I think I see where mf205i is going with his post, I've come up against this problem myself and have found out the following...... maybe a little disjointed but I'll give it a go, please bear with me.
Weldments can be awkward things,(that's being polite!) balance and the HAZ's being two major factors when considering tooling, a tough grade of carbide would on the face of it be the logical choice, but only if the other parameters can be achieved, mainly enough speed for them to work properly which is ultimately affeced by the rigidity of the workpiece and the capability of the machine to control it.
The SFM for carbides is say around the 300fpm, and out of balance forces at those speeds can get scary.
A lot of ''structural'' low carbon and hence commonly welded steels are sticky and don't like being machined, hence the high speed in order to reduce the built up edge problems etc.
Then added in to that mix you have the HAZ, and at first, interupted cut which is hard on any cutting tool, especially the outer ''skin'' and in the case of stick welding included or uncleaned off slag.
Think about first principles, the things we learnt at trade school,like minimum tool overhang and please lord NO lantern or American style toolposts!!!! I don't even like the quick chage types as they all seem to over hang the slide more than the good old four way block, allright you gotta shim, but there's one less interface to consider as well............. if you can lock the topslide if it's not being used, and the cross slide once you've set the cut, it all helps.
So HSS or Cobalt? I'm lucky that over the years I've aqquired sizable stocks (for me) of ''cobalt'' of all sorts of types, and there is a lot of variation, not long ago I bought some Chi' import M42 blanks from J&L, and it's barely any different to our good old standby Eclipse H5
My fave for heavy duty has got to be Cleveland MOMAX, it's not only tougher, it's harder, try grinding a bit, and stands up well to the sort of punishment weldments dish out.
If you want something harder look for WKE 45 (probably NOS made I think by Sandvik) then you'll know what hard is and how!
Tip radius:- the bigger the better, and top rake as small as you can get away with especially in the early stages.
The method that works best for me is shut the speed down and put the feed up, lots of folk fall in to that trap, especially as we're told speed's everything these days! .050 DOC, .020 feed at 50 rpm, is exacly the same as .050 - .010 at 100rpm.
Lots of coolant, then for the first cut get as deep as you can to get under the hard skin and general rubbish.
Tool life won't be high and you could well need a fresh tool before that first cut is done, but after that life gets better...... sometimes you just gotta grit your teeth and sweet talk the lathe through it.
Sorry if I've rattled on a bit, but I hope this might be of help to you, take care. Sami.
P.S. Ask if you'd like any further help, or is that jumbled ramblings?