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X-ray investigation of a heterogeneous steel weld
Materials Science & Engineering A 682 (2017) 248–254
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
X-ray investigation of a heterogeneous steel weld ⁎
Lyudmila L. Lyubimova, Roman N. Fisenko, Roman B. Tabakaev , Alexander A. Tashlykov, Alexander S. Zavorin
crossmark
National Research Tomsk Polytechnic University, 30 Lenin Avenue, Tomsk, 634050, Russia
A R T I C L E I N F O
A BS T RAC T
Keywords: Technical diagnostics Heterogeneous steel weld X-ray analysis Microhardness Residual stresses
The welding process has a significant impact on the chemical composition, structure and mechanical properties of materials in heat-affected zones. The existing approaches to the diagnostics cannot predict the properties of the welded joint nor do they guarantee its reliable performance throughout the projected service life. This becomes especially relevant when investigating the welds from heterogeneous steels applied in power engineering. The main goal of this paper is to offer modern approaches to the complex technical diagnostics of steam pipelines using X-ray investigation of heterogeneous weld structures. In this work, the following research methods were used: X-ray fluorescence analysis, X-ray diffraction method, and microhardness measurement. The object of the investigation is the composite weld of 12Cr18Ni9Ti austenitic steel and 12Cr1MoV pearlitic steel. The results of this investigation show that the properties of the weldable steel changed significantly as compared to its original state. The weld structure was represented by the internal structural stress gradients despite the application of the stress relief heat treatment. The proposed approaches to the study of steel welds can be used for selecting the rational mode of post-weld heat treatment. Combined with traditional inspection methods (visual or ultrasonic inspection, etc.), these approaches can serve as a basis for technical diagnostics and the selection of constructional materials.
1. Introduction Austenitic steel 12Cr18Ni9Ti and pearlitic steel 12Cr1MoV are the main structural materials in Russia to produce ultra-high pressure boiler superheaters. With the design lifetime of about 300 thousand hours, they, however, collapse prematurely for various reasons [1–4]. Welds are damaged due to the combined impact of intrinsic defects and so-called hidden defects [5,6]. Intrinsic defects occur in the form of pinholes, long cracks, incomplete penetrations, cinder inclusions etc. during equipment production and installation. These stem from errors in the welding process, violation of accepted rules and standards, equipment failure, or poor production practices. The best way to fight these defects is to eliminate their causes. Hidden defects are connected with the microstructure of the weld material and its heat-affected zones. These result from phase, structural and chemical heterogeneity, which brings about differences in the physics, chemistry, strength and mechanical properties of various sections and zones of the welds. These defects are difficult to detect by standard diagnostic methods (visual or ultrasonic inspection, etc.) [7,8]. One of the fundamental reasons for premature failure is the ⁎
accelerated structural degradation of steel due to cyclic mechanical and temperature gradients. Structural transformations drastically affect the resistance of welds to brittle failure and the mechanical properties of the products [4,9]. An uncontrolled process of structural and phase transformations leads to unpredictable consequences, which manifest themselves in changes of current stresses with the most probable occurrence of local stress concentrators exceeding the longterm strength. The state of the phases in the weld zone, their transformation under service condition, the level of structural stresses and their redistribution still have no clear systematization. This gap in the knowledge could ultimately lead to the damage of a welded assembly at any time before the end of its service life. Based on a number of specialized scientific and regulatory documentation, the technical diagnostics of the welds should include the research of the metal composition, structure and properties with the methods of the magnetic particle and dye penetrant inspection, hardness test, ultrasonic thickness measurement, metallographic examination using replica techniques. Generally, the performance analysis of a weld involves destructive inspection techniques measuring mechanical properties, heat resistance, ductile to brittle transition temperature, brittle strength, creep speed, and rate of corrosion. These methods are very effort and time
Corresponding author. E-mail address: [email protected] (R.B. Tabakaev).
http://dx.doi.org/10.1016/j.msea.2016.11.058 Received 29 July 2016; Received in revised form 17 November 2016; Accepted 17 November 2016 Available online 17 November 2016 0921-5093/ © 2016 Elsevier B.V. All rights reserved.
Materials Science & Engineering A 682 (2017) 248–254
L.L. Lyubimova et al.
consuming. Currently, durability assessment is based on the long-term strength and fatigue of the weld material, S-N diagram, cumulative damage theory as well as the control of micro- and macrostructure. These measures, however, do not guarantee weld integrity. The assessment of weld reliability and integrity executed by different methods is sometimes controversial [10]. These methods do not provide any information on changes in the microstructure of the material during its service life under specified conditions. Consequently, a more comprehensive weld assessment is required, which, apart from the above, should include the distribution of microhardness, alloying elements and structural defects, the magnitude and sign of residual stresses as well as the phase composition of different zones and layers of the pipe wall. The chemical, structural and mechanical heterogeneity of the weld properties requires the heat treatment of a workpiece to stabilize its structure after the welding process. Heat treatment, in turn, requires the sufficient data on the structure, phase composition, level of microstresses and distribution of defects in the hazardous zones of a weld. However, these data are difficult to obtain using only the standard methods of diagnostics. Thus, the problem of identifying the desired properties of the weld structure remains unresolved. For such a problem, X-ray analysis may be a very effective method. The authors' review of recent papers suggests that the presence of X-ray is limited in this industry. Sometimes, it is not even considered as a diagnostic and forecast method, because it is allegedly difficult to apply in a production environment. However, the method does not require the destruction of the sample surface and allows repeated and long-term tests of the same sample. It can also be implemented quite successfully on a cutting sample or part of a workpiece. Thus, controlling the weld microstructure by hardness diagnostics and X-ray may substantially improve the efficiency of welding, which makes this work relevant and novel. The main goal of this paper is to offer advanced approaches to complex technical diagnostics of steam pipelines using the X-ray investigation of heterogeneous weld structures.
Fig. 1. Composite weld under study: (a) cutting scheme of transverse b, c and longitudinal d samples; (b) control points of the external surface weld; (c) control points of the internal surface weld; (d) control sample of the weld-adjacent zone (5′ and 9′) and of the heat-affected zone (4′ and 10′); АС, ВС –direction of microhardness measurement along the wall thickness; No. 1–13 – control zones.
