17-7 PH CH 900

[grouse1]

I'm seeing fatigue failure in a wave spring made of 17-7 PH in the CH-900 condition. I see what I think are an excessive number of inclusions (Al nitirde as elongated clusters ~ 100 microns, ~ 5 microns individual). What would the expectation be in terms of inclusions in a 17-7 PH ?? I would think the inclusion level should be low to very low.

Thanks

Mark

 

[israelkk]

The spec for the 17-7PH should give that info if any.
What is the application? Military, Aerosapce, commercial? What is the environment? Is it a closed (punched) wave spring or an open one as those made by Smalley?
Who is the manufacturer and what the spec of the material (AMS, ASTM etc.)?

 

[EdStainless]

Some of the specs require remelted (ESR) material which would cleaner, but much of the 17-7PH is simply AOD material. Unless there is a spec with an inclusion limit I wouldn't be surprised to see this.

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[grouse1]

Thanks for the info. If the nitrides are causing a functional decrease in the mechanical properties of this spring is it possible to eliminate the nitrogen gas in the AOD process and stick w/ just argon (or does N play some other role in 17-7 PH, I never see N levels specified in the comp.)It just seems like the purposfully added Al would want to grab that nitorgen being intoduced during the AOD process.

Thanks

Mark

 

[TVP]

grouse1,

EdStainless is correct, there are ESR grades that will have fewer inclusions, but I am not sure that what you are seeing is substantially affecting the fatigue performance. Unless they are excessively clustered, it is normal to see small, elongated nitrides (TiCN, AlN) in 17-7 spring grades. There was an excellent article by Mark Hayes of IST in the November 2006 issue of Wire Journal International (originally presented during the WAI conference in Prague, 2005) that addresses this very point. The following is an excerpt:

Any metallurgist who has looked down a microscope at a longitudinal section taken from a 17/7PH spring would not describe the steel as clean. The number of inclusions is considerable, as shown in Fig. 2. However, close observation shows that these titanium carbon-nitride [sic] and aluminum inclusions are mostly in the size range of less than 10 microns. IST believes this is a full explanation of why this grade has such good fatigue resistance when springs have been glass-bead peened.

This last point is key-- surface initiated fracture is typical in helical compression springs, hence shot peening is a typical process for highly-stressed applications. For sheet-based springs, peening is unusual, and therefore the surface condition (roughness, defects, residual stresses) is very important for determining fatigue strength. I would evaluate these aspects before concluding that inclusions are the primary concern.

 

[grouse1]

Thanks very much for the info. This has been very helpful

Mark

 

[EdStainless]

If these are not coil springs you need to look closely at the condition of the edges. When flat (or wave) springs are stamped there can be a lot of edge damage. This material is in the "C" condition when cut so small tears and cracks will result from less than perfect tooling.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[grouse1]

They are wave springs....I will take a close look at the edges....thanks again

Mark