Testing Standards to determine the cold formability of copper rod 4

[Thermalsp]

We are doing cold-heading of C15000 (zirconium copper) rod (0.5" Dia). Recently, we have orange peel on the finished product by using the rods from a newly arrived production lot, while switching back to rods from the old lot, we could get nice/smooth surface finish. After checking the chemistry, yield strength, elongation, hardness, etc. of the rods from these two lots, they all meet the specs. The microstructure (grain size) of the rods is pretty similar as well.

We want to understand the reasons causing such slightly different cold forming performance. Does anyone know the testing standards that could be performed to evaluate and/or rank the old-heading capability (cold formability) of the copper and copper alloy rods?

 

[salmon2]

I always thought surface finsih only relates to lubrication and grain size. So they are same microstructure, in term of heat treatment? And just out of curiosity, how simiar are the grain size numbers? One point of difference means double/half grain size.

 

[Thermalsp]

Thanks, salmon2.

I just visually observed the grain size under a simple optical microscope and did not see big size difference. We are waiting for the raw material supplier's testing results that I hope to get the grain size number per ASTM E-112.

 

[CoryPad]

An important contributor to this condition is crystallographic texture (preferred orientation), which describes the crystal plane arrangements within your material. You may need to determine the exact processing conditions for both of your lots and see if there were some differences such as final rolling temperature, presence or absence of a final anneal/temper, etc. You may need to restrict some process parameters to give you the appearance you need.

 

[Thermalsp]

Thanks, CoryPad.

It makes perfect sense to me. Unfortunately, the mills are located in China and Korea. We purchased the raw rods from a American distributor. We could not know the details about the processing conditions and have no controls.

 

[TVP]

Thermalsp,

Here are some of my initial thoughts:

1. There is definitely a difference in the surface of the two materials, most likely related to grain size and/or crystallographic texture (preferred orientation), but possibly related to surface roughness.

2. Surface roughness is a potential factor because hot-rolled rod must be descaled to remove the black cupric oxide that forms at high temperatures, and it is possible that the descaling processes are considerably different, with one using a more aggressive pickling action, yielding a rougher initial surface.

3. The best way to detect the orange peel condition is to perform an upsetting test on a portion of the rod and observe the surface using no magnification, or only a small magnifier (< 10x). I am not aware of any industry standards for this particular combination (copper alloy UNS C15000, cold heading quality), but it is something you may want to include in the form of a purchase order or specification. It is common in the steel wire industry to perform this type of test, usually for detection of surface defects (cracks, seams, etc.). Sample is a section of rod/wire that has L = 1.5d that is upset to 1/3 of its original length.

4. You need to specify a maximum acceptable grain size, which is probably around 50 micrometers.

 

[Thermalsp]

Many thanks, TVP. All are valueable and good points.

We did check the surface finish and even did mechanical grinding on a few rods presenting orange peel problem. We still got the similiar orange peel problem on the grinding rod.

Upsetting test is the one that we would like to do but failed to find a testing service provider. It would be greatly appreciated if you could know someone who is providing such kind of test service.

 

[TVP]

I think your surface grinding method was a good choice to eliminate the surface roughness theory. Nice job. Regarding an outside testing service provider, I would start by contacting one of the Exova labs in your area (Ontario, Canada). Here is a link to their webpage:

http://www.exova.com/laboratories/42-laboratories-by-country?c=canada

I checked the Mississauga accreditation, and they list ASTM E9, which is the test method for compression testing. This is at least a starting point. My familiarity is with steel mills performing upsetting tests, using this type of machine:

http://www.petig.com/portals/h1698029/story_docs/Brochure upset testing systems PETIG (EN).pdf

Here are some other links with info on upsetting tests specific for cold heading/cold forgeability:

http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=175&pageid=2144416591

http://www.intechopen.com/source/pd...ld_formability_for_cold_forming_processes.pdf

 

[Thermalsp]

Many thanks again, TVP. I checked with Bodycote (now Exova) 1 year ago for upsetting test service and I got NO because they do not have such equipment like Petig or Vale upset tester. I will phone them again to check, they may be able to do it now.

 

[TVP]

Even if they don't have a Vale or Petig upset tester, it is possible to evaluate rod samples using a standard ASTM E9 setup with a universal test machine (servohydraulic or mechanical) or an impact tester (Dynatup, etc.). The main thing is to create sufficient circuferential strain.

 

[Thermalsp]

Thanks, TVP. Could the universal test machine (Instron or MTS) reach the upsetting speed? In other words, does the cross-head travel speed (the specimen strain rate) have no influnce to the results?

 

[TVP]

It is possible for servohydraulic machines to achieve high velocities on the order of upsetting, but I do not believe that this is critical for your particular issue. It may not have an effect on a proposal from a test lab in terms of cost, timing, etc., but in case it does, here is some food for thought.

The deformation speed has a significant impact (no pun intended) on the amount of flow localization (intense shearing in small/narrow zones) within the specimen, and upon the final workability of the material/geometry/lubrication combination, meaning the onset of fracture. In your case, you want to evaluate the degree of surface roughening that occurs during deformation, which should occur at a relatively small critical strain if the microstructure is susceptible. This means that roughening should occur well before the workability limit is reached, and therefore even if slower speeds result in a somewhat different strain distribution within the specimen and on its outer periphery, the important issue is to achieve sufficient circumferential strain such that it exceeds the critical limit for roughening. A total WAG is that you could compress a standard specimen (L = 1.5D) to 1/2 its original height and you will be able to detect surface roughening, regardless of test speed. The workability limit should be closer to 1/3 or 1/4 of the original height, but would be somewhat sensitive to test speed. If you are interested in researching this issue, there is a great deal of information available from references by Thomason, Dodd and Bai, Kuhn, and ASM Handbook of Workability.