Can shot-peening change the free-length of a compression spring?
Can shot-peening change the free-length of a compression spring?
(OP)
Can shot-peening change the free-length of a compression spring? Keep in mind our springs are not conventional wound/coiled.
I'm seeing part-to-part & lot-to-lot variability in free-length of our finished springs. Our supplier insists that shot-peening is a likely culprit. I'm not convinced (I suspect it's loose process control during the spring mfg process).
Details: Spring 'machined' from MP35N bar stock (45-55% cold-reduced). Machining process is proprietary to supplier, but they have indicated material is aged prior to 'machining' coiled section. Last step is shot-peening with tumbling process using S070H shot at .007A intensity. Shot-peeing is outsourced to a 2nd supplier.
I have to assume all lots of springs are from same raw material lot until supplier can confirm.
I'm seeing part-to-part & lot-to-lot variability in free-length of our finished springs. Our supplier insists that shot-peening is a likely culprit. I'm not convinced (I suspect it's loose process control during the spring mfg process).
Details: Spring 'machined' from MP35N bar stock (45-55% cold-reduced). Machining process is proprietary to supplier, but they have indicated material is aged prior to 'machining' coiled section. Last step is shot-peening with tumbling process using S070H shot at .007A intensity. Shot-peeing is outsourced to a 2nd supplier.
I have to assume all lots of springs are from same raw material lot until supplier can confirm.





RE: Can shot-peening change the free-length of a compression spring?
Shot peening induce compressive stresses to the surface therefore, uneven coverage may cause dimensional changes. How the dimensional changes affected your spring properties? Is it a compression or a torsion spring?
Although the manufacturing process is proprietary to supplier he can do it in three ways, milling, disc cutting. or wire EDM. When a small gap between the coils is needed only disc and wire EDM are possible. I designed and manufactured many machined spring using EDM and milling.
Two important points consider:
If it was machined by wire EDM there is a high risk of hydrogen embrittlement. You mentioned that the spring was machined after heat treatment which I assume is for high strength ~50 RC. I recommend you ask the manufacturer to be absolutely sure that the machining process of the coils process didn't produce hydrogen. If it is then you have a problem.
If you want to check the whole lot of springs you need to load them to the maximum expected load and hold them at that position for at least 200 hours. Then check for cracks (if they still in one piece).
The correct process is to wire EDM machine the coils before heat treatment and then use a fixture during heat treatment if dimensional change is absolutely required. There is always a chance that the spring properties will still be in the required range even with small dimensional changes.
If it was machined by a grinding disc then again there is a chance of micro cracks in the grounded area. Make sure the grinding/cutting was made according MIL-STD-866C Grinding of chrome plated steels and high strength steels heat treated to 180000psi or over. This spec requires grounded surface should be NDT checked to assure no cracks are present.
RE: Can shot-peening change the free-length of a compression spring?
RE: Can shot-peening change the free-length of a compression spring?
MP35N is a corrosion resistant alloy that has higher resistance to hydrogen that most alloys.
RE: Can shot-peening change the free-length of a compression spring?
RE: Can shot-peening change the free-length of a compression spring?
RE: Can shot-peening change the free-length of a compression spring?
RE: Can shot-peening change the free-length of a compression spring?
RE: Can shot-peening change the free-length of a compression spring?
RE: Can shot-peening change the free-length of a compression spring?
Re. the amount of variation. First I want to clarify that this is more of a lot-to-lot variation issue. The part-to-part variation within a lot variation is pretty good. Magnitude of the range of overall spring length for Lot A = .004", Lot B = .002", & Lot C = .010". The max/min length values for the first two lots spanned the upper half of the .010" tolerance band, while the median of length value for non-conforming Lot C was hovering just above the lower specification limit.
The raw material lot was different for Lots A/B versus Lot C. Yield and UTS of the raw material were within 2% of each other and %EL was within 10%. Although it would be convenient to blame the raw material, I can't convince myself to do so given the meek difference in mechanical properties.
To address Swall's query, the cross-section of the 'coil' is .033 x .056".
There is a question at www.shotpeener.com relating experience with the height of wound springs changing after shot-peening, but I'm not sure if the observation is applicable to my situation. See http://w
RE: Can shot-peening change the free-length of a compression spring?
RE: Can shot-peening change the free-length of a compression spring?
There is some hope buried in Latrobe's response, but no empirical data to support it. I definitely don't want to do anything the would be detrimental to the strength of the material.
I'd like to hear from any other 'experts' on MP35N.