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  1. #1
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    SS tubing for racks, seamless or welded?

    whats the benefit of using seamless over welded SS tubing, if any? Is welded any less structurally sound, could I pop the welds bending it for racks? Or is it more of a psi thing that I dont need to worry about?

    Ps which is stronger/ better for racks, SS or 4130?
    Last edited by bleedingapple; 03-09-11 at 12:59 AM.
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    when maneuvering at speed they feel just like your typical road bike on a country road.
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    4130 is significantly stronger than most stainless steels. There are many steels that are called "stainless." As far as seamed vs. not seamed, it looks to me that the seamed that most places sell is significantly lower quality. The temptation is obvious though, it's significantly less money.

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    THE Materials Oracle Falanx's Avatar
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    Quote Originally Posted by bleedingapple View Post
    whats the benefit of using seamless over welded SS tubing, if any? Is welded any less structurally sound, could I pop the welds bending it for racks? Or is it more of a psi thing that I dont need to worry about?

    Ps which is stronger/ better for racks, SS or 4130?
    Right, first things first: How big do you want these racks? What kind of diameter of tube are you looking at? That will govern what's available on the market to you in both. 4130, or other low alloy steels, while available in a large range of sizes, are more readily available in what would be considered 'structural' sizes, measured in sizeable fractions of an inch. Stainless steel tubings exist in equivlanet sizes and larger, but then there's a gap before they reappear down at the hydraulic tube sizes - you know, round the typical 3/16" you'd see for brake pipes.

    As for seamed vs seamless, that's only really a question when you're looking to use it for hydraulic purposes, like brake lines on cars. Regulations for such applications are quite happy to accept seamed tubing, as part of the manufacturing process following wrapping a very accurate hole in steel and welding, is another drawing operation that works the weld, followed by a recrystallising anneal. That said, I purchased seamless for my car ;-)

    Go and decide what kind of tubing ODs you want first, then come back and we'll give you some more project-specific advice :-)


    Quote Originally Posted by unterhausen View Post
    4130 is significantly stronger than most stainless steels. There are many steels that are called "stainless." As far as seamed vs. not seamed, it looks to me that the seamed that most places sell is significantly lower quality. The temptation is obvious though, it's significantly less money.
    You'd be surprised, it's not. To either.

    And while 4130 can be supplied heavily cold worked and/or heat treated, most tubing in non-butted sections is basically drawn and normalised, making it not hugely stronger than cold-drawn austenitic stainless (the AISI 3xx series).
    "While my father fought for you, I learnt. While my father glorified your petty administration, I learnt. While he longed every day for our line, Adunís line, to be restored, I learnt. He sent me away to bring the Dark Templar back when the time was right!
    "And you tell me that I cannot do this? That I cannot feel the weight of the universe?
    "Damn you, Tellan! Aldaris killed my father!"

  4. #4
    tuz
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    From what I see online, the yield of SS 304 is about 1/2 that of normalized 4130 cromo. Having worked with both on a manual bender, I can attest it seems true. The tensile strength is apparently similar.

    I'm about to make a SS rack using 3/8x0.035 tubes. Should be stiff enough. The strength comes in the design (triangulation).

    As for seamless vs seamed I would not worry about it if they are from a reputable source. The prices seem similar anyway.
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    I'm just going from what I get from McMaster-Carr, there is an identifiable seam on the welded stuff. That being said, in the future I'm using the seamed for non-structural uses since it's pretty nice. The yield strength they give for stainless is 35-45ksi vs 75ksi for 4130. This seems important for racks, if you can give up almost half of the yield strength you can go with thinner 4130.

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    I think the thing with stainless is you don't have to paint of powder it which is a huge deal for one-off. One can buy a rack for what powder costs.

    Another huge factor with rack is that some of the more "full featured" designs have a lot of struts to achieve various goals. With clever arrangement of the parts you can end up supporting the horizontals well enough that the actually material properties fade a little. So for instance an inside strut to triangulate the rack against swaying would also cut the span length and ensure the SS was well supported. If you look at rack designs you will see lots of other opportunities to do similar things.

