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Investigation of impurities detected in X-ray inspection of MPG-7 fine-grained graphite blanks and details

https://doi.org/10.38013/2542-0542-2017-4-80-85

Abstract

The purpose of the research was to study the structure of MPG-7 fine-grained graphite by X-ray inspection and scanning electron microscope investigation methods. We carried out a structural and local chemical analysis of inhomogeneities occurring in graphite blanks. According to the data obtained, we changed technical documentation for the incoming quality control of MPG-7 graphite blanks and details.

For citation:


Vershinin A.V., Vershinina M.V., Belyakova E.G., Polyakov E.V., Bamburov V.G., Volkov I.V. Investigation of impurities detected in X-ray inspection of MPG-7 fine-grained graphite blanks and details. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2017;(4):80-85. https://doi.org/10.38013/2542-0542-2017-4-80-85

Introduction

Graphite-based structural materials are widely used in the design and manufacture of nozzle clusters for solid-propellant rocket engines (SPRE) [1],[2]. Such choice of material is primarily due to the ability of graphite to withstand the effects of high-temperature gas flow of erosive and aggressive combustion products of composite propellant. The most heat-stressed part of SPRE is the nozzle cluster throat. As the material for manufacturing nozzle throat inserts in SPRE designs, polycrystalline graphite is used [3]. The MPG-7 graphite used in the products of Novator Design Bureau is a fine-porous material obtained by the method of sintering pressed blocks [4]. Graphite blanks are produced in accordance with NIIGRAFIT technology at the graphite products plant (the town of Vyazma). Nozzle cluster parts made from graphite blanks must conform to stricter requirements in terms of homogeneity and the presence of defects in the material structure. In this regard, random X-ray quality control of incoming blanks and 100 % inspection of parts is carried out. In the course of random X-ray inspection of a batch of blanks and 100 % inspection of the SPRE nozzle throat insert parts made from them, areas of varying radiographic density were detected in the X-ray images, namely, areas with bright spots and inhomogeneity. For the lack of data on the nature of such anomalies (impurities), it is impossible to correctly sort out and grade graphite blanks and parts during X-ray inspection.

The purpose of this paper is establishing the nature of anomalies (impurities) observed in the blanks and parts of SPRE nozzle throat inserts, with subsequent recording of the obtained information in the technical documentation of X-ray inspection.

Experimental part

Two samples were selected as objects for the study: 30×80 mm plates 2 mm thick, with areas of varying X-ray contrast, cut out from an SPRE nozzle throat insert part (sample No. 1) and from a blank, which was essentially a hollow cylinder of MPG-7 graphite (sample No. 2). The cutting of samples was carried out in the location where defects were detected, along the axis. The X-ray testing was carried out on an ISOVOLT-160HS X-ray machine with a COMET MXR 160 tube equipped with a digital radiography system based on a PerkinElmer XRD 0822 flat planar detector. Structural examination of the sample surface was performed with a JEOL JSM-6390LA scanning electron microscope. Elemental analysis of the sample surface was performed using a JED‒2300 analyser. Two shooting modes were used: secondary electron image (SEC) and backscattered electron composition (BEC).

Results

At the first stage of the study, X-ray images of the part and blank samples were obtained in the projection parallel to the axis (sample No. 1 and sample No. 2, respectively). As can be seen from the images, sample No. 1 (Fig. 1, a) has a large number of bright areas, occupying up to half of the entire volume and distributed mainly in the right-hand part of the sample. Such increased X-ray density may be caused both by the presence of metallic inclusions in the sample and by difference in the density of areas observed in the image.

 

Fig. 1. X-ray images of samples No. 1 (a) and No. 2 (b)

 

The X-ray image of sample No. 2 (see Fig. 1, b) shows inhomogeneity with minor bright inclusions in the form of dots. Earlier such bright spots were identified as graphite inclusions of higher order [5]. The inhomogeneity observed in graphite of MPG grades has not been discussed in the scientific literature, but it can be assumed that it is associated with varying density, different multi-directional orientation of particles (texture), occurring in the course of pressing, or the presence of microstructural inhomogeneities.

