Metal Quality Control | Technological Capability

When we talk about detection limits, we mean the lower limit at which your analyzer can detect a certain element within a sample of your material. It’s important to know the detection limits of your instrument because if you are measuring below this limit, you’ll get a zero reading for that particular element. In other words, your analyzer will tell you there’s none of the element present where in fact there is, just a very small amount. This gives you a false positive result if you are screening to ensure the element isn’t present, or a false negative if the element is a requirement and you have to reject the batch because you haven’t been able to detect it.

At the very least, a false reading will cost money in wasted scrap and rework; the worst case is that the final component will fail in the field. To avoid this, you must ensure that the specification of the elements that you are analyzing is above the detection limits of your spark spectrometer. What we are seeing at Hitachi is that there are three areas where the analytical requirements of the metals industry are becoming very close to – and in some cases below – the capability of many OES instruments.

1. Incoming and raw materials screening

To improve sustainability and reduce costs, more scrap metals are re-entering the supply chain. While this is good news from an environmental standpoint, it does mean that you have to check incoming material for residual and trace elements. Many elements, such as zinc, boron and tin, have a significant effect on the properties of the final components and their presence in the wrong alloy can have disastrous effects. If you’re verifying incoming material, then you’ll need to be able to measure a wide range of trace, tramp and residual elements within the ppm range.

2. Process control

The most demanding area of process control for spark spectrometry analysis is melt chemistry control. To meet the exacting specifications of the final product, you’ll have to measure a range of elements in the parts per million (ppm) range. And when it comes to aluminum melt control, it’s even more complicated as you must ensure elements such as phosphorous, antimony and bismuth are controlled to less than 10ppm, as their presence makes melt modifiers ineffective.

3. Meeting industry standards

The third area where the compositional requirements for certain elements veer dangerously near the detection limits of many instruments is with ASTM industry standards. The most potentially problematical standard is the ASTM E415 test method for the analysis of carbon and low-alloy steel by spark atomic emission spectroscopy. This details 21 different elements that must be controlled to a given limit. Probably the most challenging is the limit for nitrogen, which essentially calls for a detection limit of below 10 ppm to comply.

How to meet low detection limit requirements

Essentially, the detection limits are set by the technological capability of your instrument. You can ensure that your instrument is operating at the lowest possible detection limits by ensuring your equipment is kept properly calibrated and maintained at regular service intervals, but ultimately, all spectrometers have a detection limit where the signal literally gets lost in the noise.

The OE series of spark spectrometers from Hitachi High-Tech contains state-of-the art detector and spark-stand technology to meet the requirements of today’s metals quality control without the need to invest in high cost instrumentation. Both instruments in the range meet detection limits of below 10 ppm in all non-gaseous elements in metals – with some detection limits nearer to the 1 ppm level.

Aluminum foundries supplying to the aluminum industries need to perform melt analysis at the highest level. The OE720 is designed to meet the exacting requirements of aluminum foundries, especially those requiring lower detection limits for phosphorous, bismuth and antimony and the control of tramp elements in aluminum.