    Third big thing about racks is that the forming of stainless tubes work hardens it which has a significant effect on the properties, and narrows the gap that some charts may indicate for comparative properties. Tubing is formed, one thing that the seamless stuff may have over the other stuff is that it is double formed - OD and ID dies. This accounts for some serious difference between CREW tubing and DOM tubing in regular 1020 kind of uses. However, as was mentioned, you can tell what you are getting by it's resistance to bending.

    I still prefer 4130, but I am considering a stainless rack, and when I do, I will go for the "good stuff". As I said, the savings on finishing are sufficiently attractive that I don't intend on hammering the cost down any further. I would get the SS that Aircraft spruce sells.

    One approach I might copy, particularly where 5/16" seemed appropriate, is to sleeve the contact points for panniers with 3/8" stainless. You get to go light with 4130 elsewhere, and yet have wearproof contact points for your panniers clips. That way you can just spray the rest of the rack.

  7. #7
    Senior Member bleedingapple's Avatar
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    so the first rack i am making is using 3/8 OD .035 ID 304 SS tubing. I get mine from Alaskan Copper and Brass from here in Portland, OR. I costs $3.20 a foot actually more money than Aircraft Spruce. This rack isn't being built to carry a ton of weight but would still like to see it be able to handle 30-50#s. I am currently using the Harbor Freight bender and while it wont last too long, so far it is working alright. My next rack I am thinking of using 7/16 - 1/2 OD and either .028 or .035 ID SS. Though I would like to see this rack handle way more weight, like 75-100+. As far as welded vs seamless, for this 2nd rack do you think it will be an issue?

    It seems like the jury is still out on the strength difference on the 4130 and SS. As was mentioned before, powder coating is not cheap, and My home oven is gas.

    Quote Originally Posted by Peterpan1 View Post
    Another huge factor with rack is that some of the more "full featured" designs have a lot of struts to achieve various goals. With clever arrangement of the parts you can end up supporting the horizontals well enough that the actually material properties fade a little. So for instance an inside strut to triangulate the rack against swaying would also cut the span length and ensure the SS was well supported. If you look at rack designs you will see lots of other opportunities to do similar things.
    I am new to designing things like this so when you say struts to prevent swaying do you mean horizontal ones in the triangle of the rack? do you have any photos of what your talking about?
    Quote Originally Posted by Ken Cox View Post
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    when maneuvering at speed they feel just like your typical road bike on a country road.
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    there are tons of pictures of racks online. Enter "porteur rack" into your favorite search engine, and you will find almost everyone that makes racks.

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    The design I was thinking of was a touring rack.

    OK, so I went to Flickr and looked up stainless porteur rack. And chosen at random from a thumbnail:

    http://www.flickr.com/photos/22380580@N02/3235417508/

    I you look at this design you can see how he did those decorative overlaps on the sides. Those act to shorten the unsupported length of tubing. The struts support the next tube over, which is a bit of a weak point with this kind of rack in general. Even a tiny piece of tubing spanning where the strut contacts the inner tube, with the outer one would help. The rise in the rack forward creates a bit of a truss effect, and the rear bar also has a truss effect. I have seen racks where the whole perimeter is doubled with spacers. Helps hold stuff on but also acts as a truss. So just look at what people are doing, and you will see how various techniques can be used to add strength, and make the precise material chosen less relevant. When carefully done, a few doublers here and there are a lot more efficient than just making all the tubing a lot larger.


    http://www.flickr.com/photos/8772455...n/photostream/
    Last edited by NoReg; 03-10-11 at 02:23 AM.

