Raise Bore Drill Steel

“Standard Strength” is the term applied to the original AISI (American Iron & Steel Institute) 4145H raise drill steel and is still used by MTI today.  This is a chromium-molybdenum alloy which has had its chemistry altered to produce a deeper hardening level below the surface O.D. during the heating, quenching and tempering cycle of heat treatment.  The ultimate tensile strength of the 4145H is 135,000 PSI with yield strength of 110,000 PSI.  It is used typically for lower torques and pull machines with steel diameters of 5-3/4″ thru 11-1/4″ O.D.

High Strength

As higher output raise drills were developed in 1971, the need arose for “high Strength” drill rods in the 10 inch and larger diameters.  The most logical choice was the common commercial grade of 4340 steel, a nickel, chromium, molybdenum chemistry steel.  However, it was discovered that in the rods over 10 inch diameter, the physical test properties of the 4340 did not extend deep enough below the O.D. to yield the strength and fatigue properties necessary.  After extensive testing, the MTI grade 4330 VMVD (vanadium modified, vacuum de-gassed) was found to be a superior steel for large diameter raise drill products.  The toughness and fatigue strength of the 4330 VMVD is far superior to that of the 4340, with an ultimate tensile strength of 150,000 PSI with a yield strength of 140,000 PSI.  It has been used by MTI since 1972, proving itself for nearly 40 years and will continue to be the MTI “high strength” steel for years to come.