5 ms·

I'd really like to see some backing of these claims. I've seen "grain flow" claiming big gains for years in various enthusiast magazines (bike, motorcycles, ca

by abe_m 2y ago

I'd really like to see some backing of these claims. I've seen "grain flow" claiming big gains for years in various enthusiast magazines (bike, motorcycles, cars, etc) as to why components are forged. Then I started working in engineering, and I can't find any support for these claims. For sure when a steel bar is worked down to become wire for a steel rope, it cannot be pulled to an elongation of 100x increasing strength. A36 steel which is a basic structural steel has an elongation at break of 23% in a 2" gauge length [1]. In every rolling mill I've been in, there is a limited amount of reduction per pass through the mill, after which the metal needs to go for thermal treatment to be annealed to remove all the cold work. Every time you anneal the material, you completely resets the elongation (internal plastic strain) and strengthening due to work hardening. If they do too much reduction in one pass or at too low of a temperature, it cracks the material and makes it weaker. For sheet metal, there is lore about the material being stronger in the rolling direction as that is the direction of grain flow. I have yet to find a source that can point to any large difference. In papers like this [2] there are claims of certain orientations of samples relative to rolling direction have different tensile properties, but when you look at the tensile charts, there is minimal difference. The yield strength in these charts isn't reported, but all three orientations look to yield at the same point. In this test the across the grain (90 degree to rolling direction) orientation had the highest tensile strength which is the opposite of the expectation of the forging "grain flow" promoters. But the magnitude of the difference isn't large, and is small relative to normal factors of safety in a reasonable design. When designing automotive components, I've only ever seen forging methods selected for efficiency of production. If a part mostly fills the envelope of a bar or plate, it is cut from bar or plate in all cases. If there is a lot of void volume in the part, the calculation will be made to determine if the cost of developing forging tooling and development will get paid back in reduced material and machining cost. I have yet to see the dimensions of the part change with manufacturing method, which would be needed if the non-forged part was significantly weaker. And finally, a lot of forged parts are subsequently heat treated. When heat treating steel all of the grains in the steel have to be destroyed and recrystallized. That is the mechanism by which heat treatment works. Depending on the exact process and part geometry, this process removes or reduces the grain flow in the finished parts. Having said that, the claim of superiority of forging persists, and I'd love to see a technical reference that shows the magnitude of the change from someone who has plausibly actually tested the effect. [1] https://matweb.com/search/DataSheet.aspx?MatGUID=d1844977c5c8440cb9a3a967f8909c3a https://matweb.com/search/DataSheet.aspx?MatGUID=d1844977c5c... [2] https://www.researchgate.net/publication/283447700_The_effect_of_rolling_direction_to_the_tensile_properties_of_AA5083_specimen https://www.researchgate.net/publication/283447700_The_effec...

