Any metallurgists out there? Want to explain why rolled threads are better than cut? - 3000GT/Stealth International Message Center
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Old 04-22-2006, 08:49 PM   #1 (permalink)
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Default Any metallurgists out there? Want to explain why rolled threads are better than cut?

So why exactly are rolled threads better than cut threads for studs/bolts? I've done a search on the forum and can't find a place where this has been discussed before...
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Old 04-23-2006, 01:48 AM   #2 (permalink)
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Default Re: Any metallurgists out there? Want to explain why rolled threads are better than cut?

Rolled threads are better because the grain structure of the metal is made to conform to the thread shape rather than cutting the thread out of a generic grain structure without specific orientation along the contours of the shape. The material being rolled is also strain-hardened, which is to say that the strength of the material is increased due to it being deformed plastically. Cut threads do not primarily deform the material, and so do not increase its strength.



This explains what but not quite how or why. I'm somewhere between unable to explain and not very interested in doing so at the moment.
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Old 04-23-2006, 02:10 PM   #3 (permalink)
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Default Re: Any metallurgists out there? Want to explain why rolled threads are better than

I just wrote my exam on this last week. When the threads have sharp angles on them, there is a higher fatigue stress concentration factor (Kf). There is a tendancy for most of the bolt load to be carried by the treads nearest to the loaded face of the nut and so the nut design influences the degree to which the stresses are concentrated in this region. Also rolled threads have lower Kf values because of work hardening and residual stresses while cut threads have higher Kf values because of their greater notch sensitivity (sharp edges). Thats all I got, any thing else is beyond the scope of usefull knowledge.
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Old 04-23-2006, 10:33 PM   #4 (permalink)
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Default Re: Any metallurgists out there? Want to explain why rolled threads are better than

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Originally Posted by Multiades
Rolled threads are better because the grain structure of the metal is made to conform to the thread shape rather than cutting the thread out of a generic grain structure without specific orientation along the contours of the shape. The material being rolled is also strain-hardened, which is to say that the strength of the material is increased due to it being deformed plastically. Cut threads do not primarily deform the material, and so do not increase its strength.



This explains what but not quite how or why. I'm somewhere between unable to explain and not very interested in doing so at the moment.

YES! on top of that from that pic you can see that the grains shown there will also be stressed longitudinally along the grain boundaries. So as a result it will end up making it easy to start a crack along a grain boundary and then have that crack propogate along those grain boundaries. By rolling, you do end up loading it along the transverse section of the grain, which will make you push against the grain itself in the short direction, and also not along the grain boundary. Take a look at a stress strain curve for steel, and then you will see the linear section when that ends it is considered the yeild. After that strain hardening occurs, which is great! This is due to the fact that you are building up the material so that a larger stress is required to produce a strain. The material gets harder and thus stronger with increased deformation!! You can see why this is great. Basically this occurs due to the introduction of more dislocations which then pile up and make the metal contorted microstructurally. This greatly improves the strength then of the material. Furthermore the post above me is also correct, the Kc values will be lower for a rolled material due to the work hardening and the general shape of the bolt due the manner in which it was formed. By having a lower stress concentration factor the stress that is applied is then spread out and not directly applied to that certain point. It in essence has blunted out the region and make it less susceptible to crack formation.
However, I do not understand why a notch wouldn't improve the properties since most metals will improve with notch strengthening techniqes. Perhaps this behaves similarly to a brittle material, or I am just way off on that topic. I will have to ask around at school and see what kind of answers I get.
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Old 05-09-2006, 07:19 PM   #5 (permalink)
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Default Re: Any metallurgists out there? Want to explain why rolled threads are better than cut?

From a practical point of view, I would argue that rolled threads are not better than cut threads. Now, assuming the threaded rod is used as a fastener (i.e. a bolt or screw) and not for motion (i.e. a ballscrew), why would you want the grain to point in any direction other than longitudinally? If you align the grain helically (in a "corkscrew" fashion), you have lower strength and a lower modulus along the screw's length.

Now, I know many people are going to say, "But Dmitry, you torque the screw down. That's why you want the grain structure to follow the screw's helix." While it is true that torque is used to tighten the screw, the induced torque applies loads along the screw's length. A fastener doesn't fail under torsion because any torque that you throw at the screw gets converted into linear motion (and in turn linear forces). If you're not aligning the grains with the applied loads, I see no reason to roll a screw.

Moreover, any machinist will tell you that hard-turned screws have a much finer surface finish than rolled screws, so the surface energy is actually lower for cut threads than for rolled threads. Throw precision grinding into the mix (to lower the surface energy even further), and a rolled screw can't hold a candle to a cut and ground screw.

And finally, strain hardening does not necessarily make a material stronger. In other words, it does not increase its ultimate strength. Strain hardening makes the material harder. For all you metallurgists out there, that means harder materials have a higher Young's modulus and possibly a larger shear modulus. Now, obviously modulus plays a role in keeping the screw threads stiff enough to sustain their applied loads, but there is a point of diminishing returns where too hard a material will not be able to withstand high dynamic loads (i.e. "shock" loads). It's all a very delicate balancing act.

