Casting hardness

[smurali1]

Hello,

We are procuring Grey cast Iron casting (GG25) from one of the supplier.

Supplier has checked in 10 spots and certified the casting to 185-200BHN using Poldi hammer. He also has certified UTS value as 305MPa.

Supplier then shipped the casting to Europe.

When European plant checked the casting in the same spots using Ernst method, we are getting 150-160 BHN. (Spec. is 170-220BHN). Re-check was conducted with two operators to remove the operator variability. Plant also reports that they used Portable Briness hardness tester to verify in one spot (where feasible) and the hardness is same.

Now, I want to understand the reason for this difference in hardness value. Any inputs on where I have to look, is appreciated.

 

[desertfox]

Hi smurali

I would be talking to your supplier.


desertfox

 

[CoryPad]

The difference is due to the testing methods. Unless you are using an actual Brinell test with a floor model and 3000 kgf, then your results will vary.

 

[arunmrao]

Use standard testing pratices to determine hardness. PPoldi is only a shop floor assist and results vary with operators and also the calibration of the test bar. Use calibration test bar close to 1.0. If you use 0.85/1.15 test bars results are skewed. however, that should not form a basis for acceptance/rejection.

Also please recall that the structure of gey iron is graphit flakes in a ferritic+ pearlitic matrix. The graphite flakes act as voids,hence large diameter indentors are preferred to measure hardness to even out the error in measurement.


If you think education is expensive, try Ignorance.
- Andy McIntyre


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[smurali1]

Hello all,

Thanks for your comments.

Now I have few more questions:

1. Will the hardness on the skin of the casting be more or less than say 1 mm below the skin?

2. Why the hardness checking surface is prepared by grinding before hardness check (for eg. Poldi or Foundrax)

I am trying to figure out if measurements at different surfaces is leading to difference in values between plant and supplier.

 

[EdStainless]

You want to grind to get a fairly smooth surface. The roughness of a casting will result in deeper indentor travel and lower measured hardness. The larger the indentor ball the less that this matters.
I don't know of anyone that would certify a Poldi reading. We used a similar method, but took a real Brinell somewhere on the part for ref.

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Plymouth Tube

 

[arunmrao]

THe skin can be hard in cast irons if the moisture content in the sand is very high.

Also Poldi instrument is not recommended. Use a floor brinell testing machine.


If you think education is expensive, try Ignorance.
- Andy McIntyre


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