Restructuring dimensionally distorted slag pots: a case study

April 10th, 2014, Published in Articles: Vector

 

by Grant Sutherland, Xpert Welding

Dimensional distortion in 160 mm thick cast-iron slag pots remained a major concern for steel furnaces with no successful solution in sight until a specialised welding company saved its client millions through a unique restructuring process.

Fig. 1: During the cutting and colapsing process.

Fig. 1: During the cutting and colapsing process.

Metal-based slag pots undergo dimensional distortion, becoming oval and unable to fit into their carriers when their temperatures reach 1600°C during tapping. They should ideally be cooled to 250°C before re-use. The slag is unable to form a protective freezing skull if the pot’s temperature is too high when the slag is poured. If the pot is too cool, on the other hand, thermal shock and cracking may occur.

This problem has persisted over many years until specialised welding company Xpert Welding proposed splitting the slag pot, removing a segment (much like removing a wedge from an orange) and welding it back together. It was proposed that this would reduce the excessive oval distortion and bring the oversized trunion outreach back to the standard size.

The solution sounded simple but many challenges were soon identified. Firstly, the responsible engineer on site did not consider the proposed solution practicable and a slag pot which had been distorted beyond re-use was allocated to the project. Having held the maintenance contract for only four years, Xpert Welding MD Grant Sutherland and his late father Ian Sutherland, who had gained 20 years’ experience on the Mufilira copper mines in Zambia, set out to prove their point.

The slag pot was measured and the segment to be cut out pop-marked as close to the spout as possible, on both sides. A line linking these two at the base show the hinge line where the sides would fall together once the segment is removed.

Fig. 2: Double V prep (X prepraration).

Fig. 2: Double V prep (X prepraration).

Although the distortions were quite severe, the cut on each side of the line was a maximum of 75 mm away, reducing the overall width of the trunion stick-out by up to 150 mm. The cut was made carefully where the distortion was identical across the width of the cut to ensure that the sides would meet again easily.

Once the initial cut-through was made, each side was profiled in turn and the required segment was removed (see Fig. 1), leaving the V-groove preparation for welding to be ground down accordingly. A full, double V-preparation was done (see Fig. 1).

A full butt joint was welded from both sides (see Fig. 3), noting that less parent material is removed on the outside and that the inside naturalises in a plug-type effect ensuring good strength and sealing for future molten slag transportation. A smaller amount of weld metal than a single-sided preparation is also required. The angular distortion can be controlled by using a balanced welding sequence.

The root face can often be dispensed with because of the large solid angle, thereby giving better penetration. The depth of penetration on each side of the plate may be unequal – as was the situation in this particular case. Back gouging was done before welding on the second side of the joint.

Fig. 3: Front and back view of pot post split and back in process.

Fig. 3: Front and back view of pot post split and back in process.

Although a double U or J preparation would have reduced the required volume of weld metal, it was too difficult to do this with chrome and other steel inclusions in the pot shell. Also, only a normal cutting torch and pencil grinder were used to cut and prepare the weld joint in situ. Once the root run was complete, NDT was enforced before continuing.

Pre-warming and the use of blankets and sand enabled controlled cooling while minimising the possibilities of laminar, cold or other cracks.

Linear and angular misalignment also posed challenges as the segment to be removed shrank in size closer to the base of the pot.

The company repaired eight pots in this way, saving the client millions of rand (in 2004). No sign of cracking or any other form of failure has been reported to date. However, further distortion of the pot shell continues and hang-up within the pot while pouring remains a concern.

Contact Grant Sutherland, Xpert Welding, Tel 013 246-1317, xpertweld@xsinet.co.za

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