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Failure study of pneumatic rock drill piston
Engineering Failure Analysis 13 (2006) 1108–1115 www.elsevier.com/locate/engfailanal
Failure study of pneumatic rock drill piston Tezcan Sekercioglu
*
Mechanical Engineering Department, Engineering Faculty, Pamukkale University, Camlik 20020, Denizli, Turkey Received 24 June 2005; accepted 14 July 2005 Available online 16 September 2005
Abstract In this study, the fracture of piston for rock drill produced from case hardening steel is investigated. In order to study the causes of the fracture, specimens prepared from the damaged pistons were subjected to experiments such as hardness, chemical analysis and metallurgical tests. The effect of microstructure on the fracture was also considered. The detailed metallurgical analysis was conducted on the fractured pistons and compared with the unfailed pistons in service. Results showed that the obtained pistons hardness values were lower than the values stated in the literature. Geometrical analysis of the pistons revealed that early failure of the structure started from a notch by which stress concentrations are created at the end of the threaded region. 2005 Elsevier Ltd. All rights reserved. Keywords: Fracture; Failure analysis; Rock drill; Microstructure
1. Introduction The hand-held pneumatic rock drill is a piston rotary unit that is designed mainly for use as a hard rock drill; however, it is equally efficient in soft and medium formations. The photograph and the schematic diagram of the rock drill are shown in Figs. 1 and 2. The drill consists of a back head group, cylinder unit, and front head group. The back head group consists of the four-position throttle, handle, and live air inlet. The cylinder unit consists of a cylinder with a reciprocating piston. The front head group consists of the chuck, retainer latch, and anvil. The drill design directs air through the drill, down the drill steel, and into the bottom of the hole to blow out rock cuttings. Four classes of rotary rock drills are as follows: The first class is a light drill, weighing about 7 kg. This class is used for drilling shallow holes in quarry operations. The second class is a light drill, weighing 12–18 kg. This class is used for light work, such as potholing and drilling *
Tel.: +90 258 212 55 32x128; fax: +90 258 212 55 38. E-mail address: [email protected].
1350-6307/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2005.07.018
T. Sekercioglu / Engineering Failure Analysis 13 (2006) 1108–1115
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Fig. 1. Photograph of the pneumatic rock drill.
Fig. 2. Schematic diagram of the pneumatic rock drill.
concrete. The third class weighs from 18 to 22 kg. This class is used for drilling in limestone and other soft rock. The fourth class is a hand-held drill, weighing from 22 to 30 kg. This class is used for drilling holes up to 20 m during quarry operations [1].
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Hand-held rotary rock drills used in quarry operations may be the dry drill, the blower drill or the wet drill. The dry drill allows very little air to pass through the drill tool while drilling; therefore, you should drill 30 s and blow 60 s. When the hammer is not running, the dry drill allows enough air to pass through for cuttings to be blown out of the hole. Drill steels for this drill come in lengths of 0.6, 1.2 and 1.8 m with tips made of carbon inserts, diamond, or star. The blower drill allows a steady supply of air to pass through the drill steel to help remove cuttings from the hole while the hammer is running. This type also permits air to pass through the drill steel when the hammer is not running. The wet drill provides a constant supply of water through the drill steel while the hammer is running [1]. The investigated pistons are made of case hardening steel. The case hardening steel (20NiCrMo2-2, EN 10084) is a low nickel–chromium– molybdenum with medium hardenability, generally supplied in the as rolled condition with a maximum brinell hardness of 280 (30 HRC). It is characterized by good core strength and toughness in small to medium sections with case hardness up to 62 HRC when carburized, hardened and tempered. This steel can also be used (uncarburised) as high tensile steel, which when suitably hardened and tempered can be utilized