kragen 2y ago
your comment is an extremely valuable contribution! disclaimer: i don't have a relevant technical reference handy, and i'm far from an expert on the area, which is vast, and i recognize you know things i don't about it. still, i do spend a lot of time reading papers with metallurgical micrographs in them†, and i think i figured out the answer to your question many years ago, so i will explain my understanding except for the part about grain orientation, anyway > In every rolling mill I've been in, there is a limited amount of reduction per pass through the mill, after which the metal needs to go for thermal treatment to be annealed to remove all the cold work. Every time you anneal the material, you completely resets the elongation (internal plastic strain) and strengthening due to work hardening. If they do too much reduction in one pass or at too low of a temperature, it cracks the material and makes it weaker. as i understand it, this is exactly right, but you say it as if it's contradictory. strain hardening increases the yield strength of metal (by making it yield). it can also change the tensile strength, but to a much smaller degree. when the metal can no longer handle stress by yielding, in particular by yielding in a way that produces further work hardening, so that the yield is distributed over the metal rather than being concentrated wherever it starts, it cracks. that's why strain hardening metal makes it more prone to cracking. in general, a given metal is more prone to cracking when you harden it, whether you harden it by cold forging, case hardening, or quenching. (peening is the exception; it inhibits crack initiation by a different method.) https://en.wikipedia.org/wiki/Work_hardening https://en.wikipedia.org/wiki/Work_hardening has an overview that talks about how this phenomenon can be either desirable or undesirable the change in yield strength from cold working can be quite large, a factor of 4 or so. it doesn't change the ultimate tensile strength much (or at all in the case of your wire rope), but there are a lot of cases where what you care about is the yield strength, not the uts, because if the part yields by more than a tiny amount, it is out of tolerance and has therefore failed (with respect to a36 steel, elongation at break, and wire rope, this is a minor detail, but it's possible to elongate it somewhat more through rolling than you can through wire-drawing. but you are certainly correct that you cannot elongate it 100×, and wire rope is mostly made by drawing, not by rolling.) there are different kinds of heat treatment, but the most common kind for steel involves a phase transition to austenite and back, which does indeed destroy the entire grain structure of the steel, losing any potential advantage of forging, precisely as you say. i'd think this would also be mostly true for hot-forging, where steel is forged while still austenitic; the relevant grain structure for strength will be the one that the steel acquires when it leaves the austenite phase. there are other kinds of heat treatment (more commonly used with things like aluminum) that don't involve fully recrystallizing the metal, and i would expect some grain structure to survive those probably none of that is telling you anything you don't already know, but perhaps it's a different way of thinking about the things you know that explains the apparent contradictions as for which direction i would expect grain orientation to make things strongest in, i really have no idea at all ______ † last night, for example, i read https://www.mdpi.com/2075-4701/8/2/91/pdf https://www.mdpi.com/2075-4701/8/2/91/pdf and https://www.jstage.jst.go.jp/article/jjspm/63/7/63_15-00089/_html/-char/en https://www.jstage.jst.go.jp/article/jjspm/63/7/63_15-00089/..., but also parts of https://pure.tue.nl/ws/portalfiles/portal/1584410/617544.pdf https://pure.tue.nl/ws/portalfiles/portal/1584410/617544.pdf, http://www.diva-portal.se/smash/get/diva2:1352113/FULLTEXT01.pdf http://www.diva-portal.se/smash/get/diva2:1352113/FULLTEXT01..., https://yadda.icm.edu.pl/baztech/element/bwmeta1.element.baztech-0da90fe1-ef98-41b2-b732-6013124164d8/c/Stanula_jamme_2022_114_1.pdf https://yadda.icm.edu.pl/baztech/element/bwmeta1.element.baz..., https://www.imerys.com/public/2022-03/Specialty-Carbons-for-Powder-Metallurgy.pdf https://www.imerys.com/public/2022-03/Specialty-Carbons-for-..., and https://backend.orbit.dtu.dk/ws/portalfiles/portal/200743982/Electrodeposition_of_Iron_with_Co_deposition_of_Carbon_On_the_Nature_of_Nanocrystalline_Fe_C_Coatings_as_published_to_Orbit.pdf https://backend.orbit.dtu.dk/ws/portalfiles/portal/200743982..., but i was maybe on a bit of an atypical metallurgy bender. none of these are more than marginally relevant to the questions at hand of forging, strain-hardening/work-hardening, and grain structure orientation

