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Metallurgical analysis of failed jaws used in joint of a water supply pipe line
Engineering Failure Analysis 26 (2012) 266–273
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Metallurgical analysis of failed jaws used in joint of a water supply pipe line Jongmin Lee ⇑, Shinho Han, Heongkee Kim, Ungi Lee Green Technology Headquarter, Korea Testing & Research Institute (KTR), Yeongdeungpo-dong 8ga 88-2, Seoul, Republic of Korea
a r t i c l e
i n f o
Article history: Received 16 July 2012 Accepted 9 August 2012 Available online 13 September 2012 Keywords: Intergranular corrosion cracking Sensitization Precipitation Martensitic stainless steel Failure analysis
a b s t r a c t In this paper, the cause of failure of the 420 stainless steel jaws in a joint was investigated. It was found that the martensitic stainless steel ASTM type 420 jaws used in connection joints of a water supply pipeline were cracked after 6 months in service. The failed parts were investigated by means of stereoscopic microscope, optical microscope, scanning electron microscope equipped with energy dispersive X-ray spectroscopy micro-analysis, inductively coupled plasma atomic emission spectroscopy, carbon–sulfur analyzer and Rockwell hardness tester, in order to identify the causes of failure and suggest preventive solutions. The study shows that failure was mainly due to intergranular corrosion cracking caused by the precipitation of chromium-rich carbides (or oxides) formed during heat treatment along grain boundaries. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Failures come from various mechanisms and related environmental factors [1]. Failure analysis is a process performed to determine the root causes or factors that led to an undesired loss of functionality [2,3]. Analysis of a failure of a metal structure or part usually requires identification of the type of failure [1]. The process is complex, draws upon many different technical disciplines, and uses a variety of observation, inspection, and laboratory techniques. During the investigation, the analyst must collect, examine and evaluate all the available data in order to determine its plausible origins and the sequence of events that led to the failure. The immediate objective is to find the root causes of the failure in order to determine the compensation for the induced damage, even if the long-term priority is to prevent similar failures in new recommendations. Usually these failure analyses involve obtaining samples for chemical analysis, mechanical tests and metallographic studies [2–5]. Diverse stainless steel pipe fittings are used in water pipes, public water supplies in buildings, hot water supply piping, hot and cold water pipes, fire-fighting pipes, drain pipes, and plants. Materials for piping valves and joints are selected considering the arrangement with other accessories and the connection with the pipes. The cracked jaws in the joint were made of martensitic stainless 420, and were connecting the pipe and the joint body (Fig. 1). The failure analysis was carried out on jaws taken from the joint, in order to investigate the failure mode and to provide some recommendations to solve the problem.
2. Materials and methods In general, martensitic stainless steels are essentially iron–chromium alloys containing 12–17% Cr. With sufficient carbon a martensitic structure can be produced by quenching from the austenitic phase region. These alloys are called martensitic ⇑ Corresponding author. Tel.: +82 2 2164 0169; fax: +82 2 2634 0035. E-mail address: [email protected] (J. Lee). 1350-6307/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2012.08.006
J. Lee et al. / Engineering Failure Analysis 26 (2012) 266–273
267
Fig. 1. Schematic of the connection joint configuration.
Fig. 2. The general appearance: (a) normal jaw and (b) failed jaw.