  10. #10
    THE Materials Oracle Falanx's Avatar
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    Quote Originally Posted by tuz View Post
    From what I see online, the yield of SS 304 is about 1/2 that of normalized 4130 cromo. Having worked with both on a manual bender, I can attest it seems true. The tensile strength is apparently similar.
    I'm about to make a SS rack using 3/8x0.035 tubes. Should be stiff enough. The strength comes in the design (triangulation).
    As for seamless vs seamed I would not worry about it if they are from a reputable source. The prices seem similar anyway.
    A lower yield with a higher work hardening rate as per most austenitic stainless (3xx series) is good for forming initially, but once you get into heavy deformations becomes a right pain. For racks, I can't see you having to make more than 90 degree bends often, and at that level of deformation on a thin section, thinnish walled tube, that's still low enough down the work hardening rate to make a 3xx steel the better choice.

    Of importance to remember is the material stiffness, and therefore the rigidity in identical section is eesentially the same. AISI 304's Young Modulus is about 2% lower than that of 4130, that's all.

    Quote Originally Posted by unterhausen View Post
    I'm just going from what I get from McMaster-Carr, there is an identifiable seam on the welded stuff. That being said, in the future I'm using the seamed for non-structural uses since it's pretty nice. The yield strength they give for stainless is 35-45ksi vs 75ksi for 4130. This seems important for racks, if you can give up almost half of the yield strength you can go with thinner 4130.
    But that's *as-delivered*. By the time you've bent it to racks, the strengths wil be essentially the same due to 3xx's higher work hardening rate.

    And the seam remains visible, that's due to the annealling heat effects prior to pickling and drawing.

    Quote Originally Posted by Peterpan1 View Post
    I think the thing with stainless is you don't have to paint of powder it which is a huge deal for one-off. One can buy a rack for what powder costs.
    Good call.

    Quote Originally Posted by Peterpan1 View Post
    Another huge factor with rack is that some of the more "full featured" designs have a lot of struts to achieve various goals. With clever arrangement of the parts you can end up supporting the horizontals well enough that the actually material properties fade a little. So for instance an inside strut to triangulate the rack against swaying would also cut the span length and ensure the SS was well supported. If you look at rack designs you will see lots of other opportunities to do similar things.

    Third big thing about racks is that the forming of stainless tubes work hardens it which has a significant effect on the properties, and narrows the gap that some charts may indicate for comparative properties. Tubing is formed, one thing that the seamless stuff may have over the other stuff is that it is double formed - OD and ID dies. This accounts for some serious difference between CREW tubing and DOM tubing in regular 1020 kind of uses. However, as was mentioned, you can tell what you are getting by it's resistance to bending.
    I think there's a recurring theme appearing here... ;-)

    Quote Originally Posted by bleedingapple View Post

    It seems like the jury is still out on the strength difference on the 4130 and SS. As was mentioned before, powder coating is not cheap, and My home oven is gas.
    Oh no it's not. By the time it's shaped and bent, 304 will be at the same strength level as 4130. There's a dirty trick to make it even stronger, but it requires liquid nitrogen...

    And don't worry about it being welded at all, okay?
    "While my father fought for you, I learnt. While my father glorified your petty administration, I learnt. While he longed every day for our line, Adunís line, to be restored, I learnt. He sent me away to bring the Dark Templar back when the time was right!
    "And you tell me that I cannot do this? That I cannot feel the weight of the universe?
    "Damn you, Tellan! Aldaris killed my father!"

  11. #11
    tuz
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    Cool that's good to know. I'd be happy to skip the paint/powder/chrome required for cromo!
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  12. #12
    Senior Member bleedingapple's Avatar
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    Here is a photo of the direction I'm headed. Does this look structurally sound or would it be good to add some struts since I'm using thinner OD tubing?
    http://www.flickr.com/photos/mcbomb/3740087319/

    Quote Originally Posted by Falanx View Post
    Oh no it's not. By the time it's shaped and bent, 304 will be at the same strength level as 4130. There's a dirty trick to make it even stronger, but it requires liquid nitrogen...

