5.

6.

.

8.

9.

T. Khan and P. Caron, "Effect of processing conditions and heat treatments on mechanical properties of single-crystal superalloy CMSX-2," Mater. Sci. Tech., ~, 486-492 (1986). T. Ohno, R. Watanabe, and K. Tanaka, "Development of a nickel-base single crysta ! superalloy containing molybdenum by an alloy designing method," J. Iron Steel Inst. Jpn., 74, No. ii, 133-140 (1988). H. Harada and M. Yamazaki, "Alloy design y' precipitation hardened nickel-base superalloys containing Ti, Ta and W," Tetsu-to-Hagane, 65, No. 7, 1059-1069 (1979). Ohtomo Akira, "Progress in materials for aircraft engine," J. Japan Soc. Heat Treat., 28, No. 2, 106-112 (1988). T. Khan, P. Caron, and M. Brun, "Progres recents dans le domaine des superalliages monocristallins pour abes de turbines," Materiaux Mechanique Electricite, Mai, No. 425, 32-36 (1988).

MORPHOLOGICAL TRAITS OF DAMAGE TO STEAM PIPES IN AGGRESSIVE MEDIA I. I. Mints and L. E. Khodykina

UDC 620.186.4:621.186.1

In steam pipes handling steam 540-560°C hot there were repeated cases of damage to curved parts of the pipes (bends). In the bends cracks form both on the outer surface of tensioned zones and on the inner surface of neutral zones. Statistical processing of the cases of destruction of steam pipe bends showed that damage to bends was found on the inner surface of the neutral zones only in power stations which used returned condensate from petrochemical production. The proportion of damaged bends in the neutral zones is about 30% of the total number.

It is known that cracks on the outer surface of tensioned zones of bends are due predominantly to creep. The morphological traits of such cracks have been investigated fairly exhaustively [1-5] but the special features of cracks on the inner surface of the neutral zones received much less attention [6]. In the present work we investigated the metal of one straight section and of five bent sections of steam pipes made of steel 12KhlMF, damaged in operation on the inner surface. The curved sections were damaged in the neutral zones. In all cases damage occurred on the horizontally situated sections of the steam pipes. The main data on the investigated pipes are presented in Table I. Destruction of the ends occurred by brittle breakaway of the tensioned zone along one or both neutral zones, destruction of the straight section by brittle breakaway of part of the pipe. Earlier on, destruction of straight pipe sections had not been encountered either in the Soviet Union or in other countries. In the investigated cases the arterial cracks developed, as a rule, along the lower generatrix of the pipes. Reticular cracking and transverse cracks on the inner surface of pipes, which are characteristic of thermal fatigue failure when the pipes are sprinkled with water, were not encountered. In all the investigated pipes, on the inner surface near the fracture, there were blind accompanying cracks parallel to the arterial crack. Most cracks developed from marks with different depth, some of them within the permissible limits of valid standards. The fractures along the lower generatrices of the neutral zones of bends and of the straight section are brittle, covered with a compact layer of scale, coarse-grained and with considerable surface roughness which increases with increasing crack growth. Foci of destruction were not discovered. The section of minimal final fracture amounts to about 10% of the wall thickness of the pipes. Analysis of the path of the cracks on transverse metallographic sections after nickelizing of the fractures showed the predominantly intergranular nature of their development. The fractures along the upper generatrix of the neutral zones of bends are also brittle, with rough surface, but less oxidized. In these fractures we find coarse All-Union Research Institute of Heat Engineering (Ural Branch). Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 27-29, September, 1991.

