SS316/SS304 vs. SS316L/SS304L

Stainless steel type 316 and type 304 are raw material grades for stainless steel. Stainless steel fasteners are sometimes referred to and ordered as their raw material grades (such as SS 316 and SS304) and sometimes referred to and ordered as their ASTM specification. The common ASTM specifications that cover stainless steel bolts are A193, A320 and F593. Type 316 stainless has a unique chemical composition which includes 16% chromium, 10% nickel and 2% molybdenum. Type 304 stainless has 18% chromium, 8% nickel and no molybdenum (this is where the “18-8” designation comes from). Stainless steel 316 is more resistant to chlorides than stainless steel 304, making it the preferred material for marine construction.

Sometimes, a type “SS316L” or type “SS304L” bolt will be specified in a particular application. The “L” at the end of these designations refers to the low carbon content of the steel. Stainless steel certified to 316L or 304L will have a maximum of 0.03% carbon, versus the 0.08% maximum that regular 316 and 304 can have. Low-carbon stainless steel bolts are used to avoid corrosion problems that are caused by welding, as the lower carbon content reduces the “sensitization effect” (the precipitation of chromium carbides at grain boundaries) caused by the high temperatures involved in welding. Since carbon is the principal hardening element in steel, low-carbon bolts have reduced hardness but are easier to weld.

The difference in carbon content does not affect the overall corrosion resistance of the steel, and low-carbon stainless steel bolts can be ordered to each of the A193, A320 and F593 specifications. When you order a low-carbon stainless steel bolt, just let us know about this requirement and our expert estimators will be sure to provide you with the correct material.