Name	Euronorm EN 10088-2	Carbon (max. %)	Nickel Ni	Chromium Cr	Molybdenum Mo	Other Elements	UNS Equiv -alent
304L	1.4307	0.03	8.00-10.00	17.50-19.50	-		S30403
316L	1.4404	0.03	10.0-13.0	16.5-18.5	2.0–2.5		S31603
316Ti	1.4571	0.08	10.5-13.5	16.5-18.5	2.0–2.5	Ti	S31635
22% Cr duplex (alloy 2205)	1.4462 1.4462	0.03 0.03	4.5-6.5 4.5-6.5	21.0-23.0 22.0-23.0	2.5-3.5 3.0-3.5	N N	S31803* S32205*
25% Cr super- duplex	1.4507 1.4410 1.4501	0.03 0.03 0.03	5.5-7.5 6.0-8.0 6.0-8.0	24.0-26.0 24.0-26.0 24.0-26.0	2.7-4.0 3.0-4.5 3.0-4.0	Cu,N N Cu,N,W	S32550 S32750 S32760
Alloy 904L	1.4539	0.02	24.0-26.0	19.0-21.0	4.0-5.0	Cu	N08904
6% Mo super- austenitic	1.4529 1.4547 -	0.02 0.02 0.03	24.0-26.0 17.5-18.5 23.5-25.5	19.0-21.0 19.5-20.5 20.0-22.0	6.0 -7.0 6.0-7.0 6.0-7.0	Cu,N Cu,N N	N08926 S31254 N08367

Table 3. Mechanical Properties taken from EN 10088-2(Cold rolled, solution annealed strip)

Popular Name	0.2%Proof Stress* MPa	Tensile Strength MPa	Elongation* %
304L	220	520-670	45
316L	240	530-680	40
22% Cr Duplex	480	660-950	20
25% Cr Super-duplex (1.4507)	570	690-940	17
Alloy 904L	240	530-730	35
6%MoSuper-austenitic (1.4547)	320	650-850	35

*Single values are minimum values.

The alloys listed are not exhaustive and further information can be found in: