by George Goodrich 

    Proper machining of test specimen is crucial for reliable results.  GGG 40.3 specification requires a "U" notch in the machined bar.  Proper machining of the "U" notch specimen is very critical before testing the impact specimen.  The way the "U" notch is machined can affect the result.   The way the "U" notch is machined can affect the result.  For example, if the "U" notch is ground instead of broaching (even if one uses a new broach tool) the impact values are two to three foot pounds higher.
     In general, the impact value increases as the "U" notch is broached vs. milled vs. ground.

     There was no difference found in the impact value of a "U" notch specimen vs. a "V" notch specimen, under the following conditions:

     The fracture appearance of the impact specimen can give some indication of the impace value.   Higher amount of crystalline (clevage) white shiny appearance in the fracture will indicate less impact.
     Investigation was conducted from samples received from a foundry which experienced wide fluctuations in impact values.  Above .011% molybdenum, impact was poor.

Conclusions:

1.  The type of machining the "U" notch is very critical for reliable accurate impact values.
2.  Grinding the notch gave the highest impact values.
3.  Presence of intercellular carbides are detrimental for impact values.
4.  Carbon, silicon, nickel and molybdenum have the most influential effect on impact value.
5.  Regression formula of chemistry vs. impact is available in the literature.

The chemistry of the sample had the following range:

     There was good correlation between silicon and aluminum on the impact values.  Higher silicon had higher aluminum and lower impact.
     The foundry was using nickel magnesium alloy for treatment and in-mold inoculation.
     As inoculation increased, silicon and aluminum nicreasd and impact decreased.
     Other conclusions on this investigation were:

1.  There was good correlation between carbon/silicon/nickel vs. field strength.
2.  There was good correlation between aluminum/silicon vs. tensile strength.
3.  There was NO good correlation between aluminum/silicon vs. % elongation.
4.  There was good correlation of carbon/nickel with impact strength.
Note:  Analysis of nickel on the sample was not provided.
5.  When molybdenum was present in the sample, when it was less than .007%, 100% of the bars passed 8 ftl lb. value.  68% of the bars passed 10 ft. lb. value.

Reviewer's Comment (P.H. Mani)
     Foundries currently producing or planning to produce in future, castings to meet the specification GGG 40.3 will benefit from this paper on the advantages of grinding the "U" notch to get two or three foot pounds increased impact values.  As opposed to broaching the "U" notch.
     The use of looking at fracture appearance is beneficial.   But because the impact sample is very small, especially at the fractured face, it is difficult to get reliable indications of impact value by looking at the fracture alone.
     Members are advised to refer to DIS Project 19, which correlates fracture appearance with impact value on a larger specimen - dynamic tear specimen.
     One should use caution in interpreting the correlation between impact properties and the chemistry of the iron.  It is the metallurgy of the iron which influences the impact properties, more than the chemistry.

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