Accurate Determination of Trace Sulfur in Superalloys and Its Application

The accurate determination technology of trace sulfur in superalloys adopts the latest infrared carbon/sulfur analyzer CS844ES, and carries out technological innovation in five aspects: the three calibrations technology at both ends, the ultrapure composite flux, the reference material traceability technology, the blank influence value determination technology, and the criteria for the accurate determination of trace sulfur. The lower limit of sulfur measurement is extended to 0.5ppm. After appraisal, it has reached the international advanced level. Two reference materials of wrought superalloy GH4169 with trace Sulfur concentration were developed by this technology. Finally, the accurate determination technology of trace Sulfur in superalloys, independent intellectual property rights and trace sulfur reference materials constitute a “Trinity” technical system. This technical system was used to measure GH4169 superalloy in the United States and China at the same time, and the results showed that there are differences in sulfur content among them. This technology is mainly applied independently and has been applied in relevant units and won many technical awards.


Introduction
Sulfur is one of the most harmful impurities that negatively affect the properties and the structure of superalloys.For instance, it tends to cause hot brittleness and reduce mechanical properties, which adversely impact wear resistance, plasticity, and weldability.In particular, the sulfur combines with components in single-crystal superalloys into sulfides, leading to stress concentrations.Generally, sulfur in superalloys is controlled to be less than 0.0005% [1][2][3] .
As a consequence of the wide application of the triple smelting technology, the content of sulfur in high-quality superalloys is already below 5 ppm [4] .In order to measure the low content of sulfur(<5ppm), continuous efforts have been made.
The trace sulfur content in superalloys was measured by glow discharge mass spectrometry (GDMS) [5-7]   .Unlike high-purity silicon, superalloys are very complex alloy systems that interfere heavily with each other during measurement and rely on standards of the same base.Therefore, glow discharge mass spectrometry (GDMS) is only semi-quantitative for superalloy traces.
Japanese scholars established a method for low content of sulfur in steel and high-purity iron as 1 ppm [8][9][10] by employing a LECO CS-444 LS carbon/sulfur analyzer with an induction furnace, fluxing aid and crucible heating at ambient atmosphere to 1003 K to remove sulfur blank as well as the measure of acid washed samples.However, the base of superalloys is nickel, which is different from the situation in pure iron and steel.Owing to its high calorific value and magnetism, which is conducive to high frequency induction, iron itself is a flux.The high-frequency induction combustion conditions differ when the bases change, resulting in obvious deviation of measurement results [11][12][13] .Therefore, the measuring method for trace sulfur amount in high-purity iron and steel cannot be used in measuring the content of trace sulfur in superalloys.
High frequency infrared carbon sulfur analyzer is generally utilised for the analysis of sulfur content in superalloys, and analysis methods of different sequences have been established, such as international standard ISO, American Society for Testing and Materials ASTM, national standard GB, aviation industry standard HB, Aeronautical Standard AETM, etc.The lower limit of these analysis methods for measuring sulfur in superalloys is only 0.0005%, that is, 5 ppm.
Song and Wang, et al [14,15] .reduced and stabilized the sulfur blank by using medical oxygen, W-Sn flux, crucible pretreatment, and achieved the measurement of sulfur content in Ni-base superalloy from 0.0001% to 0.0005%.Wei, et al [16].established a new method to analyze ultra-low sulfur (S: 0.0001%～ 0.0020%) in powder metallurgy superalloy FGH96 under combustion conditions, blank test, and other relevant test conditions.Rong, et al. [17] reduced the lower limit of sulfur in tungsten grains to 1 ppm by improving the circuit and gas path of the carbon sulfur instruments and correcting the analyzer by testing different content of sulfur.Lawrenz [18] used a CS444LS trap to determine the sulfur amount in superalloys as low as 0.0000 4%.Mekhanik et al. [19] used W-Sn flux and tin flux to determine 0.0001%~0.0009%sulfur in superalloys.Kinoshiro [20] invented a device for the determination of sulfur by high-frequency combustion ultraviolet fluorescence spectrometry.The device was verified by certified reference materials, and the detection limit could be as low as 0.0000 5% (mass fraction).The accuracy of the related reports above is difficult to be effectively guaranteed because there is no verification of the same base reference materials [21] .
Based on domestic and foreign literature, this technology adopts the latest infrared carbon/sulfur analyzer CS844ES to provide technological innovation in five aspects: the three calibrations technology at both ends, the ultrapure composite flux, the reference material traceability technology, the blank influence value determination technology, and the criteria for the accurate determination of trace sulfur.The lower limit of sulfur measurement is extended to 0.5 ppm.It has been appraised to be at the international advanced level.The technology has independent intellectual property rights, including patents, standards, papers, development technology reports, etc.Two reference materials of wrought superalloy GH4169 with trace sulfur concentration have been developed using this technology.Finally, it has formed a "trinity" of wrought superalloys trace sulfur determination technology, independent intellectual property rights, and trace sulfur reference materials.The technology is mainly self-applied and used for the determination of sulfur in various superalloys, and has been applied in relevant units and won several technical awards.

