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While my research about the untreated surface of moulded cast I read that the properties such as tensile strength, 0,2% proof stress and so on are different (much smaller) from the properties you can find in the tables. That’s because the samples have a treated surface to evaluate the material properties.

Now my question is how can I determine the thickness of the surface where my properties are much smaller?

The background is that I have a bore near the untreated surface loaded with hydraulic pressure and for safety I need a specific thickness of material around the bore. And because the material properties of the untreated surface are much smaller, I will not take this area into account to my calculations. If it is needed: the regarded cast is GJS 400.

Many thanks in advance!

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I doubt there is significant cast surface affect in ductile cast iron. Other than roughness , the only surface affect should be decarburization . Cast irons have high carbon levels of about 3 % so losing a little carbon does not make much difference. Cast steels with much lower carbon content , such as 0.2 % ,can be much more affected by mold surface carbon loss. For an inexpensive material like cast iron , I expect the component is made thicker if there is any concern about strength.

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  • $\begingroup$ Thanks for your response and sorry for my delay… You are saying make your component thicker. But how much thicker 1 mm or 2 mm or 3 mm or… And when I am limited in installation space or weight it’s not always easy to say “okay, just add x mm for safety”.Is it possible to clarify this issue - thanks! $\endgroup$
    – tueftla
    Nov 4 '19 at 7:13
  • $\begingroup$ Simpler answer ; your original information is wrong , cast surface makes no practical difference to the bulk properties of ductile iron. If you want more strength get pearlitic ductile iron. $\endgroup$ Mar 30 '20 at 18:47
  • $\begingroup$ @ blacksmith37: thanks again… have a look at my answer proposal. I’m not looking for a material that has better strength values than GJS 400. I was interested in an quantitative or qualitative statement how to get the thickness of cast surfaces and how to consider them in calculations… $\endgroup$
    – tueftla
    Apr 3 '20 at 10:52
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For the overview I drawed two cast samples. One has a cast surface with varying strength values over the depth – this curve is only qulitative. And on the right there is a sample without cast surface. The strength values are constant along the depth.

cast with surface and without

The deeper the cast surface is the more loss of the strength values arises. With the strength values I mean tensile strength and yield stress.

Cast surfaces have a specific depth which is not easy to determine generally because the depth is shape dependent.

The convection within the melting affects also the thickness of the cast surface. A well marked convection minimises the thickness of the cast surface. Therefore it is recommended to do a simulation of your cast to get proper values for your calculation… Otherwise you have to guess the thickness. It can be 0.2 mm up to ?? mm depending of the shape of your cast and the possibility to allow convection within your melting while solidification.

And now the calculation topic:

  • If you are taking the whole cast or only the inner cast (without cast surfaces) into account – it‘s up to you…
  • When you are only taking the inner cast you can use the full strength values for your calculations
  • When you want to use the whole cast it is recommended to use a reduction factor for your strength values.
  • In both cases you have to rethink proper the choice of your saftyfactor
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There is no significant loss of cast surface strength in ductile cast iron . In fact the ASM Handbook ( 9th ed.) shows higher strength near the surface , the amount of strengthening depends on grade and thickness of casting. Basically caused by the cooling rate; faster cooling = more strengthening.

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  • $\begingroup$ I’m not talking about the hardening process. When you do a moulded cast and you don’t remove the surface of your cast you have a surface like shown in [picture] (de.wikipedia.org/wiki/Gusseisen#/media/…) And this surface hasn’t the strength values like listed in the tables. They are smaller… And it’s a general cast problem – no matter if your using GJS or GJV – I think it’s also valid for GJL. $\endgroup$
    – tueftla
    Apr 6 '20 at 5:26

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