ASTM A312 TP321H HWF Fin Tube with Carbon Steel Fin ASTM A312 TP321H is a high-carbon version of the stabilized austenitic stainless steel grade 321. The "H" designation signifies its suitability for high-temperature service, guaranteeing enhanced elevated temperature strength while maintaining excellent resistance to intergranular corrosion due to its titanium stabilization. The ASTM A312 TP321H HWF (High-Frequency Welded) Fin Tube with Carbon Steel Fin is a specialized