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Tensile test reproducibility of ductile iron.

One of your important customers calls and requests that he be sent a test bar to verify his foundry is getting the tensile properties ordered. One test bar from the keel block passed, but how sure are you that the other bars will pass when the customer pulls them?

It is known that the tensile test can vary from bar to bar. The ASTM published a report in 1984 quantifying the typical variations that occur in tensile testing, but, while the work included many different metals, no cast irons were evaluated.

Test bars are highly significant to ductile iron foundries. They are often required to show the metal produced meets specified tensile requirements.

In his work on the effects of alloying elements in gray iron done for the AFS Cast Iron Division's Gray Iron Research Committee, Charles Bates, Southern Research Institute, developed data for gray iron. Parallel data for ductile iron testing has been somewhat ignored, however, resulting in no comparative reference.

In response, the AFS Cast Iron Quality Control Committee noted that, because the same tensile bar cannot be pulled repeatedly, it would initiate a similar testing program. The first stage involved formulating a procedure that would produce over time many test bars of the same composition and microstructure. Deceptively simple, obtaining enough test bars of the same material became the most time-consuming phase of the project.

The American Cast Iron Pipe Co. (ACIPCO), Birmingham, Alabama, helped resolve the problem by placing baked oil sand keel block molds in two large green sand molds. Using an appropriate gating system and pouring both molds as quickly as possible from the same ladle of iron provided the required consistency. This system produced more than 300 test bars needed to confirm test bar consistency and to conduct the other phases of the research. The keel blocks were annealed to produce a 60-40-18 iron.


Test Bar Consistency

After verifying the consistency of the bars through established statistical methods, 10 randomly selected bars were sent to each of the 12 foundry and commercial laboratories participating in this study. The labs were instructed not to take any extra care with the bars to assure the results would represent typical test procedures. The only special instruction was that they were not to machine or pull more than two bars in any one day. This requirement helped ensure that any variation would be typical of normal conditions.

The degree of variation shown in Table 1 is of direct concern to foundrymen. It means that if a foundry could produce absolutely consistent test bars, it would need to produce iron with an additional 4300 psi ultimate, 5300 psi yield and 8.27 elongation to meet the minimum specification requirements.

In other words, to compensate for the variance in testing from these laboratories at a three-standard deviation level, the foundry would need to produce iron with 64,300 psi ultimate, 45,300 yield and 26.3 elongation. The additional properties would only act to compensate for testing variance.

The committee also considered the possible causes for variations due to the test bar machining. Proving that theory, another set of 10 machined test bars was sent to each of the 12 laboratories for tensile testing. All bars were machined at the same place on each bar. The results of those tests are summarized in Table 2.

While some of these figures show improvement, statistical analysis proved only the variance reduction in elongation was significant.

In summary, this research quantified the variability that exists when comparing laboratories' results. The perplexing fact is that the variability proved to be so great, concluding that tensile testing of cast iron deserves further study.
COPYRIGHT 1993 American Foundry Society, Inc.
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Copyright 1993, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Lobenhofer, Roy W.
Publication:Modern Casting
Date:Aug 1, 1993
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