11. METALLIC & NON METALLIC
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STAINLESS STEELS
Special STAINLESS STEELS
Carpenter 20-Cb3 : A stainless steel having 20% chromium, 34% nickel, 2,5% copper and columbium
and tantalum equal to 8 times the carbon content for carbide stabilization. This alloy
has excellent resistance to corrosive conditions, especially to sulfuric acid.
THERMOWELL MATERIALS
This group of metals forms an invisible chromium oxide which serves to resist oxidation and corrosive attack
by chemicals and acids. To be effective, they need to have a minimum of 14% chromium. The 300 series
stainless steel are known as ’austenitic’ while the 400 series are known as ‘ferritic’. Austenitic stainless
steels do not become brittle at low temperature as ferritic steels do.
SS 304
: Also known as ‘18-8’ (nominally 18% chromium, 8% nickel) is the most commonly speci-
fed austenitic stainless steel. SS 304 like other 300 series stainless steel is subject to
‘carbide precipitation’ in the area of 700-1,650 ºF. This means that chromium forms car-
bides when SS 304 is held in/of is cooled slowly through the above temperature range.
The net effect is a localized depletion of chromium around the carbides, which can lead
to inter-granular corrosion from acids of other corrosives. This condition is especially
apparent where parts are welded (leading to ‘weld decay’). SS 304 has a maximum
temperature rating for continuous service of 1,650 ºF in air. Care must be taken as the
strength falls off considerably at elevated temperatures. SS 304 is widely used as a
thermowell material for lower temperature applications across industry since it is not
affected by most organic and inorganic chemicals.
SS 310
: Has higher chromium and nickel (nominally 25% chromium and 20% nickel) improved
high temperature characteristics. The SS 310 is subject to carbide precipitation in the
800 °F to 1,600 °F range. Maximum continuous service temperature in air is 2,100 °F.
The SS 310 is used where good high temperature strength is needed or in carburizing/
reducing atmospheres.
SS 316
: Another very popular all purpose austenitic stainless steel. SS 316 has nominally
18% chromium and 12% nickel, but is modifed with 2-3% molybdenum which improves
its resistance to chlorides. SS 316 is subject to carbide precipitation in the 800-1,600 °F
range.
Maximum continuous service temperature in air is 1,650 °F. Because of its increased
corrosive resistance, SS 316 is used where improved corrosion resistance is required,
especially in chlorides.
304L and 316L : Low carbon versions of SS 304 and SS 316. These alloys solve the problem of carbide
precipitations since they have a very low carbon content (0.03% maximum instead of
0.08% maximum).