Experimental Instrument
The infrared carbon/sulfur analyzer CS844ES is produced by American LECO.Compared with the previous models of LECO, such as CS-444 and CS600, several technical improvements have been made, such as the improvement the automatic cleaning device [22] , the improvement of the furnace head thermostatic heating device, which effectively reduces the adsorption of ash on sulfur dioxide [23] .At the same time, the sensitivity mode for sulfur measurement has been enhanced [24], with the carrier gas flow to be 0.8 L/min (in the standard mode: 3.0 L/min), and the analysis time to be 130 seconds (in the standard mode: only 40 seconds).Due to the slowing down of the carrier gas flow, the impurity removal function of the water removal device between the combustion furnace and the detector is fully exerted [25]   .The improvement of the technologies above significantly strengthened the ability of sulfur content measurement.

The Three Calibrations Technology at Both Ends--the Core of the Technological Innovation
This technology has been protected by an authorized patent.For the specific calibration process, please refer to the patent text [27] .Here, only the calibration process with examples in the patent will be simply describe.
The reference material AR-673(S: 0.0011%±0.0002%)wasused to calibrate the infrared carbon/sulfur analyzer CS844ES.The calibration allowable range is 0.0009 to 0.0013.The results of the three calibrations are shown in Table 1.From performing the three times of calibrations, the values after each calibration fall within their allowable differences.Seeing the data after the first calibration, the technicians would naturally stop at the first calibration and not go through the time-consuming and meaningless second and third calibrations.
After carrying out the first blank deduction, the analysis gave a second average value of 0.00102 for the high content sulfur standard.From the average value, there was a drop of 0.0011 0 -0.0010 2= 0.0000 8.The rest results were got applying the same calculation method.It is the seemingly slight drop that is overlooked by those skilled in the art in a conventional way.The slight decline inspired the innovation based on the prior art.

The Ultrapure Composite Flux--A Material Basis for the Realization of This Technology
The sulfur blank value of the common fluxes is labelled as S≤0.0005% and the actual measured value is S≈0.0005%.The sulfur blank content is higher than the trace sulfur amount in superalloys, and therefore the common fluxes cannot be used for the measurement of trace sulfur.
On the basis of the flux technology of Rong, et al [28][29][30] , combined with the measures of heating the flux at 1003K for an appropriate time and reducing the sulfur blank in the literature [8][9][10] , the ultra-pure composite flux and its application technology [31][32] were developed with a sulfur blank value S ≤ 0.0000 3%, i.e, S ≤ 0.3 ppm.
The ultra-pure composite flux was weighed and the Sulfur blank value was measured directly by infrared carbon/sulfur analyzer CS844ES.Four times of measurements were taken in total.The results are shown in Figure 1.As can be seen from Table 2, the average value of the ultra-pure composite flux sulfur blank was 0.21 ppm, with extreme values less than 0.23 ppm, achieving the degree of homogeneity of the reference material.From figure 1 it can be seen that the four measured sulfur release maps superimposed well.

The Reference Material Traceability Technology--the Key to Overcome the Lack of Reference Materials.
It involved several aspects during chemical measurement processes, and thus cannot be directly traceable to SI units.When traceability to national or international measurement benchmarks is not possible, the reliability of the measurement results can be demonstrated in other ways [33][34] .Paragraph 4.7 from <CNAS-CL01-G002-2021 Requirements on the Metrological Traceability of Measurement Results>states that it can be traced to an appropriate benchmark when traceability to SI units is not technically possible.
This technique refers to the relevant literature [35][36][37] for traceability confirmation using a benchmark potassium sulfate reagent.

