Mining Technologies International
offers two distinct grades of steel, characterized generically as "standard
strength" and "high strength". To the uninitiated, these are often
misunderstood.
Standard Strength
“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.