Testing ductile iron nodularity.

As metalcasters are well aware, nodule count in ductile iron is extremely important to the success of castings. It is this spherical graphite shape that gives ductile iron its higher strength compared to gray or vermicular and compacted graphite irons, as well as the ability to appreciably elongate before fracture.

The percent nodularity that is acceptable on a pass/fail basis varies from facility to facility, but generally metalcasters will pass a sample if the estimated percent nodularity is 85% or more. In other words, out of one hundred graphite particles, the number of particles that is determined to be sufficiently rounded equals the percent nodularity, and that percentage must meet or exceed the established internal standard.

To determine whether your ductile iron reaches those internal standards (wherever you decide to set them), nodularity coupons can be poured and examined. However, it is critical that you be mindful of how and when you collect those samples to assure their accuracy.

What Is the Coupon?

Nodularity coupons can be made in one of several methods. The American Foundry Society (AFS) offers a standard nodularity coupon. The advantage of this is that percent nodularity measurements can be made in sections of differing cooling rates. In a low modulus, rapidly cooling section, the percent nodularity can be higher than in a larger modulus, slower freezing section poured with the same iron. So, if you are pouring high modulus castings, this may be a good choice for a nodularity coupon since the concern is adequate nodularity in slowly solidified sections.

Some metalcasting facilities will use an in-house-designed pincore (the size of a person's thumb) as the coupon. These can be produced in a shell or resin bonded sand, depending on what the metalcaster finds easiest. A suitable nodularity coupon also could be part of a casting itself but would have to be removed from the casting in the mold, which can cause further delay in processing.

What is not acceptable is splash taken from the top of the mold. Splash is thin. The rapid cooling facilitates graphite nodules even at low residual magnesium levels, and the size is not sufficiently consistent to provide the control desired in the metalcasting facility.

When to Pour

The effects of the nodulizing treatment of a base iron with magnesium and rare earth elements fade over a relatively short time. If you simply wait too long--how long will depend on many processing variables unique to each metalcasting facility--this nodulizing effect fades, and the castings poured will exhibit degenerate, or non-spherical, graphite. If there is excessive deterioration in the graphite morphology, the castings will not have the required percent elongation, yield and tensile strengths.

To determine whether any of your castings have exceeded that time limit, the nodularity coupon should be poured from the last iron in the pouring ladle, after the last mold from that ladle has been teemed. Some operators will dip the nodularity coupon into the pouring basin of the last mold poured to take the sample. Other shops may pour the coupon directly from the pouring ladle at the end of the pour. It is important to be consistent and systematic. Care also should be taken to ensure the coupon is filled with clean iron and does not contain dross that can prejudice the result.

Preparing the Coupon

After cooling to a temperature that allows the coupon to be handled, the sample must be prepared for evaluation. If the sample is quenched in water to hasten cooling, the matrix structure will reflect that quench, but as long as the sample has solidified, the graphite morphology already will have been determined because the nodules are formed at solidification.

Typically, a surface at least 0.2 in. (5 mm) below the as-cast surface should be examined in order to avoid contamination effects from the mold or dross inclusions. The coupon should be polished to at least a 9 micron finish or better. A 1 micron finish is ideal. Technicians should use a double-ended grinder for rough polishing, abrasive paper (starting at 120, then 240, 400 and 600 grit) for sanding, and diamond or alumina abrasives for final polishing. The technician should use ethanol to clean the surface before examination. The better the polishing, the more accurately the percent nodularity estimate can be made since the nodules will not be mechanically distorted and appear misshapen.

Often, technicians perform such a poor sample preparation that an accurate measurement cannot be made. Technicians should be trained and experienced in both coupon preparation and percent nodularity measuring. Wall charts and other resources can be used to train operators.

Another useful step is to etch the nodularity coupon in 5% nital. This exposes carbides and pearlite in the coupon. Since the coupon typically is small and cools quickly, a small percentage of carbides may be observed in it even though any castings poured would be carbide free. Etching can reveal whether the extent of carbides is unusually high, which might indicate that the inoculation of the ductile iron was insufficient or not done at all. The coupon can provide a clue that something is amiss in the process and problems may be evident in the castings poured.

Recording Nodularity

Percent nodularity values should be recorded by treatment or ladle number and become part of a metalcasting facility's records, along with mechanical test results, spectrographic results, furnace charge makeup and pouring temperature records. The entire nodularity testing process will take approximately five minutes, depending how long it takes the sample to get into the metallographic lab.

If the percent nodularity in the coupon is acceptable, then the assumption can be made that all castings poured before the nodularity coupon would also exhibit acceptable percent nodularity. If the percent nodularity is not acceptable, then the metalcasting facility should have a procedure in place to quarantine all molds poured from that ladle until further evaluations are made to determine if they are acceptable. Typically, the procedure is to examine a section of the runner bar, or even a section cut from a casting, in the last mold poured from the suspect ladle. If the percent nodularity is acceptable in the last casting poured, the assumption can be made that all previous molds would have acceptable percent nodularity. If it is unacceptable, then the metalcasting facility should continue backwards from the last casting poured to the first to find the initial instance of an acceptable casting. Once that casting is found, it can be assumed that all previous castings have sufficient nodularity.

For More Information

"Sound Advice for Nodularity Testing," A. Alagarsamy, MODERN CASTING, July 2005, p. 31-34.

AFS Molten Metal Processing (5-L) Committee, Schaumburg, Illinois 

Fred Linebarger, Miller and Co., Rosemont, Illinois