because they are capable of developing a martensitic structure with austenitizing and quenching heat treatment [6]. In this case, the martensitic stainless steel type 420 specified in the ASTM standard for jaws was manufactured through the following process: r press working from plates, s heat treatment at 990 °C in N2 atmosphere (2 h), t air quenching, u tempering at 210 °C in the air (2 h), and v natural air cooling. Three samples were prepared for failure analysis. One was an unused sample before heat treatment (sample 1), another was an unused sample after heat treatment referred to as a normal sample (sample 2) and the other was a failed jaw (sample 3). All samples for microstructural evaluation were mechanically polished by SiC paper and diamond paste down 1 lm, and their microstructure was evidenced by picral and hydrochloric acid etching. The failed jaw was inspected visually and macroscopically by means of a stereoscopic microscope, with care taken to avoid damage to fractured surfaces. The chemical composition of the failed jaw was checked using inductively coupled plasma atomic emission spectroscopy (ICP) and a carbon–sulfur analyzer. Several Rockwell hardness measurements were made on a polished and unetched surface of an unused jaw sample. Investigations of transverse cross-sections were carried out by means of optical microscope (DMRM, Leica) and scanning electron microscope (JSM-6490LV, JEOL). The fractured surfaces were ultrasonically cleaned and observed with a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). In addition, the energy dispersive X-ray spectroscopy microprobe was used to analyze the chemical composition of the particles present on surfaces of fracture and cross-section in the failed jaws.
3. Results and discussion 3.1. Visual observations and macroanalysis of the failed part Visual observations and stereoscopic microscope observations revealed severe corrosion and fracture of the failed jaw (sample 3). In addition, brown/red1 deposits were seen on the surface of the failed jaw, and black deposits were found on fracture surface. Cracks in jaws were found approximately at the site in contact with pipes (Fig. 2b). 1
For interpretation of color in Fig. 2, the reader is referred to the web version of this article.
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J. Lee et al. / Engineering Failure Analysis 26 (2012) 266–273
3.2. Analysis of chemical composition and hardness test Chemical analysis using inductively coupled plasma atomic emission spectroscopy (ICP) and carbon–sulfur analyzer was carried out, and the results are given in Table 1. The ranges for the composition of ASTM 420 stainless steel and Korea National Standard 420J2 stainless steel are also included in Table 1. The chemical composition of the failed jaw corresponded to the specifications of ASTM 420 stainless steels provided in the ASTM A473 Standard ‘‘Standard Specification for Stainless Steel Forgings’’, and to KS 420J2 stainless steels under Korea National Standard D 3698 ‘‘Cold rolled stainless steel plates, sheets and strip’’. 3.3. Hardness test Several Rockwell hardness measurements were made on a polished and unetched surface of an unused jaw (sample 2). The average value of the test results was 56.1 HRC. Therefore, this value satisfies the specifications of ASTM 420 martensitic stainless steel (min. 50 HRC) and KS 420J2 martensitic stainless steel (min. 40 HRC). 3.4. Test of metallurgical structure 3.4.1. Metallurgical structure of the normal and failed jaws In accordance with ASTM E3 ‘‘Standard Practice for Preparation of Metallographic Specimens Test of Metal Microscopic Structure’’, we prepared the samples for measurement of cross-sectional metallurgical structure of samples 1–3. Microstructure of type 420 stainless steel before heat treatment (sample 1) consists of randomly dispersed particles of chromium carbide in a matrix of ferrite (Fig. 3a). In contrast, the microstructure of normal sample and failed sample (samples
Table 1 Chemical composition of the failed jaw.
a b
Element (wt.%)
C
Si
Mn
Cr
S
P
Failed jaw ASTM 420a STS 420J2a,b
0.30 Over 0.15 0.26–0.40
0.54 1.00 1.00
0.51 1.00 1.00
13.09 12.0–14.0 12.0–14.0
0.003 0.030 0.030
0.005 0.040 0.040
Maximum, unless range or minimum is indicated. Korea Standard for STS 420J2.
Fig. 3. Optical micrographs of etched cross-sectional surfaces of (a) sample 1, (b) sample 2, (c) sample 3 (200), and (d) surface near the fractured end (100).
