Underfill : Formed during bar rolling when the bar does not completely fill the rolling die. Also formed during rod end forging when there is insufficient material to fill the die. (Spec 11B)
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}