Here’s some more info on the NPL study of Viking swords. Key passage:

NPL’s results match descriptions of ancient sword making in Herat (now in Afghanistan) described by ninth century Arab philosopher and writer Al-Kindi. This links to a known Viking trade route down the Volga and across the Caspian Sea to Iran but until now it was not known that Vikings had brought crucible steel back to Scandinavia and integrated ancient Arab steelmaking methods with their own swordsmithing.