FS (Fanged Seal) and SS (Subsea Seal) metal seals replace elastomers in high-temperature Class X/Y (>350°F) wellheads, using Inconel 718 with silver plating. FS seals fit tubing hanger neck grooves, energized by lockdown screws; SS seals use dual-lip designs for annulus access ports. Pressure ratings reach 15,000 psi with https://globalsealstech.com/product-category/oil-gas-seals/api-6a-wellhead-equipment-seals/