Optimized Steel Analysis with Accurate XRF Spectrometry

Ferrous base materials are essential because they are the foundation for numerous applications, including construction, automotive, and manufacturing. Precisely evaluating these materials ensures compliance with their chemical standards and enables high-quality, efficient manufacturing.

Irons

Irons vary in composition and use. They fall into two primary categories:

• Hot metal, commonly known as pig iron, is the primary raw material used in steel production
• Cast irons are used to make semi-manufactured items

Metallographically, white cast iron with a cementite structure differs from grey cast iron, which contains free graphite in laminae or nodules.

These make gray cast iron inhomogeneous and difficult to examine. Alloy cast irons contain alloying elements such as nickel, chromium, manganese, copper, and so on, which improve hardness, corrosion resistance, and engineering qualities.

ARL X900 Simultaneous/Sequential X-ray Fluorescence Spectrometer. Image Credit: Thermo Fisher Scientific – Production Process & Analytics

Low Alloy Steels

This category includes steels used in various applications, including:

• Steel castings, rails, axels, boiler and ship plates, automobile bodies
• Girders, all kinds of bridge and structural sections
• Wires, nuts, bolts, and forgings of any description
• Springs, cutting steel

From a compositional standpoint, these steels are differentiated because the alloying elements often amount to less than 5 to 7 %. Typically, the major alloying elements are present at less than the following concentrations:

Mn 2%; Cr 3%; Ni 5%; Cu 1.5%; Mo 1.5%; and V 1%

High Alloy Steels

Aside from iron and carbon, high alloy steels contain significant amounts of nickel, chromium, manganese, silicon, cobalt, tungsten, molybdenum, and vanadium.

This category includes stainless steels such as 18/8, austenitic, maraging, martensitic, and all types of special stainless steels, tool steels, high-speed steels, and high-manganese steels. 

Instrumental Parameters and Conditions

The Thermo Scientific™ ARL™ X900 XRF Spectrometer can accommodate up to 24 fixed monochromator channels in addition to the goniometer or up to 32 channels without the goniometer. Optional high-counting fixed channel monochromators are available to improve analytical precision, particularly for elements like Ni, Co, and Mo.

Thermo Scientific’s revolutionary Moiré fringe goniometer, with its smart friction-free positioning system, ensures analytical speed, flexibility, and reliability. Up to nine crystals and four collimators can be installed. The two detectors (flow proportional and scintillation counters) allow for exact elemental analysis ranging from boron to californium.

The ARL X900 XRF Spectrometer can be calibrated with commercially supplied certified reference material (CRM) standards or user-provided, well-analyzed samples. Calibrations can be sent from the factory, minimizing commissioning time at the customer’s location.

Typical Performance in Steel Samples

Table 1 summarizes the limits of detection (LoD) found by repeat examination of a blank sample (pure iron RE12) using 20 and 100 seconds of counting time per element on fixed channel monochromators at high power (4200W). The detection limits are three times the standard deviation of twenty-one repeatability runs.

The counting time for fixed channels used in the steel industry is 20 seconds, providing a complete result in less than a minute. As a point of comparison, the counting time of one hundred seconds is frequently used to represent the LoD in the X-ray fluorescence technique.

Table 1. Typical limits of detection in ferrous matrix for 20 fixed monochromator channels at two different counting times. Source: Thermo Fisher Scientific – Production Process & Analytics

Typical Precision Tests

The stability of an instrument represents the precision that may be achieved. Calibration curves were obtained using many international steel standards using 50 kV and 70 mA X-ray tube settings. Thermo Scientific’s cutting-edge recommendations were followed while correcting overlap and matrix errors.

A short-term test of 11 20-second runs was carried out on several steel samples. Fixed monochromator channels provide simultaneous analysis. As a result, up to 32 elements can be detected in less than a minute, assuming they are all fitted into the spectrometer.

This comprises loading and pumping the sample into the spectrometer’s vacuum chamber. Tables 2a and 2b summarize the analytical results from short-term repeatability tests. The ARL X900 Spectrometer is outfitted with 21 fixed channels. Only the essential data is displayed.

The unique, high-counting fixed channels allow for measuring high Ni, Cr, and Mo concentrations without an attenuation filter, significantly improving analysis precision.

A long-term repeatability test of 38 hours was carried out, with one analysis of 20 seconds every second hour at X-ray tube settings of 50 kV and 70 mA (Table 3). All elements are measured using fixed-channel monochromators.

The high linearity channels for Ni, Co, and Mo allow for analysis without an attenuation filter, improving analytical precision.

Table 2a. Short term precision test on a NiFeCr high temperature alloy using 20 seconds counting time (3500 W). Source: Thermo Fisher Scientific – Production Process & Analytic

Table 2b. Short term precision test on a stainless steel using 20 seconds counting time (3500 W). Source: Thermo Fisher Scientific – Production Process & Analytics

Table 3. Precision test over 38 hours using fixed channels on a high alloy steel sample. Source: Thermo Fisher Scientific – Production Process & Analytics

Accuracy of Analysis

The accuracy of the analysis can be evaluated by measuring steel-CRMs and comparing the findings to the certificate’s suggested values. Table 4 compares five different steel alloys.

It should be noted that while an XRF spectrometer is a very accurate comparator, the accuracy of the final analysis is entirely dependent on the quality of the calibration standards used, as well as the care and reproducibility of sample preparation, which must be the same for CRMs and routine samples.

Table 4. A comparison for five different steel alloys. Source: Thermo Fisher Scientific – Production Process & Analytics

Conclusion

The ARL X900 Simultaneous-Sequential XRF Spectrometer allows for easy analysis of irons and steels. Thermo Fisher Scientific can supply appropriate steel alloy calibrations complete.

In this instance, the spectrometer’s commissioning time is minimal. These matrix types provide exceptional precision and accuracy for regular or R&D analysis, particularly when the new high-counting fixed channel monochromators are utilized for elements such as Ni, Co, and Mo.

The well-known Moiré fringe goniometer can be used with fixed channels. It performs well enough to analyze items not fitted with fixed channels.

The goniometer and fixed channels are analyzed simultaneously, so they happen simultaneously. The goniometer can serve as a backup if any of the fixed channels fail. Thermo Scientific^™ OXSAS^™ Software simplifies operations and is compatible with the current Microsoft Windows^® package.

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This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Production Process & Analytics.

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