Metallurgical failure analysis was requested on a 65 foot conveyor that had failed while undergoing repair welds. An overall view of the conveyor is shown in Photograph A below
Photograph A Overall view of the conveyor.
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 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 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.