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Should You Develop Your Own Material Specification and Skip ASTM’s?

Otherwise, there may be random impurities you may have to accept.

Image source Pixabay

The basic format for an ASTM standardized metal alloy for medical uses includes a specification of the required elements as well as one or more undesirable elements for which there is a maximum allowable amount.

For example, ASTM F2886, last approved in 2017, covers Metal Injection Molded Cobalt-28Chromium-6Molybdenum Components for Surgical Implant Applications. While the name gives part of the recipe, a more-detailed list of elements specifies that chromium must be between 27 and 30%, and molybdenum between 5 and 7%. Thus, these major required alloying elements together constitute 32 to 37% of the mix. Twelve other elements are given as maximum amounts, suggesting that they are undesirable and could be absent. Other ASTM alloys have fewer restricted elements. For example, F60 for a different cobalt alloy has specifications for only six such elements.

The greatest allowable amount of specified undesirable elements in F2886 are for silicon and manganese at 1% each. The smallest allowable amount is for boron at 0.01%. This low limit suggests that such small amounts can be important. If each of these 12 elements were present at its maximum value, they would add up to 4%. But you may ask about cobalt, the first element in the title, which usually implies it is the dominant contributor. The specification says that the amount of cobalt need not be measured but is “approximated” by subtracting the other measured quantities from 100%. Therefore, “cobalt” could be between 59 and 64%, including the undesired elements. Thus, we have 15 elements accounted for, 3 necessary and 12 unnecessary. This leaves a great many elements unaccounted for. For example, how much vanadium can there be, or tantalum? Since these elements are not specified, and cobalt is not measured, these and other elements could, in principle, be present in relative abundance.

If a full elemental analysis were undertaken that showed some unspecified element present that the buyer found objectionable, one might think they could reject that lot of alloy. But some participants in the ASTM process have been busy at ASTM trying to prevent this from happening. “Template” language has been appearing in ASTM standards as they are revised, or newly added, that states: “All commercial metals contain small amounts of elements other than those which are specified. It is neither practical nor necessary to specify limits for unspecified elements, whether residual elements or trace elements, that can be present. The producer is permitted to analyze for unspecified elements and is permitted to report such analyses. The presence of an unspecified element and the reporting of an analysis for that element shall not be a basis for rejection.” In F2886, this is Section 6.1.3. In summary, this means that you have to take it no matter what else is in it.

I have a number of issues with this statement besides that overall it is objectionable from the buyer’s perspective. First of all, I don’t believe one can say that “all” metals have unspecified amounts of unlisted elements. Secondly, “small” is not defined. And I ask whether “small” is different from “residual” and “trace,” which are also not defined. Whether it is possible to manage additional elements should depend on the element, the alloy, the amount, and the care taken by the producer. Furthermore, it cannot be correct that it is always “not necessary.” We can again note here that boron is limited to 0.01%. This seems to be a small, or residual, or trace amount, yet presumably it matters and is therefore necessary to control the amount of boron. Note that if boron had not been specified, it could have been present in much larger amounts, yet under the disclaimer the buyer would have to take the lot anyway.

Given this producer-friendly language, the buyer cannot rely on a material meeting an ASTM specification as being one they actually want to use. The solution is to not specify a material only by its ASTM number, but to add to your purchase order language that allows you to reject a lot if it contains amounts of other elements that either individually or collectively would be objectionable. Such additional specifications can protect you from a producer pointing to ASTM language that says you have to take it anyway. Alternatively, you can use your own specification and not reference ASTM at all.

William A. Hyman

William A. Hyman is a professor emeritus in the department of biomedical engineering at Texas A&M University and adjunct professor of biomedical engineering at the Cooper Union. Reach him at [email protected].

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