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Study of annealing temperature effect on stress-corrosion cracking of aluminum brass heat-exchangers tubes by microdiffraction experiments
Engineering Failure Analysis 15 (2008) 54–61 www.elsevier.com/locate/engfailanal
Study of annealing temperature effect on stress-corrosion cracking of aluminum brass heat-exchangers tubes by microdiffraction experiments Annalisa Pola *, Marcello Gelfi, Laura Eleonora Depero, Roberto Roberti Mechanical Department, University of Brescia, via Branze 38, 25123 Brescia, Italy Received 11 December 2006; accepted 7 January 2007 Available online 26 January 2007
Abstract Laboratory investigations were carried out on C68700 copper alloy heat-exchangers tubes to study the effect of stress relieving annealing at different temperature and time to prevent SCC phenomena. The analysis includes metallographic characterizations, mercurous nitrate tests and residual stress measurements by X-ray microdiffraction. A significant decrease of residual stresses was registered after standard annealing at 250 °C; similar results were obtained at 200 °C for longer time demonstrating the opportunity to achieve a good stress-relief also at lower temperature. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Stress-corrosion cracking; Stress-relief; Residual stress; X-ray analysis
1. Introduction Stress-corrosion cracking (SCC) or ‘‘season cracking’’ is a phenomenon of spontaneously cracking of susceptible alloys in the simultaneous presence of sufficiently high tensile stresses and specific corrosive environments. Sources of stresses can ensue from ordinary service or residual stresses resulting from non-uniform plastic strain during cold forming of alloys. This is the case of copper and copper alloys used for condenser and heatexchanger tubes with integral fins on which the external and/or internal surfaces have been enhanced by cold work finning [1] to increase their in service efficiency. Brass alloys have good or poor cold working behaviour depending on the amount of Zn in the alloy. Typically, brass alloys with low Zn contents have good or excellent cold working characteristics, while brasses with high Zn contents have poor cold working qualities. In general, cold working operations leave internal
*
Corresponding author. Tel.: +39 030 3715576; fax: +39 030 3702448. E-mail addresses: [email protected] (A. Pola), marcello.gelfi@ing.unibs.it (M. Gelfi), [email protected] (L. Eleonora Depero), [email protected] (R. Roberti). 1350-6307/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2007.01.004
A. Pola et al. / Engineering Failure Analysis 15 (2008) 54–61
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stresses, which can lead to stress-corrosion cracking. The damage appearance in aluminium brass tubes is characterized by cracked areas, which usually show markedly branched transgranular crack patterns. Usually, environment associated with SSC contains ammonia and ammonium compounds, but atmospheres containing sulphur dioxide or nitrites can also determine this corrosion phenomenon. Copper alloys containing 20–40% Zn are highly susceptible to SCC [2]. Thus stress-corrosion cracking can be controlled and prevented in two ways: by selecting an alloy with high resistance to the phenomena (i.e. an alloy with low Zn content) and by reducing residual stresses by annealing. This heat treatment, commonly performed at about 250 °C, is also carried out in order to minimize the amount of distortion which may occur during machining. This low temperature heat treatment is also known as ‘‘stress-relief annealing’’, and should not affect the mechanical properties of the material. The standard test used to verify excessive internal stress in tubes and components of brass and other copper alloys is the mercourous nitrate test (ASTM B 154, BS2871: part 3, BS EN ISO 196). However, this is only a qualitative test used to certificate the acceptability of components and it does not give the residual stress value. When different heat treatments and different stress-relief levels must be distinguished, other techniques, such as the X-ray diffraction (XRD), are used. XRD is one of the most common technique to calculate residual stress, since it is phase selective, noncontact and non-destructive. However, the