Vacuum vs argon technology for hydrogen measurement

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Vacuum Vacuum vs vs argon argon technology technology for for hydrogen hydrogen measurement measurement a on Fracture, PCFb,c,∗ b,c 2016, Paço de Arcos, b,c XV Portuguese Conference 2016, February A. M. Polyanskiy , K.10-12 P. Frolova a , V. A. Polyanskiyb,c,∗ b,c , Yu. A. Yakovlevb,c Portugal

A. M. Polyanskiy , V. A. Polyanskiy a

, K. P. Frolova , Yu. A. Yakovlev

RDC Electron & Beam Technology, Ltd., Bronevaya str. 6, St. Petersburg 198188, Russia a RDC Electron & Beam Technology, Ltd., Bronevaya str. 6, St. Petersburg 198188, Russia b Peter the Great St. Petersburg Polytechnic University, Polytekhnicheskaya str. 29, St. Petersburg 195251, Russia b Peter the Great St. Petersburg Polytechnic University, Polytekhnicheskaya str. 29, St. Petersburg 195251, Russia c Institute of Problems of Mechanical Engineering RAS, V.O. Bolshoy pr. 61, St. Petersburg 199178, Russia c Institute of Problems of Mechanical Engineering RAS, V.O. Bolshoy pr. 61, St. Petersburg 199178, Russia

Thermo-mechanical modeling of a high pressure turbine blade of an airplane gas turbine engine Abstract P. Brandãoa, V. Infanteb, A.M. Deusc* Abstract a Within the framework of this paper, we review the development of theUniversidade problem ofdehydrogen diagnostic for metals. Metal sample Department of Mechanical Engineering, Instituto Superior Técnico, Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Within the framework this on paper, we review the extraction development of the problem of hydrogen diagnostic for metals. Metal sample enrichment techniques of based hydrogen vacuum method Portugalused for a long time. Development of the industrial control b enrichment on hydrogen vacuum extraction method usedUniversidade for a long Development thea 1, industrial control IDMEC,techniques Department of Engineering, Instituto Superior Técnico, de Lisboa, Av. Rovisco 1049-001 Lisboa, technologies has led tobased theMechanical almost complete replacement of vacuum techniques with time. “atmospheric” ones.ofPais, As result systematic technologies has led to the almost complete replacement of vacuum techniques withmeasured “atmospheric” ones.concentration As a result systematic Portugal errors have occurred. These errors lead to multiple differences between certified and hydrogen values for c CeFEMA, Department of Mechanical Superior Técnico, Universidade de Lisboa,hydrogen Av. Rovisco Pais, 1, 1049-001 Lisboa, errors have occurred. These errors leadEngineering, to multipleInstituto differences between certified and measured concentration values for standard samples. Portugal standard samples. In this paper, we analyze reasons of systematic errors genesis observed for hydrogen measurements while applying the thermal In this paper, analyze reasons of systematic errors genesis observed for hydrogen applying the thermal conductivity cellwe technique. As a result, we demonstrated that measurements resulting measurements from samples while heating and melting in the conductivity cell technique. As a result, we demonstrated that measurements resulting from samples heating and melting in the inert gas flow depend on its heat capacity and surface temperature of the melting pot. Due to this reason, one can obtain multiple Abstract inert gas flow depend on its heat capacity and surface temperature of the melting pot. Due to this reason, one can obtain multiple errors and even negative values for measurements of a low hydrogen concentration. errors and even negative values for measurements of a low hydrogen concentration. During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, c especially  2018The TheAuthors. Authors. Published by Elsevier © 2018 Published byturbine Elsevier B.V. B.V. the high pressure (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent c 2018 The under  Authors. Published by Elsevier B.V. Peer-review responsibility of ECF22 the organizers. Peer-review under responsibility the organizers. degradation, one of which isofcreep. AECF22 model using the finite element method (FEM) was developed, in order to be able to predict Peer-review under responsibility of the ECF22 organizers. the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation Keywords: diagnostic; hydrogen analyzer; extraction in data the inert flow; thermal conductivity cellIn order to create the 3D model company,hydrogen were used to obtain thermal and mechanical for gas three different flight cycles. hydrogen diagnostic; hydrogen analyzer; extraction in the inert gas flow; thermal conductivity cell Keywords: needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a can be useful in the goal of predicting turbine blade life, given a set of FDR data. 1.model Introduction