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J. Lee et al. / Engineering Failure Analysis 26 (2012) 266–273
2 and 3) after heat treatment consists of elongated layers of particles indicated by arrows in Fig. 3b–c along grain boundary in a matrix of tempered martensite. Crack propagation along the grain boundaries was seen near the fracture end (Fig. 3d). 3.4.2. Determination of the grain size in the normal and failed jaws In accordance with ASTM E112 ‘‘Estimating the average Grain Size of Metals – Circular Intercept Procedures’’, the grain sizes were probed using a microscope (Olympus BX51M) and image analysis system (Clemex Vision PE). The grain grade indices of the cross-section of the normal sample and failed sample are shown in Table 2. It is found that the grain sizes of the two samples were similar. 3.5. Scanning electron-microscopic analyses of the surfaces of fracture and cross-section in the failed jaws The fracture surfaces were examined with the help of a scanning electron microscope (SEM) in order to characterize the fracture micromechanism(s). Dried mud patterns, which are indicated by arrows in Fig. 4, were observed clearly on the fracture surfaces of the failed sample (Fig. 4). High magnification observations of the fracture surface revealed typical intergranular fracture features described as a ‘‘rock candy’’ with the presence of particles (Fig. 5). Intergranular fracture is a fracture mode in which failure occurs as a result of separation along the metal grain boundaries (and/or failure in a near grain boundary layer) rather than through the metal grains. Intergranular fracture is often described
Table 2 Grain size of the normal (sample 2) and failed jaw (sample 3). Sample
Mean intercept distance (lm)
Grain size no. G
Normal sample (sample 2) Failed sample (sample 3)
20.58 21.53
7.93 7.80
Conditions Pattern: 3 circles Quantity of fields: 5 Length: 1000 lm
Fig. 4. SEM photograph of the fracture surface of the failed sample (sample 3) showing dried mud patterns.
Fig. 5. SEM photographs of the fracture surface of the failed sample (sample 3) showing features of intergranular, ‘rock candy’ fracture and particles (white).
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as a ‘‘rock candy’’ fracture appearance [1]. Intergranular fracture is evidence of zero or near zero ductility, and is not generally a desirable fracture mode. Intergranular fracture can usually be easily recognized, but determining the primary cause of the fracture may be difficult. Fractographic observations can readily identify the presence of large fractures of second-phase particles at grain boundaries. In addition, the SEM–EDS analysis of the fracture surfaces showed that Cr, Si, and S contents of the ‘rock candy’ shaped particle were considerably higher, while Fe content was significantly lower than in the surroundings (Fig. 6). This is believed to be due to the precipitation of Cr-rich carbides (or oxides) along the grain boundaries. Since this shape is observed on the entire fracture surface, it seems that intergranular corrosion was generated in the chromium-depleted zone near the particle, resulting in fracture. Generally, the anti-corrosiveness of metals decreases in a chromium-depleted zone, and metal exposed to a corrosive environment tends to corrode badly in the grain boundaries due to anodic reaction. The precipitation of chromium-rich carbides (or oxides) along grain boundaries is particularly damaging since it can occur in almost all operations necessary for structural materials. SEM observation of Fig. 3b and c shows particles along the grain boundaries to the direction of elongation; if highly magnified, the particles appeared to be circles of 1 lm diameter (Fig. 7). Semi-quantitative chemical analysis was carried out by EDS attached to SEM on the polished surface of normal sample and failed sample (samples 2 and 3) to qualitatively determine the particles’ chemistry and to analyze the fracture mechanism. The analysis of particles along the grain boundaries indicated by arrows in Fig. 8b and c by means of scanning electron microscope with EDS showed that particles were mainly composed of Fe, Cr, C, and O, with small amounts of Al. In particular, the contents of carbon, oxygen and chromium were significantly higher in particles than surroundings. Also, as seen in Fig. 3a, chromium carbide did not appear along the grain boundary before heat treatment, but did after heat treatment, which means that the particles were produced during the heat treatment. The chromium carbide precipitates are very high in chromium, but the matrix alloy is depleted of chromium in the grain boundaries and Fig. 9 is schematic of carbide precipitation at a grain boundary during sensitization to intergranular
Fig. 6. SEM photographs of the surface of the failure sample (sample 3) and (c and d) X-ray energy spectrum of the ‘spectrum 1 and 2’ microregion.