by abe_m 2y ago

kragen 2y ago

your comment is an extremely valuable contribution! disclaimer: i don't have a relevant technical reference handy, and i'm far from an expert on the area, which is vast, and i recognize you know things i don't about it. still, i do spend a lot of time reading papers with metallurgical micrographs in them†, and i think i figured out the answer to your question many years ago, so i will explain my understanding except for the part about grain orientation, anyway > In every rolling mill I've been in, there is a limited amount of reduction per pass through the mill, after which the metal needs to go for thermal treatment to be annealed to remove all the cold work. Every time you anneal the material, you completely resets the elongation (internal plastic strain) and strengthening due to work hardening. If they do too much reduction in one pass or at too low of a temperature, it cracks the material and makes it weaker. as i understand it, this is exactly right, but you say it as if it's contradictory. strain hardening increases the yield strength of metal (by making it yield). it can also change the tensile strength, but to a much smaller degree. when the metal can no longer handle stress by yielding, in particular by yielding in a way that produces further work hardening, so that the yield is distributed over the metal rather than being concentrated wherever it starts, it cracks. that's why strain hardening metal makes it more prone to cracking. in general, a given metal is more prone to cracking when you harden it, whether you harden it by cold forging, case hardening, or quenching. (peening is the exception; it inhibits crack initiation by a different method.) https://en.wikipedia.org/wiki/Work_hardening https://en.wikipedia.org/wiki/Work_hardening has an overview that talks about how this phenomenon can be either desirable or undesirable the change in yield strength from cold working can be quite large, a factor of 4 or so. it doesn't change the ultimate tensile strength much (or at all in the case of your wire rope), but there are a lot of cases where what you care about is the yield strength, not the uts, because if the part yields by more than a tiny amount, it is out of tolerance and has therefore failed (with respect to a36 steel, elongation at break, and wire rope, this is a minor detail, but it's possible to elongate it somewhat more through rolling than you can through wire-drawing. but you are certainly correct that you cannot elongate it 100×, and wire rope is mostly made by drawing, not by rolling.) there are different kinds of heat treatment, but the most common kind for steel involves a phase transition to austenite and back, which does indeed destroy the entire grain structure of the steel, losing any potential advantage of forging, precisely as you say. i'd think this would also be mostly true for hot-forging, where steel is forged while still austenitic; the relevant grain structure for strength will be the one that the steel acquires when it leaves the austenite phase. there are other kinds of heat treatment (more commonly used with things like aluminum) that don't involve fully recrystallizing the metal, and i would expect some grain structure to survive those probably none of that is telling you anything you don't already know, but perhaps it's a different way of thinking about the things you know that explains the apparent contradictions as for which direction i would expect grain orientation to make things strongest in, i really have no idea at all ______ † last night, for example, i read https://www.mdpi.com/2075-4701/8/2/91/pdf https://www.mdpi.com/2075-4701/8/2/91/pdf and https://www.jstage.jst.go.jp/article/jjspm/63/7/63_15-00089/_html/-char/en https://www.jstage.jst.go.jp/article/jjspm/63/7/63_15-00089/..., but also parts of https://pure.tue.nl/ws/portalfiles/portal/1584410/617544.pdf https://pure.tue.nl/ws/portalfiles/portal/1584410/617544.pdf, http://www.diva-portal.se/smash/get/diva2:1352113/FULLTEXT01.pdf http://www.diva-portal.se/smash/get/diva2:1352113/FULLTEXT01..., https://yadda.icm.edu.pl/baztech/element/bwmeta1.element.baztech-0da90fe1-ef98-41b2-b732-6013124164d8/c/Stanula_jamme_2022_114_1.pdf https://yadda.icm.edu.pl/baztech/element/bwmeta1.element.baz..., https://www.imerys.com/public/2022-03/Specialty-Carbons-for-Powder-Metallurgy.pdf https://www.imerys.com/public/2022-03/Specialty-Carbons-for-..., and https://backend.orbit.dtu.dk/ws/portalfiles/portal/200743982/Electrodeposition_of_Iron_with_Co_deposition_of_Carbon_On_the_Nature_of_Nanocrystalline_Fe_C_Coatings_as_published_to_Orbit.pdf https://backend.orbit.dtu.dk/ws/portalfiles/portal/200743982..., but i was maybe on a bit of an atypical metallurgy bender. none of these are more than marginally relevant to the questions at hand of forging, strain-hardening/work-hardening, and grain structure orientation