Crack in 303 Stainless Steel 1

[will123]

I am examining the failure of a machined part manufactured from 303 Stainless Steel, and not exactly sure of the cause. Your thoughts would be appreciated. The key points are:-
The part is a banjo type connector from a brake system on a racing car.
The design has been in use for a number of years with no previous problems. I do not know the full history of the actual failed part, but it is believed to be very new and seen limited used.
I have had a look at the fracture and found it propagated along the grain boundaries - from stereomicroscopy & microscopy of polished cross-section.
The crack extends the full thickness of the banjo (it was found due to a fluid leak, but it does not follow any stress concentrations, it simply follows the longitudinal direction of the material.
When new the part had passed a pressure test of three times the normal in-service pressure.
The brake fluid used is a super-dot 4 type.
The hardness of the material is as it should be.
There is no branching of the crack and the part is not under constant tensile loads - therefore I have ruled out SSC.
Mt first throught on looking at the failure was Hydrogen cracking, but I cannot think of the sort of hydrogen.

As I mentioned above your thoughts are appreciated and I thank you in advanced for your assistance.

 

[EdStainless]

I'll bet that the material has been changed. Either a different source or a move from 304 to 303.
A guess, how about that the crack, or at least part of it, was pre-existing. Take some new parts and air test them underwater. You might find some leaks that don't show up with fluid until they open up a little more.

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

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

 

[Goahead]

303 is a free machining stainless steel, embedding elongated sulfide inclusion (along rolling direction) to break up chips. This selection may have solved a machining problem, probably at the expense of service life.
If you look at the fracture you would probably see fatigue beach marks.

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

 

[will123]

Thank you for your comments. Certainly, I agree with respect to performance you would not necessarily opt for 303 - either 304 or 316 are better alternatives, but I have been assured that the material has not been changed. However, I am currently in the middle of sectioning an old example to compare.

Under a stereomicroscope I cannot see beach marks, in addition I would normal expect changes in crack direction as is passed geometric features, but there was no deviation even when it reached a sharp raduii. Although I have already planned to take a look at the fracture surface using an SEM tomorrow at the local university hopefully that will yield further information.

 

[metengr]

If you are reporting an intergranular fracture morphology based on viewing optical photomicrographs, have a competent metallurgist review this information and render an opinion. In addition to optical microscopy, more detailed metallurgical examination might be required. The chemical composition of the material should also be verified and compared to a previous part. Hydrogen embrittlement is not the culprit.

 

[Kenneth]

The aerospace industry has had bad experiences with 303 (and particularly 303Se) for the reasons covered by previous posters. The size/shape/distrubution of the intentional, chip-breaking "inclusions" can also vary significantly from supplier-to-supllier and lot-to-lot. I've seen small machined 303Se parts crack clear through when exposed to relatively minor deformation. Your longitudinal crack may simply be following these "free-machining" inclusions, which can appear as long "stringers" in the rolling/drawing direction. The examination reccomended by metengr would be key to determining the cause of your failure.

 

[Kenneth]

While military "specification reform" now prohibits citing the document as a "requirement," take a look at the list of materials "restricted from use on Air Force Weapons Systems" in Table I on page ten of MIL-HDBK-1587 (

http://assist.daps.dla.mil/docimages/0002/50/08/BK1587.PD4).

303, 303S and 303SE (in all tempers and forms) can be found at the bottom of the Table. The list is based on experience (mostly bad), and while the materials listed may well have ended up on this list due in part to bad design and/or material selection, it is an interesting "rouge's gallery" of materials/tempers nonetheless....

 

[DDRSS10]

Have you considered a flaw in the raw stock. I am a machinist and have seen Flaws, flakes, inclusions, cracks.ect in all types of matl. It is very possible in my opinion that this crack was there from before machining and only opened up under preasure. Just my thoughts.

Nick

"Speed costs money boys, how fast do you want to go?"