Optical emission spectrometry (OES) is the perfect solution for analyzing elements at low limits of detection like carbon, boron, and sulfur. However, conventional wisdom would tell you that effectively analyzing cast iron with an OES is a much more difficult task.
Ductile cast iron needs a specific form of carbon formation to ensure the right mechanical properties. The requirement to carry out a high-energy pre-spark sequence with OES leads to a potential analytical problem: it burns away some of the free carbon and results in inaccurate readings.
This issue leads to foundries and other companies in the metals industry to use techniques that have higher operational costs. However, accurate OES analysis of carbon in cast iron is not only achievable it is also effective and simple. It just requires the correct sample preparation.
OES expert Wilhelm Sanders explains how:
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The Importance of Cooling Time
When utilizing a spark spectrometer, white solidification is crucial for the correct carbon result. To ensure a good sample for OES, grey solidification must be avoided. Grey solidification happens when nodules form in the casted sample and prevent the best homogeneity of the carbon. The carbon needs to dissolve in as consistent a manner as possible to get an accurate reading.
If using moulds, so that rust formations from previous tests don’t slow down the cooling process, you should ensure they are clean and are made completely of thermally conductive material like copper. Cooling time is key - taking too much time will result in grey solidification and destroy any chance of an accurate carbon reading.
Grinding Paper is Key When Using Disc Grinders
Utilizing grinding machines with the wrong kind of grinding paper is a frequent mistake when preparing a sample. To analyze cobalt, steel, nickel, or titanium based metals, aluminum oxide should be used. For low aluminum concentrations, silicon carbide or Zirconium oxide can be used.
After every 5-10 samples, grinding paper should be changed to ensure you get the best performance out of your spark spectrometer and avoid contaminating your samples. If a sample’s surface is not completely metallic, cross-contamination can also happen. Any remaining oxides and other substances can begin to skew results. However, utilizing a cup wheel grinding machine is the best technique for cast iron sample preparation.
Avoiding Drift
OES analyzer sensitivity does not stay stable over the long term. Aging components as well as environmental factors can make the analytical performance drift over time. To combat this, companies either tend to focus on materials that are less likely to suffer elongation in temperature changes or try to stabilize the pressure and temperature of their testing environment.
Hitachi High-Tech OES analyzers have been designed so that you don’t have to make this choice. Their OES analyzers are more efficient, smarter and more cost-effective. Optics with a total wavelength coverage between 130 and 800 nm make it much easier to spot drift which is a result of pressure or temperature change early and will correct it before it is a problem.
Analyzing carbon with an OES accurately is all about understanding your analyzer. It is also about making sure an instrument is built to work in the way you require it to. Hitachi High-Tech has over 45 years of helping companies get the optimum results from their analysis.
This information has been sourced, reviewed and adapted from materials provided by Hitachi High-Tech Analytical Science.
For more information on this source, please visit Hitachi High-Tech Analytical Science.