CONFIDENTIAL TIBU3939-00

Base edge assemblies usually fail by cracks developing along the toe of the adapter welds. Factors that contribute to the failures are the number of loading cycles, the size of the load, the stiffness (thickness) of the base edge, and the quality of the adapter welds. The process for welding the adapters to the bucket base edge can contribute to these failures.

Welds act as stress concentrators. Undercut weld toes, depth of fusion, hydrogen embrittlement, weld contamination, and the formation of a heat affected zone during the weld process contribute to the early formation and rapid propagation of base edge cracks.

Tests recently performed at Caterpillar have revealed that the base edge assembly fatigue life can be substantially improved by the proper selection of where the last weld pass on the adapter is applied.

When the last weld pass is applied next to the base edge, the quenching action of the base edge mass will draw the heat away rapidly enough and create a heat affected zone in the base edge next to the weld. The quenching action occurs even when the base edge and adapter are preheated to 177 to 240?C (350 to 500?F). The heat affected zone will become several points harder and more brittle than the parent metal.

The last weld pass must be applied next to the outermost weld bead that is fused into the base edge. The additional heat input to the toe of the weld tempers and reduces the hardness of the base edge in the heat affected zone. The result is an improved weld.

Adapter strap failures on corner adapters will usually occur at the toe of the weld for the sidebar. The last weld pass must be applied to the adapter strap in the same sequence as to the base edge in order to improve the weld.

Note: For additional information, refer to Special Instruction, REHS0597, "Recommendations for Welding 2-Strap Bucket Adapters to Base Edges".

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