Tungsten as a spring material is no longer used as a material by PRV manufacturers. Where Tungsten steel as a spring material has been specified, it is likely that an 'Alloy Steel' (such as H12) will be offered. Tungsten springs have been largely replaced by Chrome Steel or Alloy Steel dependant on the application. Depending on manufacturer, valve size and application, even Inconel X750 may be used as a viable alternative.
*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***
*** 'Pressure-relief Valve' is the equivalent ASME/API term ***