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Failure analysis of lead-free brass valve bodies
Engineering Failure Analysis 100 (2019) 536–543
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of lead-free brass valve bodies ⁎
Gan Chunlei , Zhou Nan, Kang Yuehua, Wang Shuncheng, Zheng Kaihong
T
Guangdong Institute of Materials and Processing, Guangzhou 510650, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Lead-free brass valve body Crack Failure
This paper presents the failure analysis of the lead-free brass valve bodies that had been installed in plumbing sanitary equipment. Leakage points were found adjacent to the brazing location. Visual inspection, stereomicroscope and scanning electron microscopy coupled with an energy dispersive spectrometer (SEM/EDS) were used as the principal analytical methods for the present failure analysis. The analysis results implied that the combined action of excessive brazing residual stresses and bismuth boundary segregation was probably responsible for the failure of lead-free brass valve bodies. Finally, two important suggestions were proposed to minimize the risk of recurrence of the present leakage failure in the future.
1. Introduction Valve bodies are one of the most critical components in plumbing systems. The main advantage of using valve bodies is to control the direction, pressure and flow of fluid. Brasses which contain over 0.5 wt pct lead (Pb) exhibit excellent machinability, high corrosion resistance to drinking and industrial water, good strength and are easily worked [1–4]. Therefore, leaded brasses have been widely used as the materials of valve bodies in the piping and sanitary industry for several decades. However, in recent years the leadfree brass with alternative free-machining additive such as bismuth (Bi) has attracted special research interest and been frequently used to minimize the negative impact of Pb on the environment, health and safety [5–9]. In their application, the lead-free brass components are often subject to various kinds of failures due to worse work environment. These failure accidents can be usually caused by intergranular cracking, mechanical-environmental induced failure and cold shut formation [10–12]. In this study, the failed bismuth brass valve bodies were examined visually and by stereomicroscope, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS), respectively. Failure analysis of the typical lead-free brass valve body used in plumbing sanitary equipment was carried out. The aim of this study is to find out the exact root cause of the failure and improve the quality of the lead-free brass valve body. 2. Experimental For the purpose of the failure analysis, the leaking lead-free brass valve bodies were pressure tested once more. When the failed valve bodies pressurized with air at 0.5 MPa were immersed in water, it was found that very small air bubbles rose continuously through the leakage points adjacent to the brazing location. These detected leakage points were then marked for sample preparation. The chemical compositions of the failed lead-free brass valve body were checked by wet chemistry. Low-magnification inspection of fracture surface was implemented using a Leica DFC 295 stereomicroscope. To determine the mechanism of cracking, the failed lead-free brass valve bodies were cut in the regions of the cracks to observe the cross section. The samples were prepared by
⁎
Corresponding author. E-mail address: [email protected] (G. Chunlei).
https://doi.org/10.1016/j.engfailanal.2019.03.001 Received 29 December 2018; Received in revised form 27 February 2019; Accepted 7 March 2019 Available online 08 March 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 100 (2019) 536–543
G. Chunlei, et al.
Table 1 Chemical compositions of the as-received brass valve body. Category
Testing value HPb61–1
Content of elements (wt%) Cu
Bi
Pb
Fe
Zn
58.83 58.00–62.00
0.87 –
– 0.60–1.20
0.18 0.15
40.12 balance
mounting, grinding on successively finer grits of emery papers, followed by mirror-polishing using a diamond paste. Swab etching of the polished samples was performed in a chemical solution (100 mL HNO3 + 100 mL C2H5OH + 50 mL H2O), and then ultrasonically cleaned samples were examined using a JAX-8100 scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) for elemental microanalysis and photomicrographs were recorded. 3. Results 3.1. Chemical compositions The actual chemical compositions of the lead-free brass valve body were determined by wet chemistry. These test results are illustrated in Table 1. In order to compare with the examined results, the referenced values of HPb61–1 specified by GB/T 5232–2001 are also listed in Table 1. It can be noted that the chemical compositions of the as-received brass valve body are close to the referenced values, except for Bi as a nontoxic alternative to Pb for counterbalancing the adverse effects on machinability. As can be also shown by Table 1, the content of Zn is 40.12 wt%, which suggests that the present lead-free brass can be a duplex alloy [13]. 3.2. Visual inspection Fig. 1 shows a general view of the typical lead-free brass valve body assembly used in plumbing sanitary equipment. This brass valve body plays a key role on pressure and flow of water. In the application process, water in the lead-free brass valve body is not allowed to leak. Whereas, the present lead-free brass valve body after brazing was found to be liable to leakage. The brazing location is on both sides of the lead-free brass valve body, as indicated by the red arrows in Fig. 1. Therefore, the urgent task is to resolve the leakage of the lead-free brass valve body. In order to enhance the rate of finished products and avoid the leakage, the failure analysis was carried out to determine the primary damage root source of the lead-free brass valve body. In the present case, the sample of the lead-free brass valve body received for investigation was placed on the support stand to determine the leakage area. Magnified view of the lead-free brass valve body containing crack is shown in Fig. 2. It can be seen that flaw is discovered adjacent to the brazing location on the flat location indicated by yellow arrows. Water leaked exactly from the crack. Arrows also illustrate the extent of cracking on the part of the lead-free brass valve body. 3.3. Stereomicroscopic examination The fracture of the failed lead-free brass valve body was forcibly opened to reveal this appearance of the fracture. Fig. 3 displays the fracture surface near the failed region of the lead-free brass valve body as examined by the stereomicroscope. The preliminary stereo-microscopic observation affirmed that crack ran through the lead-free brass valve body wall showing a rather sizeable streak. Both sides of the fracture are depicted on this slide. Yellow arrows point to suspected material defects.