Formulation of benchmark potassium sulfate solutions
The sulfur content in the benchmark potassium sulfate reagent is 18.4005%.1.6304 g of benchmark potassium sulfate powder was weighed accurately in a 150ml beaker, dissolved in boiled secondary distilled water and diluted to 1000ml.This solution contained 0.3000 g sulfur at a concentration of 0.3 mg/ml.And then 20 ml (S: 6 mg) solution was transferred into a 1000 ml volumetric flask and diluted to 1000ml, which was at a concentration of 0.006 mg/ml.A certain mass of the solution was weighed into a tin sac and dried out.And then the mass of the powder was measured.Table 3 shows the results of the measurements.After traceability confirmation, most of the sulfur-containing standards met the traceability requirements, while a small number of sulfur-containing standards, such as ECISS 273-1, deviated too much from the measured values and did not meet the traceability requirements.

The Blank Influence Value Determination Technology--Preconditions for Trace Sulfur Measurements
Determination of blank values, as well as subtraction, is one of the key techniques to guarantee the accuracy and precision of trace sulfur measurements.Blank tests [38][39][40][41] are crucial for improving the authenticity and accuracy of low content sulfur measurements.The success of the measurements of low content Sulfur depends overwhelmingly on the measurement of blanks, and the more successful the measurement of blanks, the higher the credibility of the specimen measurement results.Three methods have been developed for measuring blank values: direct measurement, indirect flow measurement, and cyclic measurement.Especially, the cyclic measurement method created by Wei, et al [42][43][44] is particularly advanced.
The measured blank influence value is closer to the zero point by using a homogeneous flux with a lower blank, a crucible with a lower blank, as well as the carbon/sulfur analyzer with a lower detection limit.The blank influence value determination procedure can be found in the patent text [27] and is only briefly described here in a tabular form 2.5g of the ultra-pure composite flux was conducted in the experiment and was recognized as 1.0000g inputting into the calculation process.Below is the corresponding sulfur blank measurements.
As can be seen from table 4, the maximum of the blank influence value obtained by the third subtraction of blank is only 0.1 ppm, which is relatively low to 0.5 ppm.

The Criteria for the Accurate Determination of Trace Sulfur--Judgment Basis for Determination of Trace Sulfur
The blank influence value < 1/5 Value to be measured [27][45] .The measurement of trace sulfur (0.0000 5%~0.00050%) is affected by a number of factors.Extensive operational experience is required.Failures tend to occur if any part is not handled properly.
For example, conventionally adding fluxes will make the measurement fail.The reason is the operators generally use a spoon to add the W-Sn and iron flux and the mass of one spoon of W-Sn flux is approximately 1.5-2.0g,while 0.5-1.0gfor iron flux (lower density).That is, the total mass of the two fluxes fluctuates between 2.0g and 3.0g, leading to the inaccuracy of the measurement.Another example is that the blank value of sulfur in common W-Sn flux is S: ≤0.0001%, and S ≤ 0.0005% in common iron flux.The content of the sulfur blank in W-Sn is equal to the sulfur amount in the sample to be tested, which means the common flux cannot be applied.Moreover, the common carbon/sulfur analyzer with poor sensitivity and the blank influence value > 2ppm is not qualified for measuring the trace sulfur amount.
There are a large number of factors that affect the measurement, and the magnitude of the effect as well as the direction also varies.It shows the weak effects when measuring the conventional content (S ＞0.0005%).However, these effects become significantly severe at 0.0005%.Accordingly, in this technique, it is innovative that quantifiable blank influence values are measurable, which is hard for one skilled in the art to imagine.
Once the blank influence values do not meet the requirements, the operators will work on parameter optimizations in the operational steps, fluxing blanks, and analyzers until the blank influence values meet the technical requirements, which is < 1 / 5 of the values to be measured.
Confirmation of lower detection limit: 0.5 ppm This technology extends the lower detection limit for measuring sulfur from 0.0001 % to 0.00005 %.Although it is a modest improvement numerically, there is a great technical difficulty to overcome.The existing technology at home and abroad can only reach 0.0001 %.It is the innovative advance in this technology that the requirement for accurate sulfur determination in the triple smelting technology can be met.  5. the results of the empty run of the carbon/sulfur analyzer %

Measurement value
Average value Maximum value 0.0000 0022 -0.0000 0302 0.000 0482 -0.0000 0201 0.0000 0000 0.0000 482 It is stated that the carbon/sulfur analyzer is under a stable condition with a maximum value of 0.0000 048, that is, the lower limit of sulfur measurement of carbon/sulfur analyzer is: S ≈ 0.05 ppm.
The lower limit of sulfur measurement by CS844ES (0.05ppm) is one tenth of that in this technology (0.5ppm), which is well within the tolerance of the carbon/sulfur analyzer.

