Slab Stacks and Hydrogen


In an earlier blog I described how I failed to recognise the effect of a gas shroud whilst pouring on reducing the hydrogen content of liquid steel (Hydrogen Shroud). There was, however, another experimental part to my hydrogen project, which was aimed at assisting the Finite Element Analysis (FEA) calculations.

You see, in order to be able to calculate the diffusion of hydrogen out of a slab, I had to have some idea of how fast the slab would be cooling until for all intents and purposes no more hydrogen diffusion would take place. This was fairly simple for a single slab surrounded by and atmosphere at ambient temperature: all it required was to perform an FEA calculation for temperature first, and then use the calculated temperature at a given time to calculate the speed of the hydrogen diffusion.

Another method for assisting hydrogen diffusion from slabs was to place them in soaking pits normally used for reheating ingots prior to slab rolling. In this case knowing the slab temperature was simple, since this should merely reflect the temperature of the soaking pits themselves.

There was, however, one other method that I investigated as an alternative to the use of soaking pits, and that is to place a small number of critical slabs inside a stack, surrounded by other slabs once they had come off the slab mill and still were close to white heat (around 1100°C if I remember correctly). This way, the cooling speed could be reduced substantially over that of a single slab.

The way I measured the temperature was using a thermocouple placed inside a stainless steel hollow tube, which I positioned on top of one slab (with the aid of a long metal bar, which also prevented the thermocouple from being crushed) as the stack was being formed, and then had it covered by the subsequent slabs in the stack. On the other end the thermocouple fed into a data logger, which was placed inside a changing room locker which was shielded from the radiating heat from the stack by some cardboard.

All in all this worked fine, but to stand within 5 yards of slabs at 1100°C was far from comfortable, even when wearing protective gear. In the end I did two different configurations for stacking the slabs and called it a day after that, thinking that they hopefully would be representative for this type of slab cooling. In the report I made it seem like I had performed several more stacks, but had chosen the two that I had actually performed as representative samples.

Getting the data went without a glitch, and I had my temperature trace for calculating the dehydrogenation of the slab. As expected it worked better than a single slab, but not so well as slabs placed in a soaking pit. Besides, the formation of the stack would be depending on how the operator built it, whether there were any delays at the slab mill whilst the stack was being built, and on top of that the cooling would be affected to some extent by the ambient temperature, which during the winter nights could go well below freezing.

In short, the soaking pits allowed a far better control over the slab temperature, and that was then my advised method for reducing the hydrogen content of a critical slab. In addition, someone asked me whether giving ingots a similar soak would help – I knew this wouldn’t be the case given the far higher volume-to-surface area ratio of an ingot, but did the FEA calculation anyway to prove the point. Besides, since an ingot is a cast structure it would contain a lot more spaces between the crystals, and at times complete gas bubbles, which would trap the hydrogen and completely scupper the diffusion of hydrogen out of the ingot.

To be honest, a thought experiment probably would have come up with a similar answer, without the need for any experiments, but being able to show actual temperature traces and calculated hydrogen content graphs was somehow far more convincing to my audience than if I had merely stated “trust me, I know what I’m talking about”.

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