The H grade versions of familiar grades like 304, 316 and 321 have a minimum carbon content to produce increased high temperature strength, particularly creep strength. They must also meet a minimum grain size.
Approximate Compositions: As 304, 316 and 321 but with 0.04-0.08% C (Exact composition ranges vary between EN and ASTM standards).
Compared to 304, 316 and 321 these grades combine the following characteristics:
- Similar general corrosion resistance
- Reduced intergranular corrosion resistance
- Improved high temperature 0.2% proof strength
- Improved creep strength
Carbon is an important element in stainless steels. It is usually kept low in order to aid weldability, especially in the avoidance of chromium carbide, which gives rise to intergranular corrosion. However, carbon is an important strengthening element. In certain cases, notably at high temperature, strength may take precedence over the risk of intergranular corrosion. The minimum carbon level of 0.04% in the H grades provides this extra strength. Creep strength is enhanced with a large grain size. Therefore, it is usual for standards to specify an ASTM grain size of 7 or coarser for the H grades. H grades are now something of a rarity in general stockholders.
Applications which illustrate these features include:
Heat exchangers, gas boilers, aerospace, exhaust systems, process plant, element tubing, power generation, rocket engine parts, seals, expansion joints.