Fig. 1. The typical lead-free brass valve body assembly in the as received condition. 537
Engineering Failure Analysis 100 (2019) 536–543
G. Chunlei, et al.
Fig. 2. Appearance of the part received for investigation (the leaked position as indicated by yellow arrows near the brazing joint). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Stereomicroscopic micrograph showing the fracture surface of the failed lead-free brass valve body.
3.4. Microstructural investigation Metallurgical sample was cut from the lead-free brass valve body to reveal microstructural characteristics in the present study. The metallurgical structure was observed by SEM after the metallurgical sample was ground, polished and etched by a corrosive agent. Fig. 4 shows clearly the metallurgical structure of the lead-free brass valve body. It can be found that the metallurgical structure is a heterogeneously dual α + β phase structure with coarse particles of different sizes. The mean size of these particles was about 1.2 μm. It was further analyzed that the ratio of these coarse particles along boundary to that in the grain interior was approximately 1:2. The presence of these phases is typical for duplex brasses manufactured by extrusion and drawing showing a relatively uniform Bi-particle distribution. In addition, it's worth noting that the precipitates seem to partly aggregate along phase boundaries, which may exert a negative effect on the property of the lead-free brass.
3.5. Crack observation To better expose the crack surface, metallographic sampling of the failed lead-free brass valve body was carefully made along the direction of crack propagation. Fig. 5 displays a low-magnification image of the shape of the typical main crack which propagates from the internal to the external wall. The part between the two yellow arrows 1 and 2 is the cracking path. The crack was found to 538
Engineering Failure Analysis 100 (2019) 536–543
G. Chunlei, et al.
Fig. 4. SEM micrograph showing a typical α + β microstructure.
Fig. 5. Image of the propagation of crack through the lead-free brass valve body wall.
have extended through the wall of the failed lead-free brass valve body resulting evidently in an approximately 1.2 mm long ragged crack near the brazing location. In order to further reveal crack mode, higher magnification scanning electron microscopy was also undertaken over the entire crack propagation, with representative locations given in Figs. 6 and 7, respectively. It can be observed that there are some small dents on the inner wall surface, which can be caused by the manufacturing tool. The main crack was initiated from the big dent, as shown in Fig. 6, which is just at the corner of the valve body wall. The path of the main crack can be observed and it passes along phase or grain boundaries. By statistical analysis, the ratio among grain boundary crack length is 35.83%, the one among phase boundary crack length is 63.54% and others are approximately 0.63%. Close observations revealed that the main crack front seemed to present principally a ragged intergranular propagation mode, and many secondary cracks which looked like branches were emanated from the main crack and propagated intergranularly (where the red arrows point to in Figs. 6 and 7). These characteristics exhibited importantly the operation of fatigue mechanism. For the in-depth examination of the failed region, a SEM micrograph of a partially magnified crack zone and corresponding EDS results are shown in Figs. 8 and 9, respectively. In terms of the EDS results, the chemical composition of these particles is determined to be Bi, as a typical chemical composition of the present lead-free brass valve bodies. It is seen that coarse Bi particles with an irregular size are rather segregated between cracks in the microstructure, as can be also demonstrated in Fig. 4. The existence of Bi segregation along grain or phase boundaries can be also evidenced by considering similar observations in relevant references 539
Engineering Failure Analysis 100 (2019) 536–543
G. Chunlei, et al.
Fig. 6. Partial enlarged images close to the internal side in Fig. 5.
