Determining Percent of Nodularity for Gray and Ductile Iron

This article has been written to show how to use ultrasonic sound velocity to help determine the percent of nodularity in a gray or ductile iron casting. Quality Control departments at gray or ductile iron foundries are able to determine if there is a problem with a casting because of the “lack” of rounded nodules. The size and the density of these nodules are related to the mechanical strength of the casting.

Gray iron is made up of “flake”, layer upon layer, and this type of casting is typically used for low-stress applications. The average maximum tensile strength is approximately 40,000 pounds per square inch (PSI.) This type of casting is also desirable if reduced “ringing” or noise level is preferred. Sound velocity is reduced due to the layering of the flake.

Ductile iron has an increased alloy content (magnesium, etc.) and the foundry tries to achieve very few flakes in these castings. With increased round nodules, there is an increase in tensile strength (65,000 to 85,000 PSI).  A ductile iron casting with too many linear nodules (flake) could cause a fracture if designed for high-stress applications.

Through examination using the ASTM Standard E-494-15, “Measuring Ultrasonic Velocity in Materials,” we are able to compare the velocity of a material to determine the percent of nodularity. This examination helps to determine the quality of the casting using a Nondestructive means.

Let’s address accuracy first. It is important to be able to rely on a specific type of examination. ASTM Standard E-494 states, “Factors including techniques, equipment, types of material, and operator variables will result in variations in absolute velocity readings, sometimes as much as 5%. Relative results with a single combination of the above factors can be expected to be much more accurate (probably within a 1% tolerance).”1

Element segregation and improper temperatures during the molten and cooling staged will affect results. (See Figures 1 and 2.)

Tables for “Acoustic Properties for Metals in Solid Form” are commonly published and are freely available from several sources. These tables — see this useful example — can be very helpful in the examination under consideration here.

The American Foundry Society offers for sale charts to be used as reference photographs of micrographs. These are to be used as a comparison to determine the percent of nodularity in a foundry’s cast samples. (See Figures 3 through 6.)

Unfortunately, tables that compare the velocity of common materials with the percent of nodularity are not commonly published. Because of this, the necessary information is collected through investigation. Foundries use this information to determine the quality of their cast products without destroying the production parts. (See Figures 7 through 9.)

To properly determine the Bulk Longitudinal Wave Velocity of an unknown material (V1), the following examination procedure is performed. (ASTM-E-494 includes additional steps/information to make the following procedure clearer. Note: A qualified technician, knowledgeable in the setup and calibration of the ultrasonic testing instrument, should perform this examination.)

UT testing procedures

1. Select a sample of a known velocity (Vk) that has uniform parallel front and back surfaces. Measure the thickness (Tk) within +/-0.001 or 0.1%, whichever is greater.

2. Align the transducer over the sample and obtain a multiple signal pattern (Number of round trips Nk) of several back-surface echoes. (See Figure 10.)

3. Using a caliper or the gate on the instrument, measure the distance from the leading edge of the first back wall echo to the leading edge of the most clearly defined last back wall echo (Ak). Record this distance as Ak in the following formula (Distance between round trips of known velocity sample). Document the number of round trips Nk in the known material.

4. Select the area on the unknown velocity sample that has parallel front and back surfaces. Using a grinder, polish the surface to an equally smooth surface as the known velocity sample. Measure the thickness (T1) within +/-0.001 or 0.1%, whichever is greater.

5. Align the transducer over the unknown velocity sample and obtain a multiple signal pattern (Number of round trips, N1) of several back-surface echoes.

Using a caliper or the gate on the instrument, measure the distance from the leading edge of the first back-wall echo to the leading edge of the most clearly defined last back-wall echo.

6. Record this distance as A1 in the following formula (distance between round trips of known velocity). Document the number of round trips, N1, in the unknown material.

Calculate the value of the unknown velocity as follows:
V1 = (Ak x N1 x T1 x Vk) ÷ (A1 x Nk x Tk)
Where Ak = distance from first to Nth back echo on the known material, (inches), measured along the base line of the a-scan display;
N1= number of round trips in the unknown material;
T1 = thickness of unknown material, (inches);
Vk = velocity in known material (inches per second);
A1 =  distance from first to Nth back echo on the unknown material, (inches), measured along the base line of the a-scan display;
Nk = number of round trips in the known material; and
Tk = thickness of known material, (inches).

Once the velocity of the unknown sample has been determined it can be compared to the internal quality standard for product acceptance. This quality standard usually is determined through experimentation.

Example for quality standard

We have performed multiple velocity tests following the above procedure and compared the results with microanalysis for percent nodularity of the same samples (See Figures 7 through 10 for microanalysis procedure.) The sample micrographs are then compared to references such as Figure 3 through 6.

These investigations determined that a longitudinal velocity of .2160 in/µs gives a micro analysis of 60% nodularity, a longitudinal velocity of .2180 in/µs gives a micro analysis of 80% nodularity, and a longitudinal velocity of .2190 in/µs gives a micro analysis of 90% nodularity.

If the quality-department acceptance criteria has a minimum longitudinal velocity of .2180 in/µs, or 80% nodularity, they have a “standard” for Acceptance / Rejection criteria.  
Daniel E. Potter is the Nondestructive Testing Manager for American Castings LLC in Pryor, OK. He is ASNT Level III-certified in radiography, ultrasonics, eddy current, magnetic particle, penetrant, and visual testing through the American Society for Nondestructive Testing. He has 40 years’ experience in casting inspection. Contact him at [email protected]

References

1. ASTM-E-494-15, “Measuring Ultrasonic Velocity in Materials” American Society for Testing and Materials.

2. Casting Defects Handbook: Iron and Steel. American Foundry Society.

3. Ductile Iron Microstructures Rating Chart. American Foundry Society.

4. American Castings LLC, www.AmericanCastings.com.