Hi Elodie,
you can try to insert in material master of the material a value for internal production time field in mrp2 view. This time is used at creation of production order (and the planned order create by MRP planning for to find the explosion date.With this one the system will try to explosion a routing correct according to the automatic rule research manage in customizing.
You could to check also the customizing setting for determine operation default and the scheduling view for the work center of the default operation.
Daniele