Fracture strength of ductile cast iron 1

[Adrian2]

Dear Folks;

I am redesigning a part for a subway car that must shear off in the event of interference with the track. The part has been made of ductile cast iron in the past. We have done a chemical analysis on the cast iron that is available for our use.

We intend to machine in a closely controlled notch on the part much like the notch used in test samples for charpy or izod fracture testing machines. In this way we can predict what the shear plane area will be.

What fracture strength can be expected with ductile cast iron and what aspect of the chemical composition controls the fracture strength ?

I would like to able to specify the material's composition so that we get verifiable and consistent fracture strength.

Many thanks in advance

Adrian D.

 

[TVP]

The fracture toughness of ductile iron, like that of all metals, is determined by its microstructure. The microstructure is developed by the chemical composition, the casting process, and any subsequent thermal processing. Important microstructural features are the following:

1. Phase fractions (ferrite and pearlite)
2. Grain size
3. Nodule count
4. Degree of nodularity
5. Internodular spacing
6. Degree of homogeneity
7. Porosity (both solidication shrinkage and macroporosity)

Other things to consider:

8. Phosphorus and silicon content-- increases in these elements cause reductions in fracture toughness.

9. The fracture toughness will be a strong function of temperature. Iron with a BCC crystal structure (ferrite & pearlite) undergoes a ductile to brittle transition somewhere between -100 C and 100 C, depending on composition, microstructure, etc.

10. Section size and geometry will also strongly influence the fracture behavior. Plane-strain conditions will result in lower fracture toughness than plane-stress conditions.

There is an excellent section on the various cast irons in ASM HANDBOOK Volume 19 Fatigue and Fracture. You can obtain it directly from ASM using the following link:

http://www.asminternational.org/Tem...=Ecommerce/ProductDisplay.cfm&ProductID=10494

Other sources of information on the fracture behavior of ductile iron are included below:

http://www.astm.org/cgi-bin/SoftCart.exe/JOURNALS/TESTEVAL/PAGES/136.htm?E+mystore

http://www.tms.org/pubs/journals/MT/A/9711/abstracts-9711.A.html#8

http://www.sae.org/servlets/productDetail?PROD_TYP=PAPER&PROD_CD=2003-01-0832

http://www.sae.org/servlets/productDetail?PROD_TYP=PAPER&PROD_CD=980685