J. Lee et al. / Engineering Failure Analysis 26 (2012) 266–273
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Fig. 7. SEM photographs of the surface of the normal sample (sample 2). (a) SEM photograph of the cross-section surface showing inclusions along grain boundary in the rolling direction and (b) enlargement of (a).
Fig. 8. SEM photographs and EDS analyses of the surfaces of the normal sample (sample 2). (a) SEM photograph of the surface of the normal sample (sample 2), (b) X-ray energy spectrum of the particle ‘spectrum 1’, (c) X-ray energy spectrum of the substance ‘spectrum 2’, and (d) EDX analyses of ‘spectrum 1 and 2’.
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Fig. 9. Schematic representation of carbide precipitation at a grain boundary during sensitization to intergranular corrosion in stainless steel [11].
corrosion in stainless steel [11]. Sensitized grain boundaries in stainless steels are more prone to intergranular corrosion attack, and stressed and sensitized materials are prone to intergranular stress corrosion cracking [7–10]. The analysis of this study indicates that chromium-rich carbides are generated during heat treatment along the grain boundaries to the direction of elongation; and a chromium-depleted zone is produced in the surroundings, from which intergranular corrosion occurs, resulting in cracks. 4. Conclusions This study was conducted on a failed jaw used in a pipeline joint. Chemical analysis, mechanical testing results and microstructure identified the failure mode of the failed jaw as ASTM type 420 martensitic stainless steel. The fracture surface is characteristic of an intergranular corrosion fracture caused by the precipitation of chromium-rich carbides along grain boundaries. 5. Recommendations 1. Change the heat treatment process (temperature and time) to reduce chromium-rich precipitations. Heat to a high temperature above 1000 °C followed by quenching (rapid cooling) in water or quenching oils. During the heating stage, the carbides dissolve and their formation is suppressed by fast cooling. 2. Use ASTM type 440A, 440B, 440C, which have high carbon and chromium contents for highest hardness and corrosion resistance [12], instead of ASTM 420, and apply proper heat treatment by grade. 3. Use ASTM type 422, 414, 431 instead of ASTM 420. Molybdenum and nickel can be added to martensitic stainless steel to improve corrosion and toughness properties, as in ASTM type 422 stainless steel. Nickel also serves to maintain the desired microstructure, preventing excessive free ferrite when high chromium levels are used to improve corrosion resistance, as it is in ASTM type 414 and 431 [12].
References [1] Boyer Howard E et al. Metals handbook 10 – failure analysis and prevention. 8th ed. American Society for Metals; 1975. p. 1–10. [2] Irisarri AM, Silveria E. Study of failure of one machining tool. Eng Fail Anal 2010;17:380–6. [3] Leonel Edson Denner, Venturini Wilson Sergio, Chateauneuf Alaa. A BEM model applied to failure analysis of multi-fractured structures. Eng Fail Anal 2011;18:1538–49.
J. Lee et al. / Engineering Failure Analysis 26 (2012) 266–273 [4] [5] [6] [7] [8] [9] [10] [11] [12]
Layer John, Adler Tom et al. ASM HANDBOOK – failure analysis and prevention. 11th ed., ASM International; 2002. p. 14–65. Li Yongde, Na Xu, et al. Failure analysis of the 304 stainless steel tube in a gas analyzer. Eng Fail Anal 2012;20:35–42. Smith William F. Structure and properties of engineering alloys. 2nd ed. McGraw-Hill Inc.; 1993. p. 303–4. Corrosion of stainless steels. ASM handbook. vol. 13. Corrosion, ASM International; 1996. Fontana MG, Greene ND. Corrosion engineering. McGraw-Hill; 1984. High performance stainless steels. Nickel Development Institute, Publication No. 11021; 2000. Sedricks A John. Corrosion of stainless steels. 2nd ed. J. Wiley & Sons; 1996. p. 282–6. Jones DA. Principles and prevention of corrosion. Upper Saddle River (NJ): Prentice Hall; 1996. p. 292. Davis JR. ASM specialty handbook-stainless steels. ASM International; 1996. p. 14–7.
