Strength of cold working steel 1

[Sarikahirpara]

Cold working is the process of plastic straining below the recrystallization temperature in the plastic region of the stress strain diagram. Properties of the material obtained from hot rolling are quite different from those obtained by cold working. 
Consider the stress strain diagram of fig. Here the material has been stressed beyond the yield strength at y to some some point i, in the plastic region and then the load removed. At this point the material has a permanent plastic deformation ɛp . If the load corresponding to point i is now reapplied, the material will be elastically deformed by the amount ɛp. Thus at point i the total unit strain consists of the two components ɛp and ɛe and is given by the equation E= ɛp + ɛe. 

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https://res.cloudinary.com/engineering-com/image/upload/v1617882067/tips/Picture6_uivmzj.png

This material can be unloaded and reloaded any number of times from and to point i, and it is found that the action always occurs along the straight line i.e. approximately parallel to the elastic line Oy. Thus ɛe = σ I / E.  

The material has now a higher yield point, is less ductile as a result of reduction in strain capacity and is said to be strain harden.

From equipment’s design point of view we need to consider allowable stress and as per ASME allowable stress is minimum of yield strength/1.6 and ultimate tensile strength/2.35. now since here in the cold working process only yield strength is increasing and there is no change in ultimate tensile strength. If allowable stress of a material happens to depend on ultimate tensile strength then we are not getting any benefit of cold working in terms of allowable stress. So why unnecessarily we spend money to do cold working process for such materials where ultimate tensile stress becomes the criteria for choosing allowable stress.

 

[EdStainless]

You are raising two very different issues, and mixing them.
I am not aware of any items in the B&PV Code that take advantage of cold work other than fasteners.

In other spec systems (ASTM & AMS) there are specs for rod and bar, wire, sheet, and tube that all take advantage of cold working to various precise strength levels.

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P.E. Metallurgy, consulting work welcomed

 

[Sarikahirpara]

I wanted to highlight only one issue in the cold working process i.e. yield stress can be increased substantially compared to UTS in the cold working process. So for equipment design if allowable stress depends on UTS only then why should we do cold working. Is there any other advantage of a cold working process ?

 

[EdStainless]

Yes there are reasons to cold wrok; you improve dimensional tolerances, improve surface finish, and with many very soft alloys a bit of cold work improves how they machine (they are less gummy).

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P.E. Metallurgy, consulting work welcomed

 

[btrueblood]

Well, for once I disagree with Ed but also agree: cold work is used for pressure vessels, but as Ed says the B&PV code does not cover that, beyond stating that you can push past the code if you do the homework...

Autofrettage is a fairly..., well not exactly standard, but at least a reasonably well-understood, method of improving a pressure vessel's ultimate strength by a very specific type of cold work. It was originally done to improve the burst strength of gun barrels 'way back in the late 1800s and early 1900s, but has also been used since then to strengthen propellant tanks for spacecraft and rockets, amongst other things. Autofrettage extreme-pressure tubing is used somewhat commonly for very deep subsea drilling rigs, on the control hydraulics for the valves and such that operate down on the subsea weld head.

https://en.wikipedia.org/wiki/Autofrettage

 

[hydtools]

Improved machinablity. Improved appearance.

Ted

 

[EdStainless]

BTB, I left tubing out of the discussion, there are many exceptions in the tubing world. There is a lot of 'as-cold-drawn' tubing used and AF is its own very special case. It is used extensively for the piping on high pressure recip compressors. It only works with heavy walls and materials that have specific strain hardening relationships. The engineering to do it right is daunting.

The problem with light amounts of cold work is that you may increase the yield and there will be virtually no change in UTS. Controlling the Yield in these situations is largely guesswork. I don't know of any cold worked steel (not stainless) specs where there is a specific range of cold worked yield strength required.

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P.E. Metallurgy, consulting work welcomed

 

[gtaw]

The advantages of cold working are pretty much lost if the materials are welded or if they are subject to temperatures high enough to recrystallize. Ooops.

Best regards - Al

 

[btrueblood]

I figured you didn't mention it in your earlier post for good reasons, Ed. Mainly this one: "The engineering to do it right is daunting."

The OP talks about design to ultimate strength, when in reality a lot of designs are done to avoid yielding, or avoid fatigue which generally is a lower strength level yet. As GTAW notes, losing yield strength due to welding is a typical flaw in aluminum structures designed by newbies.

Like fasteners, spring wire is another place where heavily cold worked material is generally used, because the fatigue strength of spring wires improves when the YS is increased, but again this is very heavy cold work under tightly controlled conditions with very specific alloys, and a lot of subsequent testing gets done to verify the correct outcome. Most of that engineering was done decades ago. I've also read a paper that discusses wire drawing and talks about failure theory (weakest link or survivor bias) and statistical control, i.e. heavy draws during the process tends to "weed out" defective material, i.e. if you draw down a billet with internal flaws, the wire will likely break at some point, and better manufacturers will reject the entire billet's worth of material rather than attempt to salvage the remainder. It is interesting to look at a plot of the ultimate strength of music wire as a function of diameter - .010 music wire has a minimum tensile breaking strength of around 400,000 psi, vs. .125 diameter wire at around 200,000 psi.

Some colleagues of mine in a sister company worked for 3 years to develop and qualify a device that operates as a pressure vessel and is subjected to fatigue cycles. The design of the part was trivial, but getting the right autofrettage treatment took a lot of time, money and talent to perfect.

 

[mfgenggear]

btrueblood I liked your post, piss me off my brand new chair two cheesy brackets broke due to inadequate design. Jesus.
so to add on that any thermal treatments throws all that cold working out the window. mechanical assembly would be a good fit.

 

[dhengr]

Sarikahirpara:
I think you are looking at this issue the wrong way. It is true that cold working most metals tends to strengthen them, that is, move the apparent yield strength up the stress/strain curve. But, many metals (ductile metals) do not have a well defined and long plastic range or strain hardening range. Many of them go fairly quickly through these ranges, at a fairly high/steep rate, and on up to the ultimate strength. Thus, it requires very careful consideration and engineering judgement to really take much advantage of this apparent design strength increase. I’ve always looked at it as something of a strength increase ‘insurance policy’ when I am designing to yield, but also must recognize that I would be working closer to ultimate strength/failure, with less margin of safety remaining, to fall back on. Knowing I have this ‘insurance factor’ assuming I know the strain hardening level, and thus, the new apparent yield strength, might allow me to push my design closer to my normal yield/design stress, with some greater degree of comfort. I’ve often looked at the mill certs for the material I am using and used those Fy and Fult values for my design values; rather than using the ASTM guaranteed min. values, which is common engineering design practice.

You do generally seem to understand the mechanism around the stress/strain curve and loading a specimen above/beyond yield, unloading, and reloading up to the same point/load, generally following the same (old) E slope from a new zero load strain starting point back up to the new apparent yield point. But, you do not know the actual Fy and Fult of every piece of material before you use it to build a product/structure, and you are also unsure of the strain hardening which has taken place, if you were to try to use that improved/higher apparent Fy as some sort of a design strength.

Under very controlled and repeatable conditions and means of manufacture, with a very controlled virgin product, and with considerable testing and design history, I can see using some higher strength, by virtue of cold working. Many times, at first, cold working is just the most practical means of final manufacture of the raw material. Examples might be cold drawing of wire and bars, some tubing or cold working steel sheet and light plate, coil, etc. But, this is a fairly narrow range of materials, and certainly not run-of-the-mill commercial metal materials from just any steel supply house.

 

[mfgenggear]

Sarikahirpara

what specific material are you considering