In order to investigate the inhomogeneities in the samples detected by X-ray inspection in more detail and to determine their nature, a fractographic examination was carried out. The images of the surface and end face of sample No. 1, obtained with an electron microscope in the BEC mode, are shown in Fig. 2. The occurrence of areas with different contrast (see Fig. 2) allows assuming that the bright spots are the result of morphological impurities in the matrix, caused by the presence of chemical elements with an atomic number different from that of the main material (matrix), i.e. carbon.

 

Fig. 2. Electron microscope images of the surface (a) and end face (b) of sample No. 1, obtained in BEC mode

 

For the purposes of verifying this assumption, an energy dispersive elemental analysis of the sample surface based on the characteristic excitation spectra of X-ray lines was carried out simultaneously with obtaining electron-optical images of the mentioned samples. The areas where elemental analysis was carried out and energy dispersive spectra were obtained are indicated by markers 001–007 (see Fig. 2). According to the obtained spectra, elements Al, Si, Ti, K, Na, Cl, Ca, and Fe with content higher than 0.01 % (wt.) in (local) quantity of 2–20 % (wt.) are present in the bright areas. The results of local elemental analysis of areas 005, 006, and 007 testify to the absence of chemical impurities in the matrix material base.

Fig. 3 shows an example of energy dispersive spectra obtained in the bright area, indicated by marker 003 (Fig. 3, a), and in the area without bright impurities within a zone indicated by marker 005 (see Fig. 3, b).

 

Fig. 3. Energy dispersive spectres obtained on the surface of sample No. 1 segments (markers 003 and 005)

 

The stoichiometric ratio of the impurity atoms and their spatial arrangement within the volume of sample No. 1 allows to make the following assumption about its phase composition. The elemental impurities responsible for the observed bright areas in the X-ray and electron diffraction images appear to be carbides (Al, Ti) and/or oxycarbides (Si-O-C) formed by the carbothermic reduction of the impurity phase during the formation of graphite blanks [6].

The electron microscopic images of the investigated surface and end face of sample No. 2, obtained in SEI mode, are shown in Fig. 4. It can be seen that the sample surface is homogeneous, without any impurities.

 

Fig. 4. Electron microscope images of the surface (a) and end face (b) of sample No. 2, obtained in SEI mode

 

Fig. 5, a shows an electron microscopic image of the surface of sample No. 2 obtained in SEI mode. A chemical analysis of the sample surface was performed in the areas indicated by markers 001–004. The absence of contrasting areas in the electron microscopic image (see Fig. 5, a) also testifies to the absence of impurity elements in the basic matrix of graphite, which is confirmed by the energy dispersive spectra of its surface. An example of the energy dispersive spectrum for the area indicated by marker 002 is given in Fig. 5, b.

 

Fig. 5. Electron microscope image of the surface of sample No. 2 (a) and energy dispersive spectre for selected area (marker 002) (b)

 

According to the research data of sample No. 2, its composition does not contain impurity elements in the form of local areas with inhomogeneous density (optical contrast). Hence, the bright spots observed in the X-ray images do not differ in elemental composition from the main carbon matrix, which confirms the results obtained earlier in study [5].

In this way, the bright areas in the X-ray images can be classified as extraneous impurities (e.g., oxycarbides) dissolved or dissociated in the basic graphite matrix. The appearance of such impurities is known to significantly change the thermophysical properties of graphite, which have to be further investigated. An analysis of the morphology of sample No. 2 does not provide additional information on the nature of the contrast observed in the X-ray image.

In connection with assumptions on the phase composition of sample No. 1 and insufficiency of information regarding structural inhomogeneities of sample No. 2, physical-chemical studies of the local structure and elemental composition of the graphite matrix phase will be subsequently conducted. Obviously, for solving the issue concerning the influence of detected anomalies on the serviceability of nozzle throat inserts made of graphite, as well as new promising products of JSC Novator Design Bureau, bench tests of parts with existing defects will be carried out.