1. Introduction

© 2016 The Authors. Published by Elsevier B.V. In 1961 Fremy theofresults of investigation the hydrogen in iron. He found that steel can be Peer-review under published responsibility the Scientific Committeeof PCF 2016. occlusion In 1961 Fremy published the results of investigation ofofthe hydrogen occlusion in iron. He found that steel can be

obtained from iron alloys only if hydrogen is removed Fremy (1861). Nevertheless, hydrogen had been often used for obtained from iron alloys only if hydrogen is removed Fremy (1861). Nevertheless, hydrogen had been often used for improvement ofPressure the steel quality forCreep; the next years Method; Gesner (1901). Keywords: High Turbine Blade; Finite40 Model; Simulation. improvement of the steel quality for the next 40Element years Gesner3D (1901). Appearance of discontinuities in rolling, in other words, flocs, was one more problem at the beginning of the 20th Appearance of discontinuities in rolling, in other words, flocs, was one more problem at the beginning of the 20th century. Initially, their formation was explained by the iron hydroxides presence Bernhard (1915). Later it was found century. Initially, their formation was explained by the iron hydroxides presence Bernhard (1915). Later it was found out that the reason both of the flocs formation and decrease of the impact toughness resulting in the steels brittleness out that the reason both of the flocs formation and decrease of the impact toughness resulting in the steels brittleness is dissolved hydrogen Keiichi (1938) accumulating in the liquid metal in the open-hearth furnace. is dissolved hydrogen Keiichi (1938) accumulating in the liquid metal in the open-hearth furnace. Application of hydrogen diagnostics in the industry appeared at the beginning of the forties de Haas and Hadfield Application of hydrogen diagnostics in the industry appeared at the beginning of the forties de Haas and Hadfield (1934); Zapffe and Sims (1941). Nowadays measurements of the hydrogen concentration is performed serially for (1934); Zapffe and Sims (1941). Nowadays measurements of the hydrogen concentration is performed serially for Corresponding author. Tel.: +351 218419991. ∗* Corresponding author. Tel.: +7-921-748-0637; fax: ∗ Corresponding E-mail address: [email protected] author. Tel.: +7-921-748-0637; fax:

+7-812-321-4771. +7-812-321-4771. E-mail address: [email protected] E-mail address: [email protected] 2452-3216 2016 The Authors. Published Elsevier B.V. c© 2210-7843  2018 The Authors. Published byby Elsevier B.V. cunder 2210-7843  2018 The responsibility Authors.of Published by Scientific Elsevier B.V. Peer-review under of the Committee of PCF 2016. Peer-review responsibility the ECF22 organizers. Peer-review under responsibility of the ECF22 organizers. 2452-3216  2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. 10.1016/j.prostr.2018.12.293

