Pattern welding goes back to the Celts and Romans already. The Vikings long imported their better sword blades from the south - there were numerous Frankish royal edicts banning the sale of such weapons to the buggers, which as usual the merchants cheerfully ignored. Local craftsmen then added furniture according to local taste.

The main thing that made the Vikings such a trouble was their ships - they could usually pick their battles and avoid serious resistance until they had looted their fill and went home. Later on they started getting more ambitious of course, partly because arable land was running short in Scandinavia.

As a side note, the famous "Viking" ships were actually a fairly late developement of something like the 6th-8th cenuries AD, and the designs were being constantly improved until the structural limits of the construction method were reached around the turn of the millenium. Baltic ships first began having masts in something like late Roman times...