Q: What are the differences between the results you get from a portable alloy verification device and the results you see on a supplier’s certified materials test report (CMTR)?
A: Customers commonly request positive material identification (PMI), or alloy verification, and CMTRs to assure the valves and other chemical process equipment they are purchasing have the proper materials. It is important to understand the differences in these two requests and the capabilities of the equipment used to provide the information they require.
The chemical or heat analysis results provided on a CMTR are determined by analyzing a test sample obtained during the pouring of the raw material. These reports are meant to show that the raw material meets the requirements of the applicable material standard (such as those for the American Society of Mechanical Engineers or American Society for Testing and Materials (ASTM)). One common technique used for heat analysis is optical (or spark) emission spectroscopy (OES). With OES, atoms in a sample are excited by energy that comes from a spark formed between the sample and an electrode. This causes electrons to emit light, which is then detected by an analyzer; each element has its own emission pattern.
In contrast, portable alloy verification devices are typically used on semi-finished or finished products. This is considered product analysis as opposed to heat analysis. The range of the acceptable composition for this verification is normally greater than that for heat analysis to account for differences in the homogeneity of the finished product.
Portable alloy verification devices can provide quick and easy alloy identification for almost any size part. However, it is important to understand that several types of portable alloy verification devices are available and each type uses a different method to analyze a part’s chemical makeup.
X-ray fluorescence is a common non-destructive technique for alloy identification and is often referred to as PMI. This works much the same way as OES, but samples are bombarded with x-rays and each element gives off its own characteristic x-rays, which are in turn detected by an analyzer.
PMI analyzers are not capable of analyzing all elements of consequence in the alloys used in the process industries. For example, they do not provide information on light elements such as carbon, nitrogen, phosphorus, sulfur or silicon. This means they cannot verify the carbon content in carbon and alloy steels, nor can they distinguish between standard and low-carbon grades of stainless steel. They also cannot verify the nitrogen content in grades of stainless steels that are nitrogen-alloyed for increased strength and resistance to chloride pitting. They cannot verify that silicon, sulfur and phosphorus contents have been met.
Portable OES machines that can provide alloy identification using non-destructive methods are also available. These machines tend to be less accurate than the non-portable OES machines that foundries and mills use, especially when trying to detect trace elements. For example, one portable OES machine manufacturer reported that, to detect niobium (Nb) in carbon steel, an extra calibration step needed to be performed. Once all of the key elements were determined, the machine needed to be recalibrated to the Nb test standard so the carbon steel sample could be tested specifically for that element.
If a customer wants all components in a valve certified to an ASTM-grade chemistry, only heat analysis provided by the raw material supplier on a CMTR is sufficient. Despite what manufacturers of portable analyzers may claim and what customers may believe, these instruments can only provide alloy verification and cannot be used to check compliance to a CMTR or an ASTM specification.
Cherra Meloy is senior materials engineer, Advanced Technology Group, Emerson Process Management, Fisher (www.emersonprocess.com/ fisher). Reach her at Cherra.Meloy@emerson. com.