When drill pipes is added to increase the depth of drilling, the loosening of the threads are now performed at the bottom end of the Saver Sub as opposed to the bottom of the Kelly which is the most valuable part of the drill string. It means the the connection between the top end of the Saver Sub and Kelly is seldom used, and suffers minimal wear and tear, whereas the lower connection is used in almost all cases displacing the most wear and tear from the rotary head connection to the much cheaper Saver Sub which is expendable and reduce cost. The material of saver sub is usually 4145H/ 4145H-MOD /Non-magnetic alloy.