17-4 spring fracture

[DHollo]

I have had two springs that have fratured where the end of the coil touches the active coil. The springs are 17-4 PH CRES at the H-900 condition and the coil diameter is 0.875 inches. The springs are passivated. The Metallurgist said there was no corrosion or defect at the fracture points. Has anyone seen this type of failure before? I need to prevent it from happening again.

 

[desertfox]

Hi DHollo

Can you post a picture of the springs, we need a lot more information like spring dimensions ie number of active turns, details of spring ends, load at length data, free length, wire diameter, type of spring and duty cycle, we then might be able to help you more.

desertfox

 

[israelkk]

Are you sure it is 17-4PH H900 and not 17-7PH CH900?

 

[CoryPad]

Are the end coils closed, ground, both, neither?

The fracture site you describe is a common one. The sharp (sometimes) end can dig into the active coil above it and initiate fatigue cracks. Perhaps your applied forces are high? Also, I would look at unused springs and see how the parts look.

 

[DHollo]

Both springs broke while being compressed for the first time during assembly. There have been no issues with assemblies that have been shipped. The material is 17-4, free length is 19.017 inches, active coils 9.1, diameter of coils 0.875 inches.

 

[desertfox]

Hi DHollo

Thanks for the picture, your failure looks typical of a tensile failure along the principal planes due to shear stress as the spring was compressed, if you look at the river lines and trace backwards the lead you to the start of the crack, which in this case is at 12o'clock position of the full face spring cross section ie the bell shaped shadow area.
We need to know the wire diameter and what the compressed length of the spring was.
I would say that with only 9.1 active coils and a 19" free length, that your helix angle on the turns might be greater than say 15 degrees, that alone can have an effect on the stresses of the spring.

Regards

desertfox

 

[desertfox]

Hi DHollo

Have a look at this link I think your failure looks like this one:-

http://school.mech.uwa.edu.au/~dwright/DANotes/springs/fatigue/fatigue.html

Now I am confused because your metallurgist said there was no defect or corrosion and from what you have told us it can't be a fatigue failure either.
So with my limited knowledge of fracture surfaces I am curious to know what the metallurgist thinks of the river lines in the fracture face, because I was led to believe if you followed the river lines back it would point to where the crack intiated and for a crack to form you normally need a defect in the surface or a stress concentration point.

regards

desertfox

 

[israelkk]

Wire Diameter?
Assembled length?
Environment?
Cleaning process after heat treatment and before passivation?