Testing times

Materials World magazine
14 Oct 2014

Fred Starr recollects...

What was the first useful thing you did after you started work? Was it useful enough to be noticed by the boss? I have a distinct memory, back in 1962, of sacrificing a Saturday afternoon and evening to get the first of Dorman Long’s creep tests to its temperature target of 450°C. It needed a periodic tweak of the furnace and patience to let the temperatures settle. The following Monday, everyone was full of congratulations.

Stepping forward a mere 52 years, sees me, at the beginning of August, at the Kirkaldy Testing Museum in London, just off London Bridge, for what might well be my last opportunity for a visit. 

Pride of place in the Kirkaldy Testing and Experimental Works, as it was called when built in 1866, is the massive tensile testing machine (pictured). Built like a battleship and able to test not just specimens but massive components as well, one of its first jobs was for the girders of the world’s first steel bridge spanning the River Mississippi at St Louis. The machine’s last major effort was to assess fragments from a De Havilland Comet jet, which, through a basic design weakness, broke up in the air. Sadly, the museum, although Grade II listed, faces an uncertain future as its current lease runs out.

As well as preserving equipment from the time when the building was usefully employed, the Museum houses an incredible variety of machines for materials testing. The sight of some creep test rigs brought back memories of my lost innocence, where I gave up my own time to further the progress of science. What our lab had begun doing was part of a nationwide effort to assess steels for steam plant superheaters and turbines. In Britain, many companies and hundreds of creep and stress rupture rigs were involved, as even a medium-term test can last more than a year. The data we produced has given us reliable and efficient steam power plants, some of which are still going strong. 

Why do we want tests of such duration? If the temperature is high enough, no matter how low the stress, a material will break. The question is when. Even today we cannot accurately extrapolate from tests that last much less than 25,000 hours, so we are stuck with very long duration and costly programmes. One would think that after so much effort to improve our predictive capability, beginning with the groundbreaking paper by Larson and Miller in 1952, we would be in good shape. If anything, because of the complexity of modern alloys, the situation seems to be getting worse, so we have the call for test times of 200,000 hours.

Is this rational? Could it be that laboratories full of the massed ranks of creep testing machines are as obsolete as the Kirkaldy Test House and Experimental Works? In Europe and North America, coal-fired steam plant seems to have had its day. Plants like this date back to when global warming was not an issue, and where a new steam plant could look forward to years of steady-state operation as a baseload unit. This was the justification for long-term creep testing. But if coal, as a fossil fuel, is to have a place, it will have to fight against devastating competition from renewables, nuclear and gas.

What does the future hold? We all know about the vagaries of wind energy and the need for back-up, which in the UK is effectively 100% of the time. Nuclear will capture the baseload, accentuating the impact of the day-to-night changes in demand on the fossil sector. And what about gas? In the UK, both coal and gas are currently doing their bit to support the Grid. Two years ago, gas was providing baseload energy and coal plants were being switched on and off. It all depends on the relative costs of these two forms of fossil energy.  

Almost from the time when a new coal plant is first synchronised with the Grid, it will be ramped up and down, and stopped and started, as the demand for power changes. The stresses from thermal expansion far exceed those due to steam pressure, knocking seven bells out of superheaters and reheaters. These will be testing times for steam plant operators, who are calling for complex tests of the thermal fatigue type. 

So, no shortage of work in what used to be the creep laboratories. But a final word of advice for those young and foolish enough to enter the world of high temperature metallurgy – no matter what the praise, don’t give up your Saturday night.