Welding on a structure or weldment, which is a major load path and/or which supports a
major portion of a live structural load, is EXTREMELY dangerous. When repairing
the conveyor, the welder chose to re-weld the strut stub to the conveyor
axle without removing the strut structure from the strut stub. This provided an
immediate load and load path which led to the accident. A proper and safe procedure would have been
to merely re-weld the strut stubs to the subject conveyor axle AFTER unpinning and
removing the strut structure from the strut stubs. Had such a procedure been
followed the strut stubs could have been re-welded, the strut legs reattached
to the strut stub and then the assembly re-pinned without the welder being
physically in and under the conveyor structure.
Photograph B: Overall view of the feed end strut structure from the
accident conveyor.
When welds are made, the metal pieces being welded together and the weld bead
joining the metal being welded must be melted to achieve good
fusion, i.e., joining of the separate pieces. When the welding arc (heat) is removed, the
liquid metal weld puddle, just made by the welder, will then solidify into a solid,
continuous weld bead or tack weld. However, the hot steel in the weld bead or tack weld has very little
strength while hot, i.e., steel has approximately 50% of its room temperature
strength at 1100°F. The melting point, where the liquid metal puddle is formed occurs at temperatures greater than
2800°F. In this condition (hot) the strut
stub/axle weldment cannot support much load. The tack welds put in place by the
welder probably failed due to a high temperature creep or by mechanical overload
or a combination of both. When the strut stub/axle welds which were made by the welder were loaded, their ability to support
the load being applied by the conveyor was exceeded and the subject conveyor collapsed.
Photograph C: Close-up view of the “left” strut stub/axle weldment footprint.
In Photograph C several facts about the welding
failure are depicted.
- Re-welding appears to have been attempted on
at least one and possibly two separate occasions
following initial conveyor manufacture.
- Re-welding has been attempted without removal
of prior original weld bead and/or re-weld bead
residue.
- Re-welding was attempted by positioning the
original cut face of the strut stub against and/or
on top of prior weld metal without regard for the
poor joint fit-up gap thereby created.
- Very little new weld metal has been deposited
on the axle side of the re-weld performed by the
welder. This gives a false visual appearance of
strength and good joining (welding) between the
strut stub and the axle.
- The tack welding bead performed by the welder
joins the strut stub to less than 50% (visually
about 35 to 45%) of the available weldment area.
Thus, the weld bead strength, assuming that the
welding conducted by the welder produced a full
strength, quality weld, would have only had about
35 to 45% of the ambient temperature strength that
could have been attained with proper welding.
As stated before welding is extremely dangerous
and, if it is not done properly and correctly,
can lead to serious injury or death. Proper welding
techniques and safety protocols were ignored when
the repair welding was done on the conveyor. If the
welding job was done correctly and safety was taken
into consideration the conveyor would not have
collapsed and no injury would have resulted from the
collapse.
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Photograph A: Overall view of the conveyor