273
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Metallurgical analysis of failed jaws used in joint of a water supply pipe line Jongmin Lee ⇑, Shinho Han, Heongkee Kim, Ungi Lee Green Technology Headquarter, Korea Testing & Research Institute (KTR), Yeongdeungpo-dong 8ga 88-2, Seoul, Republic of Korea
a r t i c l e
i n f o
Article history: Received 16 July 2012 Accepted 9 August 2012 Available online 13 September 2012 Keywords: Intergranular corrosion cracking Sensitization Precipitation Martensitic stainless steel Failure analysis
a b s t r a c t In this paper, the cause of failure of the 420 stainless steel jaws in a joint was investigated. It was found that the martensitic stainless steel ASTM type 420 jaws used in connection joints of a water supply pipeline were cracked after 6 months in service. The failed parts were investigated by means of stereoscopic microscope, optical microscope, scanning electron microscope equipped with energy dispersive X-ray spectroscopy micro-analysis, inductively coupled plasma atomic emission spectroscopy, carbon–sulfur analyzer and Rockwell hardness tester, in order to identify the causes of failure and suggest preventive solutions. The study shows that failure was mainly due to intergranular corrosion cracking caused by the precipitation of chromium-rich carbides (or oxides) formed during heat treatment along grain boundaries. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Failures come from various mechanisms and related environmental factors [1]. Failure analysis is a process performed to determine the root causes or factors that led to an undesired loss of functionality [2,3]. Analysis of a failure of a metal structure or part usually requires identification of the type of failure [1]. The process is complex, draws upon many different technical disciplines, and uses a variety of observation, inspection, and laboratory techniques. During the investigation, the analyst must collect, examine and evaluate all the available data in order to determine its plausible origins and the sequence of events that led to the failure. The immediate objective is to find the root causes of the failure in order to determine the compensation for the induced damage, even if the long-term priority is to prevent similar failures in new recommendations. Usually these failure analyses involve obtaining samples for chemical analysis, mechanical tests and metallographic studies [2–5]. Diverse stainless steel pipe fittings are used in water pipes, public water supplies in buildings, hot water supply piping, hot and cold water pipes, fire-fighting pipes, drain pipes, and plants. Materials for piping valves and joints are selected considering the arrangement with other accessories and the connection with the pipes. The cracked jaws in the joint were made of martensitic stainless 420, and were connecting the pipe and the joint body (Fig. 1). The failure analysis was carried out on jaws taken from the joint, in order to investigate the failure mode and to provide some recommendations to solve the problem.
2. Materials and methods In general, martensitic stainless steels are essentially iron–chromium alloys containing 12–17% Cr. With sufficient carbon a martensitic structure can be produced by quenching from the austenitic phase region. These alloys are called martensitic ⇑ Corresponding author. Tel.: +82 2 2164 0169; fax: +82 2 2634 0035. E-mail address: [email protected] (J. Lee). 1350-6307/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2012.08.006
J. Lee et al. / Engineering Failure Analysis 26 (2012) 266–273
267
Fig. 1. Schematic of the connection joint configuration.
Fig. 2. The general appearance: (a) normal jaw and (b) failed jaw.
because they are capable of developing a martensitic structure with austenitizing and quenching heat treatment [6]. In this case, the martensitic stainless steel type 420 specified in the ASTM standard for jaws was manufactured through the following process: r press working from plates, s heat treatment at 990 °C in N2 atmosphere (2 h), t air quenching, u tempering at 210 °C in the air (2 h), and v natural air cooling. Three samples were prepared for failure analysis. One was an unused sample before heat treatment (sample 1), another was an unused sample after heat treatment referred to as a normal sample (sample 2) and the other was a failed jaw (sample 3). All samples for microstructural evaluation were mechanically polished by SiC paper and diamond paste down 1 lm, and their microstructure was evidenced by picral and hydrochloric acid etching. The failed jaw was inspected visually and macroscopically by means of a stereoscopic microscope, with care taken to avoid damage to fractured surfaces. The chemical composition of the failed jaw was checked using inductively coupled plasma atomic emission spectroscopy (ICP) and a carbon–sulfur analyzer. Several Rockwell hardness measurements were made on a polished and unetched surface of an unused jaw sample. Investigations of transverse cross-sections were carried out by means of optical microscope (DMRM, Leica) and scanning electron microscope (JSM-6490LV, JEOL). The fractured surfaces were ultrasonically cleaned and observed with a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). In addition, the energy dispersive X-ray spectroscopy microprobe was used to analyze the chemical composition of the particles present on surfaces of fracture and cross-section in the failed jaws.
