Machining considered cold working? 3

[swimmingskibble]

I recently ran into an issue where some parts were rejected for not having the heat treat certs for a part that should have been annealed. The source of the confusion is a drawing note that states "ANNEAL AFTER COLD WORKING." After talking with multiple engineers and machine shops, I've discovered that there are two view points on this. The first point of view is that cold working is the plastic deformation of a material (bending, drawing, extruding, shot peening, etc...) and since machining is only removing material, it is not considered cold working. The other point of view is that machining is considered cold working because of the micro plastic deformations that occur along the surfaces where the chip formation occurs.

Personally I think it was just a bad callout that needed to be revised, so that there is no room for interpretation. But I am curious if there is some type of an official stance that machining is considered cold working or not. So far I haven't been able to find an official ASTM or AISI spec that states one way or the other.

 

[ornerynorsk]

I can't speak to the various standards that may be in play, but in actual practice, if you have a material that is sensitive or subject to cold working, one almost needs to assume that some cold working will be present to a varying degree in a random sampling of parts. There are a number of factors, some of which would include: Is the tooling being optimally maintained, or are there dull cutters and inserts being made to "plow through" material? Is the correct coolant or cutting fluid being applied? Does the machine have sufficient horsepower and tool geometry (some materials do not tolerate light cuts well)? Etc. It is very easy to cold work some materials in the process of machining them.

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.

 

[mrfailure]

Mechanical deformation frommachining does tend to induce cold work. If you can sacrifice a part, have a metallurgical mount made and etch it. If you see machining deformation at the surface, then you know you are inducing cold work. If you do not, then have the lab take macrohardness both at the surface and in the center of the mount. If you see statistically- significant increase in hardness at the surface, then again you will know you have cold work.

 

[EdStainless]

This is a two part issue.
Was enough cold work done in machining that it is measurable? (see above posts)
Does the cold work matter? If the material has an upper hardness for annealed material does the surface now exceed that? If not then why worry.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[mrfailure]

Typo on my last response that may be significant: that was microhardness (Knoop or Vickers), not macrohardness...

 

[WKTaylor]

Swimmingskibble [???]... hope this makes sense...

In aerospace, very high strength forms of steel [low alloy, tool, etc], CRES and titanium alloys have relatively tiny differences between ultimate and yield strength resulting in low toughness and low strain to failure.

These materials are usually ‘worked’ [rough machined, welded, etc] in low strength states such as annealed or solution heat treated [not quenched]. Thick section parts may be subject to multiple annealing operations for strain/distortion control with aggressive rough machining and/or cold, warm or hot straightening as necessary in-between.

However there is an inevitable point in processing when these parts must be heat treated to the highest mechanical stress state, usually just prior to final machining. At this point, any mechanical operation [machining, drilling, reaming, grinding etc] or chemical operation [etch, cleaning, etc] can induce embedded stresses in the surface… which must be eliminated by stress relieving HT operations to prevent brittle fracture. For each of the alloy families mentioned above, there are specific processes for this purpose.

NOTE. after all Inspections are done and the part has been declared fit for finishing, these same parts are usually shot-peened to further enhance the surface by compressive forces which retard crack/corrosion initiation… then are plated/coated for added environmental resistance. At each of these stages fabrication stages, secondary heat treat operations, annealing, embrittlement relief, stress relief are done routinely… just to get to the next stage/operation. HOWEVER, EVERY operation can be a spoiler if done wrong, not done at all or done out-of-sequence. Gaaaaa!

A clear example of the complexity of processing involved, is work with ultra-high strength steel parts [IE main landing gear parts] typically 4340M and 300M steel heat treated above 270-KSI. Do anyone step wrong in a processing sequences [fail to anneal between aggressive machining operations, miss a hydrogen embrittlement relief, miss a second temper operation, etc]… and the material goes in the dumpster. Reasons are obviously complex; however, when processed correctly these alloys perform well and predictably.

A classic example of this intricate fabrication dance is given in SAE ARP1631 Manufacturing Sequence for Fabrication of High-Strength Steel Parts - 300M or 4340 Modified Low-Alloy Steels - 270,000 psi (1860 MPa) Tensile Strength and Higher. I have referred to this carefully written document many times when dealing with these/similar ferrous alloys at the limits of their heat treatment … and have developed similar/parallel processes for VHS CRES and Ti-parts. So far so good.


Regards, Wil Taylor

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[Jughandle]

Is machining considered cold working?
Both points of view that you presented are correct but one would be more correct depending on part use and geometry. What are you are making? If the note that you quoted is found on an ‘engineering’ drawing for something like a large machinery component, then it refers to the bulk section; and no, machining is not considered CW. Subtleties on how many microns below the surface contain residual stresses would usually be unimportant, that is, until the part geometry starts getting down to the same order of mag. If it’s a thin metallic strip, then yes, machining or grinding is CW. So its the universal answer: "It depends."

 

[mfgenggear]

my point of view is as follows.

i will Normalize and harden after rough machining prior to semi finish machining. never anneal after rough machining.
and that would be the incorrect procedure to do. unless absolutely necessary. an usually not necessary.
saw and or rough turn then intermediate heat treat.

#1) part semi finished machine best if core harden to 25-37 HRc prior to machining.
#2) only stress relieve after rough machining to remove stresses.
#3) depending on the final hardness requirement . final heat after after semi finish machining.
#4) do finishing machining such as grinding after hardening.

again depending on the hardness requirement. if the final hardness requirement is not over 40 HRc
then finish machine after heat treating.

 

[swimmingskibble]

Thanks.

WKTaylor, I really enjoyed your response.

Some of the posts asked about what kind of part this was about. The same callout is used within my company for various different parts (generally all made of 304SS), but the part that brought the concern to light is a little bearing housing. The housing has fairly thin walls (~0.15"), and is later overmolded with urethane. The primary purpose of the annealing is to reduce the magnetism of the material so that it doesn't attract metal particulate that could contaminate the bearings. The machine shop didn't considering machining as cold working, and they weren't able to anneal after the fact because of the tight tolerances of the surface mating with the bearing race. The engineer that designed the part wanted the manufacturer to rough machine, anneal, and then do the final machining/honing to the tight tolerances.

 

[mfgenggear]

it is plain to see that designer never worked in a shop. machining annealed material is terrible. poor finish, bad tearing, & not good especially for tight tolerances.

edited.

a simple demag will remove the magnetism.