breaking of stainless steel retaining washer

[Teunis]

Hello,

I have the following situation :

a little wheel (round 15mm.) is put on an axle (round 5) and retained by a retaining washer (Stainless steel, 1.4122, DIN6799, groove 3,2mm.) that fits on a groove in the axle.

After some while some of the retaining washers start to break brittle (no deformation and brittle break surface under the microscope).

What could be causing the breaking of the washer ?

Some more info :

One thing I found was that the groove is not deep enough. It should be 3,2 +0 -0,1. but I found it to be round 2,25. Which puts the whasher under permanent tension.

It's an outside application. (breaking occurs round 3 weeks outside)

Bending the whasher by hand also causes the whasher to break brittle. Would it be normal behaviour of this material, or should there be some kind of degradation of the material?

 

[Goahead]

There should be no degradation in 3 weeks.
You could check the hardness, it should probably be no more than 48 RC.
If you check some of the washers after one or two weeks
you might find tiny fatigue cracks.
The side thrust might be excessive.

http://www.welding-advisers.com/

 

[israelkk]

I suspect hydrogen embrittlement.

However I am not familiar with DIN designation of steels and I could not find what is the AISI/SAE/UNS compatible designation.

If this circlip ring was heat treated or aged and a chemical cleaning was done to allow passivation, there is a chance of hydrogen embrittlement due to the chemical cleaning process.

 

[EdStainless]

Your washers were prob not heat treated properly. They may be too hard (not enough temper) or simply have the wrong microstructure. They should be quenched and tempered.
Hydrogen could be a real problem with this alloy.
What carbon level do you have?
This is a 17Cr 1Mo grade.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

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

 

[metengr]

The obvious is that one can speculate all day regarding cause of failure, send the failed pieces to a competent metallurgical lab for analysis.

 

[Teunis]

Thank you all for replying. First thing I am going to do is order another batch of washer from another supplier and check if I can see any difference in breaking behaviour.

@ metngr : Surely I should send it to a competent metallurgical lab and get it analysed. However, my experience with these labs is that I must tell them what to test specifically and they will test exactly that. However my guess is that when I first consult this forum of experts I am better able to pinpoint my problem and check if I didn't overlook anything. Spending lots of dollars on a material anaylis is an action that can allways be taken.

 

[israelkk]

You can/should ask your supplier to provide a COC (certificate of compliance) or even better a COT (certificate of tests) made by the circlip manufacturer. This way you can be pretty sure that you get a certified checked circlips from a respectable source.

If the circlip is made according to some spec such as DIN/ISO/MIL etc. they should be able to supply the COC/COT documents.

 

[metengr]

Teunis;
My comment about the obvious was not meant to be sarcastic. In dealing with metallurgical labs there are those that perform only analytical tests based on specific requirements from the customer, and there are those labs that have metallurgical engineers on staff that perform routine failure analysis. You don’t need to guide the analysis, simply asking for a failure analysis is sufficient for most competent labs.

I believe it is necessary to seek qualified opinions. My only comment is that you can speculate all you want; the proof is to have a failure analysis performed by a competent metallurgical engineer to avoid repeat failures. You don't have to spend large dollars for this failure analysis.

 

[TVP]

Teunis,

In addition to the comments from metengr, you may be surprised at the number of people who ask questions regarding failure analysis, but never have the parts properly inspected/tested/analyzed.

EdStainless,

The C content for this alloy is 0.33-0.45.

Teunis,

You should definitely confirm that the fracture mode is brittle by have the part examined with a Scanning Electron Microscope (REM auf Deutsch). Hydrogen embrittlement is a concern, but the brittle fracture may be due to something as simple as incorrect heat treatment (too hard, wrong microstructure, heat treating defects, etc.). Can you confirm what the hardness requirement is according to DIN 6799?

 

[Teunis]

TVP and Metengr :
You are absolutely right with your comment about testing the material. But in the beginning I thought this would be a quicky : I allready did check with our materials engineer about this issue. He confirmed it was a brittle break and the only conclusion for him (at that time) was Hydrogen embrittlement. Only thing I wanted from the forum was a confimation of his conclusion.
However, material specs I got from the supplier were wrong. (1.4122 instead of 1.4301 (aisi304).
This surely changes a lot, but then I allready posted this message......


The DIN spec. is the spec for the part, not the material.

The material is a stainless steel 1.4122
I searched my documentation for another (aisi) coding, but couldn't find it.
Hardness should be around 49HRc, according to my data.

 

[TVP]

Teunis,

Thanks for the update. I understand that the DIN standard is for the product, not material. However, there is no requirement for hardness or strength of alloy 1.4122 according to a material standard such as DIN EN 10088-2-- this is only for chemical composition. Hardness/strength should be specified in the product standard if it is a requirement.

There is not a direct North American equivalent to this grade, but Type 420 is similar. This grade is sometimes specified in a low C and a high C range, with the high C range being similar to 1.4122. I would have the parts inspected for heat treat errors like microstructure, too high hardness, and then hydrogen embrittlement. Good luck with your investigation.