Turbine housings can crack from an improper tune which causes high EGT (exhaust temperatures). Another reason why they crack is due to improper cool down of the turbocharger. It is important to let the automobile idle for a few minutes to let the oil recirculate through the turbo charger to remove the heat from the turbo charger. This cool down period is even more important in the winter time. During this idle time exhaust system will also release heat. Turbine housings cracking is not covered by OEM manufactures due to falling under the category of miss use of the turbocharger. This is also something that is out of our control.
Most manufactures use a nickel additive in the turbine housings, the help prevent cracking of the turbine housing. This significantly reduces the chance of the turbine housing cracking. However Often the turbine housing will still crack on sharp edges in the turbine housing.
Some companies have also used pure stainless steel turbine housings, which also reduces the possibility of the turbine housing cracking. However, sharp edges of the turbine housing still tend to crack. A way to try and prevent cracking is to use a porting tool and sand the sharp edges of the turbine housing smooth, which I do to the best of my ability. However I have no control over improper cool down of the turbocharger.