3. Results and discussion 3.1. Visual observations and macroanalysis of the failed part Visual observations and stereoscopic microscope observations revealed severe corrosion and fracture of the failed jaw (sample 3). In addition, brown/red1 deposits were seen on the surface of the failed jaw, and black deposits were found on fracture surface. Cracks in jaws were found approximately at the site in contact with pipes (Fig. 2b). 1
For interpretation of color in Fig. 2, the reader is referred to the web version of this article.
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J. Lee et al. / Engineering Failure Analysis 26 (2012) 266–273
3.2. Analysis of chemical composition and hardness test Chemical analysis using inductively coupled plasma atomic emission spectroscopy (ICP) and carbon–sulfur analyzer was carried out, and the results are given in Table 1. The ranges for the composition of ASTM 420 stainless steel and Korea National Standard 420J2 stainless steel are also included in Table 1. The chemical composition of the failed jaw corresponded to the specifications of ASTM 420 stainless steels provided in the ASTM A473 Standard ‘‘Standard Specification for Stainless Steel Forgings’’, and to KS 420J2 stainless steels under Korea National Standard D 3698 ‘‘Cold rolled stainless steel plates, sheets and strip’’. 3.3. Hardness test Several Rockwell hardness measurements were made on a polished and unetched surface of an unused jaw (sample 2). The average value of the test results was 56.1 HRC. Therefore, this value satisfies the specifications of ASTM 420 martensitic stainless steel (min. 50 HRC) and KS 420J2 martensitic stainless steel (min. 40 HRC). 3.4. Test of metallurgical structure 3.4.1. Metallurgical structure of the normal and failed jaws In accordance with ASTM E3 ‘‘Standard Practice for Preparation of Metallographic Specimens Test of Metal Microscopic Structure’’, we prepared the samples for measurement of cross-sectional metallurgical structure of samples 1–3. Microstructure of type 420 stainless steel before heat treatment (sample 1) consists of randomly dispersed particles of chromium carbide in a matrix of ferrite (Fig. 3a). In contrast, the microstructure of normal sample and failed sample (samples
Table 1 Chemical composition of the failed jaw.
a b
Element (wt.%)
C
Si
Mn
Cr
S
P
Failed jaw ASTM 420a STS 420J2a,b
0.30 Over 0.15 0.26–0.40
0.54 1.00 1.00
0.51 1.00 1.00
13.09 12.0–14.0 12.0–14.0
0.003 0.030 0.030
0.005 0.040 0.040
Maximum, unless range or minimum is indicated. Korea Standard for STS 420J2.
Fig. 3. Optical micrographs of etched cross-sectional surfaces of (a) sample 1, (b) sample 2, (c) sample 3 (200), and (d) surface near the fractured end (100).
