According to AISI , the only practical difference between 304 or 316 and 304L or 316L is carbon content.
The carbon ranges are 0.08% maximum for 304 and 316 and 0.03% maximum for the 304L and 316L types.
All other element ranges are essentially the same (nickel range for 304 is 8.00-10.50% and for 304L 8.00-12.00%). .
Effect Of Carbon On Corrosion Resistance
The lower carbon ‘variants’ (316L) were established as alternatives to the ‘standards’ (316) carbon range grade to overcome the risk of intercrystalline corrosion (weld decay), which was identified as a problem in the early days of the application of these steels. This can result if the steel is held in a temperature range 450 to 850°C for periods of several minutes, depending on the temperature and subsequently exposed to aggressive corrosive environments. Corrosion then takes place next to grain boundaries.
If the carbon level is below 0.030% then this intercrystalline corrosion does not take place following exposure to these temperatures, especially for the sort of times normally experienced in the heat affected zone of welds in ‘thick’ sections of steel.
Effect Of Carbon Level On Weldability
There is a view that the low carbon types are easier to weld than the standard carbon types.
There does not seem to be a clear reason for this and the differences are probably associated with the lower strength of the low carbon type. The low carbon type may be easier to shape and form, which in turn may also affect the levels of residual stress left the steel after is forming and fitting up for welding. This may result in the ‘standard’ carbon types needing more force to hold them in position once fitted-up for welding, with more of a tendency to spring-back if not properly held in place.
The welding consumables for both types are based on a low carbon composition, to avoid intercrystalline corrosion risk in the solidified weld nugget or from the diffusion of carbon into the parent (surrounding) metal.
Dual-Certification Of Low Carbon Composition Steels
Commercially produced steels, using current steelmaking methods, are often produced as the low carbon type as a matter of course due to the improved control in modern steelmaking. Consequently finished steel products are often offered to the market ‘dual certified’ to both grade designations as they can then be used for fabrications specifying either grade, within a particular standard.
For example, for pipes:
ASTM A312 304/304L OR ASTM A312/ASME SA312 304/304L
ASTM A312 316/316L OR ASTM A312/ASME SA312 316/316L