Analysis of non-metallic inclusions in Steels with AZtecSteel

04^th November 2022 | Author: Hui Jiang

Steels are used for a wide range of purposes, with their properties optimised for a particular application. The presence of non-metallic inclusions in steel can have a significant effect on that steel’s properties. Therefore, monitoring, characterising and controlling the presence of inclusions in steels is a vital task in ensuring that the correct properties are delivered. When investigating the influence of non-metallic inclusions on the quality of steel, the chemical, size, distribution and physical characteristics of these inclusions are of great importance. As the demand for clean and ultraclean steels increases, there is also a requirement to detect smaller and smaller inclusions in larger sample areas.

This is best addressed by using the scanning electron microscope (SEM) equipped with Energy Dispersive X-ray microanalysis (EDS) and dedicated particle analysis software AZtecSteel. The use of SEM and EDS adds further precision, as morphology can be more precisely determined (on the micro- and nanometre scale) while adding chemical information to the characterisation.

What is AZtecSteel and how it works?

Fundamentally, AZtecSteel is an implementation of our AZtecFeature particle analysis platform for the SEM. It is a customised recipe, optimised specifically for steel inclusion analysis that contains optimal settings for the detection, measurement and classification of steel inclusions. It includes Inclusion Classifier – a dedicated post-processing application to process AZtecSteel results and report them to published international national Steel Cleanliness standards. The video below illustrates how easy to use the AZtecSteel Routine is.

It works by collecting electron images, thresholding them to detect inclusions. EDS data is automatically acquired from each inclusion – to measure its composition based on the X-rays it generates when hit with the electron beam. It then quantifies and classifies them. This process is repeated for every inclusion found across the entire sample area. It includes 6 dedicated classification schemes for micro to nano-sized inclusions for different steel types.

Importantly, to ensure quality and reliability, we utilise the same spectrum processing for the spectra collected by this automated approach as for our manual work, acquiring the highest quality quantified data. We call this TruQ which I will talk about in detail in a moment. Essentially TruQ ensures elements are identified correctly and composition is quantified accurately.

We have added a series of optimisations to the standard workflow to improve throughput in every aspect of the setup and run. This allows multiple runs to be managed with ease, enables additional data to be acquired from certain features as required and provides huge improvement in the speed of the communications within our system.

Hardware

However, AZtecSteel is only one part of the whole solution.

Both Oxford Instrument latest generation Ultim Max and Xplore ranges of EDS silicon drift detectors are compatible with AZtecSteel to determine the composition of inclusions found in steel.

Ultim Max and Xplore come with a variety of sensor sizes, including 40, 65,100, 170mm². A larger sensor allows more X-rays to be collected per second and so allows more data to be acquired more quickly. Given that we typically want 20,000 – 30,000 counts in a spectrum, with the larger sensor sizes, this means we can acquire the data we need in much less time compared to a smaller detector. This gives a significantly higher level of throughput.

Alternatively, we could of course acquire for the same amount of time as we would do for a smaller detector but get significantly more data, therefore allowing us to more accurately measure low concentration elements.

Accuracy

I mentioned earlier that we apply TruQ, our spectrum processing technology, to each of the spectra. This is an essential part of the analysis process as, if it weren’t applied, the operator would have to:

• process each EDS spectrum individually,

• identify the elements present in each EDS spectrum
• determine the number of counts acquired for each constituent element
• measure standards for all the elements present in the sample in order to calculate quantitative compositional results
• account for the effect of the sample matrix on the EDS result – potentially 1000s.

It is just not practical at all.

What we do instead is to use this collection of algorithms called TruQ to go from a raw spectrum to a result where we have identified all of the elements present and produced a quantified result that can be used for classification.

TruQ is actually the name for a collection of algorithms that are applied to every single spectrum. These include:

• an approach to remove the X-ray background from every spectrum using a filtering, rather than subtraction, approach
• a complete characterisation of the whole hardware and software chain in house – only made possible by the fact that we manufacture all of the EDS system at our site in High Wycombe in the UK
• a correction for pulse pile up – which becomes more of an issue at the high count rates at which it is now possible to work
• AutoID - to identify the elements present in each spectrum and a matrix correction.

All of this combined gives results that have been proven to more accurate– which is exactly what is needed for steel inclusion analysis.

Finally, on accuracy – a note on quantification. We incorporate within AZtec an option for what is often referred to as standardless analysis – this is the ability to quantify your EDS data without recording your own standards. Of course – this doesn’t mean that standards aren’t used at all – instead they have been acquired in an exceptionally rigorous approach by our in-house experts and then implemented in the system. We find that this approach is particularly useful for automated particle analysis where ease of use and throughput are key – whilst accuracy is maintained.

Hopefully, this gives you an insight into steel inclusion analysis performed with AZtecSteel. I will also be presenting a tutorial on Analysis of Non-Metallic Inclusions in Steels with AZtecSteel on the 24^th Nov – do look out for this!

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