Unspecified and Trace Elements

All steels and nickel alloys contain residual elements that are not required to be analyzed or reported on the Mill Test Certificate. It is not practical to analyze and report every element in the periodic table for every heat. There are subtle differences in specifications relative to how chemistries are reported. Understanding these differences by following the specification references can answer most questions. In this discussion we will look at the most common issues.

ASTM stainless steels are covered in "A" specifications and most chemistry related issues, beyond the defined alloy chemistry in the product specification, are controlled by the General Requirements in A480, A484 and/or A751. In AMS specifications, chemistry variations and reporting is governed by AMS 2248. Nickel alloys in ASTM specifications are covered in "B" specifications. The individual product specifications refer to ASTM B880 for chemistry allowable variations and reporting. AMS specifications for chemistry variations are detailed by AMS 2269. The requirements for ASTM and AMS are the same in most cases, but the ASTM standards provide additional clarifications.

The major element in each alloy (iron in stainless steel, nickel in nickel alloys) is often listed in the chemistry as "remainder" or "balance". In such an instance, analysis of the "remainder" element is not required and is taken to be the arithmetic difference of all reported elements from 100%. Reporting this arithmetic difference is neither required nor prohibited. The test report can simply say "balance" or "remainder". Actual chemical analysis and reporting of this element is not prohibited, however. Note that this practice could result in all reported elements totaling to more than 100% due to analytical variation.Specified elements are those for which the alloy definition has a maximum, minimum or range. An unspecified element is an element that may or may not be present, but is not included in the definition of the alloy composition. A residual element can be either a specified or unspecified element, not intentionally added, which originates from the raw materials, refractory or atmosphere during steel production. Trace elements are residual elements that may occur in very low concentrations, generally less than 0.01 weight percent.

Analyzing and reporting unspecified elements is also permitted and is fairly common. The omission of a specified element from the alloy's definition (copper limits in Type 304, for example), does not mean that the alloy must be completely free of that element. Mills often analyze for elements beyond those required by the specification because they can have an impact on a material's properties or performance.

The above referenced specifications also recognize the tolerance variations permitted in performing product or "check" analyses on finished products. For more information on this topic see the posting for Feb 16, 2015.