for various applications requiring good tensile strength and reasonable toughness. Pre hardened and tempered (uncarburized) 20NiCrMo2–2 can be further surface hardened by nitriding but will not respond satisfactorily to flame or induction hardening due to its low carbon content. It is used extensively by all industry sectors for light to medium stressed components and shafts requiring high surface wear resistance with reasonable core strength and impact properties. Typical uses are: bearings, bushings, cam shafts, differential pinions, guide pins, king pins, pistons pins, splined shafts, ratchets, sleeves, etc. Carburization is a thermochemical treatment, usually conducted at temperatures in the range 800– 940 C, in the first stage of case-hardening. This process changes the chemical composition of the surface of a low-carbon steel component so that subsequent fast cooling, by quenching, produces a hard case combined with a softer/tougher core. Quenching is normally followed by a low-temperature tempering/stress relieving treatment. In the process of carburization, controlled levels of carbon are introduced at the surface and allowed to diffuse to a controlled depth. Carburizing case hardening treatment offers a means of enhancing the strength and wear properties of parts made from relatively inexpensive easily worked materials. Generally applied to near-finished components, the processes impart a high-hardness wear resistant surface which, with sufficient depth, can also improve fatigue strength. Applications range from simple mild steel pressings to heavy-duty alloy-steel transmission components [2]. In this study, the damaged rock drill pistons are investigated. Almost two rock drill pistons are damaged every month in the quarry. The damaged pistons of rock drill were evaluated, and the causes of fracture of a piston manufactured from case hardening steel were carried out. Some properties of rock drill are given in Table 1. Also, the main dimensions and the break position of the piston are shown in Fig. 3. A number of mechanical and microstructure analyses are carried out to determine the causes of fracture.
Table 1 Some properties of pneumatic rock drill Class (10 kg) Drilling type Operating temperature (C) Operating pressure (bar) Drilled material
Light Blower drill 22 ± 5 6 Medium rock
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Fig. 3. Main dimensions (mm) and the fracture zone of the rock drill piston.
2. Techniques used in fracture analysis The three damaged drill piston specimens and the unfailed piston in service were subjected to various tests. The following experimental works were done:
visual inspection and fractography; hardness tests; chemical analysis; metallographic analysis.
3. Analysis and results 3.1. Visual inspection and fractography Although the unfailed and failed pistons have a same geometry, initial study of structure led to the view that fractures may have been caused by thread notch. It is considered that the cause of failure was unlikely to be thread notch alone, but their contribution to the failure cannot be underrated due to the presence of stress concentrations. The separated pistons 2–4 showed similar failure and did not bear any indication of fatigue crack growth when the fracture surface was examined, indicating that the failure was of a brittle type of fracture. The fracture zone is shown in Fig. 3. The distance between thread and the pistonÕs outer diameter is less for torque strength. As shown in Fig. 4, all fractures seem to start at a threaded region because of the high stress concentration at this region, which may be compounded by the poor impact energy of the material used. The reduced thickness between the threaded hole and the pistonÕs outer diameter assisted the failure. The fractured surfaces of pistons are shown in Fig. 5. According to the fractured surfaces, it was said that the failure was due to a brittle mode. 3.2. Hardness analysis Hardness analysis of fractured piston materials was carried out using a Rockwell hardness test machine. The measurements were carried out on three different surface areas. The hardness values are listed in Table 2. The hardness values of unfailed (1) and failed pistons (2–4) are different. In order to
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Fig. 4. Unfailed (1) and failed piston (2–4).
Fig. 5. Fractured surfaces of pistons.