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J. Lee et al. / Engineering Failure Analysis 26 (2012) 266–273
2 and 3) after heat treatment consists of elongated layers of particles indicated by arrows in Fig. 3b–c along grain boundary in a matrix of tempered martensite. Crack propagation along the grain boundaries was seen near the fracture end (Fig. 3d). 3.4.2. Determination of the grain size in the normal and failed jaws In accordance with ASTM E112 ‘‘Estimating the average Grain Size of Metals – Circular Intercept Procedures’’, the grain sizes were probed using a microscope (Olympus BX51M) and image analysis system (Clemex Vision PE). The grain grade indices of the cross-section of the normal sample and failed sample are shown in Table 2. It is found that the grain sizes of the two samples were similar. 3.5. Scanning electron-microscopic analyses of the surfaces of fracture and cross-section in the failed jaws The fracture surfaces were examined with the help of a scanning electron microscope (SEM) in order to characterize the fracture micromechanism(s). Dried mud patterns, which are indicated by arrows in Fig. 4, were observed clearly on the fracture surfaces of the failed sample (Fig. 4). High magnification observations of the fracture surface revealed typical intergranular fracture features described as a ‘‘rock candy’’ with the presence of particles (Fig. 5). Intergranular fracture is a fracture mode in which failure occurs as a result of separation along the metal grain boundaries (and/or failure in a near grain boundary layer) rather than through the metal grains. Intergranular fracture is often described
Table 2 Grain size of the normal (sample 2) and failed jaw (sample 3). Sample
Mean intercept distance (lm)
Grain size no. G
Normal sample (sample 2) Failed sample (sample 3)
20.58 21.53
7.93 7.80
Conditions Pattern: 3 circles Quantity of fields: 5 Length: 1000 lm
Fig. 4. SEM photograph of the fracture surface of the failed sample (sample 3) showing dried mud patterns.
Fig. 5. SEM photographs of the fracture surface of the failed sample (sample 3) showing features of intergranular, ‘rock candy’ fracture and particles (white).
270
J. Lee et al. / Engineering Failure Analysis 26 (2012) 266–273
as a ‘‘rock candy’’ fracture appearance [1]. Intergranular fracture is evidence of zero or near zero ductility, and is not generally a desirable fracture mode. Intergranular fracture can usually be easily recognized, but determining the primary cause of the fracture may be difficult. Fractographic observations can readily identify the presence of large fractures of second-phase particles at grain boundaries. In addition, the SEM–EDS analysis of the fracture surfaces showed that Cr, Si, and S contents of the ‘rock candy’ shaped particle were considerably higher, while Fe content was significantly lower than in the surroundings (Fig. 6). This is believed to be due to the precipitation of Cr-rich carbides (or oxides) along the grain boundaries. Since this shape is observed on the entire fracture surface, it seems that intergranular corrosion was generated in the chromium-depleted zone near the particle, resulting in fracture. Generally, the anti-corrosiveness of metals decreases in a chromium-depleted zone, and metal exposed to a corrosive environment tends to corrode badly in the grain boundaries due to anodic reaction. The precipitation of chromium-rich carbides (or oxides) along grain boundaries is particularly damaging since it can occur in almost all operations necessary for structural materials. SEM observation of Fig. 3b and c shows particles along the grain boundaries to the direction of elongation; if highly magnified, the particles appeared to be circles of 1 lm diameter (Fig. 7). Semi-quantitative chemical analysis was carried out by EDS attached to SEM on the polished surface of normal sample and failed sample (samples 2 and 3) to qualitatively determine the particles’ chemistry and to analyze the fracture mechanism. The analysis of particles along the grain boundaries indicated by arrows in Fig. 8b and c by means of scanning electron microscope with EDS showed that particles were mainly composed of Fe, Cr, C, and O, with small amounts of Al. In particular, the contents of carbon, oxygen and chromium were significantly higher in particles than surroundings. Also, as seen in Fig. 3a, chromium carbide did not appear along the grain boundary before heat treatment, but did after heat treatment, which means that the particles were produced during the heat treatment. The chromium carbide precipitates are very high in chromium, but the matrix alloy is depleted of chromium in the grain boundaries and Fig. 9 is schematic of carbide precipitation at a grain boundary during sensitization to intergranular
Fig. 6. SEM photographs of the surface of the failure sample (sample 3) and (c and d) X-ray energy spectrum of the ‘spectrum 1 and 2’ microregion.
