increase the fatigue life

Apply enough pressure to the inside of the cylinder that the inner wall goes beyond its yield stress and plastically deforms. Voila. The inner wall takes a compression set and you enhance fatigue life and load bearing capacity (assuming you got the material properties right).

Tom.

Yes - it is not reliable, it requires a device design made especially for autofrettage, and it requires materials which all have the appropriate compatible properties.

As to fatigue improvement by autofrettage - none per se that I know of, other than that gained by raising the base yield.

Do you think AISI 630 is an apprppriate material to be autofrettaged? This will be for waterjet cylinder.