Case Studies

Welded RHS Items

Background

One type of steel item was fabricated in two separate locations by a client company. The item consisted of pieces of steel rectangular hollow section (RHS) joined by fillet welds. Durability testing of the items, which involved cyclic loading until failure, showed that those fabricated at location 1 (L1) had superior performance to those manufactured at location 2 (L2). The client wanted to understand the reasons for the relative differences in performance.

Hypothesis 1

Differences in weld quality between L1 and L2 welds made L1 welds more resistant to cracking.

Data Collection 1

The items were examined visually, showing broadly similar features. Cracking had originated at/adjacent to fillet welds. Where cracking had extended away from the associated weld into the steel RHS, the length of this parent metal cracking was relatively greater in L1 items.

Macroscopic examination of fillet weld cross-sections from L1 and L2 items showed similar face/leg/throat dimensions and revealed no reportable defects or discontinuities. Vickers hardness traverses likewise gave similar results for L1 and L2 items, and both complied with the requirements of AS/NZS 1554.1.

Analysis 1

Because the weld geometry in L1 and L2 was similar and neither item type exhibited any weld discontinuities or defects, it is unlikely that weld quality is responsible for the observed discrepancy in performance.

Conclusion 1

The hypothesis is rejected — the evidence did not show any discernible differences in weld quality between L1 and L2 items.

Hypothesis 2

Differences in RHS material ductility between L1 and L2 items made L1 items more resistant to cracking.

Data Collection 2

The L1 and L2 arms exhibited very similar microstructures in their welds and heat-affected zones. In the RHS parent material, however, the microstructures were distinctly different. The L1 material was significantly spheroidised, with only intermittent, small pearlite colonies (Fig. 1). In contrast, the L2 material was not spheroidised and contained a considerable portion of pearlite (Fig. 2).

Tensile testing revealed meaningful differences in material properties between the L1 and L2 RHS material. While the L2 material had slightly higher strength (i.e., peak stress), the L1 material exhibited significantly greater ductility and toughness. Toughness—a measure of resistance to fracture represented by the area under a stress-strain curve—was nearly twice as high for L1 versus L2 (refer Fig. 3).

Analysis 2

The spheroidised microstructure in L1 would be expected to be more ductile than the ferrite-pearlite microstructure in L2. This was supported by the tensile test results, which showed considerably higher toughness and elongation in the L1 material. The reason for the difference in microstructure was heat treatment — L1 had been subjected to a spheroidising heat treatment while L2 had had a normalising heat treatment.

Conclusion 2

The hypothesis is accepted — the ductility of the L1 material (based on toughness and elongation) was considerably greater than that of L2, resulting in improved resistance to cracking.

Recommendation

It is recommended that all RHS material used for the items in question be subjected to spheroidising heat treatment. While spheroidised steel has lower strength versus normalised steel (controlling for composition), in the present application the ductility benefits more than make up for the strength decrease. Further, durability testing of the items did not raise any concerns about the absolute strength of the RHS material since failure occurred due to insufficient ductility rather than insufficient strength.

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