dimension of the X-ray spot (some millimetres squares) may be a problem if the component has a complex geometry, small dimensions or strongly curved surfaces. In this case, to avoid the defocusing errors and the limitations in spatial resolution, the microdiffraction technique can be used. This technique is already largely applied at the synchrotron facilities [3], but has become accessible for laboratory apparatus only few years ago [4]. It employs incidence beam collimator to reduce the spot size and allows the analysis of small quantities of materials and the examination of selected small areas. In this work microdiffraction was used to study the residual stress, which occurs on fins and internal surface of aluminium brass tubes for heat-exchanger applications, and their changes after annealing treatment. Considering that ASTM SB-359 (‘‘specification for copper and copper-alloy seamless condenser and heatexchanger tubes with integral fins’’) does not directly prescribe stress-relief anneal conditions, thus the aim of this research was to define the effect of different annealing temperature/time combinations on microstructure, hardness, and residual stresses to prevent SCC. 2. Experimental procedure The samples used for this study are finned tubes of two different diameters (1 in. and 3/4 in.) in aluminum brass UNS C68700 with the following average chemical composition: 77.5% Cu, 20% Zn, 2% Al and 0.1% As. This alloy is normally used for applications as condenser, evaporator and heat-exchanger tubing, condenser tubing plates, distiller tubing, ferrules, thanks to its excellent corrosion resistance, excellent cold workability for forming and bending. Specimens were heat treated, under ambient atmosphere, in a laboratory furnace preheated at the testing temperature. Cooling down to room temperature was carried out in the switched off open furnace. Samples have been designed according to the numbers reported in Table 1.
Table 1 Designation of the samples Number
Condition of the sample
0 1 2 3 4 5 6 7
As produced Treated at 250 °C-5 h Treated at 250 °C-3 h Treated at 250 °C-1 h Treated at 200 °C-3 h Treated at 200 °C-5 h Treated at 200 °C-10 h Treated at 200 °C-24 h
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Vickers microhardness has been measured under 100 g load, starting from either the head or the root of a fin. The metallographic analysis of H2SO4 (80 ml H2O, 5 ml H2SO4 and 10 g sodium bicarbonate) etched sample sections was carried out by means of optical microscope (Reichert-Jung MeF3). The stress-relief annealed samples have been subjected to the mercurous nitrate test. Residual stresses were finally measured by X-ray microdiffraction spectra, collected by a D/max-RAPID Rigaku microdiffractometer with Cu Ka radiation. This system is equipped with a cylindrical imaging plate (IP) detector which can measure two-dimensional (2D) X-ray diffraction from 45° to 160° (2h). 3. Results 3.1. Hardness tests Microhardness has been measured in stress relieved tubes and compared with the microhardness profile of the as-produced ones. In Fig. 1 some microhardness profiles along the thickness of 1 in. tubes are reported as an example of the results obtained. Hardness profiles in as-fabricated tubes show the highest values within the fin, due to work hardening, and a progressive decrease throughout the inner diameter. A general scatter in microhardness measurements is observed, probably due to the comparable size of the crystalline grains and microhardness indentation. Since Vickers microhardness is not affected by stress relieving at 250 °C, further measurements have not been carried out on samples stress relieved at 200 °C. 3.2. Microstructure A first set of microstructures, taken from the fin zone of as fabricated and 250 °C-5 h stress relieved samples, are shown in Figs. 2 and 3. The fin region has been firstly investigated in order to verify the effect of stress relieving at the higher temperatures and for the longest time.
Fig. 1. Microhardness of 1 in. tubes measured from the fin head.
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Fig. 2. Fin zone microstructure of 1 in. tubes in condition 0 (left) and condition 1 (right).
Fig. 3. Fin zone microstructure of 3/4 in. tubes in condition 0 (left) and condition 1 (right).
