I've probably cut thru more case-hardened security chain than most folks. We buy it in bulk from Austria, made by PEWAG, their 3/8" chain and cut into 1 meter sections for bike chains. Can tell you that stuff is damn hard! We used to "thin" the end links such that they would fit into a medium sized monobloc lock. This thinning operation was taking 20 minutes per chain on an industrial grinding machine. TOO much time, we now use a bigger ABUS monobloc lock. NO more thinning operation, it was a time killer!

One benefit of a chain versus U lock is that a thief must cut a chain TWICE, each side of the link, to break the chain. With a U-lock he only has to make one cut. With a cable, one easy cut.

We cut our 1 meter chain segments using a big oxy-acetylene torch, even w/ the gas flowing at a maximum, it takes 30 seconds per chain to cut. There are some links to a U-tube video showing us cutting w/ oxy on: BikeRegistry link

A thief MAY cut thru a case-hardened security chain using an angle grinder, but it will wipe out his cutting disk for sure and take several minutes, using an industrial grade grinder.