2. Materials and methods The object of the investigation is the weld of austenitic 12Cr18Ni9Ti steel and pearlitic 12Cr1MoV steel. The welded joint was divided into zones No. 1–13 for the research (Fig. 1d). The weld zone (No. 7) was prepared for the research of the internal and external surfaces (Fig. 1b). Zones No. 5` (12Cr1MoV) and No. 9` (12Cr18Ni9Ti) were arranged at a distance of 7.5 mm from the weld axis. Zones No. 4` (12Cr1MoV) and No. 10` (12Cr18Ni9Ti) were arranged at a distance of 14.5 mm from the weld axis. The weld structure was investigated using an EDX 2800 X-ray analyzer (SKYRAY, USA), a DRON-3 X-ray diffractometer (Russia), and a PMT-3 microhardness tester (Russia). The X-ray fluorescence analysis was performed using EDX-2800 for measuring the spectra of the characteristic radiation of the chemical elements in the test sample. The chemical elements were identified by a standard calibration curve embedded in the spectrometer software. The quantitative composition of the alloy was determined by estimating the intensity and area of the experimental spectral lines. The X-ray diffraction method is based on the classical fundamental theory for the X-ray analysis of the internal stresses and crystal grain size using a thoroughly tested mathematical apparatus. The diffraction angle (2θ) of each diffraction line, the intensity and width of its profile are defined using the DRON-3. The full profile is determined by the combined effect of geometrical and physical factors. The physical profile of the diffraction line (β) is a function of the material properties and state. It depends on the crystallite size (D ) and the residual stresses of the second kind (σII ):
D=
0. 94∙λ m1 ∙ cos θ1
⎛ n ⎞ σII =⎜ 2 ⎟ ∙E ⎝ 4∙tgθ2 ⎠
(1)
(2)
where m1 is a part of the physical broadening of the first diffraction line depending on the crystallite size, n2 is a part of the physical broadening of the second diffraction line depending on residual stress, E is Young's modulus, λ is the X-ray wavelength of CuKβ, θ1 is the diffraction angle of the first analyzed diffraction line, θ2 is the diffraction angle of the second analyzed diffraction line. The residual stresses (σI ) of the first kind were determined on the basis of Hooke's law using the equation:
∆a σI = a E
(3)
where ∆a / a is the percent elongation of the crystal lattice parameter under tension. 3. Results and discussion The X-ray investigation of the weld zone (Fig. 2) has shown the inhomogeneous decay of the supersaturated solid austenite solution occurring due to the thermal effect of welding. Structural polymorph249
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Fig. 2. X-ray diffraction pattern of the weld zone: (a) the weld-adjacent zone (No. 9′), (b) the heat-affected zone (No. 10′).
ism caused both the samples to acquire a mixed phase structure. One of the phases had a body-centered cubic (BCC) lattice and the other one was austenite (γ -Fe) with a face-centered cubic (FCC) latitude. On the external surface of zone No. 9′, the mass fraction of δ-Fe was 81% and that of γ-Fe was 19%; on the internal surface, the mass fractions of δ-Fe and γ-Fe were 83% and 16% respectively. On the external surface of zone No. 10′, the mass fraction of δ-Fe was 89% and that of γ-Fe was 11%; and the internal surface consists of δ-Fe only. In the heat-affected zone, the observed decay of austenite (a smaller proportion of γ-Fe) was more complete than in the weld-adjacent zone at a higher temperature. The inhomogeneous decay of the solid solution engendered point defects in the form of liberated impurities and their nonuniform distribution (Figs. 3 and 4). The liberation process was reversible and it was accompanied by the release of the dissolved impurities with the temperature and stress gradients. During the dissolution of impurities, vacancy pores appeared along the dissolution front, followed by microcracks and void coalescence. The final stage of these processes was the emergence of a macrocrack [11,12]. The greatest fluctuations of the chromium mass fraction were observed in the longitudinal direction of the weld axis (Fig. 4): the maximum mass fraction was reached at the external surface of weldadjacent zone (No. 9′); the minimum mass fraction was at the outer surface of the heat-affected zone (No. 10′). At the internal surface of these zones, a decrease in the chromium mass fraction from the axis to the periphery was less pronounced. One of the conventional theories defining the propensity of steel to the intergranular corrosion (IGC) is the theory of grain boundary depletion by chromium. This theory states that the chromium added to austenitic chromium-nickel steels is included in the austenitic grain as a solid solution with iron and combines with carbon and carbide metals (iron, manganese, etc.) to form complex carbides like (Cr, Fe, Mn)23С6. After some operation or during welding, the chromium redistributes between the carbides and
Fig. 3. Content of alloying elements (V, W, Mo, Ti, Co, Cu): (a) weld-adjacent zone (No. 9′), (b) heat-affected zone (No. 10′).
the austenite grain increasing the quantity of carbides. In some cases, the quantity of carbides can reach 60%. In the center of a grain, the chromium mass fraction is up to 17–19%, and at the grain boundaries, it decreases down to 8–10%. Similar data are given for steel 12Cr1MoV, initially containing, among other elements, 10–20% of 250
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its stratification by levels of microhardness. For example, at the external surface of the weld-adjacent zone, the microhardness was 1700 MPa; at the internal surface, it reached 3,400 MPa and in the middle of the wall, it amounted to 2200 MPa (Fig. 5a). The maximum (4000 MPa) and the minimum (1000 MPa) of the microhardness were reached near the middle of the wall. Within these ranges, the microhardness is highly oscillating. In the heat-affected zone, the microhardness was distributed as follows (Fig. 5b): the microhardness reached its maximum (3,750 MPa) near the middle of the pipe wall thickness and then decreased towards the external and internal surfaces down to a value about 1700 MPa. In general, the microhardness oscillated from 1400 to 3750 MPa. While being a strength parameter, microhardness also characterizes the material resistance to deformation and destruction [17]. A significant spread in the microhardness values provides a scatter of the metal strength properties, which results in superheater coils having various lifetimes under the same service conditions. Structural and mechanical inhomogeneity of butt-welds leads to significant errors in equipment reliability assessments [18]. In the process of long-term operation, the zonal distribution of microhardness along the wall thickness (Fig. 5) may lead to a decrease in high-temperature strength due to the emergence and development of ring cracks in the weld steam pipe. This assumption was confirmed by the detected stratification of the superheater wall (Fig. 6) as a result of deep oxidation at a high temperature. Fig. 7 shows the distribution of alloying element concentrations in zones No. 1–13 (as shown in Fig. 1d) obtained by X-ray method. There was a significant material heterogeneity of the sample zones. Under the impact of external factors (power, temperature), this heterogeneity of alloying elements can cause changes in the physical and mechanical properties of the material, distortion of the geometric shape with the development of stress concentrators and IGC destruction. Fig. 8 shows the X-ray diffraction pattern of the weld (zone No. 7).
