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Carpal Tunnel
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That sounds like it ought to be a contradiction in terms, Bryan. Could you elaborate a little, if it wouldn't get too technical? And thank you for an enlightening post. I must say, you have an unusal hobby; I am imagining, when someone asks you what you do for fun, you saying, "Oh, I just go out to the shed and marage some steel".  Oh! Maybe you can tell me (though I wouldn't blame you for refusing, after that little remark!): how is maraging pronounced, please?
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A similar aging process is used with many alloys, even Nickel and Titanium. The centrifuges the Pakistani wanted to build would have been some type of stainless.
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In reading an old New Scientist, an improvement for supersonic jet engines is described. It is to be made o f "nitinol" a nickel-titanium "shape memory" alloy. This is produced by a maraging process, although the article did not mention that.
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journeyman
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journeyman
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No problem, Jackie - but you're in for it. :) BTW, AWAD is the hobby, I'm a materials engineer by profession.
Metal is metal because of something called dislocations, and how they behave. These dislocations are imperfections in the molecular structure. Metal is strong because it is crystalline (ceramics are much stronger) but they are ductile (tough) because the dislocations can move (unlike ceramics, where dislocations are absolutely pinned in place). To get stronger metal, you have to pin the dislocations, but this reduces ductility and so it is a balancing act. With me so far?
The "favorable stresses" produced by aging require the dislocations to avoid the areas where precipitates are. Instead of moving in a straight line, they are required to go around. In this way, they get tangled up with themselves (work hardening) and also don't move all in the same direction as pushed by the outside force. This distributes the stress in the material and produces less yielding - hence, it is stronger. However, since the dislocations aren't absolutely pinned as in ceramics or "as-quenched" (freshly made, with no further heat treatment) martensite, the metal can absorb shock (this is "toughness").
Favorable stresses are also found in other applications, where metal is bend constantly (fatigue loading). Picture a paper clip that you bend back and forth until it breaks. That is fatigue failure. At the surface, if you shot-peen it, you introduce residual compressive stresses, which resist the bending (tensile) stresses, and improves fatigue resistance to fatigue failure.
Hope that was clear.
Cheers,
Bryan
Cheers,
Bryan
You are only wretched and unworthy if you choose to be.
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journeyman
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journeyman
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FYI
The phenomena of aging to improve strength and shape-memory are distinct. The first may actually be included into a shape-memory alloy, but maraging does not necessarily produce that effect (although that is one route). The second involves the fact that some of the metal is in a different phase (call it B) than the rest (phase A). When stressed (thermally or mechanically) phase B actually changes phase to phase C (like water going to ice, only much more subtly - it takes on a different molecular structure). When the stress is relieved, phase C changes back to phase B, and the metal returns to its normal shape. There were frames for spectacles that had such a metal, and the advertisement showed someone bending them beyond all recognition, and then letting go and they snapped back to shape. That wasn't a trick - it is really possible.
Cheers,
Bryan
Cheers,
Bryan
You are only wretched and unworthy if you choose to be.
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Dear Bryan:Now that you mention it, I remember seeing that ad on TV about the glasses, but never before heard any explanation.But the ones in the ad were "granny glasses" that did not appeal to me. And even if they had, I'm sure the price would have completely discouraged me.
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Thanks, Bryan. Now--what is shot-peen, please?
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Carpal Tunnel
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Jackie and bryan-- i have got to tell you, i love these digressions into technical terms and definintions.. in most cases, i have 0.05% knowledge about the subject-- and after reading the posts, i feel like my knowlegde has been boosted to 10%-- but since i don't use it, or need it, it quickly, and massively falls back.. and i am left knowing .1% about something! not much, but i have doubled my knowledge!
i don't know what shot-peening is , but i am going to guess-- a peen is what a hammer has -- as in a "ball peen hammer" and to peen something is to shape or temper it by hitting it with a peen..
peening metal is one way to make it stronger.. and tin plate (a pie pan, maybe) or copper trays are often finished with a peened surface (it gives them a dimmpled look) to make these soft materials stronger.. but i think you can peen a surface, not have a dimple look, and still have made the metal stronger.. it is the peening, and not the dimples that have the effect of making the metal stronger.
so i think shot peening is just a specific type of peening..
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journeyman
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journeyman
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Helen,
Your guesses are definitely close to the mark. In the sense of peened soft metals, what you are doing is cold-working it all the way through, which tangles up dislocations and makes them less likely to move. Shot peening is indeed a specific type, using hard balls (usually metal or ceramic) fired at high speed to work the surface of a metal. This does do cold work, but only on the surface. There is also laser peening, which takes advantage of the thermal stresses induced by very high, localized heat and consequent superhigh cooling rates to induce the same stresses as shot peening, only with less mess, deeper, and more uniformly. Even better is hydropeening, which a water curtain is heated with a laser, and the superheating causes detonations on a very small scale, also inducing those favorable compressive stresses.
Cheers,
Bryan
Cheers,
Bryan
You are only wretched and unworthy if you choose to be.
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Early in WWII, my wife had a job in a defense plant, X-raying some very critically important castings to detect defects. Quite often she was able to detect places where the molder had attempted to hide a defect, by peening metal into the defect.
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