Another casting failure 6

[emonje]

Continuing from this thread:

http://www.eng-tips.com/viewthread.cfm?qid=418786

Taking the sound advice & opening a new thread as it's a different part.

This casting failed over the holiday break.

Here are the shots of fracture surface:

All these ridges and facets, does this look like "rock candy" fracture? I'm thinking of asking for Al and Boron content tests.

Why is there this different layer on the outside?? Brittle fracture inside then ductile failure on this shinier layer??

Other side survived.

The casting will be in town from the site next week, trying to get my head around & see if there's any obvious hint what happened here.

Thanks for looking & your comments highly appreciated.

 

[emonje]

An update, just got hands on some preliminary mechanical tests results:

Material is too brittle, for Charpy our specs ask for 40J at -40 degrees C. Also Tensile strength is some 200 MPa below our minimum requirements.
Chemical analysis meets specs, so may be they didn't tamper it to high enough temperature?

 

[EdStainless]

Was it heat treated at all?
Maybe you should try to HT some samples as see what you get.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[emonje]

It was supposed to be quenched & tempered, the foundry is being very sketchy with what they did. We are not going to use them anymore.

 

[bobjustbob]

Looking at this from a foundry perspective. With a 700MPA yield I am assuming this is a 120ksi/100ksi (tensile/yeild). How thick is the part? It looks pretty beefy from the pictures. What specification did you order it to? The chemistry looks like a 4320, which is a good match for the section and mechanical requirements. Where did you take the mechanical samples from? Was there any evidence of porosity in the tensile and charpy specimens?

The reason I ask, material properties out of castings are not directly comparable to the test bar results. Test bars are over rigged to ensure no imperfections in the metal. If you ordered to a specification that allowed for standard 1 1/4 x 1 1/4 sized test bars to represent such a large casting that may not be appropriate. You will see a large difference between the part and test bar properties. If you ordered to a spec that required and equivalent round test coupon like API or ASTM A1001 the properties will be better represented but still can differ from the actual part depending on where you took your test coupon. The API and ASTM A1001 require the test coupon to be taken at (1/4)T (API) or (1/8)T and (3/8)T for A1001. You should not be taking the samples out of the direct center of the section thickness.

Any porosity in the sample coupons will give you low ductility and low impact values. That porosity may be completely acceptable per the specification. That is why we should be careful when comparing mechanical results out of parts.

Now 31 ft-lbs at -40° out of a 120ksi material is pretty stout for a 4320 material if you expect that out of a test coupon 4" or above. If, the mechanical coupons show no evidence of porosity at the fracture surface and the test coupon were taken near the 1/4T section, from the tensile results it looks like the quench was not adequate to completely through harden.

On the weld repair. 309 is not the proper electrode to weld repair this material. Any weld repair done should have underwent PWHT no matter what filler metal was used.

 

[Tmoose]

How cold does the part get in service, and was it at the time of failure ?

 

[emonje]

Rarely gets below 10 degrees Celsius where it works. It was a hot day, above 20 degrees C when it failed.

 

[priyam07]

40J at -40C is really high spec, this part must have been tempered wrong. Not sure if it met the hardness spec, cant see in the image? Possibly temper embrittled going by the low CVN values, SEM would identify any intergranular fracture on the surface to confirm.

Any microstructure results you have would help to understand further.