Stress Relieving and Pre-Fabricated Nodes for BE Dragline Booms

A previous ACARP project (C11052) identified weld quality and residual stresses as two major factors contributing to poor fatigue life of BE boom clusters compared to Marion clusters. The traditional fabrication methods that have been used for decades without much improvement are responsible for both these factors. Surprisingly, according to the current practice, the main chord butt welds that have comparatively more favourable conditions from residual stress and weld quality view points are stress relieved, while the clusters that generate more than 90% of fatigue problems are not stress relieved. Stress relieving is expected to improve fatigue life of clusters significantly, and as part of this project sound procedures for cluster stress relieving have been developed and successfully trialled on a BE boom.

Stress relieving in-situ can have other effects such as structural deformations, material property changes, etc. These effects were comprehensively examined though the laboratory and field trials undertaken as part of the project. The results indicate that stress relieving of BE boom clusters can be undertaken safely to improve the fatigue performance of the clusters. 

The investigations found that stress relieving also had other benefits such as tempering the weld material and also reducing the hardness of welds. These improvements will also contribute to improving the resistance to initial growth of cracks under load by improving the toughness of the steel near the welds. 

The following specific conclusions were made from the cluster stress relieving investigation:

• Stress relieving can significantly reduce the unfavourable axial tensile residual stresses in a cluster. Following the recommended procedure in the report, the residual stress levels can be reduced at least by 40%.
• Stress relieving the clusters as recommended can increase the fatigue life of clusters at least by 50% and possibly up to 100%. 
• The cost of stress relieving a pair of clusters (i.e. A and B chords) varied between $3-5k. During the field trials at Blackwater mine, stress relieving of six pairs of clusters were undertaken with no addition to the overall shutdown duration.
• Stress relieving also has a beneficial effect on the hardness of welds, reducing them to acceptable levels. This will also contribute to longer fatigue life of clusters.
• The procedure adopted for stress relieving of clusters generally conforms to AWS D1.1:2000 Structural Welding Code – Steel [6].
• The adopted stress relieving procedure has no significant influence on the microstructure of the steel used in the boom. The material tests showed that the influence of stress relieving on the other material properties such as the yield strength of the parent material is small, and therefore should not significantly affect the strength of the boom.
• Based on the findings of the investigation, stress relieving of clusters is recommended for all new clusters in BE booms.

When clusters are replaced after several repairs, the conventional approach is to cut off ~2m length of main chord covering the cluster and replace it with a new main chord segment, and then weld all the lacing pups in-situ to complete the cluster. This approach results in poor weld quality and high levels of residual stresses, and also contributes to longer shutdown durations. Using the latest techniques in surveying and accurate fabrication methods, it is possible to prefabricate the clusters under shop conditions, stress relieve them under controlled conditions, and complete only the attachment welds under field conditions at less critical locations.

The project investigated the feasibility of pre-fabricating the clusters with higher weld quality standards, stress relieving them under well controlled conditions and then attaching them to the booms during shutdowns.  This procedure can reduce the downtime of the machines and also significantly improve the fabrication quality of the clusters. The investigation concluded that the pre-fabrication of clusters is technically feasible to implement on a dragline boom.  However, a field trial could not be undertaken as an opportunity to implement it on an actual dragline boom could not be found. Some of the measuring techniques were trialed on a cluster manufactured in the laboratory.