Casting Answers & Advice.

AFS Technical Dept.

Des Plaines, Illinois 

Q & A

Q Since the mechanical properties of gray iron castings are directly related to the chemistry, is there a method to reliably predict their tensile strength?

Background: Since the early 1950s, iron foundries have been using the McElwee-Schneidewind equation to predict the tensile strength of gray iron castings. However, the correlation between the actual and predicted tensile strength was not consistent, especially at the lower tensile strengths. AFS Research in 1992 concluded that: tensile strength is primarily related to graphite flake length; flake length is related to carbon equivalent, cooling rate and percent of alloying elements; and some alloying elements have a negative affect on pearlite fineness and strength. This research produced a tensile strength equation that closely predicts the actual casting properties.

Recommendation: The new formula for predicting the tensile strength of gray iron is:

UTS (ksi) = No. 1 x No. 2

where No.1 = 101.1193 - 20.3283 x [%total carbon + %silicon (Si)/4 + %phosphorous (P)/2] + 4.3887/cast bar radius;

No. 2 = 1.000 [+ or -] the following factors for alloying elements:

+ 0.1371 x %Si;

-0.0021 x [%manganese-1.7%sulfur(S)];

-0.3132 x %S;

+0.3562 x %chromium;

+ 0.0282 x %nickel;

+0.1107 x %copper;

+0.6297 x %molybdenum;

-5.2985 x %titanium;

-0.2305 x %tin.

The equation can be computerized and is useful for most fully pearlitic irons. Because the predicted and actual values are on parallel lines with this equation, even if they don't exactly coincide, a constant difference should be a useful indicator of iron consistency.

For additional information, see "Cast Facts: New Formula Equates Tensile Strength of Gray Iron, "modern casting, October 1992, p. 35; and "Composition, Section Size and Microstructural Effects on the Tensile Properties of Pearlitic Gray Cast Iron," AFS Research Report No. 5, Sept. 1991.

Q Is there a nondestructive test available to evaluate the graphite structure in a ductile iron casting?

Background: The current procedures for evaluating graphite nodularity verify a heat of metal prior to pouring but do not qualify a casting. With the industry moving toward more in-mold treatment, there is an increased need for a nondestructive testing method that can be used to verify casting graphite structure and correlate that to casting properties.

An AFS Research Project reported in the 1980 AFS Transactions examined the use of ultrasonic testing to measure the graphite structure and predict principal mechanical properties. According to this report, resonant frequency or ultrasonic velocity measurements can be used to predict the effect of small changes in graphite form on the tensile strength, offset yield strength and other properties of ductile iron castings of known similar matrix structure containing up to 70% non-nodular graphite. To make similar predictions where matrix structure also varies, measurement of eddy current, coercive force or hardness must be used in conjunction with either resonant frequency or ultrasonic velocity.

Recommendations: While the basic test is described above, considerable qualification is required. Although users of this technology have indicated that matrix variations do not influence the test results as significantly as variations in graphite structure, the influence is sufficient to require the use of standard test castings for each part number that is being produced. If the matrix is known to have 100% ferrite or 100% pearlite, then the two nondestructive tests--ultrasonic velocity or resonant frequency--can be used without supplementing the testing. However, if carbides are present, the test results will be altered undesirably. Also, if the ferrite (or the pearlite) is in the structure as a result of heat treatment, the readings will not be the same as 100% ferrite (or 100% pearlite) produced as-cast.

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