0026-0673/91/0910-0689512.50

© 1992 Plenum Publishing Corporation

689

TABLE i Pipe

Do ' So' mm

I No0]

• °C

MPa! i

' 560 560

! Crumbling of straight sec14,0 79,0 I tio n of bent outlet pipe 133'25 F + C 14,0 ] 90,0 1 Through crack in tensioned !31 0 F + c + (15-20%) B ]zone and blind crack (up to] ' |38 ~ deep) in natural zone] 14,0 I 97,6 ] Crumbling of tensioned zone] 31,6 F + C I0,0 I 45 0 ] along both neutral ones ~ 12 0 F + C + (5-10%) B

1

273X32

2

273X32

3

273X32

4

194X 12

560 540

5

273X32

560

6

273X20

540

0

' / T h e same 14,0 I 70,3 / B l i n d crack

Damage

2 nun deep i n

h, inn]

~ ' ] 29,5 I

neutral zone I0,0 179,0 Crumbling of tensioned zone~ 20,5 along both neutral ones I

Structure

+ c + (5-1o~)

F + C + (15-20%) B

HB of zones ten-

146

!43

I

128

--

156

straight section

I

126 135 143

135

130

163

I56

-137

Notation: D o ) outer pipe diameter, S o ) wall thickness of the pipe; t) operating temperature, p) steam pressure, N) operating time, h) wall thickness at the fracture, F) ferrite, C) carbides, B) bainite. fan-shaped scars diverging from the inner surface, and fibers arranged perpendicularly to the front of crack growth. The section of minimal final fracture amounts to about 25% of the wall thickness. The investigations were carried out with specimens cut out of sections near the fracture of the bends. We used optical and transmission microscopy, and also electron fractography of operational fractures after they had been cleaned by the method of [7]. The results of metallographic analysis showed that failure mostly developed by the formation and growth of zones of cracking oriented at angles of about 50-70 ° to each other. In their turn, the zones of cracking, independently of their thickness, are a set of systems of intergranular cracks. These systems are oriented to each other at the same angles as the zones of cracking, and the cracks of which they are composed are approximately parallel to each other and oriented predominantly perpendicularly to the surface of pipes. Some isolated microcracks have ramifications at the same angles as the systems of cracking (Fig. la). In the investigation of unetched sections it was established that in the zones of cracking there are, in addition to the systems of cracks, also systems of oxidation which are the sets of intergranular corrosion channels oriented in the same directions as the cracks. The cracks and corrosion channels are surrounded by "clouds" of oxidation consisting of a multitude of disperse oxide particles. Special etching for chemical inhomogeneity brings out clearly visible traces of intergranular local oxidation, viz., continuous intergranu!ar channels (Fig. ib) and isolated shallow, predominantly intergranular oxides (Fig. ic). The existence of many zones of oxidation without cracking provides grounds for assuming that the development of cracks was preceded by corrosion processes. In one bend (Table i, pipe 2) we found a deep (up to 90% of the wall thickness), cavitylike crack, comparatively wide at the source (up to 0.025 mm), narrowing down as it grew, having a zigzag path with angles of turning ~70 ° . The general direction of crack development was perpendicularly to the surface of the pipe. The crack ends in a narrow intergranular channel alternating lengthwise with crumblings about one grain in size. A special feature of this crack is the considerable difference in the structure of its lips: a comparatively smooth relief on one side, and a strongly corroded relief with attendant crumblings on the other. On the crack lip with smoothed relief we discovered a strip of metal several grains wide, consisting of columnar ferritic crystallites, extended in the direction perpendicular to the front of crack growth. In the region of developed columnar crystallites the metal is decarburized. At the very edge of the crack in a strip up to one grain wide, the columnar crystallites are strongly deformed, as indicated by the bright deformation relief. The mean microhardness of the deformed metal is 466 H, of the decarburized columnar crystallites 160 H, of the equiaxial ferrite grains far from the crack 193 H. In the crack there are round gray inclusions with 0.01-0.04 mm diameter with high microhardness (752 H); this is wustite and iron oxide FeO (Fig. 2a). Inclusions of wustite may form when access of oxygen to the metal surface is difficult or when the medium causes reduction of the iron oxides Fe=O 3 or FeaO 4 to FeO. T h e reducing agent in the medium can be hydrogen [8]. According to the data

690

Fig. i. Damage to the metal of failed pipes: a) systems of cracking (×50); b) intergranular corrosion channels (×i000); c) intergranular oxides (×i000).

