Compare 304H and 304L stainless steel to choose the right carbon variant for high-temperature strength or corrosion-sensitive welding, and learn what to specify on the RFQ.
Many buyers treat 304H and 304L as interchangeable because both share the familiar 18-8 chemistry of grade 304. They are not. The single letter refers to a controlled carbon difference that changes how the steel behaves at temperature and in the heat-affected zone of a weld, and choosing the wrong variant can shorten service life or force costly rework. This guide explains where each variant pays off and which details belong on your purchase order.
What the H and L Really Mean
The H grade carries a higher carbon range, typically held toward the upper end around 0.04 to 0.10 percent, so the steel retains more strength at elevated temperature and resists creep under sustained load. The L grade is the opposite: carbon is held below 0.030 percent so that during welding there is not enough free carbon to combine with chromium and form chromium carbide at the grain boundaries. In plain terms, H buys you heat strength and L buys you weld corrosion resistance. Both still meet the general 304 family, which is why a casual glance at the chemistry looks the same.
When 304H Is the Safer Choice
If the part will run above roughly 425 degrees Celsius for long periods, such as boiler casings, furnace parts, heat exchanger tubes, or exhaust manifolds, the extra carbon of 304H helps it hold mechanical strength instead of softening. The same higher carbon that aids high-temperature strength is exactly what makes 304H more vulnerable to sensitization in slow cooling, so it is normally supplied in a solution-annealed condition and should not be left in service where it will be heated intermittently through the sensitization range unless stabilized or low-carbon logic applies.
When 304L Is the Safer Choice
For welded assemblies that will see corrosive service afterward, particularly in chemical, food, and marine environments, 304L is the conservative pick. Because there is little carbon available to form carbides, the weld heat-affected zone keeps its chromium in solution and resists intergranular attack. If a structure is large, heavily welded, and difficult to re-anneal after welding, 304L avoids the post-weld sensitization risk that 304H would carry.
What to Put on the RFQ
Always state the variant by full designation rather than writing only 304. Specify the carbon range you need, the condition such as solution annealed, the applicable standard such as ASTM A240 for plate or A312 for pipe, and the service temperature if known. If you are unsure, tell the supplier the maximum operating temperature and whether the finished assembly will be welded; that information alone lets a conscientious mill recommend H or L correctly and prevents the common error of supplying general 304 where a controlled variant was intended.