As produced tubes have been observed for the comparison. No recrystallization can be observed in the work hardened zone; however a somewhat different appearance seems detectable, as stress relieved grains show less disturbed metal in comparison to as fabricated grains. A second set of microstructures is reported in Figs. 4 and 5 and shows the inner diameter zone for the as fabricated tubes and 250 °C-5 h stress relieved tubes. Microstructure observation aimed in this case at investigating the metallurgical appearance of the region that is firstly affected by the stress-corrosion attack in the mercurous nitrate test. No significant differences are observed between inner diameter as-fabricated and stress relieved samples. The inner diameter microstructure, therefore, is not affected by the finning enhancement of the outer diameter.
Fig. 4. Inner zone microstructure of 1 in. tubes in condition 0 (left) and condition 1 (right).
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Fig. 5. Inner zone microstructure of 3/4 in. tubes in condition 0 (left) and condition 1 (right).
3.3. Mercurous nitrate test Accelerated stress-corrosion tests according to the mercurous nitrate procedure have been carried out by an external laboratory and results are shown in Table 2. The stress-corrosion attack was reported to affect always the inner surface of the tubes. Specimens stress relieved at 200 °C-24 h were not affected by the mercurous nitrate test, as well as all specimens stress relieved at 250 °C-5 h and 3 h. 3.4. Microdiffraction analysis Fig. 6 shows the 2D image collected in 5 min from the fin of a brass tube as fabricated by using a beam collimator of 800 lm. Debye rings appear smooth, indicating the presence of small grains and not uniform in intensity because of texture, due to the cold working operations. Table 2 Results of the mercurous nitrate test Sample
Temperature (°C)
Time (h)
Results
1 2 3 4 5 6 7
250 250 250 200 200 200 200
5 3 1 3 5 10 24
Not cracked Not cracked Cracked Cracked Cracked Cracked Not cracked
Fig. 6. 2D microdiffraction image collected from 3/4 in. tube in condition 0.
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Fig. 7. XRD patterns from 3/4 in. tube (upper) and 1 in. tube (lower) condition 0. The intensities are in arbitrary units.
Samples after heat treatment at 250 °C show similar 2D images, confirming the absence of recrystallization as observed by the metallographic analysis. Integrating in b the 2D image, powder diffraction pattern can be observed and analysed by the appropriate software to obtain phase identification, quantitative analysis, crystallite size determination, residual stress measurements, etc. [5]. Fig. 7 shows integrated diffraction patterns from 3/4 in. tube (upper line) and 1 in. tube (lower line) as fabricated. The vertical bars are the peaks of a-brass phase (JC-PDF card. no. 501333). The experimental peaks positions are shifted to higher 2h angles with respect to the tabulated ones. This difference can be due to the presence of a small amount of Al in the structure and to the different content of Cu. Residual stresses were measured on brass samples as fabricated (type 0) and on two samples which pass the mercurous nitrate test (samples 1 and 7, annealed at 250 °C and 200 °C, respectively) in order to quantify the stress relaxation due to the thermal treatments. Only few microns in depth are probed depending on the penetration of the X-rays into the material. After sectioning, stresses were calculated on both external and internal surfaces of the tubes. X-ray diffraction analysis generally needs a flat surface in order to prevent defocusing errors which can occur during the tilting of w angle. However, curved samples can be measured provided that the X-ray beam size is about four time smaller respect to the curvature of the specimen. In this case, it means that the beam spot must be smaller than 3 mm and 2.4 mm for 1 in. and 3/4 in. samples respectively. On the other hand, the spot should be longer enough to allow a sufficient number of crystallites to diffract. For these reasons the measurements on the internal diameter were performed by using a beam collimator of 800 lm. The second problem is related to the measurement of external surface which is characterized by fins with a thickness of about 0.5 mm. In this case, to avoid edge effects the irradiated area was reduced by using a collimator of 300 lm. A video microscope allowed a very accurate adjustment of the sample position in the X-ray beam, to focus the head of fin (Fig. 8).
Fig. 8. Image of the sample and increasing magnifications to focus a single fin.
