Fred Starr recollects: bolting for cover
Fred Starr recollects…
Cautiously, the door of the conference room opened. The lady who ran the typing pool leaned in. ‘Could you please stop shouting – the secretaries are in hysterics!’ Our meeting had become more than argumentative. It was the British Gas Standards Department versus frontline metallurgists from the company’s R&D divisions. Process engineers from our liquefied natural gas (LNG) plants, for whose benefit we were having the meeting, sat in the middle, trying to make sense of the pseudo-technical to-and-fro.
The topic over which time and money, if not blood, was spilt? Whether or not British Gas needed a formal set of rules, or an ‘industry standard’, covering 18/8 stainless steel nuts and bolts for service at cryogenic temperature. A typical LNG facility houses the energy of at least one Hiroshima bomb. Bolting up of pipework flanges was critical. Escape of LNG could result in a vapour cloud explosion – the very worst.
We would have been spared much acrimony if the vital issue of what could happen to the bolts, when in service, had been properly explained when the draft standard was circulated. My initial reaction was that more paraphernalia would add costs and slow construction at LNG sites. All metallurgists know that ordinary ferritic steels are extremely brittle below room temperatures, but stainless steel 18/8 austenitics are different. What was the problem?
I don’t think the real concern ever emerged at the meetings, or even during visits to the LNG sites. It was my former workmate, Steve Hull, who had eloped to the Standards Department, who put me in the picture. To get the necessary strength in a relatively soft alloy such as 18/8 stainless, bolts had to be made from cold rolled bar stock. Furthermore, when bolts were tightened up, there was more plastic strain where the nut grips. At LNG temperatures (-173°C), cold worked austenite can transform to martensite, increasing its length and volume.
The mechanical properties of the bolts are not seriously impaired, but the dimensional changes from the martensitic transformation are quite marked. The Japanese used the same phenomenon to give Samurai swords their vicious curvature. In LNG plants this behaviour would slacken off the bolts, but only after the pipework had cooled down – a painful and expensive embarrassment. This wasn’t made clear in the preamble to the standard, itself full of unnecessary clauses. I believe I killed the whole thing off when I saw the mechanical device proposed to measure bolt strain. All that was really needed was a recommendation to cryo-soak 18/8 stainless bolts in liquid nitrogen, or to use a higher nickel austenitic that would be more stable at LNG temperatures.
If our discussions could become so heated, one wonders what might be happening at meetings in France and Finland over the steels for the nuclear reactors at Flamanville and Olkiluoto. At a very late stage in construction, segregation of carbon – and, one presumes, other alloying elements – has been detected in the pressure vessel forgings. Well, blow me down! Given that ingot weights are around 300 tonnes, who on Earth could be surprised? My very first failure investigation, at the ripe old age of 21, at Dorman Long, Middlesbrough, was on how segregation was giving problems in the rolling of steel sections. And they were made from four-tonne ingots.
Even more pressing is that recent tests on existing French nuclear plants show similar anomalies. Segregation has been detected, and the steels are not compliant with the formal specification. One doubts that the French nuclear fleet will be shut down, much as the eco-warriors might demand. At the worst, there will be minor changes to start-up and shutdown procedures, limiting thermal stress.
In all probability, when the need for such massive forgings was first envisaged, steelmakers and forge masters would have been fully cognisant of the impact of segregation. Specifications, one expects, would have been written in the light of
Rules, it has been said, are for the guidance of the wise and enslavement of fools. They should not be used by those lacking proper engineering judgement as an excuse to bolt for cover, stopping everything at the first sign of a problem. But common sense, I am sure, will prevail in France.