[14–16]. 4. Discussion According to all the aspects mentioned in the above results, it was manifested that brazing residual stresses and bismuth segregation between boundaries existed in this case by macro and micro morphology observations and EDS analysis. The failure of leadfree brass valve bodies was thereby considered to be caused by the combined effects of brazing residual stresses and bismuth boundary segregation. In the following sections, we will discuss in particular the effects of brazing residual stresses and bismuth segregation. 4.1. Effect of the brazing residual stress Brazing is a precision forming method, which has been extensively used in joining complex shape parts and thin wall accessories in plumbing sanitary industry. However, it can frequently lead to the formation of residual stresses in the vicinity of the joining zone. In the present investigation, lead-free brass valve bodies were found cracked when thermal processing such as brazing was performed. More than10% of a certain batch of lead-free brass valve bodies was found to fail. From the above fact, it is easy to understand that brazing can generate residual stresses contributing to the formation of these cracks. The brazing residual stress plays an important role in the formation of crack. As shown in Figs. 5 and 6, surface flaws such as dents which result from the manufacturing defects can act as stress raiser. In this case study, it was apparent that the dent at the corner of the valve body wall would lead to the initiation of the crack due to local stress concentration. Moreover, the inclined form of the initiation site hints further that prior presence of surface flaw can assist in the crack forming. On the other hand, the residual stresses imposed during brazing, added to the working stresses, can boost stress amplitude probably triggering fatigue crack growth under the gradual water pressure applied. So, the cracks that were developed from the turning of the internal wall started to continually spread. 4.2. Effect of bismuth segregation It is possible that brazing residual stresses of the lead-free brass valve body have contributed to the formation of these cracks, but 540
Engineering Failure Analysis 100 (2019) 536–543
G. Chunlei, et al.
Fig. 7. Partial enlarged images close to the external side in Fig. 5.
Fig. 8. Local enlarged SEM micrograph of a typical crack zone showing coarse particles along the fracture surface and within the cracks.
it is noteworthy also to mention that the failure occurred probably as a result of bismuth grain or phase boundary segregation. As is well known, the diffusion coefficient of Bi is smaller than Pb in copper at the same process conditions [17], which can result in a higher concentration of bismuth during solute distribution in the liquid interface. On the other hand, the melting temperature
541
Engineering Failure Analysis 100 (2019) 536–543
G. Chunlei, et al.
Fig. 9. Corresponding EDS results of coarse particles in Fig. 8 (Yellow arrow A). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(271.3 °C) of bismuth is much lower than copper‑zinc, and the solubility of bismuth in copper‑zinc is very small. So bismuth segregates to the solid/liquid interface. As a result, bismuth tends to form a bismuth film along phase or grain boundaries with cooling of the casting [15,16]. Furthermore, the metallurgical observation demonstrates also discernible bismuth precipitates on the phase boundaries, as can be displayed in Fig. 4. Bismuth rich phase between grain or phase boundaries can enhance the machining and engraving qualities of the lead-free brass valve body. Nevertheless, once overheating incidents occurred in metal working processes, bismuth particle melting and coalescence could cause hot-shortness. Even at temperature below its melting temperature, bismuth can still conduce to intergranular brittle fracture [18]. Consequently, the existence of coarse bismuth particles on the crack face during brazing could become an additional weakness supplying conditions for crack initiation and crack growth. 4.3. Effect of the synergy Further analysis shows that the occurrence of the leakage failure of lead-free brass valve bodies can be possibly interpreted, taking into consideration a plausible result of the combination of brazing residual stresses and bismuth boundary segregation. As described above, the failure of lead-free brass valve bodies during the usual working conditions can be considered to undergo three processes: i) Some surface flaws proximal to the brazing location were formed by mechanical damage and became the stress concentration sources; ii) Under the coactions of the excessive brazing residual stress and bismuth segregation on grain or phase boundaries in the failed lead-free brass valve body, cracks tended to initiate from these small dents; iii) Once this kind of fatigue cracks propagated along the boundaries of bismuth segregation and penetrated the wall of the lead-free brass valve body, leak event would inevitably occur. Therefore, the synergistic effect of the excessive brazing residual stress and bismuth boundary segregation may induce cracks in conjunction with secondary crack branches that give rise to phase or grain boundary decohesion and splitting resulting in the final leakage failure. 