Tag Archives: Hydrogen

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”.

When Is a Project Finished?

That was the question asked in all earnest when I was at Iscor’s Steelmaking Technology department. The situation was thus : for every major project, you had to write a project brief, stating the initial problem setting, the desired outcome and the steps proposed to achieve the latter. So far so good.

However, there must be something especially bureaucratic n the Afrikaner mind, because once you had stated that you were going to take steps 1 to 14 to resolve a problem, you were expected to complete all those steps. Never mind that it’s not always clear in advance what steps will have to be taken – you could, for instance, stumble upon an intermediate finding that looked so promising that it took on a life of its own and generated a whole new set of action steps. Or a finding quite early on could make it clear that steps 6 to 9 are no longer required because of something you’ve learned in step 5.

To the Afrikaner mindset the first instance, where one action step generates a whole new subset of its own, would require you to write a whole new project brief, in essence creating a completely new project. In the case of the latter, it is more likely than not that steps 6 to 9 would still be undertaken, or at least you would have a fight on your hands convincing your boss that it was indeed a good management of resources to drop them.

But the instance I’m thinking of was a project on the cleanliness of steel, where one action step made such a difference that the goal stated in the project brief was already achieved. This is what caused the question to be asked : now that we’ve achieved the aim, do we continue pouring resources into this project, or do we cut it short ? I can’t quite remember what exactly was decided in the end, but I suppose you could make a case for both types of action.

Cutting the project short once you’ve achieved your aim frees up finite resources in manpower and time on the production lines for other projects. Whereas continuing your project as originally planned makes it possible (at least in principle) that you’ll discover further means of improving the steel cleanliness.

In a way, the hydrogen project I described in an earlier blog was a victim of this type of box ticking. I had done my trial attempting to enrich the steel with hydrogen, had failed, and that was that action step taken care of. If anybody at this stage had said “hey, that’s funny …” maybe further steps could have been added to what was officially a completed project brief, but somehow that opportunity was overlooked.

Presumably as much my fault as the system’s (after all, I had by then started working on inclusions in DWI tinplate, and had lost interest in the hydrogen-in-steel issue), but a project leader more inclined to follow his nose rather than completing the paperwork might just possibly have found an answer to a question that hadn’t been asked, which is “How do you keep the hydrogen content of your steel low in the first place?”

Hydrogen Shroud

During some of my time at Iscor, I had been studying the effects of hydrogen on armour plate steel, and the means to try and get rid of it. That’s why the following experiment is rather odd, firstly because it didn’t yield the expected result, and secondly because no action followed once I realised what had actually happened. Here’s what happened.

I had been doing a literature survey, and done some trials and FEA analysis to find means of getting hydrogen out of solid steel, but the next planned trial was rather hard to get through the system: it was my intention to INCREASE the hydrogen content of one ingot and see how the properties would be affected. In order to do so, I had a cylinder of argon enriched with 5% hydrogen (the highest concentration you could go without a major danger of explosive flammability) which I intended to aim at the liquid steel flow at the time it was poured into the ingot mould.

It took quite a bit of arguing my case before I could convince the powers-that-be to allow me to perform an action that would possibly affect the steel properties for the worst. That’s why the actual trial took place quite late in the day, and once I had already moved on to my next project, which concerned itself with the cleanliness of tinplate steel. And when it ended in what looked like a damp squib, no-one took any notice, and possibly valuable lessons were not learned.

The way the hydrogen was being injected into the steel was by means of a gas shroud, i.e. a ring was placed at the bottom of the ladle where the liquid steel was poured from so that the liquid steel stream was surrounded by a mantle of argon enriched with hydrogen, and the idea was that some of the hydrogen would be absorbed in the liquid steel before it collected in the ingot mould.

The result ? Instead of an enrichment I had actually achieved a LOWER hydrogen content in the targeted ingot. Relief all round that they wouldn’t have to downgrade or scrap an expensive ingot, and me scratching my head, not quite sure what had happened. It was only on later reflection that I realised what had happened: molecular hydrogen gas probably does not split into its individual hydrogen atoms, and as such never entered the liquid steel. What I HAD actually done was to exclude another source of hydrogen, which is dampness in the air.

Consider the mechanism when a water molecule meets liquid steel, especially a turbulent stream when pouring into a mould : water’s oxygen atom readily reacts with iron to form FeO, thereby creating two free hydrogen atoms which can then enter the steel at the same time the oxygen reacts with the steel. The lesson that should have been learnt was that you should try and prevent hydrogen entering the liquid steel at the time you pour it into the mould by applying a gas shroud. Plain argon would have done the job, the hydrogen enrichment being totally unnecessary.

Surely preventing the steel from entering the steel in the first place is far better than attempting to diffuse hydrogen out of solid steel at a later stage. But somehow my realisation of what had happened came too late, and the hydrogen project had already been closed. I still don’t know why I didn’t bring it up with my boss though, because at that time I was still in the same department.

Local Expert

Twice in my working life I had the epithet “expert”or “guru” bestowed upon me, in both cases unwarranted as far as I’m concerned.

The first time was at Iscor when I was given a project to study hydrogen embrittlement in steel, and as a start I performed a literature survey which I issued as an internal report. The second part, which was supposed to be the practical part, with a number of trials attached to them never came to much, and the report for the second part was never written. Nevertheless, from the moment I had done the literature survey, I was referred to as “our expert on hydrogen embrittlement” – when I questioned the use of the word “expert” I was told that no-one else knew more about the topic than I did, so that made me the local expert.

That was my first encounter with this definition of the word “expert”, and it made me realise that, unless you’re a globally recognised expert, the word only has a relative merit, one that could easily be toppled by someone coming into the company who knew more about it.

The second time was in Corus and later Tata Steel, when I was gradually building up a reputation for being able to handle databases and building useful websites, which earned me the title of “IT guru”. Granted that this time round the reputation was grounded in somewhat more solid foundations, but even then people tend to forget how large a field IT is and how being an advanced user in one part does not imply being an expert in all IT areas.

The number of times I had to let people down who approached me with the words “you’re our IT guru, so can you help me with Excel?”. In fact, concentrating on database design and web development makes it far more likely that you won’t need all those “advanced” Excel features because you know a more clever way to store, handle and display data.

But even when sticking to the normal IT tools and programming languages, my exposure was limited. A reasonably good knowledge of SQL in a number of flavours, a decent knowledge of Visual Basic, Java and C#, and some exposure to WebFocus and iWay Client, and in the end a good feel of how to build information systems and structure them using a combination of the above tools. It might make me employable in the IT market (even though I’ve never tried and am not likely to), but I’m very much aware of my limitations, such as a total lack of knowledge of other programming languages such as Python or C++, or my failure to have kept up with recent developments on JavaScript and JQuery.

So in the end, if someone is introduced to you as “our local expert”, you’ll have to put this statement into context, and only time will tell what degree of expertise is covered under this banner.