304 and 304/L (UNS S30400 & UNS S30403) are austenitic alloys of stainless steel bars. These alloys are the most familiar and most frequently used alloys in the stainless steel family.  The L in 304/L refers to a lower carbon content to allow for greater corrosion protection when welding.

Austenitic steels have austenite as their primary phase (face centered cubic crystal). These are alloys containing chromium and nickel (sometimes manganese and nitrogen), structured around the Type 302 composition of iron, 18% chromium, and 8% nickel. Austenitic steels are not hardenable by heat treatment. The most familiar stainless steel is probably Type 304, sometimes called T304 or simply 304. Type 304 surgical stainless steel is austenitic steel containing 18-20% chromium and 8-10% nickel.