5. Conclusion According to the SEM observations and the detailed analysis above, it can be reasonably concluded that the root cause of the failure of lead-free brass valve bodies is probably attributed to the combined action of excessive brazing residual stresses and bismuth boundary segregation. 6. Suggestions In order to prevent the similar failure in the future, the following actions are recommended: i) An appropriate heat treatment may be carried out in order to anneal out excessive residual stresses which can be caused during brazing; ii) The casting process of the lead-free brass valve body must be well monitored and controlled to avoid bismuth boundary segregation. 542
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Acknowledgments This work was financially supported by planning project of Guangdong academy of sciences (Grant No. 2018GDASCX-0117), Science and technology planning projects of Guangdong province (Grant Nos. 2017A050503004, 2017A070701029 and 2014B010105009) and Science and technology planning project of Guangzhou (Grant No. 201607010211). The authors wish to express special thanks to Guangdong hengbao precision technology Co., Ltd. for valuable contribution to sample preparation. References [1] F. Schultheiss, D. Johansson, V. Bushlya, J.M. Zhou, K. Nilsson, J.E. Ståh, Comparative study on the machinability of lead-free brass, J. Clean. Prod. 149 (2017) 366–377. [2] W.A. Badawy, S.S. El-Egamy, A.S. El-Azab, The electrochemical behaviour of leaded brass in neutral Cl− and SO4− media, Corros. Sci. 37 (1995) 1969–1979. [3] G.A. Pantazopoulos, A.I. Toulfatzis, Fracture modes and mechanical characteristics of machinable brass rods, Metallogr. Microst. Anal. 1 (2012) 106–114. [4] L.V. Zhuravel', V.A. Yatsenko, A.G. Ivashkevich, Effect of lead on plastic deformation of brass, Met. Sci. Heat Treat. 18 (1976) 247–248. [5] N.B. Emelina, A.N. Alabin, N.A. Belov, Influence of bismuth and lead on the formation of the structure of experimental alloys with a Cu-30%Zn based composition during crystallization, deformation, and thermal treatment, Russ. J. Nonferrous Met. 51 (2010) 476–482. [6] T.M. Harold, Replacing lead in brass plumbing castings, Adv. Mater. Process. 1 (2002) 75–77. [7] H. Imai, S.F. Li, H. Atsumi, Y. Kosaka, A. Kojima, J. Umeda, K. Kondoh, Mechanical properties and machinability of extruded Cu-40%Zn brass alloys with bismuth via powder metallurgy process, Trans. JWRI 38 (2009) 25–30. [8] L.V. Whiting, P.D. Newcombe, M. Sahoo, Casting characteristics of red brass containing bismuth and selenium, Trans. Am. Foundrymen's Soc. 103 (1996) 683–691. [9] D.T. Peters, New bismuth/selenium red brass alloys solve lead concerns, Mod. Cast. 87 (1997) 57–59. [10] Y. Seung-Jae, C. Yoon-Seok, K. Jung-Gu, O. Han-Jun, C. Choong-Soo, Stress corrosion cracking properties of environmentally friendly unleaded brasses containing bismuth in Mattsson's solution, Mater. Sci. Eng. A 345 (2003) 207–214. [11] Y. Jang, S. Kim, S. Han, Effect of misch metal on elevated temperature tensile ductility of the Cu-Zn-Bi alloy, Met. Mater. Trans. A 36 (2005) 1060–1065. [12] V. Vazquez, A. Juarez-Hernandez, A. Mascarenas, P. Zambrano, M.A.L. Hernandez-Rodriguez, Cold shut formation analysis on a free lead yellow brass tap, Eng. Fail. Anal. 17 (2010) 1285–1289. [13] A.A. Fadhil, M.S. Ghattas, B.A. Iskander, S.A. Ajeel, T.A. Enab, Structural characterization and detecting processes of defects in leaded brass alloy used for gas valves production, Alex. Eng. J. 57 (2018) 1301–1311. [14] M. Martinez-Hernandez, A. Juarez-Hernandez, C. González-Rivera, M.A.L. Hernandez-Rodriguez, Bismuth segregation and crack formation on a free lead yellow brass tap, Eng. Fail. Anal. 28 (2013) 63–68. [15] B. Joseph, F. Barbier, M. Aucouturier, Embrittlement of copper by liquid bismuth, Scr. Mater. 40 (1999) 893–897. [16] B. Joseph, F. Barbier, G. Dagoury, M. Aucouturier, Rapid penetration of liquid Bi along Cu grain boundaries, Scr. Mater. 39 (1998) 775–781. [17] S. Divinski, M. Lohmann, C. Herziga, Grain boundary diffusion and segregation of Bi in Cu: radiotracer measurements in B and C diffusion regimes, Acta Mater. 52 (2004) 3973–3982. [18] Sircar S. Free machining aluminum alloy containing bismuth or bismuth-tin for free machining and a method of use. US patent no. 6409966, Reynolds Metals Company, Richmond, VA, USA, 2002.
