Whoa, you guys type faster than I do!
This repeats some of what Chris just stated, with a different spin...now I got to go think about heat treating...
Wrought iron and steel are very different animals, I do not think it is advisable to extrapolate from one to the other. What might be a welcome crack arrestor in iron could be an unwelcome stress riser in steel.
The slag stringers are a natural outcome of the method used to refine freshly smelted bloomery/tatara iron and steel, repeated folding, which has the primary purpose of removing the slag and homogenizing the metal. The more folding done, the less slag there is and the grain of the slag inclusions becomes finer. The edge of a sword would be severely compromised by a large slag stringer crossing there, or in the metal just behind the edge, so the metal used there is more refined. In the body of the sword, supported by more surrounding mass and not at the stressful cutting edge of the blade, such a degree of refinement is not needed so they used material that had less labor and materials invested in it. The slag stringers are in the middle because it is a more efficient way to make a blade and they can do no harm there, no other argument is required. This Moro spear nicely illustrates the technique.
The wootz/crucible steel method trades in the ‘repeated folding’ refinement for a liquid separation, but the goal is the same - homogenous metal and little or no slag.
It is entirely possible that the Ulfberht Co. got a nice price on imported ‘pre-refined’ steel for a couple years and when the economic situation shifted they continued on with the usual stuff, but I have yet to read the Williams article and see what it actually says. The Guardian article is goofy, but a good conversation starter!
Here’s another +Ulfberht+ (as opposed to +Ulfberh+t) analysis -
Metallografisk analys av inläggningar i vikingatida svärdsklinga,
inv. nr SHM 907 Go, Hogrän sn, Ålands
Av Mille Törnblom