Determination of Sulfur and Carbon in Steels

Table of Contents

Introduction
The Inductar CS Cube
Measuring Principle
Calibration
Analysis of Steel
Summary and Discussion

Introduction

Steel is considered to be a part of everyone's daily life. Whether it is used in the building industry, energy technology, transportation sector or the domestic home, steel products can be found everywhere. In order to fulfill all the various requirements in these different applications, the properties of steel need to be individually customized. This can be attained by varying the micro-structure or the chemical composition. For instance, it is possible to tune mechanical properties by alloying iron with more or less carbon atoms. In the case of vehicles, improved high strength high carbon steels are required, but for packaging materials more ductile low carbon steels are used. Moreover, steel properties, for example, brittleness, are influenced by impurities such as sulfur atoms.

determining elements in steel

Therefore, the accurate determination of these elements in steel is of immense importance. The method of choice for the quantification of sulfur and carbon concentrations in steel is the combustion method. This method determines the sulfur and carbon concentrations in an extremely accurate manner, in the range from mass percent levels down to trace parts-per-million. Thus, elemental analysis is capable of qualifying and certifying specific compositions of steels.

The German manufacturer Elementar Analysensysteme GmbH, based on over 100 years of experience in elemental analysis of organic materials, developed a new, advanced and simple to operate CS combustion analyzer for inorganic materials, the inductar CS cube.

determining elements in steel

The Inductar CS Cube

The inductar CS cube was developed, combining reliable data quality with enhanced ease of use, based on Elementar’s comprehensive knowledge in automated sampling, efficient combustion processes and gas detection techniques.

The inductar CS cube is available with a prolonged solid-state high-frequency induction furnace, which allows it to reach sample temperatures of up to 2000 °C, permitting high precision analysis of ceramic and metallic materials. The instrument uses Elementar’s reliable autosampler, which has years of established experience in daily lab operation of organic elemental analyzers and allows unattended round-the-clock operation. Moreover, easy and quick maintenance is a well-known advantage of all Elementar analyzers.

All parts are labeled and can be changed tool free within minutes. For instrument control, Elementar has produced a new industry standard in user-friendliness, allowing for intuitive and fast data input, reporting and analysis. The operator is guided through the operation via beneficial apps, for instance, a step-by-step calibration wizard. It is also possible to further customize the software in order to allow for alternative lab equipment communication and easy connection to LIMS.

A diagram of the inductar CS cube is displayed in Figure 1.

Figure 1. Diagram of the inductar CS cube.

Measuring Principle

In contrast to comparable instruments, the ceramic crucibles, which are filled with samples in the form of chips, particles or drillings, are inserted automatically from the sample carousel at the top of the analyzer into the combustion tube with the help of a robotic arm. The oxygen carrier gas travels under pressure from the top to the bottom of the combustion tube. By doing this, a sheath gas flow is developed, which decreases adhesion of debris and dust on the combustion tube. Additionally, spattering of debris is minimized by the high walls of the ceramic crucible. This leads to a clean dust-free combustion.

The samples are heated to extremely high temperatures by a solid-state induction furnace. Sulfur and carbon atoms merge with oxygen to carbon monoxide, sulfur dioxide and carbon dioxide. The gas flow is purified by a heated dust trap and moisture traps before it passes a wide-range IR detector, which defines the sulfur dioxide concentration within the gas flow.

After SO2 detection, the gas enters a third furnace, where carbon monoxide and sulfur dioxide are further oxidized to carbon dioxide and sulfur trioxide. Sulfur trioxide is removed from the gas flow by an ideal trap allowing for carbon dioxide to be measured quantitatively by a second wide-range IR detector.

The ceramic crucible is finally removed automatically from the instrument.

The inductar CS cube conforms with the international standards ASTM E1019 on “Standard test methods for determination of sulfur, nitrogen, carbon and oxygen in steel, iron, nickel and cobalt alloys by various combustion and fusion techniques”, ISO 15349-2 on “Unalloyed steel, determination of low carbon content, part 2: Infrared absorption method after combustion in an induction furnace” and ISO 15350 on “Steel and iron, determination of total carbon and sulfur content, Infrared absorption method after combustion in an induction furnace.”

Determination of Sulfur and Carbon in Steels

Calibration

The inductar software offers an intuitive multi-point calibration with appropriate reference materials. Multipoint calibration is employed instead of single-point calibration in order to include the influence of varied sample matrices on the calibration line. In the following example, a wide-range calibration was carried out using four certified reference materials (CRMs) of various carbon and sulfur concentrations. Thus, all CRMs were weighed to the nearest 1 mg into ceramic crucibles. Approximately 2 g W/Sn accelerator of a constant and low blank value was added to each. These samples were examined twice using varied sample amounts by the inductar CS cube.