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Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of lead-free brass valve bodies ⁎
Gan Chunlei , Zhou Nan, Kang Yuehua, Wang Shuncheng, Zheng Kaihong
T
Guangdong Institute of Materials and Processing, Guangzhou 510650, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Lead-free brass valve body Crack Failure
This paper presents the failure analysis of the lead-free brass valve bodies that had been installed in plumbing sanitary equipment. Leakage points were found adjacent to the brazing location. Visual inspection, stereomicroscope and scanning electron microscopy coupled with an energy dispersive spectrometer (SEM/EDS) were used as the principal analytical methods for the present failure analysis. The analysis results implied that the combined action of excessive brazing residual stresses and bismuth boundary segregation was probably responsible for the failure of lead-free brass valve bodies. Finally, two important suggestions were proposed to minimize the risk of recurrence of the present leakage failure in the future.
1. Introduction Valve bodies are one of the most critical components in plumbing systems. The main advantage of using valve bodies is to control the direction, pressure and flow of fluid. Brasses which contain over 0.5 wt pct lead (Pb) exhibit excellent machinability, high corrosion resistance to drinking and industrial water, good strength and are easily worked [1–4]. Therefore, leaded brasses have been widely used as the materials of valve bodies in the piping and sanitary industry for several decades. However, in recent years the leadfree brass with alternative free-machining additive such as bismuth (Bi) has attracted special research interest and been frequently used to minimize the negative impact of Pb on the environment, health and safety [5–9]. In their application, the lead-free brass components are often subject to various kinds of failures due to worse work environment. These failure accidents can be usually caused by intergranular cracking, mechanical-environmental induced failure and cold shut formation [10–12]. In this study, the failed bismuth brass valve bodies were examined visually and by stereomicroscope, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS), respectively. Failure analysis of the typical lead-free brass valve body used in plumbing sanitary equipment was carried out. The aim of this study is to find out the exact root cause of the failure and improve the quality of the lead-free brass valve body. 2. Experimental For the purpose of the failure analysis, the leaking lead-free brass valve bodies were pressure tested once more. When the failed valve bodies pressurized with air at 0.5 MPa were immersed in water, it was found that very small air bubbles rose continuously through the leakage points adjacent to the brazing location. These detected leakage points were then marked for sample preparation. The chemical compositions of the failed lead-free brass valve body were checked by wet chemistry. Low-magnification inspection of fracture surface was implemented using a Leica DFC 295 stereomicroscope. To determine the mechanism of cracking, the failed lead-free brass valve bodies were cut in the regions of the cracks to observe the cross section. The samples were prepared by
⁎
Corresponding author. E-mail address: [email protected] (G. Chunlei).
https://doi.org/10.1016/j.engfailanal.2019.03.001 Received 29 December 2018; Received in revised form 27 February 2019; Accepted 7 March 2019 Available online 08 March 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 100 (2019) 536–543
G. Chunlei, et al.
Table 1 Chemical compositions of the as-received brass valve body. Category
Testing value HPb61–1
Content of elements (wt%) Cu
Bi
Pb
Fe
Zn
58.83 58.00–62.00
0.87 –
– 0.60–1.20
0.18 0.15
40.12 balance
mounting, grinding on successively finer grits of emery papers, followed by mirror-polishing using a diamond paste. Swab etching of the polished samples was performed in a chemical solution (100 mL HNO3 + 100 mL C2H5OH + 50 mL H2O), and then ultrasonically cleaned samples were examined using a JAX-8100 scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) for elemental microanalysis and photomicrographs were recorded. 3. Results 3.1. Chemical compositions The actual chemical compositions of the lead-free brass valve body were determined by wet chemistry. These test results are illustrated in Table 1. In order to compare with the examined results, the referenced values of HPb61–1 specified by GB/T 5232–2001 are also listed in Table 1. It can be noted that the chemical compositions of the as-received brass valve body are close to the referenced values, except for Bi as a nontoxic alternative to Pb for counterbalancing the adverse effects on machinability. As can be also shown by Table 1, the content of Zn is 40.12 wt%, which suggests that the present lead-free brass can be a duplex alloy [13]. 3.2. Visual inspection Fig. 1 shows a general view of the typical lead-free brass valve body assembly used in plumbing sanitary equipment. This brass valve body plays a key role on pressure and flow of water. In the application process, water in the lead-free brass valve body is not allowed to leak. Whereas, the present lead-free brass valve body after brazing was found to be liable to leakage. The brazing location is on both sides of the lead-free brass valve body, as indicated by the red arrows in Fig. 1. Therefore, the urgent task is to resolve the leakage of the lead-free brass valve body. In order to enhance the rate of finished products and avoid the leakage, the failure analysis was carried out to determine the primary damage root source of the lead-free brass valve body. In the present case, the sample of the lead-free brass valve body received for investigation was placed on the support stand to determine the leakage area. Magnified view of the lead-free brass valve body containing crack is shown in Fig. 2. It can be seen that flaw is discovered adjacent to the brazing location on the flat location indicated by yellow arrows. Water leaked exactly from the crack. Arrows also illustrate the extent of cracking on the part of the lead-free brass valve body. 3.3. Stereomicroscopic examination The fracture of the failed lead-free brass valve body was forcibly opened to reveal this appearance of the fracture. Fig. 3 displays the fracture surface near the failed region of the lead-free brass valve body as examined by the stereomicroscope. The preliminary stereo-microscopic observation affirmed that crack ran through the lead-free brass valve body wall showing a rather sizeable streak. Both sides of the fracture are depicted on this slide. Yellow arrows point to suspected material defects.
