Sammy,
Some may confuse using a pneumatic chipping hammer with peening...but if the user is deforming the weld surface to hide porosity or other discontinuities then yes, it would be called peening. However for light slag removal, these chipping hammers are acceptable. See AWS D1.1 C-5.27 regarding their explaination of acceptable/unacceptable peening.
In the design section of the code it also addresses peening in paragraph 2.20.6.6(3), and explanations given there as well.
Under interpass cleaning on my WPSs, I usually just state to remove slag....you can go so far as to specify how you want it done, or with what tools...ect....