Actually it does make sense small cracks do concentrate stresses at the head of the V which break through the crystal structure a layer at a time.
As a simple demo get a piece of paper and pull on it, you will find it pretty hard to tear it , now just nick the edge of the paper and try again, you should find it yields quite easily.
The tensile strength of steel would be a lot higher if it wasn't full of imperfections. incidentally there are two crystal structures you get with steel face centered cubic and body centered cubic
http://en.wikipedia.org/wiki/C...
now the interesting thing about this is that when you cool down steel rapidly you get one form and slowly you get the other form so if your quenching something more than a foot thick you get both types of crystals since heat just isn't removed fast enough from the centre. the larger crystal structure is in the centre and the smaller crystal structure is on the outside. This means the inside is trying to be bigger than the outside. so the outer surfaces are massively stressed. like a bomb stressed seriously. You can't cut through steel stressed like this with a saw as the cutting would unbalance the stresses and it would blow apart, so you have to do something called plunge grinding which is done in a massive lathe with a grinding wheel taking it down equally on all sides.
This is what happens when you produce a roll for a cold rolling mill the outside is very hard with a softer core. Usually the forces are lower than the uts of the steel but sometimes it isn't and you get catastrophic failures. generally this happens in the quenching tank where its safe you normally hear a few bangs as lumps of steel spawl off from the outside followed by a boom as the roll breaks apart and goes crashing down to the bottom of the tank.
Rarely they fail after the heat treatment, joe the hardness tester where i used to work was nearly killed by one. he'd hardness tested it around 12pm (the hardness was abnormally high around 890 vickers) an hour later it blew apart a ton and a half of journal end was launched across the factory floor missing his legs by inches. he was off work for a week after with the shock. We also had a used roll blow up in a storage warehouse one weekend and it took a wall out, this had been in service and had been worn down to below a serviceable size. Even after all that time it was still stressed ...
back on topic , it seems reasonable that by removing the sites for cracks to occur the uts of the steel will be much higher, normally the way round the problem is to make the thing bigger that way the forces applied will not break the cross section of course that makes it heavier and harder to work with which is why its a specialized area like drilling where this has been applied, with the plating thicknesses used the cost will be way higher than for the regular steel pipes, i'd expect probably more than 10x the cost but the rig would be able to drill deeper and that's what matters, and the return on that makes the drilling costs look like peanuts.
