ASTM A401 corrosion 2

[Jimmy5060]

would a high sodium environment (salty air) cause corrosion quickly enough for a spring made from ASTM A401 (chrome silicon) to fail after a fairly small number of cycles? The springs are painted with liquid paint via a dip procedure. Any help would be great.

 

[metengr]

immy5060;
Several questions that could help to provide a possible answer;

1. What is the condition of the paint on the surface of the spring? Does it exhibit rust spots or is it peeling? How long was the spring in service?

2. What is the geometry/dimensions of the spring? Coil or is it a leaf spring or some other design?

3. Can you describe the appearance of the failed end of the spring?

 

[TVP]

metengr asked good questions that will help identify the problem, but I am going to go out on a limb and say that yes, the salty air is enough to cause premature failure. The liquid dip coatings used for small springs have essentially no corrosion resistance in environments containing Cl- ions. Also, the coverage and adhesion of these coatings is usually quite poor.

 

[Jimmy5060]

There are a few other factors that may also be contributing to the premature failure. We shot peen these springs in a tumble wheelabrator, so the inside of the coil spring is not shot peened whereas the outside of the spring is. They are shipped overseas in containers that are fumagated before shipment. ( fumugated with what I am working on that answer) I also am waiting to find out if they are shipped in a loaded position or are assembled when they reach their destination. I can get the specs of the paint if that will be helpful, it is an enamal based paint and is baked on the spring after application. Does any of this change anything in the consideration of failure or does everything here add together to cause the failure?

 

[metengr]

Jimmy5060;
As TVP stated, the environment can play a big role in contributing to the failure. However, the lack of coverage for shot peening can also contribute to the failure. Shot peening induces local compressive stresses on the surface of the spring to reduce susceptibility to fatigue crack initiation - this is a good thing. Keep in mind however that coverage is most important during shot peening because as you introduce a beneficial compressive stress, you also introduce a residual tensile stress that could be affecting the fatigue life of the spring. Normally, the residual tensile stress occurs beneath the shoot peened surface. In your case because of uncontrolled coverage with this process, you might be introducing unwanted residual tensile stresses outside of the coverage area. If you shot peen a spring, you should provide maximum surface area coverage.

 

[swall]

Jimmy5060--is this a tension spring or compression spring, and where is the failure point?

 

[Jimmy5060]

It is an extension spring and the failure point is inconsistent and seems fairly random.

 

[swall]

The typical failure point in a tension spring is in the hook--either 180 degrees from the hook end or in the hook to body bend radius.If you are seeing random failure points, this suggests two possibilities.1) Coiling cracks were present. These would be random in quantity, but would tend to be located on the spring I.D. as well as on the hook radius inside bends.2) SCC is occuring at the areas of highest residual stress and/or at a corrosion pit.The areas of highest residual stress would be once again, the spring body I.D. and the hook radius inside bends.If small SCC cracks develop, subsequent cycling would lead to premature failure. What is the specified stress relief cycle for these springs?

 

[Jimmy5060]

Immediately after coiling is completed the springs go into an oven (on a conveyor) that is between 700 & 800 degrees fahrenheit for a time between 45 & 60 minutes. After cooling the springs are subsequently shot peened in a tumbler style wheelabrater. These springs are big enough and take up enough space in the wheelabrater that there is virtually no shot getting to the inside coils of the spring. Also there are no hooks on this spring threaded inserts are used to fasten the spring to the machinery.

 

[Jimmy5060]

All the information was very helpful thanks a bunch for all the help.

 

[swall]

Your stress relief is typical for Cr-Si. Should be adequate unless the wire size is quite large. At this point, you need to have a metallurgist look at the fractures to better ascertain the failure mode.