Fig. 1. The typical lead-free brass valve body assembly in the as received condition. 537
Engineering Failure Analysis 100 (2019) 536–543
G. Chunlei, et al.
Fig. 2. Appearance of the part received for investigation (the leaked position as indicated by yellow arrows near the brazing joint). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Stereomicroscopic micrograph showing the fracture surface of the failed lead-free brass valve body.
3.4. Microstructural investigation Metallurgical sample was cut from the lead-free brass valve body to reveal microstructural characteristics in the present study. The metallurgical structure was observed by SEM after the metallurgical sample was ground, polished and etched by a corrosive agent. Fig. 4 shows clearly the metallurgical structure of the lead-free brass valve body. It can be found that the metallurgical structure is a heterogeneously dual α + β phase structure with coarse particles of different sizes. The mean size of these particles was about 1.2 μm. It was further analyzed that the ratio of these coarse particles along boundary to that in the grain interior was approximately 1:2. The presence of these phases is typical for duplex brasses manufactured by extrusion and drawing showing a relatively uniform Bi-particle distribution. In addition, it's worth noting that the precipitates seem to partly aggregate along phase boundaries, which may exert a negative effect on the property of the lead-free brass.
3.5. Crack observation To better expose the crack surface, metallographic sampling of the failed lead-free brass valve body was carefully made along the direction of crack propagation. Fig. 5 displays a low-magnification image of the shape of the typical main crack which propagates from the internal to the external wall. The part between the two yellow arrows 1 and 2 is the cracking path. The crack was found to 538
Engineering Failure Analysis 100 (2019) 536–543
G. Chunlei, et al.
Fig. 4. SEM micrograph showing a typical α + β microstructure.
Fig. 5. Image of the propagation of crack through the lead-free brass valve body wall.
have extended through the wall of the failed lead-free brass valve body resulting evidently in an approximately 1.2 mm long ragged crack near the brazing location. In order to further reveal crack mode, higher magnification scanning electron microscopy was also undertaken over the entire crack propagation, with representative locations given in Figs. 6 and 7, respectively. It can be observed that there are some small dents on the inner wall surface, which can be caused by the manufacturing tool. The main crack was initiated from the big dent, as shown in Fig. 6, which is just at the corner of the valve body wall. The path of the main crack can be observed and it passes along phase or grain boundaries. By statistical analysis, the ratio among grain boundary crack length is 35.83%, the one among phase boundary crack length is 63.54% and others are approximately 0.63%. Close observations revealed that the main crack front seemed to present principally a ragged intergranular propagation mode, and many secondary cracks which looked like branches were emanated from the main crack and propagated intergranularly (where the red arrows point to in Figs. 6 and 7). These characteristics exhibited importantly the operation of fatigue mechanism. For the in-depth examination of the failed region, a SEM micrograph of a partially magnified crack zone and corresponding EDS results are shown in Figs. 8 and 9, respectively. In terms of the EDS results, the chemical composition of these particles is determined to be Bi, as a typical chemical composition of the present lead-free brass valve bodies. It is seen that coarse Bi particles with an irregular size are rather segregated between cracks in the microstructure, as can be also demonstrated in Fig. 4. The existence of Bi segregation along grain or phase boundaries can be also evidenced by considering similar observations in relevant references 539
Engineering Failure Analysis 100 (2019) 536–543
G. Chunlei, et al.
Fig. 6. Partial enlarged images close to the internal side in Fig. 5.