Conclusion

As a result of the studies conducted, it was established that MPG-7 graphite blanks, used in the products of JSC Novator Design Bureau, feature areas with anomalous radiographic density, characterised by high content of elemental impurities, up to 20 % (wt.). The inhomogeneity of MPG-7 graphite observed in the X-ray image is homogeneous in its chemical composition and is not identified as an impurity phase by electron microscopy. Based on the obtained data, the technical documentation for incoming quality control of blanks and parts made of MPG-7 graphite was amended to clarify the nature of detectable impurities.

References

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2. Ершов А. М. Создание тугоплавких материалов соплового блока твердотопливных ракетных двигателей: Дисс. ... канд. техн. наук. Екатеринбург: УрФУ, 2008. 135 с.

3. Ершов А. М., Иванов В. М., Полухин Е. О., Мельников В. Н. Графит ПРОГ-2400С – материал нормированного уноса массы // Оборонная техника. 1979. Т. 10. С. 67–70.

4. O'Driscoll W. G. Features and behaviour of carbon // Nuclear Engineering. 1958. Vol. 3. No. 32. Pp. 479–485.

5. Вершинина М. В., Белякова Е. Г. Структурные особенности волокнистых наполнителей углепластиков // Материалы XX межрегион. отрасл. научн.-техн. конф. «Люльевские чтения», ЮУрГУ, Екатеринбург, 2016. С. 155–156.

6. Соединения переменного состава и их твердые растворы / Г. П. Швейкин, С. И. Алямовский, Ю. Г. Зайнулин и др. Свердловск: УНЦ АН СССР, 1984. 291 с.


About the Authors

A. V. Vershinin
Novator Design Bureau, Joint Stock Company
Russian Federation

Vershinin Aleksandr Vadimovich – Candidate of Physical and Mathematical Sciences, Leading Engineer. Science research interests: structure and properties of carbon materials.

Ekaterinburg



M. V. Vershinina
Novator Design Bureau, Joint Stock Company
Russian Federation

Vershinina Marina Vadimovna – Head of Department. Science research interests: nondestrucitve inspection.

Ekaterinburg



E. G. Belyakova
Novator Design Bureau, Joint stock company
Russian Federation

Belyakova Elena Germanovna – Doctor of Engineering Sciences, Bureau Chief. Science research interests: chemistry and technology of non-metallic material.

Ekaterinburg



E. V. Polyakov
Institute of Solid State Chemistry of the UB RAS
Russian Federation

Polyakov Evgeniy Valentinovich – Doctor of Chemical Sciences, Chief Research Fellow of Federal State government-financed research institution. Science research interests: sorption, thermodynamics, kinetics, microelements, radionuclides, forms of state.

Ekaterinburg



V. G. Bamburov
Institute of Solid State Chemistry of the of the UB RAS
Russian Federation

Bamburov Vitaliy Grigor'evich – Doctor of Chemical Sciences, Professor, Corresponding member of RAS, Chief Research Fellow, Federal State government-financed research institution. Science research interests: physicochemistry of rare and rare-earth elements, processes and equipment for obtaining chemical compounds.

Ekaterinburg



I. V. Volkov
Institute of Solid State Chemistry of the of the UB RAS
Russian Federation

Volkov Il'ya Vladimirovich – Candidate of Chemical Sciences, Research Fellow, Federal State government-financed research institution. Science research interests: element and isotope analysis of solutions, electron microscopy, analytical instrument engineering.

Ekaterinburg



Review

For citation:


Vershinin A.V., Vershinina M.V., Belyakova E.G., Polyakov E.V., Bamburov V.G., Volkov I.V. Investigation of impurities detected in X-ray inspection of MPG-7 fine-grained graphite blanks and details. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2017;(4):80-85. https://doi.org/10.38013/2542-0542-2017-4-80-85

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