Table 2 Hardness values of fractured piston materials HRC
Piston Piston Piston Piston
1 2 3 4
1
2
3
Mean
57 54 56 55
58 54 54 54
58 56 56 55
57.66 54.66 55.33 54.66
obtain maximum fatigue strength, the pistonÕs outer surface hardness may be maximum 58–62 HRC [3– 5]. The hardnesses were observed as 54 HRC for the damaged pistons, which is a lesser than the values stated in the literature. 3.3. Chemical analysis Chemical analysis of 20NiCrMo2-2 case hardening steel according to EN 10084 is shown in Table 3. The chemical composition of the piston materials was determined by spectroscopy chemical analysis. The chemical compositions of piston material are listed in Table 4. It was understood from the chemical composition that the material was case hardening steel. The composition of piston materials contains low C and Cr, Ni and Mo content, which cause the structure to quench in a tough mode. The alloying additions improve the hardenability of the steel. Chromium improves corrosion resistance, while manganese contributes to the
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Table 3 Chemical analysis of 20NiCrMo2-2 case hardening steel according to EN 10084 Element (%)
20NiCrMo2-2
C
Si
Mn
P
S
Cr
Ni
Mo
0.17 0.23
– 0.40
0.65 0.95
– 0.035
– 0.035
0.4 0.7
0.4 0.7
0.15 0.25
Table 4 Chemical analysis of the piston materials Element (%)
Piston Piston Piston Piston
1 2 3 4
C
Si
Mn
P
S
Cr
Ni
Mo
0.226 0.214 0.189 0.205
0.294 0.245 0.264 0.272
0.829 0.735 0.980 0.752
0.007 0.017 0.007 0.017
0.025 0.030 0.019 0.027
0.550 1.149 0.529 1.180
0.453 0.132 0.545 0.119
0.201 0.229 0.184 0.225
deoxidation of the melt and also improves machinability. Nickel reduces distortion and cracking upon quenching. Although the alloy elements in pistons 1 and 3 were normal in composition according to EN 10084 standard, the chemical compositions of pistons 2 and 4 were abnormal. The amount of Cr in pistons 2 and 4 was double. Also, the amount of Ni in pistons 2 and 4 was very low according to related standard. 3.4. Metallographic analysis The metallographic specimens were first ground, polished and etched using standard techniques in order to examine the inner structure. A light optical microscope was used in the investigations. The microstructures of the failed piston material show that they are different structures, which is depicted in Figs. 6 and 7. It can be understand from the figures that the pistons were carburized and then differently cooled in the oil ambient. Figs. 6 and 7 show an example of retained austenite in the carburized case of 20NiCrMo2-2 low-alloy steel. The retained austenite is white and lies between the plate martensite. However, there are also a
Fig. 6. Microstructures of piston 1 (etched with nital, 200·): (a) core, (b) surface region.
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Fig. 7. Microstructures of piston 4 (etched with nital, 200·): (a) core, (b) surface region.
Fig. 8. (a) Microcracks in the core of piston 2 (unetched, 200·). (b) Microcracks in the core of piston 4 (unetched, 200·).
few white particles of cementite in the structure. Excessive retained austenite in the case hardening steels is usually detrimental to life, because it may transform to fresh martensite and cause cracking in the part, or reduce the wear resistance. Also, the microcracks were observed in pistons 2 and 4 (Fig. 8). Microcracks may be formed during the heat treatment or operation.
4. Conclusion In this research, the influences of microstructure, chemical composition and hardness of the pistons were investigated. From the experimental observations, the following conclusions may be made: 1. The chemical composition of piston material should be selected according to the standard. 2. In order to same microstructure, carburising heat treatment should be made same conditions such as time, case depth. In all cases, the austenite and accompanying martensite should be fine and evenly distributed. The re-heat quenching may lead to refined structures. 3. The hardness of some pistons is low. In order to obtain maximum fatigue strength, the pistons outer surface hardness may be increased to 60–62 HRC.
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4. On the examination of fractured parts, it can be concluded that all fractures started at a threaded region due to high stress concentrations. The thread fillet radius can be enlarged in design stage. Also, the piston diameter can be increased at the threaded region.
Acknowledgments The author gratefully acknowledges the support of ASTAS Co. and AR DOKUM Co. for this study.
References [1] [2] [3] [4] [5]
Available from: http://www.tpub.com/content/engine/14081/css/14081_394.htm. The Contract Heat Treatment Association, Datasheet for Non-heat-treaters, Carburising and Carbonitriding, 1996. Decker KH, Kabus K. Decker Maschinenelemente Tabellen und Diagramme. Mu¨nchen: Carl Hanser Verlag; 2002. Totten GE, Howens MAH. Steel heat treatment handbook. New York: Marcel Dekker; 1997. Parrish G. Carburizing: microstructures and properties. OH: ASM International; 1999.