Fig. 2. Inclusions of wustite near a cavitylike crack (a) and the cellular substructure of the metal near the inner surface of failed pipes (b): a)

x300; b) x70,O00. of [9], c o l ~ n a r crystallites can form during intense decarburization of metal heated to below Ac I in an oxidizing atmosphere and especially in the presence of moist hydrogen. Local analysis showed that the content of residual hydrogen in the metal of a failed bend is considerably higher than the characteristic content of low alloy steel in the initial state [i0]. Thus a number of morphological peculiarities of the crack and the results of the investigations [6, I0] indicate that failure of the investigated bend occurred under the corrosive effect of the medium in the presence of hydrogen. In addition to zones of cracking, zones of oxidation, and cavitylike cracks in the metal of the neutral zones (predominantly near the inner surface) of all the investigated bends we found intergranular damage in the form of discontinuities with different dimensions and morphology. In some bends these are creep pores 1-5 Dm in size, analogous to the ones found in the metal of tensioned zones, in the others they are octahedral discontinuities filled with oxides and 5-10 Dm in size. The color of the oxides in these discontinuities corresponds to

691

Fig. 3. Fractograms of the operational fracture surfaces of pipes: a) grain boundaries; b) accumulation of discontinuities; c) stone-like fracture without corrosive relief; a) ×6000; b) ×9000; c) ×5500. the color of the corrosion channels. The discontinuities as well as the creep pores are usually isolated and oriented. It can be seen from the nature of the disposition of the oriented discontinuities that they form predominantly at places where the boundaries of austentic and ferritic grains coincide with each other. The distribution density of the oxidized discontinuities is greater than of the creep pores. As a rule, the metal of the neutral zones of one bend contains either creep pores only or coarse oxidized discontinuities only. In the metal of the tensioned zones of bends, including those investigated earlier [i-5], coarse oxidized discontinuities were not found. In the metal of the destroyed straight section of the pipe intergranular discontinuties of either kind were not discovered. The lack of coarse discontinuities filled with oxides in the metal of the tensioned zones of the bends is probably due to the composition of the corrosive medium, and the lack of intergranular crumbling in the straight section is due to insufficient development of creep at low stresses. Transmission investigations of the metal of failed pipes were carried out on foils cut out near the fracture surface (para!lelly to the fracture) and at the inner surface of the pipes (parallelly to the surface). Near the inner surface there is a cellular substructure with nonuniform dimensions, with blurred subboundaries and different dislocation density within the cells (Fig. 2b). There are many places with greatly nonuniform density of chaotically distributed dislocations. In addition to isolated nucleating micropores of median size 0.13 ± 0.02 ~n, characteristic of development of creep, we found much finer submicropores with weak contrast, and accumulations of them. Submicropores are situated either on the~boundaries of carbide segregations or on dislocations. When an electron beam acts for a long time on a foil, the dimensions of the submicropores change: they either dissolve, or they grow, and then a coarser flat discontinuity forms at the place of accumulation of fine ones. As a rule, creep micropores are insensitive to electron beams. There were hardly any submicropores and accumulations of them in the metal of the tensioned zones of the bends, including those destroyed as a result of creep along the outer surface. According to the data of [ii] the formation of submicropores has to do with the fact that in the crystal lattice of the metal atomic hydrogen is dissolved, and this diffuses within the metal, accumulates on the boundary of carbide segregations or on dislocations, molizates and creates sections of high pressure. In these sections submicropores form. We will henceforth call the submicropores hydrogen micropores. 692