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Table 3 Residual stresses on fin head Stress (MPa) 1 in. 0 1 in. 1 3/4 in. 0 3/4 in. 1
109 ± 11 112 ± 11 154 ± 22 114 ± 24
FHWM 0.932 0.808 1.035 1.055
Table 4 Residual stresses on the internal diameter
1 in. 0 1 in. 1 1 in. 7 3/4 in. 0 3/4 in. 1 3/4 in. 7
Stress (MPa)
FHWM
289 ± 5 122 ± 4 139 ± 5 263 ± 10 172 ± 3 171 ± 5
1.26 1.22 1.14 1.35 1.23 1.21
The residual stresses on the internal surface of tubes were evaluated using the sin 2w technique [6] on the a-brass 420 peak with more than 10w tilts, from 0° to 50°. For the fins analysis it was studied the shift of 311 peak, because better defined respect to the 420 peak. Peak positions were determined by fitting a parabola on the diffraction data after LPA correction, background subtraction and Ka2 stripping. The elastic constants used for the stress analysis were E = 117 GPa and m = 0.29 [7]. Residual stresses results are reported in Tables 3 and 4. On the fin head the residual stresses are always in compression. The measurements were affected by some errors probably because of the curved surface and to the very small spot size, which limited the number of diffractions grains. On the contrary, the internal diameter of the tubes results always in tension. Note that the stresses of the deformed surface layers after cold working (condition 0) exceeds bulk yield strength of the alloy, which is about 190 MPa, as already reported in literature for other materials [8]. After annealing at 250 °C-5 h and 200 °C-24 h, there is a definite decrease of residual stresses, especially in the case of 1 in. tubes. The stress levels reached at the end of these heat treatments are similar, concluding that they are both effective for SCC prevention. In Tables 3 and 4 are also reported the breaths at half height of the diffraction peak (FHWM) used for residual stress measurement, which can be directly related to material properties such as hardness, cold working and yield strength [9,10]. In fact, the broadening is primarily the result of two related phenomena: a reduction of the ‘‘crystallite’’ or coherent diffracting domain size, and an increase of microstrains. Results showed a small decrease of peak breath, probably indicating a substructural change in dislocation density and a rearrangement into cellular patterns, except in the case of 3/4 in. sample on fin head. 4. Conclusions Metallographic investigations on the effect of stress relieving at 250 °C for 1, 3 and 5 h, and at 200 °C for 3, 5, 10 and 24 h on fin enhanced tubes made of C68700 copper alloy allows to conclude that: – microstructure is not significantly changed, and no recrystallization is observed in the work hardened zones and – hardness throughout the thickness is not strongly modified with respect to the as finned state. So the improved strength due to the cold working is retained.
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These observation was in a sense confirmed by microdiffraction analysis, which also permit to assess the residual stress state present in these samples: compressive on the head of the fins and tensile on the internal diameter. A significant decrease of tensile residual stresses was registered after annealing at 250 °C for 5 h as already reported in literature. Similar results were also obtained by heat treatment at 200 °C for 24 h, demonstrating the possibility to obtain a good stress-relief with lower temperature for longer time. This is consistent with the results of mercurous nitrate tests. The lower heat treatment temperature gives the opportunity to stress-relieve the heat-exchangers tubes directly into the plant by circulating superheated steam, with a consequent time and energy saving. Also, microdiffraction appears to be a very useful technique for non-destructive residual stress analysis on samples with complex geometry and/or small dimensions and, in general, it can be considered as an important tool to support metallurgical research and failure analysis. References [1] [2] [3] [4] [5] [6] [7] [8]
Loconsolo V, Nobili L. Manuale degli ottoni. Milano: Consedit sas; 1995. Thompson DH, Tracey AW. Trans Amer Inst Min Met Eng 1949:185. Shimura T, Misaki H, Umeno M, Takahashi I, Harada J. J Cryst Growth 1996;166:786. Zontone F, D’Acapito F, Gonella F. Nucl Instr Meth B 1999;147:416. Uekusa H. Rigaku J 1999;16(2). Noyan IC, Cohen JB. Residual stress measurement by diffraction and interpretation. New York: Springer-Verlag; 1987. Internet: www.matweb.com. Preve´y PS. In: Friel J, Van der Voort G, editors. Developments in materials characterization technologies. Material Park, OH: ASM International; 1996. p. 103–10. [9] Hornbach PDJ, Prevey PS, Mason PW. Proceedings: First international conference on induction hardened gears, May 15–17. Indianapolis, IN: Gear Reaserch Institute; 1995. p. 69–76. [10] Preve´y PS. In: Young WB, editor. Process and materials selection. Metals Park, OH: ASM; 1987. p. 11–9.
