Interesting article on the metallurgical analysis of a few Viking swords:

I found this interesting. I tried checking the original scholarly article, but I can only read the abstract, and the abstract says nothing about the usability of the weapons.

Vikings (800–1050 CE) are famous for being fearsome seafarers and their weapons represented an indispensable tool in their plundering raids. Sword from the Viking age often showed pattern-welding, made by welding together thin strips of iron and steel that were twisted and forged in various ways, producing a decorative pattern on the surface. In this work we present a neutron diffraction study of three swords from the Viking age belonging to the National Museum of Denmark. This non-invasive approach was used to allow us to characterize the blades in terms of composition and manufacturing processes involved. The study shows how the effects of past conservation treatments can either help or obstruct the extraction of archaeological information.
The article is correct in stating that the BEST way to take advantage of the different qualities of iron and steel is to steel line an iron core (as the Japanese did, and some higher quality European swordsmiths did), but that requires a hugely advanced metal technology that may not have been available to the Vikings of (at least) the earlier period. The second best - often used when highly refined iron/steel is not available - is to twist steel of differing carbon content together, as these Viking swordsmiths did.

Anyway, for what it's worth...