Cracked exhaust manifold - when and why does it happen?

I have noticed the rear exhaust manifold on my 3.2l AJ16 is cracked. While I am not 100% it was ok when I bought the car, it certainly has got worse. What are the conditions that cause the problem? It’s it extrem temperatures, or is it fast heating and cooling?
I noticed shortly after a long motorway drive during which I pushed quite hard at times reaching speeds of 150mph (that’s about all it will do, although I was still away from the rev limiter). I am pretty sure we won’t be allowed to drive that fast for much longer here in Germany, they are pushing for a 130km/h speed limit, but that’s a topic for the pub.
Would that motorway drive have done it?

What are your experiences trying to weld these manifolds?

Cheers,
Harald

Just seems to be a problem that crops up from time to time, I have a set of manifolds from one of my AJ6 engines that I have had repaired, they are just sitting on a shelf in the garage. A good welder can do the job, they just need to be brought up to a good temperature prior to welding and the cool down needs to be controlled, they would probably need a light scim across the mating face afterwards.

Thanks Robin!
I have a spare engine and will take the manifold off that. Hopefully it is not cracked. But if it is, I’ll have to go down the welding route. I seem to remember I have some cast iron welding rods somewhere. Should probably practice on a manifold from the scrapyard. Or as you suggest find a professional to do it.

Cheers,
Harald