The investigation of the influence of vacuum in furnace during welding flux heat treatment of moisture content in flux and diffusible hydrogen content in SAW weld metal has been conducted. The parameters of welding flux heat treatment in vacuum similar to manufacturer with moisture content in flux and diffusible hydrogen content in SAW weld metal are estimated. The data about the hydration speed of agglomerated welding flux on air are obtained. Variations of flux heat treatment efficiency are proposed.