Fig. 4. Content of alloying chromium (Cr).
chromium in the composition of carbides. However, after the operation at a temperature of 545°С for 150,000 h, the mass fraction of chromium in the composition of carbides increased up to 25–40%. Due to the depletion of grain boundaries by chromium, a local IGC could occur. There is information about the minimum chromium content of the austenite to ensure its corrosion resistance [13]. Previous studies have reported [3,14–16] that up to 80–85% of boiler failures at thermal power stations result from damaged pipe heating surfaces. The pipes supplied for the heating surfaces acquire anisotropic properties due to a complex manufacturing chain (drawing, rolling, annealing, etc.). The heterogeneity and discontinuity of the structure provide the scattering of mechanical properties along the length and the thickness of the pipe wall. To confirm this thesis, we have measured the microhardness along the thickness of the pipe wall made of 12Cr18Ni9Ti steel (as in Fig. 1d). The results of this investigation are presented in Fig. 5. Fig. 5 shows that the microhardness was different along the direction from the internal and external surfaces to the center of the pipe wall. The distribution of microhardness along the wall thickness shows
Fig. 5. The microhardness along the pipe wall thickness of 12Cr18Ni9Ti steel: (a) weld-adjacent zone (5′ and 9′), (b) heat-affected zone (4′ and 10′); weld-adjacent zone (No. 9′) heataffected zone (No. 10′); АС, ВС is the direction of microhardness measurement along the wall thickness.
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Fig. 6. Stratification structure of superheater pipe (according to central heating and power plant from one of the Russian regions): (a) the superheater pipe in the initial state; (b, c) the superheater pipe after various stages of deep oxidation.
minor fluctuations in the compositions of solid solutions unstable to the thermal influence of welding. The distribution of the chromium content in the weld (zone No. 7) is shown in Fig. 9. The maximum mass fraction of chromium was observed at the external surface of the weld at point 7a; the minimum mass fraction of chromium was observed at the external surface of the weld at point 7b. The chromium content fluctuations at the internal surface were negligible. Content (Figs. 7 and 9) and phase inhomogeneities (Fig. 8) under service conditions result in nonequilibrium stresses at the grain and interphase boundaries. Fig. 10 shows the values of the residual stresses of the first kind at various points of the weld (No. 7). These data indicate that the weld had uncompensated internal stresses. For example, at the external surface at the phase boundary the tensile stresses from alpha-iron amounted to 440 MPa, while the compressive stresses from the γ-phase were −69 MPa. Thus, a weld is a structure with structural stress gradients. A typical limiting state for a weld is a fragile crack. Various factors affect the initiation of a brittle failure: shape defects, cracks, lack of penetration, foreign inclusions, etc. Inhomogeneous internal structural stresses and their nonequilibrium distribution are a significant risk factor for brittle failure [19–22]. The magnitude of these stresses can reach the value of yield strength [23]. The most favorable effect by stresses was observed at the internal surface of the weld. In this case, the compressive stress can be a deterrent to the development of steel fatigue and corrosion cracking (Fig. 10). The obtained internal stress values are in good agreement with the data of [24,25]. Fig. 11 shows the X-ray results of the weld zone (12Cr1MoV steel, zone No. 4′ and 5′). The distribution of residual stresses in the zones No. 4′ and 5′ is shown in Fig. 12. The result of welding process was the structural inhomogeneity (Fig. 11) and the associated inhomogeneity of the residual stresses (Fig. 12). Residual stresses of the second kind in the zone No. 4′ amounted to 55 MPa and reached 285 MPa in the zone No. 5′. Residual stresses of the first kind had a compressing character. Residual stresses of the first kind may be deducted from, or added to, the stresses caused by external forces. Being added to the stresses caused by external forces, they can change the operational strength of the product. Fig. 11 shows that the microcracking more likely takes place in the body of the grain rather than in the periphery of the grain. The result of the stress gradient will be the intergranular damage in the weld-adjacent zone (No. 5′) with the appearance of longitudinal cracks. Thus, the effect of the surface damage mechanisms is entirely determined by the stresses occurring in the steel [20]. One of the main factors leading to the damage is the presence of a significant tensile
Fig. 7. Heterogeneity of alloying elements in the composite weld under study: (a–f) mass fractions of Ti, W, Mo, Cu, Co, V.
It can be seen that the internal surface of the weld was represented by α- and γ-phases of Fe in approximately equal amounts. There were traces of the third phase at the external surface, most likely due to
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Fig. 8. X-ray diffraction pattern of the weld from 12Cr18Ni9Ti and 12Cr1MoV steels: (a) internal surface of the weld, (b) external surface of the weld.
Fig. 9. Chromium content in the weld zone (zone designation according to Fig. 1b).
Fig. 10. Distribution of residual stresses of the first kind in weld zone No. 7.
