SAE 8620 is a low (relatively) carbon grade suited to case hardening (carburizing and nitriding). when you say equivalent what do you mean? as far as world equivalents it often seems that exact matches dont exist. You can likely use the key to steel and the world guide to equivalent irons and steels to get to an appropriate foreign grade. If you want to replace this with a different grade I'm not much help. (its time for someone like unclesyd or metman wh has bunches of real world experience to chime in)
It will be about the same, as the only differnce is Carbon content. Would need to add hardenability agents to change its cooling rate responce, e.g. Cr, Ni, B. See 4330 with considerability higher Ni.
