I found a great article on shot peening cast iron car parts.
It down loads a PDF of the articlehttps://www.diva-portal.org
› get › FULLTEXT01
From the article:
Shot peening of cast iron.
One ought to think for a material like cast iron that is so widely used for so many
years (over 2000 years), everything has already been done. Probably this is true, but
a lot of work on cast iron has been done by companies and the results have not
become public. Thus there exists a gap in knowledge in the scientific community
today about cast irons. During the last 15 years of research on cast iron, the main
area of interest has been in the solidification process and how to affect it to get the
best mechanical properties (static). Surprisingly few articles discuss how the fatigue
properties vary under different loading for the different cast irons available today.
Even fewer articles are found in the topic of shot peening of cast iron. Most of the
articles deliberate a nodular cast iron with various microstructures under different
peening conditions [55-61]. One of the earliest publications concerning shot
peening and cast iron, showed that the benefits of shot peening on the fatigue life
of nodular casting  is three times better than blasting. A more academic study
 on the shot peening effects on fatigue strength showed increased fatigue
strength of 7 to 30 % depending on microstructure. Another article also dealing
with shot peening of nodular cast iron with different microstructures resulted in 30
to 115 % increase in fatigue strength . Additional work on different nodular cast
iron subjected to shot peening showed an increase in fatigue strength between 20
to 50 % once again depending on the microstructure . A 60 % improvement on
the fatigue strength was shown on a Cu-Ni austempered nodular cast iron .
As a closing comment I would like to state that even though the existent of articles
on grey cast iron, or compacted graphite iron, is absent in the context of shot
peening and fatigue strength, it can be done and result in increased fatigue strength
with the right parameters.
And, they only dumb questions are those that go unasked!