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DIAGNOSIS OF CAUSES OF FAILURE OF COMPONENTS AND SUBASSEMBLIES OF CHEMICAL EQUIPMENT M. B. Chizhmakov and M. B. Shapiro UDC 66.023:620.193:660.077 A diagnosis of the character and causes of failure of the chemical industry equipment working at high temperatures and pressure, under the action of aggressive media, requires the adoption of contemporary methods of the physicochemical investigation of metals. A correct determination of the causes of failure of components and subassemblies of the chemi- cal equipment makes possible for the designer to follow whether the structural material chosen by him and the calculation method of the equipment are suitable for the service con- ditions. The technologist, using these results, can introduce changes in the technological pro- cess of equipment manufacture, if the cause of failure was the deviation from the recommended technology or imperfect technology. For the users the investigation results of the causes of failure make it possible to arrive at conclusions concerning the methods of operation and the process parameters. In addition an analysis of the character and causes of failure of machines and equipment makes possible to predict their service life. As an investigation of the causes of equipment failure in the chemical industry of one of the foreign firms, failure of material takes place above all on account of local corrosion \[i\]. An analysis of equipment failure has shown that 27.5% is caused by general corrosion, 23.7% by corrosion cracking, 14.6% by intercrystalline corrosion and corrosion of welds, 14.3% by pitting corrosion, and 13% by other types of corrosion. In addition in \[i\] it is noted that the relative level of material damage by corrosion of the equipment metal is 2~ times higher than the average level in the entire sphere of metal use. It should be noted that in many cases the determination of the type and causes of failure of the base metal and welds of the chemical industry equipment is very difficult and requires a complex approach for solving this problem. In a complex examination of the metal of a damaged apparatus, it is possible to determine the main cause of failure and factors contributing to it. The direction of the investigation and selection of the appropriate methods are determined by the features of the material failure and the operating conditions of the equipment. The investigation should be started by a visual examination of the damaged equipment in order to determine the location of failures, corrosion damage, existence of cracks, etc. It is useful to set up a scheme of the equipment failure~ It is also useful to familiarize oneself with the technical documentation, and work scheme in order to detect possible devia- tions in the operation conditions of the equipment. In many cases it is necessary to deter- mine the chemical composition of steel and its agreement with GOST; here attention should be paid to the content in steel of harmful additions and gases. It is necessary to check the mechanical properties of the metal in order to establish an agreement between the properties and the technical norms. In the material being investigated, it is necessary to determine the level of residual stresses, which are dangerous in the case of corrosion cracking, corrosion fatigue, and also in the case of a complex stressed state. In addition, it is possible to carry out color radiography of damaged subassemblies and parts which makes possible to detect cracks in the metal and the character of their location and also to obtain important information on the type of metal damage. Defectoscopy of the metal is necessary when the fracture of the part changes as a result of corrosion damage in interaction with the aggressive medium. A metallographic investigation of the metal structure yields valuable information, since in service important changes can take place in the metal structure caused by re- crystallization processes when the apparatus works at elevated temperatures. An analysis 1989. Translated from Khimicheskoe i Neftyanoe Mashinostroenie, No. 12, pp. 33-34, December, 0009-2355/89/1112- 0719512.50 9 1990 Plenum Publishing Corporation 719 of the microstructure makes it possible to detect the metallurgical defects (the presence of non-metallic inclusions, sigma phase, ferrite, etc.) as well as the defects of welds. Metallographic investigations are also important in determining the failure caused by inter- crystalline corrosion or by corrosion cracking, since this makes possible a unique determina- tion of the fracture character. However, in many cases for determining the causes of failure of chemical industry apparatus, components, and subassemblies it is necessary to use modern physical investigation methods. One of the most effective methods for the investigation of failure is fractographic analysis which is realized by the raster or transmission electron microscopy method~ The fractographic method can be used for detecting brittle failure and its connection with crystallographic planes or interphase boundaries of structural components~ In the case of ductile fracture the pitting relief or the zone of stretching on the fracture is determined by the fractographic method. In the case of fatigue fracture an analysis is carried out of the fatigue grooves or track traces. A detailed fractographic analysis provides valuable information on the fracture type. By means of the fractographic analysis it is possible to determine the fracture source and the crack propagation trajectory. By the fractographic method is determined also the degree of action of the aggressive medium or hydrogenation on the fracture process. In addition, the features of the fracture structure give additional information on the charac- ter of failure. Thus, a typical feature of the fracture under the action of a constant load is the existence on the fracture surface of a large number of cracks. In ductile fracture the shape of pits is largely determined by the character of the loads applied. A wide scatter of pits with regard to dimensions shows the nonuniformity of the structure of the material being examined. The intercrystalline character of frac- ture, when the corrosion products are absent on the fracture, suggests the segregation of harmful contaminants on the grain boundaries or the existence of film separations on them. A mixed surface relief containing intergranular failure and another type of relief suggests a nonuniformity of harmful contaminants on the grain boundaries. The presence of corrosion products on the surface of the intergranular fracture indi- cates cracking of the metal as a result of corrosion under stress. There are several other signs which permit, from the details of the fracture relief, determination