Mining in focus
Prevent heat cracks
on drill rods
Drill rig operators need to understand and prevent heat check cracking on drill
rods, writes Chris Drenth of drilling company Boart Longyear.
H
How does heat check
cracking develop on wireline
coring rods?
Heat check cracking can easily be
identified visually in the field. Heat
[24] MINING MIRROR JANUARY 2019
check cracks are unique in that they
follow the axis of the rod (longitudinal
or ‘axial’ orientation) and are located near
the female or box-end shoulder, and are
associated with a shiny, polished wear area.
This section of the box always protrudes
slightly more than any other area on a
wireline drill rod and grows or ‘bulges’
under high drilling loads.
eat check cracking is the
engineering term describing
the brittle cracking failure of
steel, wherein a thin surface layer has
become excessively hard and brittle as
the result of rapid cycles of frictional
heating and cooling.
The frictional heating is the result of
‘rubbing’, ‘contact’, or ‘drag’ against a
mating surface. Friction heat can build to
exceed the transformation temperature of
the steel (or ‘austenite transformation start
temperature’, ~750°C/1350°F), followed
by rapid cooling from surrounding
steel or cooling fluid, hardening, and
embrittlement. When this cycle repeats
frequently, the heating and cooling
create rapid expansion and contraction,
which leads to fatigue failure, seen as
perpendicular cracks that propagate from
the surface.
While this phenomenon is well
documented in engineering texts, the
problem has been prominent in the oil
and gas exploration industry since the
1940s. The API (American Petroleum
Institute) describes heat check cracking
as, “Formation of surface cracks formed
by the rapid heating and cooling of the
component”. (API ‘RP 7G-2, Recommended
Practice for Inspection and Classification
of Used Drill Stem Elements’, and ‘RP 96,
Deepwater Well Design and Construction’.)
A 1992 IADC/SPE paper on heat
check cracking described full-scale
simulations to prove that the heating and
hardening are easily achieved, but that
fatigue cracking only results from the rapid
heating and cooling associated with each
rotation of the drill string.
‘Box bulging’ is the result of the
interference fit between the pin and the
box (which is responsible for keeping the
joint closed under deceleration) and the
compression of the box shoulder under
torsion and any radial loading from the
thread-form. The positive load flank angles
of traditional thread-forms, such as Q™
threads, generate radial load components
Oil and gas tool joint with heat
check hardening and cracking.
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