Fig. 11. Content of alloying elements (V, W, Mo, Ti, Co, Cu) in 12Cr1MoV steel: (a) weld-adjacent zone (No. 5′), (b) heat-affected zone (No. 4′).
stress [21].
and chemical heterogeneity of the weld and its zones (Figs. 3, 6, 8, and 9). This indicates that the non-uniform fields of internal stresses may begin to form and failure conditions may emerge as well. 3. The phase, chemical and structural heterogeneity led to the zone distribution of the microhardness along the wall thickness (Fig. 4). There were zones with increased microhardness (Hv=400, Fig. 4a), local decrease in the ductility and impact hardness. These zones were prone to micro-damaging, which could cause brittle cracks. Zones with lower microhardness (Hv=100, Fig. 4A) had low creepresistance. Cracks in these zones may be associated with the presence of stress concentrators in the neighboring areas. The zone alternation of the high and low microhardness led to the stratification of the pipe wall structure (Fig. 6). 4. Structural and phase inhomogeneity causes high internal structural stresses in the weld zones due to thermal residual stresses, which are stress concentrators and internal sources of brittle cracks. 5. Phase transformations as a result of the welding process, a sharp heterogeneity of the properties in the weld zones as well as stress concentrators determine the need for the recovery of properties by a
4. Conclusions 1. The thermal influence of welding led to a significant stratification of the structure of the unstable austenite solid solution and theoretically unpredictably affected the metal structure of the welded construction. This impact was expressed in the form of different polymorphic α-γ-δ- transformations. The austenite taken directly from the weld zone underwent an α-γ-conversion, wherein the concentrations of the phases were an approximately equal (Fig. 7). The weld-adjacent zone and the heat-affected zone had the γ and δ phases in essentially different quantities at the internal and external surfaces (Fig. 2a, b). This phase heterogeneity led to a difference in the physical and mechanical properties along the wall thickness, which contributed to the emergence of the local stress concentrations and created preconditions for the emergence of microcracks. 2. Thermal non-uniformity due to the welding influence and the inhomogeneous decay of the γ-austenite solid solution led to a heterogeneous distribution of alloying elements, the electrochemical
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Fig. 12. Distribution of residual stresses in weld zones No. 4′ and 5′: (a) residual stresses of the second kind; (b) residual stresses of the first kind. [9] E.S. Gorkunov, Yu.V. Subachev, A.M. Povolotskaya, S.M. Zadvorkin, The influence of a preliminary plastic deformation on the behavior of the magnetic characteristics of high-strength controllably rolled pipe steel under an elastic uniaxial tension (compression), Russ. J. Nondestr. Test. 51 (9) (2015) 563–572. [10] Yu.M. Gofman, Estimation of the reliability of testing welded joints of steam pipelines of thermal power plants, Russ. J. Nondestr. Test. 39 (3) (2003) 230–231. [11] K.B. Sankara Rao, M. Valsan, S.L. Mannan, Strain-controlled low cycle fatigue behaviour of type 304 stainless steel base material, type 308 stainless stell weld metal and 304-308 stainless steel weldments, Mater. Sci. Eng. A 130 (1990) 67–82. [12] L.L. Lyubimova, A.A. Makeev, A.S. Zavorin, A.A. Tashlykov, A.I. Artamontsev, B.V. Lebedev, R.N. Fisenko, Consideration of intrastructural stresses in the processes connected with the effect of structural nonuniformity on corrosion damages inflicted to heattransfer tubes, Therm. Eng. 61 (8) (2014) 600–605. [13] C. Hoffmann, A.J. McEvily, The effect of high temperature low cycle fatigue on the corrosion resistance of austenitic stainless steels, Metall. Trans. A 13 (5) (1982) 923–927. [14] E.Y. Priymak, V.I. Gryzunov, Special features of behavior of the metal of convection superheater in the process of high-temperature operation, Metal. Sci. Heat. Treat. 53 (3–4) (2011) 136–140. [15] Q. Chu, M. Zhang, J. Li, Y. Chen, H. Luo, Q. Wang, Failure analysis of a steam pipe weld used in power generation plant, Eng. Fail. Anal. 44 (2014) 363–370. [16] M.R. Shayan, K. Ranjbar, E. Hajidavalloo, A. Heidari Kydan, On the failure analysis of an air preheater in a steam power plant, J. Fail. Anal. Prev. 15 (6) (2015) 941–951. [17] L. Tushinskii, I. Kovensky, A. Plokhov, V. Sindeyev, P. Reshedko, Coated Metal: Structure and Properties of Metal-coating Compositions, Springer Berlin Heidelberg, Berlin, 2002. [18] G. Cam, S. Erim, Ç. Yeni, M. Koçak, Determination of mechanical and fracture properties of laser beam welded steel joints, Weld. J. 78 (6) (1999) 193–201. [19] A.B. Popov, Specific features of the stressed state and estimation of the lifetime of stamped-and-welded elbows used in hot reheat steam lines of 500-MW power units, Therm. Eng. 57 (11) (2010) 982–988. [20] A.B. Popov, Main factors causing damage to the high-temperature heating surfaces used in power-generating boilers, Therm. Eng. 58 (2) (2011) 101–108. [21] S.A. Kharchenko, N.B. Trunov, N.F. Korotaev, S.L. Lyakishev, Measures for ensuring reliable operation of the welded joint connecting the reactor coolant circuit's header to the shell of a steam generator used at a VVER-1000 reactorbased nuclear power station, Therm. Eng. 58 (3) (2011) 208–214. [22] M. Bouras, A. Boumaiza, V. Ji, N. Rouag, XRD peak broadening characterization of deformed microstructures and heterogeneous behavior of carbon steel, Theor. Appl. Fract. Mech. 61 (1) (2012) 51–56. [23] N.I. Kamenskaya, A study of residual stresses and microstructure of joints welded with the use of lumped sources of welding heat, Metal. Sci. Heat. Treat. 57 (1) (2015) 48–51. [24] E.A. Grin, A.V. Zelenskii, Studying the stressed state and operating characteristics of high-pressure deaerators' metal and assessment of their longevity, Therm. Eng. 56 (2) (2009) 103–112. [25] P. Lehto, H. Remes, T. Saukkonen, H. Hänninen, J. Romanoff, Influence of grain size distribution on the Hall–Petch relationship of welded structural steel, Mater. Sci. Eng. A 592 (2014) 28–39.