The Traceability Technology of Sulfur Content(0.5ppm)
Benchmark reagents potassium sulfate (the content of sulfur is 18.4005%) were used to prepare the solution.And the solution was weighed and dried out (sulfur content concentration:6.0ppm).The measurement results are shown below.As can be seen in Table 6, the target sulfur content was 0.50 ppm, and an average of 0.46 ppm could be measured, meeting the accurate detection of the sulfur amount which is 0.5 ppm.In figures 3 and 4, the release curves of 0.5 ppm, compared to the benchmark (0 ppm), have obvious sulfur release peaks, which means that the carbon/sulfur analyzer can accurately distinguish between 0.5 ppm and 0.0 ppm.

Practical Measurements of the Deformed Superalloy GH4169 Using this Technique
Because the amount of sulfur in the high purity GH4169 is around 1 ppm and belongs to the trace level, the conventional sulfur measurement method has not been applicable.This technique must be used to accurately determine the sulfur content.
Using this technology, GH4169 with high purity was measured from different manufacturers at home and abroad.The results are presented in Table 9.According to table 9, it can be seen that the sulfur content in both ATI USA and domestic high-purity GH4169 is extremely low, around 1.0ppm.but there are differences in sulfur content among them.
"Trinity" technical system to measure the content of trace sulfur This technology owns independent intellectual property rights, including one authorized patent, one enterprise standard, several papers and several technical reports.
This technology was applied to develop two high-purity alloy GH4169 reference materials, and the sulfur amounts are: High purity alloy GH4169 spectral reference material, S: 0.0000 50%±0.000032% High purity alloy GH4169 chemical reference material, S: 0.0000 35%±0.000026% Finally, the accurate determination technology of trace sulfur in superalloys, independent intellectual property rights and trace sulfur reference materials constitute a "Trinity" technical system.

Identification, Application and Awards of This Technology
In December 2021, the Quality Science and Technology Department of the China National Aero Engine Group held a technical appraisal, which identified the precise determination of trace sulfur technology as reaching the "internationally advanced level".
This technique is mainly applied in our corporation to detect the sulfur content in various types of superalloys and develop highly purealloy GH4169 reference material.
In July 2021, this technology stood out from 416 entries and was awarded the third prize when participating in the third China General Aviation Innovation ＆ Entrepreneurship Competition.

Conclusion
The accurate determination technology of trace sulfur in superalloys adopts the latest infrared carbon/sulfur analyzer CS844ES, and carries out technological innovation in five aspects: (1) The three calibrations technology at both ends--the core of this technological innovation.
(2) The ultrapure composite flux--the material basis for the realization of this technology.
(3) The reference material traceability technology--the key to overcome the lack of reference materials.
(4)The blank influence value determination technology--preconditions for trace sulfur measurements.
(5) The criteria for the accurate determination of trace sulfur--judgment basis for determination of trace sulfur.
The lower limit of sulfur measurement is extended to 0.5ppm.After appraisal, it has reached the international advanced level.Finally, the accurate determination technology of trace Sulfur in superalloys, independent intellectual property rights and trace sulfur reference materials constitute a "Trinity" technical system.
This technical system was used to measure GH4169 superalloy in the United States and China at the same time, and the results showed that there are differences in sulfur content among them.

7 Figure 2 .
Figure 2. The sulfur release curve of the empty run of the carbon/sulfur analyzer Table5.the results of the empty run of the carbon/sulfur analyzer %

Figure 3 .
Figure 3. Release profile for potassium sulfate 0.47 ppm

Table 1 .
The results of the three calibrations %

Table 2 .
the measurements of the sulfur blank in the ultra-pure composite flux % Figure 1.Overlay of four measured Sulfur release maps

Table 3 .
Result of traceability experiment

Table 4 .
The measurement process of blank influence value determination

Table 7 .
Measurement data for blank validation

Table 8 .
Measurement results of GH4169 from different corporations

Table 9 .
Measurement of sulfur in high purity GH4169 produced by different manufacturers