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quality control of all kinds of metals and alloys. The influence of hydrogen on the mechanical properties of metals is studied in detail. Dozens of works devoted to this problem are published every year. An important feature of the industrial research is that the critical levels of hydrogen concentration or content are very low, only 4 million parts of the hydrogen mass fraction lead to the flocs formation. Therefore, vacuum methods of charging the sample Jordan and Eckman (1926); Brown (1945); Scafe (1945) were widely used, since all gases evolved from a heated in vacuum metal sample were pumped into a calibrated volume, then pressure was measured in this volume, and, finally, a component analysis of the gas mixture with hydrogen was perfomed. The main lack of vacuum methods was the long duration of the measurements. Manufacturers of industrial measuring equipment proposed to use a fast melting method in a inert carrier gas flow for measuring hydrogen concentration. To measure the hydrogen concentration in the carrier gas, it was proposed to use the thermal conductivity cell Stewart and Squires (1955); Hulsberg (1954); Willenborg (1941) developed earlier for chromatographs Olof (1927); Willenborg (1936). The operationg principle of this detector is based on the measurement of the differential difference in the thermal conductivity of a pure inert carrier gas and a carrier gas mixed with hydrogen released from a metallic sample upon its heating. Reducing the duration of hydrogen analysis from one hour to ten minutes was crucial for the industry. Cheaper and faster “atmospheric” analyzers have displaced vacuum ones from factory laboratories. Nowadays about a dozen different “atmospheric” analyzers of hydrogen and the only one vacuum analyzer are produced serially. The lack of an alternative in the industry gradually led to the fact that metrological certifications, calibrations and scientific research started to be carried out mainly by applying “atmospheric” analyzers. It turns out that regular interlaboratory comparisons realized by certified laboratories are not carried out with regard to hydrogen. The measuring data of hydrogen concentration obtained by different laboratories diverge several times ´ et al. (2018), especially for standard samples with a hydrogen concentration less Nolan and Pitrun (2004); Konopelko than 1 ppm Hassel et al. (2013). A wide range of measurement results is also observed for aluminum alloys Andronov et al. (2017). Apparently, there is an unaccounted reason for the occurrence of significant errors in the measurement of small concentrations of hydrogen by using modern analyzers. One of the possible reasons is the hydrogen detector. In this regard in this paper we analyse the detector operation. 2. Analysis of the hydrogen detector operation At the present time, two basic principles of detection are widely used in “atmospheric” hydrogen analyzers. The older one is the classical thermal conductivity cell. Some analyzers use hydrogen oxidation on a cell obtained from cuprous oxide to water, followed by detection of water in a carrier gas by an infrared sensor. Infrared method has a large number of stages, problematic in terms of the occurrence of systematic measurement errors. Thus, standard argon or nitrogen of special purity, which is used as a carrier gas, contains about 10 ppm of water, hydrogen, hydrocarbons that can not be removed by industrial methods. In this regard, it is difficult to expect a 100% removal of these gases inside the analyzer before hydrogen treatment on copper oxide, and the level of “useful” hydrogen concentration has the same order of 5-20 ppm. An additional experimental analysis of the most important stages of treatment of the carrier gas is required to take into account possible errors. Let us consider the operation of a simpler classical detector, i.e. the thermal conductivity cell. Its scheme is shown in Fig. 1. The thermal conductivity cell is a measuring bridge, exposed by constant reference voltage U. The current, the value of which depends on the precision resistors 1 and 2, heats the platinum sensitive resistive elements 3 and 4. Cooling of platinum elements is carried out by blowing by a clean carrier gas and a carrier gas with hydrogen released from the metal sample when it is heated. The crucible and a sample can be heated with either without contact by means of high-frequency current or by electric current by applying a special contact electrode. An approximate scheme of the extraction chamber is shown in Fig. 2. After a metal sample has been blown in the extraction chamber, the carrier gas is fed to a purification from various substances that are released along with hydrogen and can contaminate the sensitive platinum unit of the thermal conductivity cell. Heat exchange of the gas with the powdered sorbent takes place during the purification process. Glass tubes with a sorbent are placed on the front panel of the hydrogen analyzers. It is believed that the temperature

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3

U

1

2 u

+ Ar H2

Ar

H2

Ar

4

+ Ar

3

Fig. 1. Schematic of the thermal conductivity cell.

of the gas mixed with hydrogen is equalized with the temperature of the carrier gas that passes through the same sorbent located in the nearby tube. After purification by the sorbent gases are fed to the detection thermal conductivity cell (see Fig. 1), which actually measures the potential difference in the diagonal of the electrometric bridge. This difference is linearly related to the difference in the mean temperatures of platinum elements. The hydrogen analyzers record the dependence of the potential difference on time. This function becomes an extraction curve after calibration. The area between the extraction curve and the background line is equal to the amount of released hydrogen. For calibration, either a standard sample is used or hydrogen is introduced from the calibrated volume through the porous membrane into the carrier gas. The hydrogen pressure in the calibrated volume is known in advance. One can estimate the average temperature difference between the sensing elements, which occurs while realizing the standard measurement of the hydrogen concentration. The electric power in both sensitive elements of the thermal conductivity cell is dissipated into the gas that blows it and can be written as: qIAr = KAr S Pt (T PtAr − T Ar )   qIΣH2 = KΣH2 S Pt T PtΣH2 − T ΣH2 ,

(1)

where lower subscript Ar refers to the characteristics of the flow of the pure carrier gas (argon), lower subscript ΣH2 refers to the characteristics of the flow of the mixture of a carrier gas with hydrogen; qI is the electric power of the current passing through the helix of the thermal conductivity cell; K is the coefficient of heat transfer; S Pt is the area of the blown surface of the platinum element; T Pt is the average temperature of the platinum element, T is the temperature of gas or gas mixture. In the case of a dynamic gas flow, the thermal conductivity cell measures the difference in the heat transfer coefficient based on the heat transfer from the platinum sensitive element heated by the electric current to the gas flow that blows it, and does not measure directly the thermal conductivity. Inside the thermal conductivity cell, the gas flow is laminar. The heat transfer coefficient reads: λ K = 0.5 (Gr · Pr)0.25 , d

(2)

Fig. 2. Schematic of the extraction chamber of a hydrogen analyzer. 1 - metal specimen, 2 - crucible (usually graphite), 3 - quartz glass tube, 4 grounded pedestal for the crucible, 5 - tube and pedestal sealsl.

