Microcracking of electroless nickel over kovar substrate when exposed to 320 C

A. EN deposits have a very low ductility (1-3%). They also suffer a volume shrinkage derived form the formation of a crystal structure at that temperature range which is dependent on the P level. Even though, thin deposits like yours shouldn't be a problem. A high initial stress in the plating due to improper chemistry and/or operating parameters of the bath will definitely change the situation. Try with a freshly made EN bath at its optimum recommended parameters.
Do the cracked edges look adhered to the substrate? If not, there is also a problem with your preplate cycle resulting in lack of adhesion.

A. We regularly precipitation harden via 400C baking electroless nickel without noting the microcracks you've noted. This is especially noteworthy given the fact that you're seeing this on high phosphorus electroless nickel with is the least crystalline in nature, has the highest ductility, and, in general, exhibits the lower stress. This indicates that it's not likely the temperature and shouldn't be the EN coating. This would then suggest the substrate - what happens when you bake an uncoated part?