[14–16]. 4. Discussion According to all the aspects mentioned in the above results, it was manifested that brazing residual stresses and bismuth segregation between boundaries existed in this case by macro and micro morphology observations and EDS analysis. The failure of leadfree brass valve bodies was thereby considered to be caused by the combined effects of brazing residual stresses and bismuth boundary segregation. In the following sections, we will discuss in particular the effects of brazing residual stresses and bismuth segregation. 4.1. Effect of the brazing residual stress Brazing is a precision forming method, which has been extensively used in joining complex shape parts and thin wall accessories in plumbing sanitary industry. However, it can frequently lead to the formation of residual stresses in the vicinity of the joining zone. In the present investigation, lead-free brass valve bodies were found cracked when thermal processing such as brazing was performed. More than10% of a certain batch of lead-free brass valve bodies was found to fail. From the above fact, it is easy to understand that brazing can generate residual stresses contributing to the formation of these cracks. The brazing residual stress plays an important role in the formation of crack. As shown in Figs. 5 and 6, surface flaws such as dents which result from the manufacturing defects can act as stress raiser. In this case study, it was apparent that the dent at the corner of the valve body wall would lead to the initiation of the crack due to local stress concentration. Moreover, the inclined form of the initiation site hints further that prior presence of surface flaw can assist in the crack forming. On the other hand, the residual stresses imposed during brazing, added to the working stresses, can boost stress amplitude probably triggering fatigue crack growth under the gradual water pressure applied. So, the cracks that were developed from the turning of the internal wall started to continually spread. 4.2. Effect of bismuth segregation It is possible that brazing residual stresses of the lead-free brass valve body have contributed to the formation of these cracks, but 540
Engineering Failure Analysis 100 (2019) 536–543
G. Chunlei, et al.
Fig. 7. Partial enlarged images close to the external side in Fig. 5.
Fig. 8. Local enlarged SEM micrograph of a typical crack zone showing coarse particles along the fracture surface and within the cracks.
it is noteworthy also to mention that the failure occurred probably as a result of bismuth grain or phase boundary segregation. As is well known, the diffusion coefficient of Bi is smaller than Pb in copper at the same process conditions [17], which can result in a higher concentration of bismuth during solute distribution in the liquid interface. On the other hand, the melting temperature
541
Engineering Failure Analysis 100 (2019) 536–543
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Fig. 9. Corresponding EDS results of coarse particles in Fig. 8 (Yellow arrow A). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(271.3 °C) of bismuth is much lower than copper‑zinc, and the solubility of bismuth in copper‑zinc is very small. So bismuth segregates to the solid/liquid interface. As a result, bismuth tends to form a bismuth film along phase or grain boundaries with cooling of the casting [15,16]. Furthermore, the metallurgical observation demonstrates also discernible bismuth precipitates on the phase boundaries, as can be displayed in Fig. 4. Bismuth rich phase between grain or phase boundaries can enhance the machining and engraving qualities of the lead-free brass valve body. Nevertheless, once overheating incidents occurred in metal working processes, bismuth particle melting and coalescence could cause hot-shortness. Even at temperature below its melting temperature, bismuth can still conduce to intergranular brittle fracture [18]. Consequently, the existence of coarse bismuth particles on the crack face during brazing could become an additional weakness supplying conditions for crack initiation and crack growth. 4.3. Effect of the synergy Further analysis shows that the occurrence of the leakage failure of lead-free brass valve bodies can be possibly interpreted, taking into consideration a plausible result of the combination of brazing residual stresses and bismuth boundary segregation. As described above, the failure of lead-free brass valve bodies during the usual working conditions can be considered to undergo three processes: i) Some surface flaws proximal to the brazing location were formed by mechanical damage and became the stress concentration sources; ii) Under the coactions of the excessive brazing residual stress and bismuth segregation on grain or phase boundaries in the failed lead-free brass valve body, cracks tended to initiate from these small dents; iii) Once this kind of fatigue cracks propagated along the boundaries of bismuth segregation and penetrated the wall of the lead-free brass valve body, leak event would inevitably occur. Therefore, the synergistic effect of the excessive brazing residual stress and bismuth boundary segregation may induce cracks in conjunction with secondary crack branches that give rise to phase or grain boundary decohesion and splitting resulting in the final leakage failure. 