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difference in Hot Rolled and Cold Rolled material
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(Mechanical)
(OP)
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12 Sep 16 14:44I know that the Cold Rolled material has a higher Yield Strength due to the work hardening during the forming process. However is that strength throughout the material or mostly around the surface area? And if so how deep would you expect the work hardening to effect? The question came up regarding 9" -7 shafts that we purchase to make splined hubs from. We have specified the material as Cold Rolled however our vendor is having problems sourcing CR and would like to substitute HR instead. If 9" material is bored and splined for a 2" shaft does the work hardening of Cold Rolling even have an effect on the inner part of the material?
(Mechanical)
12 Sep 16 16:34I would definitely not substitute hot processed stock for an application like the one you're describing, for a couple of reasons:
-CRS yield strength is through the full depth, not just a surface layer
-CRS stock is much more dimensionally accurate, meaning that if you're making large quantities of parts there will be a difference in your cycle times and machining costs
(Mechanical)
(OP)
12 Sep 16 18:11"-CRS yield strength is through the full depth, not just a surface layer"
This is the thing I was most interested in knowing. I figured that the forming would give a work hardening the material down to X% of the thickness but didn't know if it would have an effect through the whole stick. Especially for a 9" shaft.
I was also aware of the looser tolerances of HRS but that is of secondary importance to strength.
Thank you for your answer.
Jim
(Industrial)
12 Sep 16 19:58On a side note, one thing to be aware of is that many larger sizes of shafting are supplied "turned and polished", which likely have their origin in HR stock. Consult with the distribution center and get certs with the material.
It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
(Materials)
12 Sep 16 20:00Cold roll 9'' bar? I cannot imagine how powerful the rolling machine needs to be to do the rolling, even COLD forge 9'' material is hardly believable!
regardless of hot or cold working, for a 9'' bar, I would not expect an uniform deformation from the center to the surface. Even for an annealed 9'' bar, expect difference in microstructure/mechanics from center to surface.
(Aerospace)
14 Sep 16 04:36I agree with others that any 9" dia steel bar is likely "cold finished" and not "cold rolled". I have not seen cold rolled or cold drawn steel bar available in sizes much beyond about 4" dia. 9" dia cold finished steel bar would just be slightly larger hot rolled steel bar that has been machined to remove the surface scale produced by hot rolling. If you are machining away the outer portion of the 9" bar stock when manufacturing the hub, there is no need to pay extra for turned and polished raw material. You can achieve the same result by using hot rolled bar stock that is slightly larger, just enough to allow at least 1/8" of the hot rolled surface material to be removed.Attached is a catalog description of (rather than ) cold finished steel bar that discusses the difference in how small and large size materials are processed. Also note the difference in material properties at the bottom.
(Materials)
15 Sep 16 16:27paintballJim,
However is that strength throughout the material or mostly around the surface area?
The deformation profile in rolled material is dependent on the size of the rod compared to the size of the rolls. When the size of the rod is large relative to the roller diameter more deformation is concentrated near the surface. So to answer your question it depends on your vendor's process. The same principle applies to hot rolling.
(Mechanical)
19 Sep 16 16:33Well I screwed this one up.
paintballJim, when I read your initial post I failed at reading comprehension and thought you were talking about machining a splined shaft, 9" long, from 2" CR round bar. Now that I've read it again, that's a pretty blatant misread.
My first post in this thread would be correct if you were using 2" round bar, but at 9" diameter, everyone else that contradicted my post has it right.
(Mechanical)
(OP)
20 Sep 16 13:40Thank all of you for your information. It goes along with what I suspected. Someone years ago specified this as Cold Rolled material and the local machine shop provided the finished part to us as such. That shop has gone and the shop with the contract has a new purchasing agent who called and said he was unable to source cold rolled in the size required and they have apparently been sending us hot rolled material all along.
This caused quite a fuss about the relative strengths of HR and CR and should we select a different, stronger material. My position was that this material has been sufficient for several years so there is no need to run out and change it today because we found out we weren't getting what we thought. I also reasoned that cold working only affects the material to a certain depth (as UConnMaterials indicated) and by machining a hub out of the shaft we were probably beyond the affected zone where we needed the strength the most anyway.
Thank you again for your input.
(Aerospace)
22 Sep 16 05:18paintballJim,
Good to hear the purchasing manager at your current vendor brought this situation to your attention before any of your products involved experienced a problem. Some vendors are reluctant to disclose these types of issues due to the potential financial liability involved with recalling/replacing a large number of non-conforming products. Hopefully this experience has shown you the importance of being very explicit with your purchase orders/specifications, and making sure there is a QA procedure to verify your vendor delivers exactly what you ordered.
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