Failure study of pneumatic rock drill piston Tezcan Sekercioglu
*
Mechanical Engineering Department, Engineering Faculty, Pamukkale University, Camlik 20020, Denizli, Turkey Received 24 June 2005; accepted 14 July 2005 Available online 16 September 2005
Abstract In this study, the fracture of piston for rock drill produced from case hardening steel is investigated. In order to study the causes of the fracture, specimens prepared from the damaged pistons were subjected to experiments such as hardness, chemical analysis and metallurgical tests. The effect of microstructure on the fracture was also considered. The detailed metallurgical analysis was conducted on the fractured pistons and compared with the unfailed pistons in service. Results showed that the obtained pistons hardness values were lower than the values stated in the literature. Geometrical analysis of the pistons revealed that early failure of the structure started from a notch by which stress concentrations are created at the end of the threaded region. 2005 Elsevier Ltd. All rights reserved. Keywords: Fracture; Failure analysis; Rock drill; Microstructure
1. Introduction The hand-held pneumatic rock drill is a piston rotary unit that is designed mainly for use as a hard rock drill; however, it is equally efficient in soft and medium formations. The photograph and the schematic diagram of the rock drill are shown in Figs. 1 and 2. The drill consists of a back head group, cylinder unit, and front head group. The back head group consists of the four-position throttle, handle, and live air inlet. The cylinder unit consists of a cylinder with a reciprocating piston. The front head group consists of the chuck, retainer latch, and anvil. The drill design directs air through the drill, down the drill steel, and into the bottom of the hole to blow out rock cuttings. Four classes of rotary rock drills are as follows: The first class is a light drill, weighing about 7 kg. This class is used for drilling shallow holes in quarry operations. The second class is a light drill, weighing 12–18 kg. This class is used for light work, such as potholing and drilling *
Tel.: +90 258 212 55 32x128; fax: +90 258 212 55 38. E-mail address: [email protected].
1350-6307/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2005.07.018
T. Sekercioglu / Engineering Failure Analysis 13 (2006) 1108–1115
1109
Fig. 1. Photograph of the pneumatic rock drill.
Fig. 2. Schematic diagram of the pneumatic rock drill.
concrete. The third class weighs from 18 to 22 kg. This class is used for drilling in limestone and other soft rock. The fourth class is a hand-held drill, weighing from 22 to 30 kg. This class is used for drilling holes up to 20 m during quarry operations [1].
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T. Sekercioglu / Engineering Failure Analysis 13 (2006) 1108–1115
Hand-held rotary rock drills used in quarry operations may be the dry drill, the blower drill or the wet drill. The dry drill allows very little air to pass through the drill tool while drilling; therefore, you should drill 30 s and blow 60 s. When the hammer is not running, the dry drill allows enough air to pass through for cuttings to be blown out of the hole. Drill steels for this drill come in lengths of 0.6, 1.2 and 1.8 m with tips made of carbon inserts, diamond, or star. The blower drill allows a steady supply of air to pass through the drill steel to help remove cuttings from the hole while the hammer is running. This type also permits air to pass through the drill steel when the hammer is not running. The wet drill provides a constant supply of water through the drill steel while the hammer is running [1]. The investigated pistons are made of case hardening steel. The case hardening steel (20NiCrMo2-2, EN 10084) is a low nickel–chromium– molybdenum with medium hardenability, generally supplied in the as rolled condition with a maximum brinell hardness of 280 (30 HRC). It is characterized by good core strength and toughness in small to medium sections with case hardness up to 62 HRC when carburized, hardened and tempered. This steel can also be used (uncarburised) as high tensile steel, which when suitably hardened and tempered can be utilized for various applications requiring good tensile strength and reasonable toughness. Pre hardened and tempered (uncarburized) 