New formula equates tensile strength of gray iron.

An AFS research project has identified the effects of alloying elements on the tensile strength of pearlitic gray cast iron. Conducted at the Southern Research Institute (Charles Bates, principal investigator), its chief findings included that:

* tensile strength is primarily related to graphite flake length;

* flake length is related to carbon-silicon (summarized as the carbon equivalent), cooling rate and percent of other alloying elements;

* some alloying elements have an effect on pearlite fineness and strength.

This work resulted in an equation for predicting tensile strength of pearlitic gray irons based on chemical analysis and cooling rate (or cast-test bar diameter).

The equation was generated and modified in the eight-year research project. It was based on 716 test bars supplied by SRI's laboratory and four production foundries using iron melted in electric and cupola furnaces.

Since 1950, foundrymen have been using the McElwee-Schneidewind equation|1~ to predict the tensile strength of gray cast iron. Fig. 1 shows the correlation between the actual tensile strength of 48 cupola melted bars and those predicted by the equation that predicted strength values about 4.7 ksi higher than the experimental values. More importantly, the equation over-predicts strength at the lower tensile strengths (i.e., 25 ksi).

The final form of the equation in the research project is presented in Table 1. The readily computerized equation is valid only for irons within the analysis range (Table 2) and for test bars cooled in the mold until black.

Table 1. Gray Iron Tensile Strength Equation
UTS (ksi) = No. 1 x No. 2
No. 1 = 101.1193 - 20.3283 x (%TC + %Si/4 +
%P/2) + 4.3887/cast bar radius
No. 2 = 1.000
       +0.1371   x   %Si;
       -0.0021   x   (%Mn -1.7 x %S);
       -0.3132   x   %S;
       +0.3562   x   % Cr;
       +0.0282   x   %Ni;
       +0.1107   x   %Cu;
       +0.6297   x   %Mo;
       -5.2985   x   %Ti;
       -0.2305   x   %Sn (Sn term should probably be omitted in
most cases because this research assumes essentially fully
pearlitic iron before alloying additions.)
Table 2. Equation Limitations
Cast bar diameter   0.875   -   2.0 in
%TC                 3.04    -   3.58
%Si                 1.59    -   2.46
%Mn                 0.21    -   0.98
%S                  0.027   -   0.164
%P                  0.21    -   0.14
%Cr                 0.020   -   0.55
%Ni                 0.030   -   1.62
%Cu                 0.050   -   0.85
%Mo                 0.010   -   0.78
%Sn                 0.008   -   0.114
%Ti                 0.004   -   0.050

Correlation of the actual and predicted tensile strength of the same 48 bars using the AFS equation is shown in Fig. 2. They had an observed strength 2.76 ksi lower than their predicted strength. The data is parallel to the line and an adjustment in the constant from 101.1193 to 99.04 results in an excellent fit about the line. The parallel data allows adjustments to handle systematic errors in tensile testing or chemical analysis.

This work has performed with essentially fully pearlitic irons and is the reason tin, a potent pearlite stabilizer, displays a negative coefficient or multiplier.

The equation is useful and foundrymen are encouraged to evaluate it. Even if predicted and actual strengths don't coincide, a consistent difference should be a useful indicator of iron consistency. Charge materials, melting and inoculant practices, pouring temperatures and other factors can induce some differences.

References

1. R.G. McElwee, R. Schneidewind, "Composition and Properties of Gray Iron, Parts I and II," AFS Transactions, vol 58, pp. 312-332 (1950).

2. C.E. Bates, J.R. Tucker, K.S. Starrett, "Composition, Section Size and Microstructural Effects on the Tensile Properties of Pearlitic Gray Cast Iron," AFS Research Report No. 5, Sept 1991.

3. ICRI Newsletter, vol 1-22, Sept 5, 1991.