Individual parts (.ipt) made using AutoDesk Inventor can be exported to IGES format (.igs, .iges) using the default Export function found in the program. Other CAD software with support of Inventor formats may also be able to help you with ipt to igs conversion and you can also find some dedicated CAD converters which should certainly help you convert ipt to igs.
Open IPT file Open IGS fileA CAD viewer program with conversion capabilities
A CAD converter with huge format support
A CAD visualizaton, analyzation and translating system
A 3D CAD translation software
A dedicated CAD converter