20NiCrMo2–2 can be further surface hardened by nitriding but will not respond satisfactorily to flame or induction hardening due to its low carbon content. It is used extensively by all industry sectors for light to medium stressed components and shafts requiring high surface wear resistance with reasonable core strength and impact properties. Typical uses are: bearings, bushings, cam shafts, differential pinions, guide pins, king pins, pistons pins, splined shafts, ratchets, sleeves, etc. Carburization is a thermochemical treatment, usually conducted at temperatures in the range 800– 940 C, in the first stage of case-hardening. This process changes the chemical composition of the surface of a low-carbon steel component so that subsequent fast cooling, by quenching, produces a hard case combined with a softer/tougher core. Quenching is normally followed by a low-temperature tempering/stress relieving treatment. In the process of carburization, controlled levels of carbon are introduced at the surface and allowed to diffuse to a controlled depth. Carburizing case hardening treatment offers a means of enhancing the strength and wear properties of parts made from relatively inexpensive easily worked materials. Generally applied to near-finished components, the processes impart a high-hardness wear resistant surface which, with sufficient depth, can also improve fatigue strength. Applications range from simple mild steel pressings to heavy-duty alloy-steel transmission components [2]. In this study, the damaged rock drill pistons are investigated. Almost two rock drill pistons are damaged every month in the quarry. The damaged pistons of rock drill were evaluated, and the causes of fracture of a piston manufactured from case hardening steel were carried out. Some properties of rock drill are given in Table 1. Also, the main dimensions and the break position of the piston are shown in Fig. 3. A number of mechanical and microstructure analyses are carried out to determine the causes of fracture.
Table 1 Some properties of pneumatic rock drill Class (10 kg) Drilling type Operating temperature (C) Operating pressure (bar) Drilled material
Light Blower drill 22 ± 5 6 Medium rock
T. Sekercioglu / Engineering Failure Analysis 13 (2006) 1108–1115
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Fig. 3. Main dimensions (mm) and the fracture zone of the rock drill piston.
2. Techniques used in fracture analysis The three damaged drill piston specimens and the unfailed piston in service were subjected to various tests. The following experimental works were done:
visual inspection and fractography; hardness tests; chemical analysis; metallographic analysis.
3. Analysis and results 3.1. Visual inspection and fractography Although the unfailed and failed pistons have a same geometry, initial study of structure led to the view that fractures may have been caused by thread notch. It is considered that the cause of failure was unlikely to be thread notch alone, but their contribution to the failure cannot be underrated due to the presence of stress concentrations. The separated pistons 2–4 showed similar failure and did not bear any indication of fatigue crack growth when the fracture surface was examined, indicating that the failure was of a brittle type of fracture. The fracture zone is shown in Fig. 3. The distance between thread and the pistonÕs outer diameter is less for torque strength. As shown in Fig. 4, all fractures seem to start at a threaded region because of the high stress concentration at this region, which may be compounded by the poor impact energy of the material used. The reduced thickness between the threaded hole and the pistonÕs outer diameter assisted the failure. The fractured surfaces of pistons are shown in Fig. 5. According to the fractured surfaces, it was said that the failure was due to a brittle mode. 3.2. Hardness analysis Hardness analysis of fractured piston materials was carried out using a Rockwell hardness test machine. The measurements were carried out on three different surface areas. The hardness values are listed in Table 2. The hardness values of unfailed (1) and failed pistons (2–4) are different. In order to
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Fig. 4. Unfailed (1) and failed piston (2–4).
Fig. 5. Fractured surfaces of pistons.