Study of annealing temperature effect on stress-corrosion cracking of aluminum brass heat-exchangers tubes by microdiffraction experiments Annalisa Pola *, Marcello Gelfi, Laura Eleonora Depero, Roberto Roberti Mechanical Department, University of Brescia, via Branze 38, 25123 Brescia, Italy Received 11 December 2006; accepted 7 January 2007 Available online 26 January 2007
Abstract Laboratory investigations were carried out on C68700 copper alloy heat-exchangers tubes to study the effect of stress relieving annealing at different temperature and time to prevent SCC phenomena. The analysis includes metallographic characterizations, mercurous nitrate tests and residual stress measurements by X-ray microdiffraction. A significant decrease of residual stresses was registered after standard annealing at 250 °C; similar results were obtained at 200 °C for longer time demonstrating the opportunity to achieve a good stress-relief also at lower temperature. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Stress-corrosion cracking; Stress-relief; Residual stress; X-ray analysis
1. Introduction Stress-corrosion cracking (SCC) or ‘‘season cracking’’ is a phenomenon of spontaneously cracking of susceptible alloys in the simultaneous presence of sufficiently high tensile stresses and specific corrosive environments. Sources of stresses can ensue from ordinary service or residual stresses resulting from non-uniform plastic strain during cold forming of alloys. This is the case of copper and copper alloys used for condenser and heatexchanger tubes with integral fins on which the external and/or internal surfaces have been enhanced by cold work finning [1] to increase their in service efficiency. Brass alloys have good or poor cold working behaviour depending on the amount of Zn in the alloy. Typically, brass alloys with low Zn contents have good or excellent cold working characteristics, while brasses with high Zn contents have poor cold working qualities. In general, cold working operations leave internal
*
Corresponding author. Tel.: +39 030 3715576; fax: +39 030 3702448. E-mail addresses: [email protected] (A. Pola), marcello.gelfi@ing.unibs.it (M. Gelfi), [email protected] (L. Eleonora Depero), [email protected] (R. Roberti). 1350-6307/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2007.01.004
A. Pola et al. / Engineering Failure Analysis 15 (2008) 54–61
55
stresses, which can lead to stress-corrosion cracking. The damage appearance in aluminium brass tubes is characterized by cracked areas, which usually show markedly branched transgranular crack patterns. Usually, environment associated with SSC contains ammonia and ammonium compounds, but atmospheres containing sulphur dioxide or nitrites can also determine this corrosion phenomenon. Copper alloys containing 20–40% Zn are highly susceptible to SCC [2]. Thus stress-corrosion cracking can be controlled and prevented in two ways: by selecting an alloy with high resistance to the phenomena (i.e. an alloy with low Zn content) and by reducing residual stresses by annealing. This heat treatment, commonly performed at about 250 °C, is also carried out in order to minimize the amount of distortion which may occur during machining. This low temperature heat treatment is also known as ‘‘stress-relief annealing’’, and should not affect the mechanical properties of the material. The standard test used to verify excessive internal stress in tubes and components of brass and other copper alloys is the mercourous nitrate test (ASTM B 154, BS2871: part 3, BS EN ISO 196). However, this is only a qualitative test used to certificate the acceptability of components and it does not give the residual stress value. When different heat treatments and different stress-relief levels must be distinguished, other techniques, such as the X-ray diffraction (XRD), are used. XRD is one of the most common technique to calculate residual stress, since it is phase selective, noncontact and non-destructive. However, the dimension of the X-ray spot (some millimetres squares) may be a problem if the component has a complex