Use an abrasive wheel; in particular use the thinnest cut off wheel possible for your angle grinder. To minimize heat buildup with abrasives, simply rock the wheel back and forth a bit and use light pressure. More sparks coming out of the cut equal less heat staying in the part. After cutting, let the part air cool.

Always wear proper eye and face protection when using abrasive wheels, other important PPE may include dust masks, hearing protection and proper clothing. Be sure your cut off wheels are properly RPM rated for your grinder.