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where Gr is the Grasshoff number, Pr is the Prandtl number, λ is the coefficient of thermal conductivity, d is the characteristic channel size. The characteristic size of the channel can be obtained the following way (3): d=

S , P

(3)

where S is the channel cross-sectional area, P is the perimeter of the channel cross-section. The Grasshoff number and the Prandtl number can be found by applying Eqns. (4, 5), respectively:   gd3 T − T gs Gr = 2 , T gs ν Pr =

(4)

αC , λ

(5)

where g is the acceleration of gravity, ν is the coefficient of kinematic medium viscosity, T − T gs is the temperature difference between the heated or cooled object and the medium, T gs is the cooling gas temperature, α is the coefficient of dynamic gas viscosity, C is the gas heat capacity. By applying all the coefficients characterizing the flow and given by Eqns. (3) –(5), we can rewrite Eqn. (2) to calculate the heat transfer coefficient in the following way: K = 0.5

0.25   0.25   λ αC gd3 1 ˜ gs T − T gs 0.25 . = k T − T gs d λ ν2 T gs

(6)

To determine the temperature difference between a helix, blown by a flow of pure argon, and a helix blown by a flow of a mixture of argon with hydrogen, we take into account that qIAr = qIΣH2 = qI , T Ar = T ΣH2 = T gs . Then, the electric power dissipated by the gas in both sensitive platinum elements can be found by applying Eqn. (7) for k˜ gs = k˜ Ar , T Pt = T PtAr and k˜ gs = k˜ ΣH2 , T Pt = T PtΣH2 :  1.25 qI = k˜ gs S Pt T Pt − T gs .

(7)

The equation for the temperature difference between the helices of the thermal conductivity reads: ∆T = T PtAr − T PtΣH2 =



1.25

qI − ˜kAr S Pt



1.25

     ˜ k qI Ar 1.25  . = (T PtAr − T Ar ) 1 −  k˜ ΣH2 S Pt k˜ ΣH2 

(8)

Let us use the following values of the parameters under normal conditions (according to specific measuring in3 struments): volumetric flow rate WAr = 0.26 dm min , analysis time t = 5 min. Then the maximum volume concentration of hydrogen in the carrier gas is QV H2 = 51.3 vppm for its concentration in the metal equal to 0.5 ppm and for the sample mass m sp = 5 . We can use a simple superposition for the coefficients due to the smallness of the hydrogen concentration.   k˜ ΣH2 = QV H2 k˜ H2 + 1 − QV H2 k˜ Ar .

(9)

The operating temperature of the platinum element is a closed parameter. Obviously, the higher the temperature, the greater the sensitivity of the cell to the hydrogen concentration. Let us take the limiting values 20◦C and 600◦C for the temperature of the gas medium; 700◦C and melting point 1770◦C for the operating temperature of the platinum sensing element. In this case the maximum temperature difference according to Eqn. (8) is ∆T = 0.19◦ C.