geometry, small dimensions or strongly curved surfaces. In this case, to avoid the defocusing errors and the limitations in spatial resolution, the microdiffraction technique can be used. This technique is already largely applied at the synchrotron facilities [3], but has become accessible for laboratory apparatus only few years ago [4]. It employs incidence beam collimator to reduce the spot size and allows the analysis of small quantities of materials and the examination of selected small areas. In this work microdiffraction was used to study the residual stress, which occurs on fins and internal surface of aluminium brass tubes for heat-exchanger applications, and their changes after annealing treatment. Considering that ASTM SB-359 (‘‘specification for copper and copper-alloy seamless condenser and heatexchanger tubes with integral fins’’) does not directly prescribe stress-relief anneal conditions, thus the aim of this research was to define the effect of different annealing temperature/time combinations on microstructure, hardness, and residual stresses to prevent SCC. 2. Experimental procedure The samples used for this study are finned tubes of two different diameters (1 in. and 3/4 in.) in aluminum brass UNS C68700 with the following average chemical composition: 77.5% Cu, 20% Zn, 2% Al and 0.1% As. This alloy is normally used for applications as condenser, evaporator and heat-exchanger tubing, condenser tubing plates, distiller tubing, ferrules, thanks to its excellent corrosion resistance, excellent cold workability for forming and bending. Specimens were heat treated, under ambient atmosphere, in a laboratory furnace preheated at the testing temperature. Cooling down to room temperature was carried out in the switched off open furnace. Samples have been designed according to the numbers reported in Table 1.
Table 1 Designation of the samples Number
Condition of the sample
0 1 2 3 4 5 6 7
As produced Treated at 250 °C-5 h Treated at 250 °C-3 h Treated at 250 °C-1 h Treated at 200 °C-3 h Treated at 200 °C-5 h Treated at 200 °C-10 h Treated at 200 °C-24 h
56
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Vickers microhardness has been measured under 100 g load, starting from either the head or the root of a fin. The metallographic analysis of H2SO4 (80 ml H2O, 5 ml H2SO4 and 10 g sodium bicarbonate) etched sample sections was carried out by means of optical microscope (Reichert-Jung MeF3). The stress-relief annealed samples have been subjected to the mercurous nitrate test. Residual stresses were finally measured by X-ray microdiffraction spectra, collected by a D/max-RAPID Rigaku microdiffractometer with Cu Ka radiation. This system is equipped with a cylindrical imaging plate (IP) detector which can measure two-dimensional (2D) X-ray diffraction from 45° to 160° (2h). 3. Results 3.1. Hardness tests Microhardness has been measured in stress relieved tubes and compared with the microhardness profile of the as-produced ones. In Fig. 1 some microhardness profiles along the thickness of 1 in. tubes are reported as an example of the results obtained. Hardness profiles in as-fabricated tubes show the highest values within the fin, due to work hardening, and a progressive decrease throughout the inner diameter. A general scatter in microhardness measurements is observed, probably due to the comparable size of the crystalline grains and microhardness indentation. Since Vickers microhardness is not affected by stress relieving at 250 °C, further measurements have not been carried out on samples stress relieved at 200 °C. 3.2. Microstructure A first set of microstructures, taken from the fin zone of as fabricated and 250 °C-5 h stress relieved samples, are shown in Figs. 2 and 3. The fin region has been firstly investigated in order to verify the effect of stress relieving at the higher temperatures and for the longest time.
Fig. 1. Microhardness of 1 in. tubes measured from the fin head.
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Fig. 2. Fin zone microstructure of 1 in. tubes in condition 0 (left) and condition 1 (right).