post-weld heat treatment. However, in this case, the recovery operation can be extremely difficult because its efficiency is determined not only by the thermal cycle, but also by the structure. The thermal cycle of the property recovery will lead to its own features of structural and phase transformations as well as the redistribution and relaxation of stresses. In order to ensure the efficiency of the post-weld heat treatment, the microstructure must be controlled. The approaches proposed in this paper are a means to improve the operational reliability of a heterogeneous steel weld. These approaches may be implemented in the technical diagnostics during the post-weld heat treatment. Acknowledgments The reported study was funded by RFBR according to research project № 15-08-99544 А and within the framework of the strategic plan for the development of National Research Tomsk Polytechnic University as one of the world-leading universities (project № SAU– 25/2016). References [1] S.V. Panin, P.O. Maruschak, I.V. Vlasov, B.B. Ovechkin, Impact toughness of 12Cr1MoV steel. Part 1 – influence of temperature on energy and deformation parameters of fracture, Theor. Appl. Fract. Mech. 83 (2016) 105–113. [2] S.V. Panin, P.O. Maruschak, I.V. Vlasov, V.P. Sergeev, B.B. Ovechkin, V.V. Neifeld, Impact toughness of 12Cr1MoV steel. Part 2 – influence of high intensity ion beam irradiation on energy and deformation parameters and mechanisms of fracture, Theor. Appl. Fract. Mech. 83 (2016) 82–92. [3] H. Shokouhmand, B. Ghadimi, R. Espanani, Failure analysis and retrofitting of superheater tubes in utility boiler, Eng. Fail. Anal. 50 (2015) 20–28. [4] V.F. Rezinskikh, B.E. Shkol'nikova, G.A. Urusova, Promising steels for the superheaters of supercritical-pressure boilers, Therm. Eng. 47 (10) (2000) 899–903. [5] S. Lugin, Detection of hidden defects by lateral thermal flows, NDT E Int. 56 (2013) 48–55. [6] F. Berto, Fatigue and fracture assessment of notched components by means of the strain energy density, Eng. Fract. Mech. 167 (1) (2016) 176–187. [7] O. Yasniy, T. Vuherer, V. Yasniy, A. Sobchak, A. Sorochak, Mechanical behaviour of material of thermal power plant steam superheater collector after exploitation, Eng. Fail. Anal. 27 (2013) 262–271. [8] F. Castro Cerda, C. Goulas, I. Sabirov, S. Papaefthymiou, A. Monsalve, R.H. Petrov, Microstructure, texture and mechanical properties in a low carbon steel after ultrafast heating, Mater. Sci. Eng. A 672 (2016) 108–120.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
X-ray investigation of a heterogeneous steel weld ⁎
Lyudmila L. Lyubimova, Roman N. Fisenko, Roman B. Tabakaev , Alexander A. Tashlykov, Alexander S. Zavorin
crossmark
National Research Tomsk Polytechnic University, 30 Lenin Avenue, Tomsk, 634050, Russia
A R T I C L E I N F O
A BS T RAC T
Keywords: Technical diagnostics Heterogeneous steel weld X-ray analysis Microhardness Residual stresses
The welding process has a significant impact on the chemical composition, structure and mechanical properties of materials in heat-affected zones. The existing approaches to the diagnostics cannot predict the properties of the welded joint nor do they guarantee its reliable performance throughout the projected service life. This becomes especially relevant when investigating the welds from heterogeneous steels applied in power engineering. The main goal of this paper is to offer modern approaches to the complex technical diagnostics of steam pipelines using X-ray investigation of heterogeneous weld structures. In this work, the following research methods were used: X-ray fluorescence analysis, X-ray diffraction method, and microhardness measurement. The object of the investigation is the composite weld of 12Cr18Ni9Ti austenitic steel and 12Cr1MoV pearlitic steel. The results of this investigation show that the properties of the weldable steel changed significantly as compared to its original state. The weld structure was represented by the internal structural stress gradients despite the application of the stress relief heat treatment. The proposed approaches to the study of steel welds can be used for selecting the rational mode of post-weld heat treatment. Combined with traditional inspection methods (visual or ultrasonic inspection, etc.), these approaches can serve as a basis for technical diagnostics and the selection of constructional materials.
1. Introduction Austenitic steel 12Cr18Ni9Ti and pearlitic steel 12Cr1MoV are the main structural materials in Russia to produce ultra-high pressure boiler superheaters. With the design lifetime of about 300 thousand hours, they, however, collapse prematurely for various reasons [1–4]. Welds are damaged due to the combined impact of intrinsic defects and so-called hidden defects [5,6]. Intrinsic defects occur in the form of pinholes, long cracks, incomplete penetrations, cinder inclusions etc. during equipment production and installation. These stem from errors in the welding process, violation of accepted rules and standards, equipment failure, or poor production practices. The best way to fight these defects is to eliminate their causes. Hidden defects are connected with the microstructure of the weld material and its heat-affected zones. These result from phase, structural and chemical heterogeneity, which brings about differences in the physics, chemistry, strength and mechanical properties of various sections and zones of the welds. These defects are difficult to detect by standard diagnostic methods (visual or ultrasonic inspection, etc.) [7,8]. One of the fundamental reasons for premature failure is the ⁎
accelerated structural degradation of steel due to cyclic mechanical and temperature gradients. Structural transformations drastically affect the resistance of welds to brittle failure and the mechanical properties of the products [4,9]. An uncontrolled process of structural and phase transformations leads to unpredictable consequences, which manifest themselves in changes of current stresses with the most probable occurrence of local stress concentrators exceeding the longterm strength. The state of the phases in the weld zone, their transformation under service condition, the level of structural stresses and their redistribution still have no clear systematization. This gap in the knowledge could ultimately lead to the damage of a welded assembly at any time before the end of its service life. Based on a number of specialized scientific and regulatory documentation, the technical diagnostics of the welds should include the research of the metal composition, structure and properties with the methods of the magnetic particle and dye penetrant inspection, hardness test, ultrasonic thickness measurement, metallographic examination using replica techniques. Generally, the performance analysis of a weld involves destructive inspection techniques measuring mechanical properties, heat resistance, ductile to brittle transition temperature, brittle strength, creep speed, and rate of corrosion. These methods are very effort and time
Corresponding author. E-mail address: [email protected] (R.B. Tabakaev).
http://dx.doi.org/10.1016/j.msea.2016.11.058 Received 29 July 2016; Received in revised form 17 November 2016; Accepted 17 November 2016 Available online 17 November 2016 0921-5093/ © 2016 Elsevier B.V. All rights reserved.