of the features of external factors (stress, medium, etc.) leading to material failure. An analysis of the data on the type of fracture obtained by the fractographic method makes it possible in most cases to determine the special features of metal fracture and the causes of failure of parts and subassemblies of the chemical equipment. However, there are also cases when the fractographic method gives incomplete information on the cause of metal failure and only a general picture of the failure topography~ Besides, the character of failure is often affected by structural changes accumulating in service, i.e., recrys- tailization, separation of dispersed excess phases, changes in the density and the character of the distribution of dislocations, etc. In this case information on the type and possible causes of failure can be obtained by means of transmission electron microscopy~ For the diagnosis of the causes of failure of the chemical industry apparatus, sub- assemblies, and parts local methods of microanalysis, such as x-ray spectral analysis and Auger-spectroscopy are being used successfully. These methods permit a microanalysis of very small volumes of metal. Thus, with the x-ray spectral analysis method it is possible to determine the composition of the matrix and inclusions, the degree of nonuniformity of the distribution of elements in the specimen being examined, and to carry out a topographi- cal analysis of the character of element distribution, the composition of corrosion products by the interaction of metal and the aggressive medium. In addition, this method can be used to determine the composition of surface films on corrosion-resistant steels and alloys. Auger-spectroscopy also permits solving these problems and in addition determining the presence of segregation of harmful contaminants on grain boundaries, establish the areas of local composition changes, impoverishment by chromium near the carbides or the grain boundaries, as well as determining the composition of fine inclusions (less than i ~m) in steels and alloys. 720 In addition to the microanalysis methods described above, for the investigation of the damaged apparatuses other methods can be used (x-ray photoelectron spectroscopy, secondary ion mass spectrometry, laser mass spectrometry, etc.). However, in most cases they are less effective than the x-ray spectral analysis or Auger-spectroscopy. Thus, on the basis of these considerations the following sequence can be recommended for determining the causes of failure of parts and subassemblies of the chemical industry taking into account the fractographic analysis described in \[2\]. i. Construction of the failure scheme with detection of the fracture source, struc- tural stress concentrators, and presence of signs of surface damage. 2. Determination of the agreement between the part dimensions and specifications of the drawing, and between the chemical composition and mechanical properties of the metal and the technological conditions. This permits checking not only the correctness of heat treatment, but also the maintenance of equipment operation in work at elevated temperatures. 3. Determination of fracture orientation in relation to the direction of the action of the principal stresses. 4. Determination of the type and degree of the macroplastic strain and its localiza- tion as a whole and in the vicinity of the fracture. 5. Determination of the type of failure in the fracture, i.e., plastic, brittle, fatigue, intercrystalline, transcrystalline; existence of signs of intercrystalline cor- rosion, corrosion cracking, and pitting. 6. Determination on the fracture surface of sharply distinguished macroscopic areas, differinginstructure and color, indicating the duration of failure in time, as well as other macroscopic signs, i.e., cuts, steps, fatigue grooves, etc. 7. Exposure on the fracture of corrosion products, oxides and others, and their con- nection with the source of failure. 8. Exposure of cracks close to and away from the fracture, evaluation of their loca- tion, number, and direction. 9. Investigation of the macro- and microstructure of the metal, their agreement with the given semifinished product, detection of structure faults, often a stonelike fracture, presence of ferritic phase in austenitic steels and of the sigma-phase in austenitic and austenitic-ferritic steels. I0. Use of microanalysis methods for the identification of nonmetallic inclusions present in the source of the fracture of inclusions on which micropores are formed in duc- tile fractures, and also segregations along the grain boundaries and film segregations. ii. Study of the fine structure (distribution of dislocations, packing defects) for exposure of changes appearing in the material of the equipment in service. In a number of cases, for example, for the equipment from austenitic or austenitic-ferritic steel it is useful to determine whether in the material appears a tendency to intercrystalline cor- rosion as a result of long service at elevated temperatures. Examples of a complex investigation of failure of components and subassemblies of chemical equipment in particular using fractographic analysis are given in \[3-5\]. It should be noted that each work concerned with thedetermination of the causes of failure of equipment in service conditions is a separate investigation. Accumulation and systematization of experience in the field of diagnostics of the character and causes of failure of parts and subassemblies of equipment, and statistical processing of results, are important tasks of materials scientists, corrosion specialists, technologists, and designers in the field concerned. This work must be carried out in the institutes of the relevant field, familiar with the special operational features of the equipment. It is necessary to increase work on the diagnostics of equipment failure. An important role shold here be attached to the equipment of scientific research institutes and laboratories, and to exchange of experience between the institutes of the relevant fields. I. LITERATURE CITED Ya. M. Kolotrykin, Metal and Corrosion \[in Russian\], Metallurgiya, Moscow (1985). 721 . 3. 4. 5~ To A. Gordeeva and I. P. Zhegina, Analysis of Fracture in Assessing the Reliability of Materials \[in Russian\], Mashinostroenie, Moscow (1978). Mo B. Chizhmakov and M. B. Shapiro, Use of Contemporary Physical Methods for the Investigation of Corrosion-resistant Steels and Alloys \[in Russian\], TsINTIKhImnefte- mash, Moscow (1986). M. B. Shapiro, Yuo P. Surkov, M. B. Chizmakov, and A. L. Belinskii, "Investigation of the failure of water heating pipes in ammonia manufacture," Khim. Neft. Mashinostro, 30-32 (1978). Fractography and Atlas of Fractograms. Reference Book \[in Russian\], Metallurgiya, Moscow (1982)o 722