5. Conclusion According to the SEM observations and the detailed analysis above, it can be reasonably concluded that the root cause of the failure of lead-free brass valve bodies is probably attributed to the combined action of excessive brazing residual stresses and bismuth boundary segregation. 6. Suggestions In order to prevent the similar failure in the future, the following actions are recommended: i) An appropriate heat treatment may be carried out in order to anneal out excessive residual stresses which can be caused during brazing; ii) The casting process of the lead-free brass valve body must be well monitored and controlled to avoid bismuth boundary segregation. 542
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Acknowledgments This work was financially supported by planning project of Guangdong academy of sciences (Grant No. 2018GDASCX-0117), Science and technology planning projects of Guangdong province (Grant Nos. 2017A050503004, 2017A070701029 and 2014B010105009) and Science and technology planning project of Guangzhou (Grant No. 201607010211). The authors wish to express special thanks to Guangdong hengbao precision technology Co., Ltd. for valuable contribution to sample preparation. References [1] F. Schultheiss, D. Johansson, V. Bushlya, J.M. Zhou, K. Nilsson, J.E. Ståh, Comparative study on the machinability of lead-free brass, J. Clean. Prod. 149 (2017) 366–377. [2] W.A. Badawy, S.S. El-Egamy, A.S. El-Azab, The electrochemical behaviour of leaded brass in neutral Cl− and SO4− media, Corros. Sci. 37 (1995) 1969–1979. [3] G.A. Pantazopoulos, A.I. Toulfatzis, Fracture modes and mechanical characteristics of machinable brass rods, Metallogr. Microst. Anal. 1 (2012) 106–114. [4] L.V. Zhuravel', V.A. Yatsenko, A.G. Ivashkevich, Effect of lead on plastic deformation of brass, Met. Sci. Heat Treat. 18 (1976) 247–248. [5] N.B. Emelina, A.N. Alabin, N.A. Belov, Influence of bismuth and lead on the formation of the structure of experimental alloys with a Cu-30%Zn based composition during crystallization, deformation, and thermal treatment, Russ. J. Nonferrous Met. 51 (2010) 476–482. [6] T.M. Harold, Replacing lead in brass plumbing castings, Adv. Mater. Process. 1 (2002) 75–77. [7] H. Imai, S.F. Li, H. Atsumi, Y. Kosaka, A. Kojima, J. Umeda, K. Kondoh, Mechanical properties and machinability of extruded Cu-40%Zn brass alloys with bismuth via powder metallurgy process, Trans. JWRI 38 (2009) 25–30. [8] L.V. Whiting, P.D. Newcombe, M. Sahoo, Casting characteristics of red brass containing bismuth and selenium, Trans. Am. Foundrymen's Soc. 103 (1996) 683–691. [9] D.T. Peters, New bismuth/selenium red brass alloys solve lead concerns, Mod. Cast. 87 (1997) 57–59. [10] Y. Seung-Jae, C. Yoon-Seok, K. Jung-Gu, O. Han-Jun, C. Choong-Soo, Stress corrosion cracking properties of environmentally friendly unleaded brasses containing bismuth in Mattsson's solution, Mater. Sci. Eng. A 345 (2003) 207–214. [11] Y. Jang, S. Kim, S. Han, Effect of misch metal on elevated temperature tensile ductility of the Cu-Zn-Bi alloy, Met. Mater. Trans. A 36 (2005) 1060–1065. [12] V. Vazquez, A. Juarez-Hernandez, A. Mascarenas, P. Zambrano, M.A.L. Hernandez-Rodriguez, Cold shut formation analysis on a free lead yellow brass tap, Eng. Fail. Anal. 17 (2010) 1285–1289. [13] A.A. Fadhil, M.S. Ghattas, B.A. Iskander, S.A. Ajeel, T.A. Enab, Structural characterization and detecting processes of defects in leaded brass alloy used for gas valves production, Alex. Eng. J. 57 (2018) 1301–1311. [14] M. Martinez-Hernandez, A. Juarez-Hernandez, C. González-Rivera, M.A.L. Hernandez-Rodriguez, Bismuth segregation and crack formation on a free lead yellow brass tap, Eng. Fail. Anal. 28 (2013) 63–68. [15] B. Joseph, F. Barbier, M. Aucouturier, Embrittlement of copper by liquid bismuth, Scr. Mater. 40 (1999) 893–897. [16] B. Joseph, F. Barbier, G. Dagoury, M. Aucouturier, Rapid penetration of liquid Bi along Cu grain boundaries, Scr. Mater. 39 (1998) 775–781. [17] S. Divinski, M. Lohmann, C. Herziga, Grain boundary diffusion and segregation of Bi in Cu: radiotracer measurements in B and C diffusion regimes, Acta Mater. 52 (2004) 3973–3982. [18] Sircar S. Free machining aluminum alloy containing bismuth or bismuth-tin for free machining and a method of use. US patent no. 6409966, Reynolds Metals Company, Richmond, VA, USA, 2002.
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