    And don't worry about it being welded at all, okay?
    Thats great to hear! Also if I can use welded that will save me so much. Another couple questions about 304 SS. I was reading that 304L was better because of rust forming at the heat affected zone if you use plain 304, is this true? is there any way to prevent this with plain 304? Also as far as cooling goes after I've brazed a joint, do I let it air cool or quench it?
    Quote Originally Posted by Ken Cox View Post
    "You can cheat death a thousand times, but death only has to win once."
    Quote Originally Posted by 11.4 View Post
    when maneuvering at speed they feel just like your typical road bike on a country road.
    Quote Originally Posted by carleton View Post
    "Hey, a fixie!!"
    "tzzzzzzzzzzz...."
    "awwww."

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    Are you making it so it bolts together? My advice is to go with a basic design, but have a fallback where you can build it out a bit to add some stiffness should it prove too flexy. I just "write around it". If you have something you are writing where you get stumped on some fact or idea, but you can't find a supporting source. You just write around it. Find some way of saying the same thing differently. I can't always get a structure right the first time, but I can approach it so that I can make it right if it turns out a little shy. But it helps to start with the end in sight.

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    Senior Member bleedingapple's Avatar
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    Quote Originally Posted by Peterpan1 View Post
    Are you making it so it bolts together? My advice is to go with a basic design, but have a fallback where you can build it out a bit to add some stiffness should it prove too flexy. I just "write around it". If you have something you are writing where you get stumped on some fact or idea, but you can't find a supporting source. You just write around it. Find some way of saying the same thing differently. I can't always get a structure right the first time, but I can approach it so that I can make it right if it turns out a little shy. But it helps to start with the end in sight.
    No I am brazing the whole thing. That was just a design I liked and seemed a good starting point. When you say basic design what do you mean? When I google porteur racks there are all kinds of designs so not sure what is considered basic. I'm new to designing things like this so please bare with me.

    My idea for struts should it turn out too flexie is to have them come from the front horizontal rail to diagonally to each front leg. does that seem like something that will work?
    Quote Originally Posted by Ken Cox View Post
    "You can cheat death a thousand times, but death only has to win once."
    Quote Originally Posted by 11.4 View Post
    when maneuvering at speed they feel just like your typical road bike on a country road.
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    "Hey, a fixie!!"
    "tzzzzzzzzzzz...."
    "awwww."

  15. #15
    THE Materials Oracle Falanx's Avatar
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    Quote Originally Posted by bleedingapple View Post

    Thats great to hear! Also if I can use welded that will save me so much. Another couple questions about 304 SS. I was reading that 304L was better because of rust forming at the heat affected zone if you use plain 304, is this true? is there any way to prevent this with plain 304? Also as far as cooling goes after I've brazed a joint, do I let it air cool or quench it?
    Right. Most commercial stainless steels of the 3xx type suffer from 'weld-decay' if left too long in the temperature region 600-650 degrees. The 3xx family contains between 17 and 28% chromium, depending on number, but for resistance to normal environmental corrosion you only need at bout 12-14%. That's all fine and good if all the chromium stay in solution, but at >600 degrees for extended periods, some of the chromium combines with the little carbon in the steel (all stainlesses except knife steel types tend to have very, very low carbon contents - of the order of <0.1% ), to form mixed chromium carbides. This happens most especially at grain boundaries because the metal's crystal is disordered there, and diffusion happens faster. As a result, metal directly adjacent to grain boundaries becomes depeleted of chromium and locally drops below that 12-14% minimum.

    Corrosion occurs along the lines of grain boundaries and severely damages the steel.

    A 3xx series steel with 'L' following the number means that it contains less than 0.04% carbon, which lowers the risk of this phenomenon - less carbon, means less chromum tied up in carbides at grain boundaries. Notice the word 'lowers', not 'removes'. These steels may still suffer from weld decay if you are not careful, but it's much easier to avoid the problem.

