60-45-15 Ductile Iron - Abnormally Low Yield Strength 4

[BLaroux]

Hello All, I'm wondering if anyone knows of a weakening mechanism that could lower the yield strength of 60-45-15 Ductile Iron by more than half without making it overly brittle.

We have several large (100-500lb) ductile iron castings that are displaying a small amount of plastic flow at an applied stress of around 20,000 psi. Deformation is occuring outwards around stud holes in a 0.63" thick flange section. (flange faces only make contact in circular regions extending ~3/4" out from the center of the 5/8" studs)

Castings were procured, tested, and certified IAW MIL-C-24707/5... and are in the feritizing annealed state. Test reports for the heat indicate: C% = 3.68, Si% = 2.30, P% = 0.034, Tensile = 66,500psi, Yield = 46,500psi, El = 22%.

Castings have been in service for several years at a maximum operating temperature of 120 F. Exposure to water was minimal, and there is no visual evidence of corrosion or heat tint. I have doubts about the usual embrittlement mechanisms being at fault because of the plastic nature of the deformation.

Is there any possible heat treatment/thermal history that could lower this metal's yield to less than 20,000 psi? The literature I have inicates that the lowest it can get is 32,000psi with a full ferritizing anneal and very poor nodularity.

Creep is unlikely, and there is no evidence to suggest severe carbon depletion... could the low yield be the result of graphite floatation during solidification of the casting? Largest section thickness is around 3", but it is unknown what orientation the casting was poured in. Literature suggests that floatation can decrease strength, but does not say by how much.

Any and all suggestions as to a cause are appreciated... right now I'm leaning towards a short term high temp. event (despite the lack of heat tint), or graphite floatation.

Thanks in advance!

 

[metengr]

BLaroux;
I read thru your post. Have you actually confirmed mechanical properties on any of the failed castings? It wasn't clear to me in your post. If you have not done this testing, I would send one of the failed castings out for failure analysis to a metallurgical lab.

 

[arunmrao]

Have you sectioned the castings and checked the mechanical properties both in the flange area and thick section. Perhaps this might give you an indication. Also was the YS property singularly tested without testing for TS and Elongation.

My doubt is the presence of microshrinkage which has led to lower properties being observed. Please confirm.

If microshrinkage is confirmed,it could be due to inadequate feeding or mold wall movement.

 

[BLaroux]

Metengr,

Destructive testing is not an option - raised areas on castings are being stoned down by hand and machinery is being re-built using original parts. (very time critical application, but failure would not be dangerous)

Determination of raised areas was done by dyechem transfer in the field both with and without paper gaskets (not the most accurate method... the quality of the indications could have been better.(dyechem applied to one surface, flange bolted together at 75 ft-lbs for each stud, flange unbolted leaving indications on opposing surface). The possibility that this deformation is elastic in nature has not been totally eliminated. In the event it does turn out to be elastic then it is merely an issue of torque, pressure, tolerances, and gasket type.

If it turns out that the deformation is plastic (currently the leading thought), I need to have an explaination ready... hence the original post.

Arunmaro,

Again, confirmation of micro shrinkage is tough without access to the actual parts... however I can say that there were no visual indications of porosity currently or during manufacture.

Also, the yield strength appreas to have been tested concurrently with the other two properties. I like this micro shrinkage idea though... I'll look into it further.

Thanks for the quick posts!

 

[ibf]

I had a similar problem in the past , and the root cause was poor nodularity (in shape , size and count) , associated to some "dross" (magnesium sulfides / oxides).

I strongly suggest you to perform metallography to check for nodularity / dross , as well as review the innoculation and nodulization techniques with your vendor.

 

[BLaroux]

Thanks

They are allowing us to remove a small 1/4" sample & send out for metallography and chemical analysis. Although a full yield test would be preferable, hopefully microstructure and chemistry will say what (if anything) is going on.

I'll post conclusive results if/when we get them.

 

[EdStainless]

I haven't tensile tested any ADI for years, but I recall that the curves have no straignt section. That is, there is measurable deformation even at very low loads.
The Yield strength is an engineering deffinition, it is not a physical limit.
I suspect that may have creep, but poor graphite morphology could also be a factor.
Is there any way to spread the load more?

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[BLaroux]

EdStainless,

Yes, we have a few possible solutions that involve spreading the load more... design change is one, adding more gaskets is another. For now, stoning the face flat and re-assembling seems to stop the problem. (we were getting leakage because the raised areas were preventing proper pressure from being applied to the rest of the gasket area) This suggests that the deformation is plastic in nature, but we're going to measure the face of an un-sanded flange in the un-stressed state to be sure.

The site below says creep is not really a problem until 575 F... a far cry from the 120 F operating temp... but the deformation is measured in thousandths, so I'm still considering it a possibility.

Thanks

http://www.ductile.org/didata/Section3/3part1.htm#Tensile Strength

 

[JStephen]

If that 20,000 PSI is an averaged loading, is it possible you have concentrated stresses considerably higher?

 

[BLaroux]

Chemistry and microstructure tests came back OK. Carbon, Silicon, and Phosphorous are within the limits of ASTM A395. Metallography showed no evidence of dross, inclusions, microshrinkage or poor nodularity. The sample's matrix was as expected for the annealed condition.

Turns out that the 'proportional limit' in shear of this alloy, even without anything wrong, is down around 21 ksi... so it is definitely possible that there was some small amount of deformation due to over-torque of the studs (especailly if there was concentrated stress)---> thx Edstainless & JStephen.