Fred Starr recollects: The perils of poor ductility

Materials World magazine
26 Nov 2012

What do we need in our leaders, whether in Government or opposition? We want them strong. But we also look for a sort of ductility – the capacity to respond to difficult situations with firmness and humour.  

For metallurgists and engineers, ductility is the ability of metals to deform plastically when stresses get above the expected loads. A ductile person will get stronger and more, not less, trustworthy under stress – what might be called the work hardening of experience.  

Are there other concepts from materials science that could apply to political life? The last thing we want is diamond-hard brittleness in a personality – someone who seems dependable until the real crunch comes. Creep, we all know, is the continued deformation under load, until the cracks emerge. A political creep is someone who is subservient to the party caucus, always modifying his or her views in the hope of gaining preferment. It leads to the common opinion that they are all out for themselves.  

So, we want ductility in materials and people, but how much? Just as we only have a general idea of what we want in a person, there is no specific value in any textbook that applies to all materials. National standards will give figures, but these only mean that, given the requirements for tensile and yield strength, this is much as that specific alloy will allow. There are minimum standards for ductility, but never any maximums.  

Those working with advanced ceramics rave about minute amounts of ductility and levels of toughness that were described by a speaker at one conference as ‘almost as good as cast iron’, as he stood flexing a toughened zirconia ceramic hoop. ‘If it’s so good, why doesn’t he drop it from the balcony?’, I pondered – but I kept this mischievous thought to myself. One only stoops to asking show-stopping questions when one is after the same bit of research funding.  

I practise what I preach. When building what was at the time the world’s highest temperature metallic heat exchanger, using dispersion strengthened Fecralloy tubing, at the first opportunity I got hold of a tube and flung it up into the air. Although the alloy was a 20% chromium, ferritic stainless steel, to which was added 4% aluminium, with a transition temperature of more than 100°C, when the tube crashed onto the concrete it stayed in one piece. I knew then, that with a bit of care, we had a project. 

The lowest ductility that seems to be acceptable in the engineering world is about 5%, and this is for nodular iron crankshafts in car engines. Five per cent was also the into-service ductility of spun cast stainless steel headers on the early steam reforming plants, which were used to make town gas. Although these cast headers were an austenitic stainless steel, the grain boundaries were crammed with coarse precipitates of chromium carbides, and it was these that were undermining the basic toughness of the austenitic. There was worse to come. The headers ran at 750°C, containing hydrogen and superheated steam at pressure. After a few thousand hours, chromium carbides precipitated, en masse, within the grains themselves. Ductility fell to just 0.5%. It was a disaster waiting to happen. On at least two occasions, boiler water got into the header, causing a massive temperature drop, with the header splitting as if it had been sawn in half. At Coleshill in the West Midlands, the steam reformer was wrecked, and every car plant in Birmingham and Coventry was shut down as gas supplies ran out. Elsewhere, while working on a steam reformer as a shift engineer, I too let water into the header. Fortunately, metallurgy was on my side. Our headers were of a wrought alloy, Incoloy 800, a high nickel super-stainless. Its 20% ductility saved me, and saved the day.  

So we can now match ductility figures with people. Sir Winston Churchill was a tough 20%. But what of the present batch who fill the parliamentary benches? Averaging close to the barely acceptable 5%? But, I think, as in all walks of life, just a few suffering from creep!