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Fig. 7. SEM photographs of the surface of the normal sample (sample 2). (a) SEM photograph of the cross-section surface showing inclusions along grain boundary in the rolling direction and (b) enlargement of (a).
Fig. 8. SEM photographs and EDS analyses of the surfaces of the normal sample (sample 2). (a) SEM photograph of the surface of the normal sample (sample 2), (b) X-ray energy spectrum of the particle ‘spectrum 1’, (c) X-ray energy spectrum of the substance ‘spectrum 2’, and (d) EDX analyses of ‘spectrum 1 and 2’.
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Fig. 9. Schematic representation of carbide precipitation at a grain boundary during sensitization to intergranular corrosion in stainless steel [11].
corrosion in stainless steel [11]. Sensitized grain boundaries in stainless steels are more prone to intergranular corrosion attack, and stressed and sensitized materials are prone to intergranular stress corrosion cracking [7–10]. The analysis of this study indicates that chromium-rich carbides are generated during heat treatment along the grain boundaries to the direction of elongation; and a chromium-depleted zone is produced in the surroundings, from which intergranular corrosion occurs, resulting in cracks. 4. Conclusions This study was conducted on a failed jaw used in a pipeline joint. Chemical analysis, mechanical testing results and microstructure identified the failure mode of the failed jaw as ASTM type 420 martensitic stainless steel. The fracture surface is characteristic of an intergranular corrosion fracture caused by the precipitation of chromium-rich carbides along grain boundaries. 5. Recommendations 1. Change the heat treatment process (temperature and time) to reduce chromium-rich precipitations. Heat to a high temperature above 1000 °C followed by quenching (rapid cooling) in water or quenching oils. During the heating stage, the carbides dissolve and their formation is suppressed by fast cooling. 2. Use ASTM type 440A, 440B, 440C, which have high carbon and chromium contents for highest hardness and corrosion resistance [12], instead of ASTM 420, and apply proper heat treatment by grade. 3. Use ASTM type 422, 414, 431 instead of ASTM 420. Molybdenum and nickel can be added to martensitic stainless steel to improve corrosion and toughness properties, as in ASTM type 422 stainless steel. Nickel also serves to maintain the desired microstructure, preventing excessive free ferrite when high chromium levels are used to improve corrosion resistance, as it is in ASTM type 414 and 431 [12].
References [1] Boyer Howard E et al. Metals handbook 10 – failure analysis and prevention. 8th ed. American Society for Metals; 1975. p. 1–10. [2] Irisarri AM, Silveria E. Study of failure of one machining tool. Eng Fail Anal 2010;17:380–6. [3] Leonel Edson Denner, Venturini Wilson Sergio, Chateauneuf Alaa. A BEM model applied to failure analysis of multi-fractured structures. Eng Fail Anal 2011;18:1538–49.
J. Lee et al. / Engineering Failure Analysis 26 (2012) 266–273 [4] [5] [6] [7] [8] [9] [10] [11] [12]
Layer John, Adler Tom et al. ASM HANDBOOK – failure analysis and prevention. 11th ed., ASM International; 2002. p. 14–65. Li Yongde, Na Xu, et al. Failure analysis of the 304 stainless steel tube in a gas analyzer. Eng Fail Anal 2012;20:35–42. Smith William F. Structure and properties of engineering alloys. 2nd ed. McGraw-Hill Inc.; 1993. p. 303–4. Corrosion of stainless steels. ASM handbook. vol. 13. Corrosion, ASM International; 1996. Fontana MG, Greene ND. Corrosion engineering. McGraw-Hill; 1984. High performance stainless steels. Nickel Development Institute, Publication No. 11021; 2000. Sedricks A John. Corrosion of stainless steels. 2nd ed. J. Wiley & Sons; 1996. p. 282–6. Jones DA. Principles and prevention of corrosion. Upper Saddle River (NJ): Prentice Hall; 1996. p. 292. Davis JR. ASM specialty handbook-stainless steels. ASM International; 1996. p. 14–7.
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