Surprising results gleaned about how rockdrill buttons wear down may influence the next generation of buttons hitting our shores. Sarah Belfield writes for Australian Mining
Newly published analyses of how rockdrill buttons wear are revealing unexpected results likely to influence not only what tomorrow’s buttons will look like but also the path that drilling productivity improvements will take. Rockdrill buttons, also called inserts, are hard, rounded nubs mounted onto the crown of a rock drill.
The shape, number and pattern of the buttons depends on factors such as the rock type being drilled and the type of drilling being carried out.
Most rockdrill buttons used in the mining industry are generally made of tungsten carbide grains held together by a cobalt binder. This combination, also known as cemented carbide, arose several decades ago but has stood the test of time for various reasons.
Swedish-based researchers who have examined used rotary-percussion rockdrill buttons of this composition have found the material actually undergoing wear is not virgin cemented carbide but a seamless blend of carbide, cobalt and the rock that was being drilled.
They found the rock could partly cover the button surface, or form an intermixed layer with the cobalt binder, or migrate further down into the button to create narrow, convoluted rock channels.
Ulrik Beste, now with high-speed steel producer Erasteel Kloster, and Staffan Jacobson, a member of Uppsala University’s tribomaterials research group, told Australian Mining that the characteristics of this new carbide-cobalt-rock blend are what should guide the search for lower wear rates in buttons.
Jacobson said another option would be to try to find ways to avoid letting rock combine with the button’s cemented carbide.
The pair’s finding that rock integrates with the upper layers of rockdrill buttons flies in the face of conventional industry wisdom. It was generally thought that during drilling, cobalt was forced out from between deforming carbide grains, leaving those grains exposed. But this conclusion may have stemmed from a misinterpretation, according to Jacobson.
“The action is not cobalt getting out of the [button] material, but rock material coming in instead,” he said. Beste, whose doctoral work forms the basis of the button wear analysis, said it took about three years to convince Sandvik, the company funding his project, that rock really had integrated with button material.
He and Jacobson had to present their findings clearly using a strong argument.
Beste’s work was also something of a curiosity for conference attendees when he presented details of his work in late 2004, after his doctoral work had drawn to a close.
“When I presented this paper at the International Conference on the Science of Hard Materials, I had many people coming up to me afterwards. They were really surprised and thought it was a really interesting paper,” he said.
Jacobson said that initially the two researchers themselves had trouble comprehending the phenomenon as well.
“It took us quite some time to realise what we were seeing, because we had never read anything about it in the literature. When we first noticed it, we didn’t really understand that it was common.”
It was only after examining many different used-button samples that they spotted a pattern.
Something else people had trouble accepting was Beste and Jacobson’s evidence that, on a very small scale, temperatures were high enough to melt tiny pockets of the rock that had become part of the rockdrill button. But the signs are that disbelief is now a thing of the past.
Following his doctoral work, Beste joined Atlas Copco Secoroc, which designs and manufactures drilling tools. While precise details were confidential for commercial reasons, Beste was able to say that Atlas Copco Secoroc was “following a number of very interesting research tracks to provide the market with better cemented carbide [buttons]”.
Beste said that once rock had blended with button material, the major factor in button wear was the crushing of carbide grains so that fragments broke away from parent grains. It meant any bid to make buttons last longer would need to limit this effect.
“This can be achieved in a number of ways and we have some ideas,” Beste said.
He said another approach to increasing the longevity of buttons could be to narrow the difference in properties between the binder material and the tungsten carbide grains, because it was this currently-large mismatch in properties that led to rock integration in the first place.
Beste said high speed steel may prove to be a binder well-suited to this purpose.
Ultimately, work such as Beste and Jacobson’s is done with a view to the future of drilling.
In the case of rockdrill buttons, improvements in wear rates will intertwine with the trend towards automation, according to Beste.
“A lower and predictable wear rate is extremely important, especially when drilling becomes more and more automated,” he said.
“In the future, with robots in underground mining, no-one will accept that a [button] can break after one metre or 2000 metres – it must be much more predictable than that.”
Ulrik Beste Erasteel Kloster
Prof. Staffan Jacobson
The Ångström Laboratory Uppsala University, Sweden
+46 18 471 3088