Now, maybe I've missed something, but as far as fasteners go, I would have to argue that cut threads are superior to rolled threads.


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- Dmitry
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Old 05-09-2006, 07:36 PM   #6 (permalink)
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Default Re: Any metallurgists out there? Want to explain why rolled threads are better than cut?

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And finally, strain hardening does not necessarily make a material stronger. In other words, it does not increase its ultimate strength. Strain hardening makes the material harder. For all you metallurgists out there, that means harder materials have a higher Young's modulus and possibly a larger shear modulus. Now, obviously modulus plays a role in keeping the screw threads stiff enough to sustain their applied loads, but there is a point of diminishing returns where too hard a material will not be able to withstand high dynamic loads (i.e. "shock" loads). It's all a very delicate balancing act.
I will have to politely disagree to a certain point. Strain hardening has NOTHING to do with your modulus. Your modulus value is determined more or less by the atomic bonds of the material, and is only present during ELASTIC deformation.. So explain to me how a PLASTIC deformation IE strain hardening will change the value of youngs modulus whatsoever? Strain hardening will make that specific material that you are dealing with stronger, by raising the point of where it will continue to yeild again since you have raised the yeild point, or the point of where PLASTIC deformation will resume. Think of it this way, you plastically deform a material, it strain hardens, then you unload it. It will creep back up the youngs modulus upon a reload and it will have to reach the "new yeild point" generated from the strain hardening in order to continue to plastically deform. (disregard fatigue for this discussion as that is another subject entirely and assume we are below the endurance limit for this case scenario so it is all more simple). Esesntially you can keep it at that load for eternity and nothign will happen (again disregard corrosion, fatigue, environment effects etc) plastically. Does that make sense?
Take a look at a stress strain curve for a high n value steel. You will see that as you continue to plastically deform the material it will just keep raising the amount of force needed to deform it any further, up until the ultimate tensile strength.
My guess is that very very little work hardening occurs to begin with as I imagine the chemistry for these are along the lines of a high strength low alloy nature. I could be wrong on that topic however.
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Old 05-09-2006, 07:54 PM   #7 (permalink)
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Default Re: Any metallurgists out there? Want to explain why rolled threads are better than cut?

Lustful, maybe I'm wrong, but here's how I understand it. Modulus is the rate at which strain increases per a given stress, for small increments. The region in which the slope is constant is known as the elastic region (as you have mentioned), and the value of its slope is the modulus.

Now say you reach plastic deformation at 300 MPa at 1% strain. In other words, upto that point lies the elastic region. Let's say you apply 350 MPa of stress, reaching an arbitrary strain value (let's say 1.5%), and in turn strain-hardening the material. If you test the material again, your elastic region in the stress strain curve now extends to 350 MPa at 1% strain. I'm sure you can agree that the slopes (and in turn, modulus) of the stress strain curves would be different for plots of 300 MPa at 1% strain and 350 MPa at 1% strain. Maybe my understanding of the test is flawed, but that was my impression of how modulus was measured.


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Old 05-09-2006, 08:05 PM   #8 (permalink)
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Default Re: Any metallurgists out there? Want to explain why rolled threads are better than cut?

VERY CLOSE.... but let me see if I can reword this for you. If you do end up loading it up so the material will plastically deform IE 350 MPa, and then unload you will have stretched the material a certain amount, and effectively have work hardened it. However, here is where we differ. Upon a reload, you will creep back up the SAME slope or the modulus, remember the modulus is atomic bond related and a little with microstructure as well. So you will still have the same slope, but you will end up going to a higher stress in order to reach the plastic deformation regime again. As a result you will also end up going to a higher strain since the slope remains constant (approximately, but very very close) before plastic deformation occurs again, IE higher than 350 MPa will be needed to cause a plastic deformation to occur.

I will see what I can get from work, they may not let me smuggle any data out. I might find something in a textbook, but it will probably be useless in real applications
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Old 05-09-2006, 08:12 PM   #9 (permalink)
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Default Re: Any metallurgists out there? Want to explain why rolled threads are better than cut?

I understand my mistake now. But cut threads are still better than rolled threads
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Old 05-09-2006, 09:46 PM   #10 (permalink)
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Default Re: Any metallurgists out there? Want to explain why rolled threads are better than cut?

Cut vs. rolled threads are typically a primary concern in fatigue situations, where cracking initiates anywhere between the contact point between the mating surfaces (half flank) and the root. Hardening the surface of the threads via cold work is what gives you the added fatigue resistance. Rolled vs. cut threads are a moot point if you are going to overload the fastener and yield the core of the material. Everything else stated... about grain boundary orientation, stress concentration at the sharp corners, etc, is also correct, but is the visual side effect of the work hardening. In high performance fasteners you will not have actual grain boundary orientation because you have a martensitic microstructure (likely through hardened) rather than the ferritic as shown in the pictures above. Good and correct info here though... glad we have so many knowledgeable people on the board!

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