Flat weld-on flanges to GOST 12820-80 are made of steel 20 or 09Г2С, with the grade confirmed by a mill certificate showing chemical composition and mechanical properties. Bore and thickness are matched to the customer's pipe — outside diameter and wall thickness — otherwise the weld sits with a step and the root stays in the gap. The weld bevel and bore diameter are checked with gauges rather than judged by eye from the drawing.
The execution is chosen by design temperature: steel 20 for moderate service, 09Г2С for minus 40 °C and below with confirmed impact toughness. The heat number is transferred to the marking of every item, so a flange on site traces back to its certificate. The sealing face is protected, and uncoated surfaces carry primer or anti-corrosion grease.