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September 12

Heat treating oxygen removal

Hi. To remove oxygen during Heat treating, knifemakers tear up little pieces of paper/tissue and put it in the foil pouch[1]. It's not perfect, since paper isn't pure carbon and contains contaminants, but it's cheap and easy and gets the job done.

Mass-production industrial users use vacuum furnace for heat treating, so this step isn't necessary. But prototyping industrial users also use the exact same foil pouch technique as the knifemakers.

Question 1. What do prototyping industrial users use for oxygen removal in foil pouches?

My guess is graphite, since it's cheap, porous, and (relatively) pure.

Question 2. Putting standard industrial practice and cost aside, what is the most ideal material for this task? Specifically, material X of volume V is placed in a air-tight container containing excess air and sealed. The container is heated to 500 °C for 1 hour. Which X is capable of reacting with the largest mass of oxygen in the container during this 1 hour?

My guess is either pure solid Lithium, Beryllium, or Boron. Or maybe Diamond. My reasoning is that X must be solid, since we're looking for high density. X must also have low atomic weight, so my 4 guesses are the lowest atomic weight solids with high density.Satoshit1 (talk) 18:08, 12 September 2023 (UTC)[reply]

Iron near melting point

Iron says that this element's melting point is 1538°, presumably at standard pressure. If you heat a piece of iron to 1500°, I assume it experiences some changes other than the temperature itself — blacksmiths heat iron because it's easier to work at high temperatures — but in general, will the piece of iron behave like the same piece at room temperature, or is it likely to exhibit many changes to its physical properties? I know that ice exhibits changes when it's melting, but perhaps those are the result of existing in an environment above the melting point; if the iron's being held in an ordinary room, you won't see the edges smoothing out as the surface melts, as with a piece of crushed ice. Nyttend (talk) 19:52, 12 September 2023 (UTC)[reply]

When iron is heated it first glows red, then orange, yellow, and finally white. The ideal heat for most forging is bright yellow-orange. Blacksmiths are often seen hammering on metal that is glowing red, which the article Red heat suggests may be about 814°C or 1497°F, which would be a waste of effort if the metal were not already softened at this temperature. This report describes the changes in atom packing and loss of magnetism as iron melts and eventually flows as a liquid. ~~~~ Philvoids (talk) 21:42, 12 September 2023 (UTC)[reply]

Heated iron is less brittle and more ductile, allowing it to be hammered, flattened by rollers, and worked in other ways. BTW, the extensive hammering by blacksmiths mainly serves the purpose of turning pig iron into wrought iron by working pockets of excess slag out, making it less brittle also at normal temperatures. --Lambiam 08:40, 13 September 2023 (UTC)[reply]

See Heat treating for various lasting effects heating a piece of iron can have. --Lambiam 09:01, 13 September 2023 (UTC)[reply]

Sorry Lambiam, but you've misunderstood the chemistry and production of wrought iron. It is a very low carbon form of iron, less than 0.05% (compare steels: around 0.1–2.1%, pig iron: 3.8–4.7%). Hammering does reduce the slag content, but doesn't change the carbon content. It's also worth pointing out that the last plant producing wrought iron commercially closed in 1973. Any genuine wrought iron today is either recycled or produced on a craft scale. Apart from this, for the last half century all "wrought iron" has been mild steel (0.05% to 0.25% C). At first glance really low carbon mild steel looks like wrought iron, but the chemical compositions are different (chiefly sulphur, phosphorous and silicon) and wrought iron is produced at a lower temperature rather than being cast, hence the slag inclusions. Martin of Sheffield (talk) 10:04, 13 September 2023 (UTC)[reply]

When I wrote "wrought iron" I meant "wrought iron", and not some "functional equivalent" produced by other methods than working the iron. I did not make a reference (also not implicitly) to the chemistry of wrought iron. Hammering is a traditional step in the process of producing wrought iron, and its purpose is as I wrote. See also this page: "Iron Making: Refining into Wrought Iron". --Lambiam 12:13, 13 September 2023 (UTC)[reply]

Agreed wrought iron is wrought by blacksmiths and others which does reduce the slag content. However no amount of hammering can change pig iron into wrought iron. They are chemically different, therefore statement "the extensive hammering by blacksmiths mainly serves the purpose of turning pig iron into wrought iron" is incorrect. If you read the link that you gave you will see that there are two processes, "fining" (a type of bloomery) and hammering. During the fining process long bars are dipped into the metal iron and lifted up into the hot air blast. The oxygen in the air oxidises the carbon in the iron to CO (and subsequently CO2 of course). As this occurs the now purer iron collects as a spongy mass on the end of the rods and can be withdraw as "bloom". In passing, it should be noted that iron production prior to the 14thC regarded pig iron as waste, the bloomery furnace was controlled to ensure that the iron didn't melt as it was reduced from the ore.
After the fining, the pig iron has been converted into a spongy mass of wrought iron. The process of working the iron to consolidate it can now begin using a variety of hammers from the simple blacksmith's up to mechanical hammers. Martin of Sheffield (talk) 14:47, 13 September 2023 (UTC)[reply]

Oh, [snort], of course I knew it changed colour; sorry I made it sound like I didn't realise that. I meant to ask "other than temperature and colour". Nyttend (talk) 19:23, 13 September 2023 (UTC)[reply]

Many physical properties vary with temperature (density, mechanical strength, electrical or thermal conductivity, etc.). For most solids, many properties vary "only a little bit" when far from the phase transition. "Most solids" , "many properties" and "only a little bit" is of course extremely precise; here’s an example. The (volumetric) thermal dilation coefficients of solids are usually in the

10^{-6}-10^{-4}K^{-1}

range. That is low compared to that of an ideal gas, which is (at room temperature)

\left(300K\right)^{-1}\approx 3\times 10^{-3}K^{-1}

.

The way I read the question, you are asking if those changes are more pronounced near a phase transition. Well, it depends on the type of phase transition. If the phase transition is discontinuous ("first-order"), then the physical properties show a "jump" near the critical temperature, but the curves just before and just after are smooth. If the phase transition is continuous ("second-order"), there are usually lots of changes in the properties near the critical point.

That might seem tautological. After all, if you have a "phase transition", it means something happens; if not a discontinuity in the value itself, then a discontinuity in the derivative of the value. However, it turns out that a great many of second-order phase transitions follow very similar scaling laws. (If you can give a good account of why that is, you are next year’s favorite for the Nobel prize.) The article about this is critical exponent (very well-written, but the topic is intrinsically complex - you need some math knowledge to read past the lead). Tigraan^{Click here for my talk page ("private" contact)} 13:26, 15 September 2023 (UTC)[reply]

I didn't know the term "phase transition". Thank you, yes, this is what I was trying to ask. Nyttend (talk) 03:31, 16 September 2023 (UTC)[reply]