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3. Discussion The carrier gas and the carrier gas with hydrogen released from the sample are conditioned before being supplied to the thermal conductivity cell. Since gases inside the analyzers are flowing, the conditioners are, in fact, heat exchangers, and the heat exchange between the gas medium and solid surfaces is slow. So, it is extremely difficult to ensure the conditioning of the gas flow with the accuracy of maintaining the temperature about fraction of degree. Where can the difference in the temperatures of gas flows come from? It is obvious that the sample and the crucible, heated up to a high temperature, will heat the carrier gas, which flows in direct contact with the walls of the crucible (see Fig. 2). The amount of heat absorbed by the gas depends on the level of the crucible walls grayness and the level of the quartz glass tube inner walls grayness, which tend to be contaminated with metal sprays and vapors released when the samples are heated to a high temperature. Calculation can be based on the concept of the heat capacity of the crucible, sample and carrier gas flowing near the sample during the analysis of hydrogen concentration. The evaluation shows that even if the carrier gas takes only a half of the energy spent to heat the sample, the carrier gas will overheat hundreds of degrees. The empty crucible will heat the gas about two to three times less than the crucible with the sample, due to approximately two to three times less heat capacity. In this regard, the “blank experience”, which would allow to set the background values of hydrogen, is impossible. Hydrogen is contained even in the degassed metal either inside or on the surface GOST (1999). Therefore, it is impossible to produce a “zero” metal sample with zero hydrogen conctent. This point of view is confirmed both in our Polyanskii et al. (2011) and in independent studies Hassel et al. (2013). In our work Polyanskii et al. (2011) we described the measurements of the hydrogen content in 54 calibration samples carved from one hydrogen content standard of the aluminum alloy D16. The measurements were carried out in a certified factory laboratory by a certified hydrogen analyzer RH402 (LECO manufacturer). Three measurements demonstrated zero result and one demonstrated negative concentration of dissolved hydrogen. On the one hand, this means only 8 % of unsuitable measurements, which must be discarded. On the other hand, the negative concentration in the measurement protocol requires explanation. Our analysis of the thermal conductivity call operation makes it possible to give such explanations. The “failure” of the extraction curve below the background can be caused by the closer fitting of a particular sample to the crucible. Because of this, the heat transfer process of the gas will go faster and the gas will warm up a little more. It is also possible that crucibles have different graying coefficients. Because of this, the relative proportions of the heat emitted and transferred to the carrier by the crucible will be different, and, consequently, the gas will be heated differently by heat transfer from the surface of the crucible. This effect was observed by independent researchers. Experimental measurements of hydrogen concentration in empty crucibles, probably obtained by using the Juwe H-mat 221 analyzer are described in Hassel et al. (2013). The report does not specify exactly what analyzer was used, but among the “atmospheric” analyzers listed there, only the H-mat 221 possessed the sensitivity necessary for measurements. It is important that all the extraction curves in Hassel et al. (2013) become negative when the empty crucible is heated. They are located at a different value below the initial background. This value depends on the frequency of the crucible using. Thus, the corresponding integral with respect to the background gives a negative value. This result confirms our estimates and assumptions. There is one more consideration explains the numerous problems that arise when “atmospheric” hydrogen analyzers are operated. The relative order of the measured quantity is 10−6 . Moreover, the situation can not be fundamentally changed while extraction of hydrogen into the carrier gas, since the mass of the carrier gas mass and the standard sample mass are comparable, for example, 1g and 5g. Our estimation allow us to explain the wide scatter observed when measuring the hydrogen concentration in certi´ et al. (2017). fied standard samples in various certified laboratoriesKonopelko 4. Conclusion In this paper we analyzed the reasons of the occurrence of systematic errors in measuring the hydrogen concentration in metals by applying modern equipment. These errors can be seven to eight times more than the real concentration.

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It is shown that the principle of hydrogen detection, based on its high thermal conductivity in the gas phase, does not allow to avoid the influence of many environmental factors, sample composition, the crucibles state and conditions of measurement on its result. It is shown that this influence can be decisive and significantly change the measured value of hydrogen concentration, especially when hydrogen concentration is less than 1 ppm. Thus, the conclusion of the fundamental report of the European program Hassel et al. (2013) that there are no reliable standard samples with a certified hydrogen content less than 1 ppm must be supplemented with the conclusion that the “atmospheric” hydrogen analyzers are not suitable for measurements in metal samples with a hydrogen concentration less than 1 ppm. Acknowledgements This research was supported by Siemens AG References Abarca, A. N., 2015, High Precision Flow Compensated Thermal Conductivity Detector for Gas Sensing with Read-out Circuit: Master thesis. – Delft, 97p. Patent 695,264 U.S. Process of manufacturing steel, T. Andrew, K. B. Thomas, published 11.03.1902 Andronov, D. Yu., Arseniev, D. G., Polyanskiy, A. M., Polyanskiy, V. A., Yakovlev, Yu. A., 2017, Application of multichannel diffusion model to analysis of hydrogen measurements in solid, International Journal of Hydrogen Energy, 42(1), 699–710 Bernhard, O., 1915, Leitfaden f¨ur Giessereilaboratorien – Berlin, Heidelberg: Springer, 44p., ISBN 978-3-662-40626-7 Patent 2,387,878 U.S. Apparatus for determining hydrogen in steel / W. D. Brown, published 30.10.1945 de Haas, W. 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E., 1941, Hydrogen embrittlement, internal stress and defects in steel, Trans. AIME. 145, 225–271