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consuming. Currently, durability assessment is based on the long-term strength and fatigue of the weld material, S-N diagram, cumulative damage theory as well as the control of micro- and macrostructure. These measures, however, do not guarantee weld integrity. The assessment of weld reliability and integrity executed by different methods is sometimes controversial [10]. These methods do not provide any information on changes in the microstructure of the material during its service life under specified conditions. Consequently, a more comprehensive weld assessment is required, which, apart from the above, should include the distribution of microhardness, alloying elements and structural defects, the magnitude and sign of residual stresses as well as the phase composition of different zones and layers of the pipe wall. The chemical, structural and mechanical heterogeneity of the weld properties requires the heat treatment of a workpiece to stabilize its structure after the welding process. Heat treatment, in turn, requires the sufficient data on the structure, phase composition, level of microstresses and distribution of defects in the hazardous zones of a weld. However, these data are difficult to obtain using only the standard methods of diagnostics. Thus, the problem of identifying the desired properties of the weld structure remains unresolved. For such a problem, X-ray analysis may be a very effective method. The authors' review of recent papers suggests that the presence of X-ray is limited in this industry. Sometimes, it is not even considered as a diagnostic and forecast method, because it is allegedly difficult to apply in a production environment. However, the method does not require the destruction of the sample surface and allows repeated and long-term tests of the same sample. It can also be implemented quite successfully on a cutting sample or part of a workpiece. Thus, controlling the weld microstructure by hardness diagnostics and X-ray may substantially improve the efficiency of welding, which makes this work relevant and novel. The main goal of this paper is to offer advanced approaches to complex technical diagnostics of steam pipelines using the X-ray investigation of heterogeneous weld structures.
Fig. 1. Composite weld under study: (a) cutting scheme of transverse b, c and longitudinal d samples; (b) control points of the external surface weld; (c) control points of the internal surface weld; (d) control sample of the weld-adjacent zone (5′ and 9′) and of the heat-affected zone (4′ and 10′); АС, ВС –direction of microhardness measurement along the wall thickness; No. 1–13 – control zones.
2. Materials and methods The object of the investigation is the weld of austenitic 12Cr18Ni9Ti steel and pearlitic 12Cr1MoV steel. The welded joint was divided into zones No. 1–13 for the research (Fig. 1d). The weld zone (No. 7) was prepared for the research of the internal and external surfaces (Fig. 1b). Zones No. 5` (12Cr1MoV) and No. 9` (12Cr18Ni9Ti) were arranged at a distance of 7.5 mm from the weld axis. Zones No. 4` (12Cr1MoV) and No. 10` (12Cr18Ni9Ti) were arranged at a distance of 14.5 mm from the weld axis. The weld structure was investigated using an EDX 2800 X-ray analyzer (SKYRAY, USA), a DRON-3 X-ray diffractometer (Russia), and a PMT-3 microhardness tester (Russia). The X-ray fluorescence analysis was performed using EDX-2800 for measuring the spectra of the characteristic radiation of the chemical elements in the test sample. The chemical elements were identified by a standard calibration curve embedded in the spectrometer software. The quantitative composition of the alloy was determined by estimating the intensity and area of the experimental spectral lines. The X-ray diffraction method is based on the classical fundamental theory for the X-ray analysis of the internal stresses and crystal grain size using a thoroughly tested mathematical apparatus. The diffraction angle (2θ) of each diffraction line, the intensity and width of its profile are defined using the DRON-3. The full profile is determined by the combined effect of geometrical and physical factors. The physical profile of the diffraction line (β) is a function of the material properties and state. It depends on the crystallite size (D ) and the residual stresses of the second kind (σII ):
D=
0. 94∙λ m1 ∙ cos θ1
⎛ n ⎞ σII =⎜ 2 ⎟ ∙E ⎝ 4∙tgθ2 ⎠
(1)
(2)
where m1 is a part of the physical broadening of the first diffraction line depending on the crystallite size, n2 is a part of the physical broadening of the second diffraction line depending on residual stress, E is Young's modulus, λ is the X-ray wavelength of CuKβ, θ1 is the diffraction angle of the first analyzed diffraction line, θ2 is the diffraction angle of the second analyzed diffraction line. The residual stresses (σI ) of the first kind were determined on the basis of Hooke's law using the equation:
∆a σI = a E
(3)
where ∆a / a is the percent elongation of the crystal lattice parameter under tension. 3. Results and discussion The X-ray investigation of the weld zone (Fig. 2) has shown the inhomogeneous decay of the supersaturated solid austenite solution occurring due to the thermal effect of welding. Structural polymorph249
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Fig. 2. X-ray diffraction pattern of the weld zone: (a) the weld-adjacent zone (No. 9′), (b) the heat-affected zone (No. 10′).
ism caused both the samples to acquire a mixed phase structure. One of the phases had a body-centered cubic (BCC) lattice and the other one was austenite (γ -Fe) with a face-centered cubic (FCC) latitude. On the external surface of zone No. 9′, the mass fraction of δ-Fe was 81% and that of γ-Fe was 19%; on the internal surface, the mass fractions of δ-Fe and γ-Fe were 83% and 16% respectively. On the external surface of zone No. 10′, the mass fraction of δ-Fe was 89% and that of γ-Fe was 11%; and the internal surface consists of δ-Fe only. In the heat-affected zone, the observed decay of austenite (a smaller proportion of γ-Fe) was more complete than in the weld-adjacent zone at a higher temperature. The inhomogeneous decay of the solid solution engendered point defects in the form of liberated impurities and their nonuniform distribution (Figs. 3 and 4). The liberation process was reversible and it was accompanied by the release of the dissolved impurities with the temperature and stress gradients. During the dissolution of impurities, vacancy pores appeared along the dissolution front, followed by microcracks and void coalescence. The final stage of these processes was the emergence of a macrocrack [11,12]. The greatest fluctuations of the chromium mass fraction were observed in the longitudinal direction of the weld axis (Fig. 4): the maximum mass fraction was reached at the external surface of weldadjacent zone (No. 9′); the minimum mass fraction was at the outer surface of the heat-affected zone (No. 10′). At the internal surface of these zones, a decrease in the chromium mass fraction from the axis to the periphery was less pronounced. One of the conventional theories defining the propensity of steel to the intergranular corrosion (IGC) is the theory of grain boundary depletion by chromium. This theory states that the chromium added to austenitic chromium-nickel steels is included in the austenitic grain as a solid solution with iron and combines with carbon and carbide metals (iron, manganese, etc.) to form complex carbides like (Cr, Fe, Mn)23С6. After some operation or during welding, the chromium redistributes between the carbides and
Fig. 3. Content of alloying elements (V, W, Mo, Ti, Co, Cu): (a) weld-adjacent zone (No. 9′), (b) heat-affected zone (No. 10′).
