The flange material is ASTM A737 Gr C , and to use ASME B16.5 to specify the flange class, I do not know what material table corresponds to this material. Can somebody shed light, please.

Med ali,

Do you mean material composition?
You can find it on ASTM code
It's Pressure Vessel Plates, High-Strength, Low-Alloy Steel and the composition for A737, Gr C is below(First row is element, second is heat analysis, last one is Product analysis)

Carbon, max 0.22, 0.24
Manganese 1.15–1.50, 1.07–1.62
Phosphorus, max 0.025, 0.025
Sulfur, max 0.025, 0.025
Silicon 0.15–0.50, 0.10–0.55
Vanadium 0.04–0.11, 0.03–0.12
Columbium, max 0.05, 0.05
Nitrogen, max 0.03, 0.03