Near the fracture the nature of the cellular substructure is the same as at the inner surface. Besides that we can see dislocation loops and many decarburized sections free of dislocations and of disperse carbides. The number of creep micropores at the fracture is small, the same as at the inner surface, whereas the number of hydrogen micropores is considerably larger. This has probably to do with the intensified dissolution and diffusion of hydrogen as a result of local plastic deformation in the region of crack development. The results of fractographic analysis showed that on the fracture surfaces of all the investigated pipes there were structural components of metal (grain boundaries, bainitic sections, carbide particles) and a corrosive relief (Fig. 3a). This is unambiguous testimony to the substantial role of the corrosive medium in the development of failure. The fracture is predominantly intergranular, the mechanism of failure is brittle as well as ductile. On the surface of ductile intergranular facets we find traces of local plastic deformation of nearboundary sections: ridges from detachment and shallow pits. On the brittle facets there is barely any deformation relief. On the fracture surface there are many discontinuities with various dimensions and shapes (octahedral and round) and also accumulations of them (Fig. 3b). One of the bends had, in addition to the above-described relief, also local sections of intergranular stonelike fracture on whose boundary there is no corrosive relief (Fig. 3c). According to the data of [12] such topography of the surface is typical of destruction in the presence of hydrogen. Thus in the metal of steam pipes destroyed from the inner surface there developed two mutually accelerating processes: intergranular oxidation (as a result of the action of the corrosive medium in the presence of hydrogen) and creep. A comparison of the morphological features of the fine structure of the metal of bends destroyed predominantly by creep on the outer surface of the tensioned zones [1-5] and the joint effect of corrosion and creep along the inner surface of the neutral zones permits the assumption that in the latter case the influence of the corrosive action was predominant. Corrosion damage of steam pipes can be due to organic compounds contained in returned condensate from petrochemical production that get into the boiler; this lowers the pH of the medium in the steam duct. Cases of corrosion damage to elements of steam turbines in power stations in a steam atmosphere at 560°C were described in [13]. We believe that our results may be useful in identifying cases of breakdown of parts of equipment operating under creep and in a corrosive medium. LITERATURE CITED i.

I. I. Mints, T. G. Berezina, L. E. Khodykina et al., "Structure, damageability, and properties of steam-pipe bends after lengthy operation," Teploenergetika, No. i0, 49-51

2.

T. G. Berezina and L. A. Ashikhmina, ',Special traits of the structure and the nature of failure of steam pipe ends of steel 12KhlMF in operation under conditions of creep," Teploenergetika, No. i0, 51-54 (1981). N. I. Slobodchikova, G. M. Novitskaya, G. N. Sokolova, and L. S. Panenkova, Analysis of failure of bends of steam pipes for live steam of plant, in: Transactions of the AllUnion Institute of Heat Engineering (1987), pp. 60-65. Yu. M. Gofman and L. Ya. Loser, "Pore formation in metal operating at elevated temperatures under stress," Metalloved. Term. Obrab. Met., No. 4, 43-45 (1987). I. I. Mints, L. E. Khodykina, N. G. Shul'gina, and N. V. Ashmarina, "Investigation of the peculiarities of failure in creep of heat-resistant Cr-Mo-V steels," Metalloved. Term. Obrab. Met., No. 7, 33-36 (1989). A. B. Vainman, O. D. Smiyan, K. N. Kalinyuk et al., "Investigation of the causes of brittle failure of a bent section of a steam pipe for live steam made of steel 12KhlMF," Elektricheskie Stantsii, No. 5, 43-47 (1989). I. I. Mints, N. V. Shengeli, and R. Z. Shron, "Method of cleaning operational fractures for electron-fractographic analysis," Zavod. Lab., 51, No. 2, 60-61 (1985). F. Todt, Corrosion and Corrosion Control [Russian translation], Khimiya, Leningrad (1967). E. Houdremont, in: Special Steels [Russian translation], Vol. i, Metallurgiya, Moscow (1959), p. 311. O. D. Smiyan and S. I. Girnyi, "One case of brittle failure of a steam pipe," Fiz. Khim. Mekh. Mater., 25, No. 6, 102-103 (1989). G. G. Uhlig and R. U. Revi, in: Corrosion and Corrosion Control. Introduction to Corrosion Science and Technique Khimiya, Leningrad (1989), pp. 150-152.

(1981).

3.

4. 5.

6.

7. 8. 9. I0. ii.

693

12. 13.

694

In: Fractography and Atlas of Fractograms: Handbook [Russian translation], Metallurgiya, Moscow (1982), p. 325. O. I. Martynova and O. A. Povarov, "The influence of impuritie~ dissolved in steam on the formation of an actively corrosive liquid phase in throughflow parts of turbines," Teploenergetika, No. 4, 19-22 (1984).