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Laser clad rolls reduce costs

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LASER-clad work rolls in a processor leveller unit has considerably improved the strip shape presentation to a butt welder at BHPSteel’s Spring Hill works at Port Kembla.

This has dramatically reduced wear, lowering maintenance costs and maintaining roll quality, weld alignment and scarfed weld appearance.

The Coupled Pickle Cold Mill (CPCM) is part of the production line for Colorbond, Zincalume and Galvabond steel brands.

Before the cladding could be trialed in the CPCM it had to be thoroughly tested by the principal materials engineer for BHP Steel, Paul Betts.

Betts first learned about the LaserBond cladding process in April 2000. Brian Probets of HVOF and Laser Technologies, described the process as a low heat surface cladding process that provides a metallurgical bond with extremely low dilution.

“A five-axis robot system is used to manipulate a high-powered 6kW laser beam across the surface of the rolls. A thin layer of the hard surfacing material is metallurgically bonded to the base metal in the process.

“The controlled heat minimises the decomposition of the hard particles that is inherent in other welding processes,” Probets said.

“The rapid quenching of the clad layer as it leaves the laser beam provides a fine, defect-free deposit and a very small heat effected zone. In short, a very high performance surface with no undesirable effects on the substrate.”

Betts said it was the sort of surface enhancement process that BHP Steel had been looking for a long time, and agreed to comprehensively test the LaserBond claddings.

Samples of different types of LaserBond claddings were given to Betts for testing in BHP’s laboratories at Port Kembla.

“First we did metallographic tests by taking a sections through the coatings, polishing them up and examining the microstructure using an optical microscope,” he said. “We also did some chemical analysis.

“The bond is important because we have heavy loading in our various applications. However, this process achieved full fusion without diluting. That was quite important, and it was free of defects.”

Hitting a sample with a ball pane hammer, and sectioning through to see if the coating was cracking or starting to spall, also tested bond and toughness.

“Spalling is when the cladding peels away from the substrate. There was no evidence of this in the samples.

“We were quite excited by the coating, the quality was good and dilution was minimal. Compared to other coatings we had tested it was a better coating.”

From the results Betts saw a huge potential for the LaserBond cladding: “If the product can survive here with our many applications it can survive anywhere,” he said.

Probets then contacted John Hefko, responsible for mechanical planning for the CPCM and Hefko agreed to trial the rolls in the mill.

“We needed quality, long wearing rolls to maintain correct strip shape to the butt welder,” Hefko said.

“The CPCM is in three sections and the LaserBond rolls are in the entry section where the steel coils are welded together to form a continuous length strip.

“Processed coils exiting the CPCM will eventually be seen in the market place as Colorbond roofing and fencing, purlins, white goods, and garage doors etc.

“Part of our process is to butt weld the tail end of the outgoing coil to the head end of the in-coming coil,” Hefko said.

The company decided to try the LaserBond clad rolls in the processor leveler. Standard rolls used there are 1700 mm long x 90 mm diameter, and of hardened carbon steel.

Despite the hardening, the standard rolls showed signs of wear in about four weeks.

“The process leveller gives the correct strip shape for the butt welding operation. It’s critical for producing a quality weld with acceptable gauge reduction for processing in our Five Stand Cold Reduction Mill. We trialed two LaserBond clad rolls in the processor leveler. Here rolls are exposed to greatest load and wear is most severe.

“Worn work rolls cause variation in alignment of strip front and tail ends. This produces high material along the weld line after the scarfing process,” Hefko said.

“This high material gets rolled into the parent metal during the cold gauge reduction process. A stress fracture may occur along the weld line resulting in a weld break in our mill and long expensive delays.”

After an eight-week trial Hefko visually compared the LaserBond rolls with the adjacent unclad rolls.

“There was no visible wear on the LaserBond clad rolls but there was plenty of wear on the standard rolls,” he said.

“We change a cartridge of seven rolls every three months and we are looking at increasing the life to six months based on the successful trial of the LaserBond clad rolls.”

“As work rolls wear the amount of intermesh to gain strip shape must increase to compensate for this wear. Less wear makes the machine more consistent and reliable.

“Further down the line at the welder itself are the welder strip support rolls and carry-over rolls. Here the strip is curved and rubs on the ends of support rolls, causing a bottle neck wear pattern and strip support rolls would wear out in four weeks. A cavity underneath the rolls allows scale dust from the strip to fall between a ledge and wear areas on the support rolls. The accumulated scale and wear prevents reliable operation of the butt-welder .

“With the LaserBond clad rolls in place the reduced wear on the roll surface reduced the amount of scale dust getting into the cavity.

“Our first set of carry-over rolls went in November 2001. We normally changed these every six weeks but after 18 months, the original LaserBond clad rolls are still going strong with negligible wear,” Hefko said.

“Following the successful trials we have installed LaserBond clad rolls in other parts of the CPCM including anti-coil break rolls, tension-leveler unit pass line rolls with other applications are planned for the future.”

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