Fig. 3. Fin zone microstructure of 3/4 in. tubes in condition 0 (left) and condition 1 (right).
As produced tubes have been observed for the comparison. No recrystallization can be observed in the work hardened zone; however a somewhat different appearance seems detectable, as stress relieved grains show less disturbed metal in comparison to as fabricated grains. A second set of microstructures is reported in Figs. 4 and 5 and shows the inner diameter zone for the as fabricated tubes and 250 °C-5 h stress relieved tubes. Microstructure observation aimed in this case at investigating the metallurgical appearance of the region that is firstly affected by the stress-corrosion attack in the mercurous nitrate test. No significant differences are observed between inner diameter as-fabricated and stress relieved samples. The inner diameter microstructure, therefore, is not affected by the finning enhancement of the outer diameter.
Fig. 4. Inner zone microstructure of 1 in. tubes in condition 0 (left) and condition 1 (right).
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Fig. 5. Inner zone microstructure of 3/4 in. tubes in condition 0 (left) and condition 1 (right).
3.3. Mercurous nitrate test Accelerated stress-corrosion tests according to the mercurous nitrate procedure have been carried out by an external laboratory and results are shown in Table 2. The stress-corrosion attack was reported to affect always the inner surface of the tubes. Specimens stress relieved at 200 °C-24 h were not affected by the mercurous nitrate test, as well as all specimens stress relieved at 250 °C-5 h and 3 h. 3.4. Microdiffraction analysis Fig. 6 shows the 2D image collected in 5 min from the fin of a brass tube as fabricated by using a beam collimator of 800 lm. Debye rings appear smooth, indicating the presence of small grains and not uniform in intensity because of texture, due to the cold working operations. Table 2 Results of the mercurous nitrate test Sample
Temperature (°C)
Time (h)
Results
1 2 3 4 5 6 7
250 250 250 200 200 200 200
5 3 1 3 5 10 24
Not cracked Not cracked Cracked Cracked Cracked Cracked Not cracked
Fig. 6. 2D microdiffraction image collected from 3/4 in. tube in condition 0.
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Fig. 7. XRD patterns from 3/4 in. tube (upper) and 1 in. tube (lower) condition 0. The intensities are in arbitrary units.
Samples after heat treatment at 250 °C show similar 2D images, confirming the absence of recrystallization as observed by the metallographic analysis. Integrating in b the 2D image, powder diffraction pattern can be observed and analysed by the appropriate software to obtain phase identification, quantitative analysis, crystallite size determination, residual stress measurements, etc. [5]. Fig. 7 shows integrated diffraction patterns from 3/4 in. tube (upper line) and 1 in. tube (lower line) as fabricated. The vertical bars are the peaks of a-brass phase (JC-PDF card. no. 501333). The experimental peaks positions are shifted to higher 2h angles with respect to the tabulated ones. This difference can be due to the presence of a small amount of Al in the structure and to the different content of Cu. Residual stresses were measured on brass samples as fabricated (type 0) and on two samples which pass the mercurous nitrate test (samples 1 and 7, annealed at 250 °C and 200 °C, respectively) in order to quantify the stress relaxation due to the thermal treatments. Only few microns in depth are probed depending on the penetration of the X-rays into the material. After sectioning, stresses were calculated on both external and internal surfaces of the tubes. X-ray diffraction analysis generally needs a flat surface in order to prevent defocusing errors which can occur during the tilting of w angle. However, curved samples can be measured provided that the X-ray beam size is about four time smaller respect to the curvature of the specimen. In this case, it means that the beam spot must be smaller than 3 mm and 2.4 mm for 1 in. and 3/4 in. samples respectively. On the other hand, the spot should be longer enough to allow a sufficient number of crystallites to diffract. For these reasons the measurements on the internal diameter were performed by using a beam collimator of 800 lm. The second problem is related to the measurement of external surface which is characterized by fins with a thickness of about 0.5 mm. In this case, to avoid edge effects the irradiated area was reduced by using a collimator of 300 lm. A video microscope allowed a very accurate adjustment of the sample position in the X-ray beam, to focus the head of fin (Fig. 8).