Table 2 Hardness values of fractured piston materials HRC
Piston Piston Piston Piston
1 2 3 4
1
2
3
Mean
57 54 56 55
58 54 54 54
58 56 56 55
57.66 54.66 55.33 54.66
obtain maximum fatigue strength, the pistonÕs outer surface hardness may be maximum 58–62 HRC [3– 5]. The hardnesses were observed as 54 HRC for the damaged pistons, which is a lesser than the values stated in the literature. 3.3. Chemical analysis Chemical analysis of 20NiCrMo2-2 case hardening steel according to EN 10084 is shown in Table 3. The chemical composition of the piston materials was determined by spectroscopy chemical analysis. The chemical compositions of piston material are listed in Table 4. It was understood from the chemical composition that the material was case hardening steel. The composition of piston materials contains low C and Cr, Ni and Mo content, which cause the structure to quench in a tough mode. The alloying additions improve the hardenability of the steel. Chromium improves corrosion resistance, while manganese contributes to the
T. Sekercioglu / Engineering Failure Analysis 13 (2006) 1108–1115
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Table 3 Chemical analysis of 20NiCrMo2-2 case hardening steel according to EN 10084 Element (%)
20NiCrMo2-2
C
Si
Mn
P
S
Cr
Ni
Mo
0.17 0.23
– 0.40
0.65 0.95
– 0.035
– 0.035
0.4 0.7
0.4 0.7
0.15 0.25
Table 4 Chemical analysis of the piston materials Element (%)
Piston Piston Piston Piston
1 2 3 4
C
Si
Mn
P
S
Cr
Ni
Mo
0.226 0.214 0.189 0.205
0.294 0.245 0.264 0.272
0.829 0.735 0.980 0.752
0.007 0.017 0.007 0.017
0.025 0.030 0.019 0.027
0.550 1.149 0.529 1.180
0.453 0.132 0.545 0.119
0.201 0.229 0.184 0.225
deoxidation of the melt and also improves machinability. Nickel reduces distortion and cracking upon quenching. Although the alloy elements in pistons 1 and 3 were normal in composition according to EN 10084 standard, the chemical compositions of pistons 2 and 4 were abnormal. The amount of Cr in pistons 2 and 4 was double. Also, the amount of Ni in pistons 2 and 4 was very low according to related standard. 3.4. Metallographic analysis The metallographic specimens were first ground, polished and etched using standard techniques in order to examine the inner structure. A light optical microscope was used in the investigations. The microstructures of the failed piston material show that they are different structures, which is depicted in Figs. 6 and 7. It can be understand from the figures that the pistons were carburized and then differently cooled in the oil ambient. Figs. 6 and 7 show an example of retained austenite in the carburized case of 20NiCrMo2-2 low-alloy steel. The retained austenite is white and lies between the plate martensite. However, there are also a
Fig. 6. Microstructures of piston 1 (etched with nital, 200·): (a) core, (b) surface region.
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Fig. 7. Microstructures of piston 4 (etched with nital, 200·): (a) core, (b) surface region.
Fig. 8. (a) Microcracks in the core of piston 2 (unetched, 200·). (b) Microcracks in the core of piston 4 (unetched, 200·).
few white particles of cementite in the structure. Excessive retained austenite in the case hardening steels is usually detrimental to life, because it may transform to fresh martensite and cause cracking in the part, or reduce the wear resistance. Also, the microcracks were observed in pistons 2 and 4 (Fig. 8). Microcracks may be formed during the heat treatment or operation.
4. Conclusion In this research, the influences of microstructure, chemical composition and hardness of the pistons were investigated. From the experimental observations, the following conclusions may be made: 1. The chemical composition of piston material should be selected according to the standard. 2. In order to same microstructure, carburising heat treatment should be made same conditions such as time, case depth. In all cases, the austenite and accompanying martensite should be fine and evenly distributed. The re-heat quenching may lead to refined structures. 3. The hardness of some pistons is low. In order to obtain maximum fatigue strength, the pistons outer surface hardness may be increased to 60–62 HRC.
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4. On the examination of fractured parts, it can be concluded that all fractures started at a threaded region due to high stress concentrations. The thread fillet radius can be enlarged in design stage. Also, the piston diameter can be increased at the threaded region.
Acknowledgments The author gratefully acknowledges the support of ASTAS Co. and AR DOKUM Co. for this study.
References [1] [2] [3] [4] [5]
Available from: http://www.tpub.com/content/engine/14081/css/14081_394.htm. The Contract Heat Treatment Association, Datasheet for Non-heat-treaters, Carburising and Carbonitriding, 1996. Decker KH, Kabus K. Decker Maschinenelemente Tabellen und Diagramme. Mu¨nchen: Carl Hanser Verlag; 2002. Totten GE, Howens MAH. Steel heat treatment handbook. New York: Marcel Dekker; 1997. Parrish G. Carburizing: microstructures and properties. OH: ASM International; 1999.