    Normally it won't ever cause you concern for something like this. If you're very concerned about long-long-longevity, you can try to buy AISI321 which is weld-stabilized with titanium additions, or 317LTi, (a non standard grade) but you'll be bloody lucky. Stainlesses containing molybdenum (the 315/6/7/8) family are rather expensive.
    "While my father fought for you, I learnt. While my father glorified your petty administration, I learnt. While he longed every day for our line, Adunís line, to be restored, I learnt. He sent me away to bring the Dark Templar back when the time was right!
    "And you tell me that I cannot do this? That I cannot feel the weight of the universe?
    "Damn you, Tellan! Aldaris killed my father!"

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    I don't mean basic relative to all porteurs, but basic relative to your design. So on that one you showed, if the perimeter wasn't stiff enough, a few short struts between the two perimeter levels would create a truss, much stiffer. If the tubes that make the platform weren't stiff enough a few pieces connecting them would make them share the load better. If the verticals were wobbly a strut tying them together would keep them more in column. But the trick is to come up with a design that allows these kind of changes to flow. So that you don't a have stage one looks great, stage 2 looks like stage one with a lot of hasty additions. It's not a big mystery, just plan ahead. Or get a design you like of some complexity, and strip it back, then if it isn't strong enough, you could go for the full build.

    A lot of porteurs just have a flat rack, that would be basic, a second level tied in as a truss would be stage 2, etc...

  17. #17
    Senior Member bleedingapple's Avatar
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    Quote Originally Posted by Falanx View Post
    Normally it won't ever cause you concern for something like this. If you're very concerned about long-long-longevity, you can try to buy AISI321 which is weld-stabilized with titanium additions, or 317LTi, (a non standard grade) but you'll be bloody lucky. Stainlesses containing molybdenum (the 315/6/7/8) family are rather expensive.
    any idea how much more the 321 SS is than the 304/304L? Also, on 3/8" OD x .035 ID SS tube, how much time under the heat would you say I have before I start causing problems?

    2011-03-11 20.35.17.jpg

    this is a drawing of what I am looking to make for a rack. The side view shows a bar in it that I was thinking of adding for strength. Also the front view shows the diagonal supports I was talking about adding. With this design do you think these added supports are needed? Seeing these images where is it you were talking I should add the trusses?

    also any thoughts on the quenching vs. air cooling?
    Quote Originally Posted by Ken Cox View Post
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    Quote Originally Posted by 11.4 View Post
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    "Hey, a fixie!!"
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    You say you are welding? Or did you mean brazing. Basically you don't have to worry about time, you just have to do it right, depending on which process you choose.

    I'm not saying you need a truss but it you build it as you intend to, see if it seems ok, then you could add one. This design you have, the supports may end up too wide spread or the top too narrow, that kind of structure is normally used on touring racks with far narrower platform.

    You don't want to quench the metal. This technique is used to harden tool steel, springs etc... All it achieves in this kind of situation is to make metal brittle, or cause it to crack. Just let the material cool at room temperature. More a problem with 4130 where the carbon content is sufficient to cause undesirable effects

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    THE Materials Oracle Falanx's Avatar
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    Quote Originally Posted by Peterpan1 View Post
    You say you are welding? Or did you mean brazing. Basically you don't have to worry about time, you just have to do it right, depending on which process you choose.
    He does if he's welding *stainless*. Refer to my previous statement about temperatures and compositions. Unless it's a stabilised, like 321.