the austenite grain increasing the quantity of carbides. In some cases, the quantity of carbides can reach 60%. In the center of a grain, the chromium mass fraction is up to 17–19%, and at the grain boundaries, it decreases down to 8–10%. Similar data are given for steel 12Cr1MoV, initially containing, among other elements, 10–20% of 250
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its stratification by levels of microhardness. For example, at the external surface of the weld-adjacent zone, the microhardness was 1700 MPa; at the internal surface, it reached 3,400 MPa and in the middle of the wall, it amounted to 2200 MPa (Fig. 5a). The maximum (4000 MPa) and the minimum (1000 MPa) of the microhardness were reached near the middle of the wall. Within these ranges, the microhardness is highly oscillating. In the heat-affected zone, the microhardness was distributed as follows (Fig. 5b): the microhardness reached its maximum (3,750 MPa) near the middle of the pipe wall thickness and then decreased towards the external and internal surfaces down to a value about 1700 MPa. In general, the microhardness oscillated from 1400 to 3750 MPa. While being a strength parameter, microhardness also characterizes the material resistance to deformation and destruction [17]. A significant spread in the microhardness values provides a scatter of the metal strength properties, which results in superheater coils having various lifetimes under the same service conditions. Structural and mechanical inhomogeneity of butt-welds leads to significant errors in equipment reliability assessments [18]. In the process of long-term operation, the zonal distribution of microhardness along the wall thickness (Fig. 5) may lead to a decrease in high-temperature strength due to the emergence and development of ring cracks in the weld steam pipe. This assumption was confirmed by the detected stratification of the superheater wall (Fig. 6) as a result of deep oxidation at a high temperature. Fig. 7 shows the distribution of alloying element concentrations in zones No. 1–13 (as shown in Fig. 1d) obtained by X-ray method. There was a significant material heterogeneity of the sample zones. Under the impact of external factors (power, temperature), this heterogeneity of alloying elements can cause changes in the physical and mechanical properties of the material, distortion of the geometric shape with the development of stress concentrators and IGC destruction. Fig. 8 shows the X-ray diffraction pattern of the weld (zone No. 7).
Fig. 4. Content of alloying chromium (Cr).
chromium in the composition of carbides. However, after the operation at a temperature of 545°С for 150,000 h, the mass fraction of chromium in the composition of carbides increased up to 25–40%. Due to the depletion of grain boundaries by chromium, a local IGC could occur. There is information about the minimum chromium content of the austenite to ensure its corrosion resistance [13]. Previous studies have reported [3,14–16] that up to 80–85% of boiler failures at thermal power stations result from damaged pipe heating surfaces. The pipes supplied for the heating surfaces acquire anisotropic properties due to a complex manufacturing chain (drawing, rolling, annealing, etc.). The heterogeneity and discontinuity of the structure provide the scattering of mechanical properties along the length and the thickness of the pipe wall. To confirm this thesis, we have measured the microhardness along the thickness of the pipe wall made of 12Cr18Ni9Ti steel (as in Fig. 1d). The results of this investigation are presented in Fig. 5. Fig. 5 shows that the microhardness was different along the direction from the internal and external surfaces to the center of the pipe wall. The distribution of microhardness along the wall thickness shows
Fig. 5. The microhardness along the pipe wall thickness of 12Cr18Ni9Ti steel: (a) weld-adjacent zone (5′ and 9′), (b) heat-affected zone (4′ and 10′); weld-adjacent zone (No. 9′) heataffected zone (No. 10′); АС, ВС is the direction of microhardness measurement along the wall thickness.
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Fig. 6. Stratification structure of superheater pipe (according to central heating and power plant from one of the Russian regions): (a) the superheater pipe in the initial state; (b, c) the superheater pipe after various stages of deep oxidation.
minor fluctuations in the compositions of solid solutions unstable to the thermal influence of welding. The distribution of the chromium content in the weld (zone No. 7) is shown in Fig. 9. The maximum mass fraction of chromium was observed at the external surface of the weld at point 7a; the minimum mass fraction of chromium was observed at the external surface of the weld at point 7b. The chromium content fluctuations at the internal surface were negligible. Content (Figs. 7 and 9) and phase inhomogeneities (Fig. 8) under service conditions result in nonequilibrium stresses at the grain and interphase boundaries. Fig. 10 shows the values of the residual stresses of the first kind at various points of the weld (No. 7). These data indicate that the weld had uncompensated internal stresses. For example, at the external surface at the phase boundary the tensile stresses from alpha-iron amounted to 440 MPa, while the compressive stresses from the γ-phase were −69 MPa. Thus, a weld is a structure with structural stress gradients. A typical limiting state for a weld is a fragile crack. Various factors affect the initiation of a brittle failure: shape defects, cracks, lack of penetration, foreign inclusions, etc. Inhomogeneous internal structural stresses and their nonequilibrium distribution are a significant risk factor for brittle failure [19–22]. The magnitude of these stresses can reach the value of yield strength [23]. The most favorable effect by stresses was observed at the internal surface of the weld. In this case, the compressive stress can be a deterrent to the development of steel fatigue and corrosion cracking (Fig. 10). The obtained internal stress values are in good agreement with the data of [24,25]. Fig. 11 shows the X-ray results of the weld zone (12Cr1MoV steel, zone No. 4′ and 5′). The distribution of residual stresses in the zones No. 4′ and 5′ is shown in Fig. 12. The result of welding process was the structural inhomogeneity (Fig. 11) and the associated inhomogeneity of the residual stresses (Fig. 12). Residual stresses of the second kind in the zone No. 4′ amounted to 55 MPa and reached 285 MPa in the zone No. 5′. Residual stresses of the first kind had a compressing character. Residual stresses of the first kind may be deducted from, or added to, the stresses caused by external forces. Being added to the stresses caused by external forces, they can change the operational strength of the product. Fig. 11 shows that the microcracking more likely takes place in the body of the grain rather than in the periphery of the grain. The result of the stress gradient will be the intergranular damage in the weld-adjacent zone (No. 5′) with the appearance of longitudinal cracks. Thus, the effect of the surface damage mechanisms is entirely determined by the stresses occurring in the steel [20]. One of the main factors leading to the damage is the presence of a significant tensile
Fig. 7. Heterogeneity of alloying elements in the composite weld under study: (a–f) mass fractions of Ti, W, Mo, Cu, Co, V.
