Crack detection in 430FR bars

[dgallup]

Search is down at the moment so I'm starting a new thread even though I'm sure there have been relevant posts in the past.

We buy parts machined from solenoid quality 430FR (ASTM A 838 alloy type 2) bar. Several parts have cracked completely through from one end to the other right through the center axis of the bar. There is one large flange on the part which the crack does not go completely through the OD. There is no doubt in my mind that the crack is a seam in the bar stock which was present before machining. We are 100% sorting parts at the moment but would like the supplier to check their remaining bar stock inventory.

What sort of non destructive testing is available to find an internal crack in magnetic material like this? The bar stock is approximately 7/8" OD.

 

[CoryPad]

Ultrasound

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[taylordm]

In the steel making industry one of the newer technologies out is called an Eddy check. Herkules roll grinders use this to check for cracks in mill rolls. They are a much larger diameter but the same principle should apply. Could be helpful, but your best bet is still the UT scan

 

[dgallup]

Thanks for the replies. From what I've found, eddy current testing can only find cracks that reach the surface or are very close to the surface. These cracks are right down the center and don't reach the surface. Ultrasonic crack detection seems to be the only method. I have found a shop that uses modal testing to find cracks in finished parts.

The steel supplier is doing metallurgical tests on the cracked parts and we don't have the results yet. However, in a phone conversation, they said the cracks could have been created when some of the bar stock was straightened. I don't see how straightening could create centerline cracks. The centerline is the neutral axis during a straightening operation and there should be zero stress & strain. Any cracks from straightening should be on the surface in my opinion. I'm no metallurgist, but I think centerline cracks would be more likely do to improper heating/cooling during hot rolling. Any comments?

 

[CoryPad]

You need an independent laboratory doing this work. As you stated, straightening isn't going to initiate cracking. Two possible causes: 1) centerline segregation of impurities (occurs during solidification); or 2) central bursts (occur during diameter reduction).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.