    Quote Originally Posted by Peterpan1 View Post
    You don't want to quench the metal. This technique is used to harden tool steel, springs etc... All it achieves in this kind of situation is to make metal brittle, or cause it to crack. Just let the material cool at room temperature. More a problem with 4130 where the carbon content is sufficient to cause undesirable effects
    Er.. not quite right, actually. In these sections and sizes, yes welded, quenched 4130 will harden, but that merely returns us to the difference between hardness and hardenability. He'll be building a big network of heatsinks in the shape of a rack, here. Secondlyu, 3xx series stainless steels are non-transformable, so they won't harden. They'll soften if anything. Unles she refridgerates them in liquid nitrogen before deforming them, if they're the metallurgically unstable ones - 301-304, 315/316.
    "While my father fought for you, I learnt. While my father glorified your petty administration, I learnt. While he longed every day for our line, Adunís line, to be restored, I learnt. He sent me away to bring the Dark Templar back when the time was right!
    "And you tell me that I cannot do this? That I cannot feel the weight of the universe?
    "Damn you, Tellan! Aldaris killed my father!"

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    "He does if he's welding *stainless*. Refer to my previous statement about temperatures and compositions. Unless it's a stabilised, like 321."

    My concern is that in the real world he has to worry about whether it is properly shielded or not. With enough heat and time he could convert it into a puddle, but unless he shields it properly he will damage it in almost any process. That means back purging, and the process does mater, at least again in the real world, you could back purge and stick weld, it just never probably happens.


    "Er.. not quite right, actually. In these sections and sizes, yes welded, quenched 4130 will harden, but that merely returns us to the difference between hardness and hardenability. He'll be building a big network of heatsinks in the shape of a rack, here."

    I don't think I get what you are saying, with this series of steels he can harden them. One can speculate how likely that is, given a certain structure, but he needs to know it is possible, and he needs to know there is probably no reason to ever attempt it. People report hardening in from shield gas postflow, let alone quenching. They also report hardening in clusters which you would think would be an unlikely place. Aircraft guys are very concerned about that stuff.


    " Secondlyu, 3xx series stainless steels are non-transformable, so they won't harden. They'll soften if anything. Unles she refridgerates them in liquid nitrogen before deforming them, if they're the metallurgically unstable ones - 301-304, 315/316."

    That is why I said, or crack. You can pretty much crack anything if you dump it in water.

  21. #21
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    Quote Originally Posted by Peterpan1 View Post
    My concern is that in the real world he has to worry about whether it is properly shielded or not. With enough heat and time he could convert it into a puddle, but unless he shields it properly he will damage it in almost any process. That means back purging, and the process does mater, at least again in the real world, you could back purge and stick weld, it just never probably happens.
    Very valid, yes.


    Quote Originally Posted by Peterpan1 View Post
    I don't think I get what you are saying, with this series of steels he can harden them. One can speculate how likely that is, given a certain structure, but he needs to know it is possible, and he needs to know there is probably no reason to ever attempt it. People report hardening in from shield gas postflow, let alone quenching. They also report hardening in clusters which you would think would be an unlikely place. Aircraft guys are very concerned about that stuff.
    No, I'm agreeing with you there. But we've had this discussion before, about the difference between hardening, and how hard; and hardenability, and how deep. . I'm not disputing that they notice hardening, but in low-hardenability steels, any increase in hardness A) isn't massive B) doesn't really affect the notch-sensitivity of the steel. I know aircraft guys are often very concerned about things like that, and working in the aerospace industry I can tell you now, they're mostly design-stress-analysis guys, who don't know a huge deal about actual material science.

    TL, DR; They're panicking over nothing. Really.

    Quote Originally Posted by Peterpan1 View Post
    That is why I said, or crack. You can pretty much crack anything if you dump it in water.
    The quench rate in hardenable steels is important because you're transforming from a ductile phase to a hard, brittle phase with substantial thermal strain and little time for regimented microstructural change. Not all alloys have the same CoTE, not all the same working temperature range.

    Aluminium alloys have huge CoTE, and low ductility, brazing brasses and bronzes too through some phase regions, but austenitic steels are thermally very well-behaved.

    In a 3xx steel, there's no phase change and that stuff is ductile down to cryogenic temperatures. The total thermal strain from 800 degrees to zero in a 3xx steel is about 0.01%.

    I 'm not saying it's impossible to crack it. I'm saying it's not bloody easy ;-)
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