It can be seen that the internal surface of the weld was represented by α- and γ-phases of Fe in approximately equal amounts. There were traces of the third phase at the external surface, most likely due to
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Fig. 8. X-ray diffraction pattern of the weld from 12Cr18Ni9Ti and 12Cr1MoV steels: (a) internal surface of the weld, (b) external surface of the weld.
Fig. 9. Chromium content in the weld zone (zone designation according to Fig. 1b).
Fig. 10. Distribution of residual stresses of the first kind in weld zone No. 7.
Fig. 11. Content of alloying elements (V, W, Mo, Ti, Co, Cu) in 12Cr1MoV steel: (a) weld-adjacent zone (No. 5′), (b) heat-affected zone (No. 4′).
stress [21].
and chemical heterogeneity of the weld and its zones (Figs. 3, 6, 8, and 9). This indicates that the non-uniform fields of internal stresses may begin to form and failure conditions may emerge as well. 3. The phase, chemical and structural heterogeneity led to the zone distribution of the microhardness along the wall thickness (Fig. 4). There were zones with increased microhardness (Hv=400, Fig. 4a), local decrease in the ductility and impact hardness. These zones were prone to micro-damaging, which could cause brittle cracks. Zones with lower microhardness (Hv=100, Fig. 4A) had low creepresistance. Cracks in these zones may be associated with the presence of stress concentrators in the neighboring areas. The zone alternation of the high and low microhardness led to the stratification of the pipe wall structure (Fig. 6). 4. Structural and phase inhomogeneity causes high internal structural stresses in the weld zones due to thermal residual stresses, which are stress concentrators and internal sources of brittle cracks. 5. Phase transformations as a result of the welding process, a sharp heterogeneity of the properties in the weld zones as well as stress concentrators determine the need for the recovery of properties by a
4. Conclusions 1. The thermal influence of welding led to a significant stratification of the structure of the unstable austenite solid solution and theoretically unpredictably affected the metal structure of the welded construction. This impact was expressed in the form of different polymorphic α-γ-δ- transformations. The austenite taken directly from the weld zone underwent an α-γ-conversion, wherein the concentrations of the phases were an approximately equal (Fig. 7). The weld-adjacent zone and the heat-affected zone had the γ and δ phases in essentially different quantities at the internal and external surfaces (Fig. 2a, b). This phase heterogeneity led to a difference in the physical and mechanical properties along the wall thickness, which contributed to the emergence of the local stress concentrations and created preconditions for the emergence of microcracks. 2. Thermal non-uniformity due to the welding influence and the inhomogeneous decay of the γ-austenite solid solution led to a heterogeneous distribution of alloying elements, the electrochemical
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post-weld heat treatment. However, in this case, the recovery operation can be extremely difficult because its efficiency is determined not only by the thermal cycle, but also by the structure. The thermal cycle of the property recovery will lead to its own features of structural and phase transformations as well as the redistribution and relaxation of stresses. In order to ensure the efficiency of the post-weld heat treatment, the microstructure must be controlled. The approaches proposed in this paper are a means to improve the operational reliability of a heterogeneous steel weld. These approaches may be implemented in the technical diagnostics during the post-weld heat treatment. Acknowledgments The reported study was funded by RFBR according to research project № 15-08-99544 А and within the framework of the strategic plan for the development of National Research Tomsk Polytechnic University as one of the world-leading universities (project № SAU– 25/2016). References [1] S.V. Panin, P.O. Maruschak, I.V. Vlasov, B.B. Ovechkin, Impact toughness of 12Cr1MoV steel. Part 1 – influence of temperature on energy and deformation parameters of fracture, Theor. Appl. Fract. Mech. 83 (2016) 105–113. [2] S.V. Panin, P.O. Maruschak, I.V. Vlasov, V.P. Sergeev, B.B. Ovechkin, V.V. Neifeld, Impact toughness of 12Cr1MoV steel. Part 2 – influence of high intensity ion beam irradiation on energy and deformation parameters and mechanisms of fracture, Theor. Appl. Fract. Mech. 83 (2016) 82–92. [3] H. Shokouhmand, B. Ghadimi, R. Espanani, Failure analysis and retrofitting of superheater tubes in utility boiler, Eng. Fail. Anal. 50 (2015) 20–28. [4] V.F. Rezinskikh, B.E. Shkol'nikova, G.A. Urusova, Promising steels for the superheaters of supercritical-pressure boilers, Therm. Eng. 47 (10) (2000) 899–903. [5] S. Lugin, Detection of hidden defects by lateral thermal flows, NDT E Int. 56 (2013) 48–55. [6] F. Berto, Fatigue and fracture assessment of notched components by means of the strain energy density, Eng. Fract. Mech. 167 (1) (2016) 176–187. [7] O. Yasniy, T. Vuherer, V. Yasniy, A. Sobchak, A. Sorochak, Mechanical behaviour of material of thermal power plant steam superheater collector after exploitation, Eng. Fail. Anal. 27 (2013) 262–271. [8] F. Castro Cerda, C. Goulas, I. Sabirov, S. Papaefthymiou, A. Monsalve, R.H. Petrov, Microstructure, texture and mechanical properties in a low carbon steel after ultrafast heating, Mater. Sci. Eng. A 672 (2016) 108–120.
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