MATERIALS • PROCESSES • FINISHES
fantastic jump in the steel’s properties: it’s no longer full of bifilms and the steel now achieves its full strength. Plus, it wouldn’t suffer hydrogen embrittlement, as it wouldn’t suffer the corrosion we currently see because there are no bifilms to transport it.” This begs the question, why isn’t
this method widespread throughout metal production? Campbell says he has worked with several reputable companies to install his system, namely Rolls Royce and Cosworth, but there is industry scepticism around the existence of bifilms altogether. “People look at cracks which have
The two formats for bifilms (left) harmless (right) pre-cracked network
OUT WITH THE OLD “The main problems with metals are the entrainment defects of the liquid state: bifilms and bubbles,” Campbell says. “They’re both rather similar, but there’s more air in the bubble. Now, there’s a lot of continuous casting, which is better than how steel was made before, but it’s still not perfect. Special steels are generally cast as an ingot which is top-poured because that is the cheapest way to fill an ingot mould. More expensive ingots, which are said to be better, are poured through a conical pouring basin and filled from the bottom.” These are currently the two principal ways of casting steel,
but according to Campbell these techniques are “fundamentally bad.” “With the conical pouring basin, the
basin takes in air and concentrates it, putting in about 50% air into the metal which maximises the population of bifilms,” he explains. So, what is the alternative? “The real way to cast steels is what
I call the contact pour situation, where the gap between the basin and the running system is eliminated,” Campbell says. “This is a system of melting and casting in which the liquid metal is never poured. It either moves horizontally or uphill and then fills the ingot, rather than being poured in. With this, you suddenly get a
(Left) top-pour casting (middle) bottom gating casting (right) contact pour casting methods
caused failures and they just say, oh yes, it’s a fatigue crack,” he shrugs. “We just give something a name and think that we’ve explained it. The trouble with bifilms is that they’re very, very thin oxide films that come together as closed cracks – they don’t show up on radiology. It’s hard work to reverse decades of thinking.” And how is it that Campbell is able
to identify the existence of bifilms, when others can’t? “It’s easier for me because I’ve realised they’re there,” he answers. “You can design inspection techniques to find them, and then you see them immediately. We have to slim down the metal to make it ultra- sensitive to x-rays, then they start to show up clearly. But most people wouldn’t have bothered to slim down the metal that much.” Now that Campbell has retired from
the University of Birmingham, he has made it his mission to try to change the narrative on bilfims within the metal production space. “The future is interesting, because
we could solve problems with our metals that we’ve lived with for years, like hydrogen embrittlement, in which hydrogen pressure vessels explode unexpectedly, or pipelines which explode due to stress, corrosion and cracking,” he concludes. “These things need not happen, with simple changes to eliminate bifilms we would live in a happier, more efficient world. The whole business of bifilms in metals is affecting our lives all the time, not only because of the huge inefficiencies of working with metals, which are less good than they could be, but some of them are so bad as to be dangerous, and none of them need be. All of them could be better.”
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