Fig. 8. Image of the sample and increasing magnifications to focus a single fin.
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Table 3 Residual stresses on fin head Stress (MPa) 1 in. 0 1 in. 1 3/4 in. 0 3/4 in. 1
109 ± 11 112 ± 11 154 ± 22 114 ± 24
FHWM 0.932 0.808 1.035 1.055
Table 4 Residual stresses on the internal diameter
1 in. 0 1 in. 1 1 in. 7 3/4 in. 0 3/4 in. 1 3/4 in. 7
Stress (MPa)
FHWM
289 ± 5 122 ± 4 139 ± 5 263 ± 10 172 ± 3 171 ± 5
1.26 1.22 1.14 1.35 1.23 1.21
The residual stresses on the internal surface of tubes were evaluated using the sin 2w technique [6] on the a-brass 420 peak with more than 10w tilts, from 0° to 50°. For the fins analysis it was studied the shift of 311 peak, because better defined respect to the 420 peak. Peak positions were determined by fitting a parabola on the diffraction data after LPA correction, background subtraction and Ka2 stripping. The elastic constants used for the stress analysis were E = 117 GPa and m = 0.29 [7]. Residual stresses results are reported in Tables 3 and 4. On the fin head the residual stresses are always in compression. The measurements were affected by some errors probably because of the curved surface and to the very small spot size, which limited the number of diffractions grains. On the contrary, the internal diameter of the tubes results always in tension. Note that the stresses of the deformed surface layers after cold working (condition 0) exceeds bulk yield strength of the alloy, which is about 190 MPa, as already reported in literature for other materials [8]. After annealing at 250 °C-5 h and 200 °C-24 h, there is a definite decrease of residual stresses, especially in the case of 1 in. tubes. The stress levels reached at the end of these heat treatments are similar, concluding that they are both effective for SCC prevention. In Tables 3 and 4 are also reported the breaths at half height of the diffraction peak (FHWM) used for residual stress measurement, which can be directly related to material properties such as hardness, cold working and yield strength [9,10]. In fact, the broadening is primarily the result of two related phenomena: a reduction of the ‘‘crystallite’’ or coherent diffracting domain size, and an increase of microstrains. Results showed a small decrease of peak breath, probably indicating a substructural change in dislocation density and a rearrangement into cellular patterns, except in the case of 3/4 in. sample on fin head. 4. Conclusions Metallographic investigations on the effect of stress relieving at 250 °C for 1, 3 and 5 h, and at 200 °C for 3, 5, 10 and 24 h on fin enhanced tubes made of C68700 copper alloy allows to conclude that: – microstructure is not significantly changed, and no recrystallization is observed in the work hardened zones and – hardness throughout the thickness is not strongly modified with respect to the as finned state. So the improved strength due to the cold working is retained.
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These observation was in a sense confirmed by microdiffraction analysis, which also permit to assess the residual stress state present in these samples: compressive on the head of the fins and tensile on the internal diameter. A significant decrease of tensile residual stresses was registered after annealing at 250 °C for 5 h as already reported in literature. Similar results were also obtained by heat treatment at 200 °C for 24 h, demonstrating the possibility to obtain a good stress-relief with lower temperature for longer time. This is consistent with the results of mercurous nitrate tests. The lower heat treatment temperature gives the opportunity to stress-relieve the heat-exchangers tubes directly into the plant by circulating superheated steam, with a consequent time and energy saving. Also, microdiffraction appears to be a very useful technique for non-destructive residual stress analysis on samples with complex geometry and/or small dimensions and, in general, it can be considered as an important tool to support metallurgical research and failure analysis. References [1] [2] [3] [4] [5] [6] [7] [8]
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