The measurement response, the intensities of CO2 and SO2 signals of the two IR detectors, plotted over the analysis time is displayed in Figure 2. The software automatically calculates the calibration curve by plotting the resulting peak areas against the absolute carbon and sulfur masses of the CRMs, followed by a linear regression (see Figure 3). Calibration need not be carried out on a daily basis. It is possible to use the factory calibration curve over an extended period of time. Small day to day differences can be accounted for by applying a daily factor.

Linear wide-range calibration curves for carbon and sulfur

Figure 2. Linear wide-range calibration curves for carbon and sulfur.

CO2 and SO2 signals during the analysis

Figure 3. CO2 and SO2 signals during the analysis.

Analysis of Steel

In order to show the performance of the inductar CS cube, different steel samples were analyzed. The results are presented in Table 1.

The ceramic crucibles were preheated at 1100 °C for one hour in ambient atmosphere, which is particularly important for the analyzes of small carbon concentrations. A clean crucible tong was used for further handling.

The steel samples were cleaned in Acetone and then dried before amounts of 0.5 g to 1 g were weighed to the nearest 1 mg into the preheated ceramic crucibles. About 2 g W/Sn accelerator was added to help the combustion. The crucibles were placed on a free autosampler position and analysis was carried out.

The sample mass was then transferred automatically from the balance into the software before the analysis. The carbon and sulfur concentrations were calculated by the software using equations 1 and 2.

Equation 1 is the calibration line, calculated from the linear relation between the absolute carbon and sulfur mass of certified standards and the peak area in Figure 3.

     Eq (1) y = s • x + b
     Eq (2) c = (y - b) / (s • m)

The concentration (c) is calculated using equation 2 and is considered to be a function of the sample mass (m), peak area (y), slope of the calibration line (s) and value b, which is where the calibration line and y-axis intercept.

Table 1. Analyzes of the carbon and sulfur content of different steel types using the inductar CS cube.

SUBSTANCE C (%) S (%)
maraging steel

C: 0.0018 ± 0.0003
S: 0.0025 ± 0.0003
0.0020
0.0022
0.0021
0.0021
0.0021
0.0025
0.0026
0.0024
0.0024
0.0026
average
SD
0.0021
0.0001
0.0025
0.0001
sulfur steel

C: 0.181 ± 0.001
S: 0.126 ± 0.003
0.181
0.183
0.182
0.181
0.181
0.125
0.125
0.122
0.125
0.122
average
SD
RSD
0.182
0.001
0.6
0.124
0.002
1.5
carbon steel

C: 0.277 ± 0.009
S: 0.0111 ± 0.0006
1.286
1.288
1.285
1.281
1.286
0.0109
0.0114
0.0109
0.0115
0.0111
average
SD
RSD
1.285
0.003
0.2
0.0111
0.0003
2.5

SUBSTANCE C (%) S (%)
dynamosteel

C: 0.0043 ± 0.0004
S: 0.0029 ± 0.0004
0.0042
0.0041
0.0041
0.0042
0.0042
0.0031
0.0030
0.0031
0.0028
0.0029
average
SD
0.0041
0.0001
0.0030
0.0001
unalloyed steel

C: 0.0550 ± 0.0017
S: 0.0213 ± 0.0010
0.0537
0.0542
0.0541
0.0542
0.0539
0.0214
0.0212
0.0213
0.0215
0.0213
average
SD
RSD
0.0541
0.0002
0.4
0.0214
0.0001
0.5
unalloyed steel

C: 0.067 ± 0.0023
S: 0.336 ± 0.0081
0.0671
0.0671
0.0673
0.0666
0.0668
0.342
0.341
0.343
0.334
0.335
average
SD
RSD
0.0670
0.0003
0.4
0.339
0.004
1.2

Summary and Discussion

The concentrations of carbon and sulfur in different steels can be successfully determined through combustion method. Exceptional linearity with a multi-point calibration allows an accurate analysis of carbon concentrations in a range of a few ppm to 2%. The inductar CS cube elemental analyzer is perfectly ideal for quality control, process control or for the certification of steels due to its low requirements on sample homogeneity and preparation as well as its fast and accurate results.

The inductar CS cube conforms to the international standards ASTM E1019, ISO 15349-2 and ISO 15350.

The inductar CS cube elemental analyzer

CO2 and SO2 signals during the analysis

This information has been sourced, reviewed and adapted from materials provided by Elementar Analysensysteme GmbH.

For more information on this source, please visit Elementar Analysensysteme GmbH.

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