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Failure analysis of a lancing tool used in a bridge slot filter pipe machine
Engineering Failure Analysis 105 (2019) 237–245
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Failure analysis of a lancing tool used in a bridge slot filter pipe machine
T
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D.G. Papageorgioua,b, G. Manolasa, C. Medreaa, , I. Chicinaşc a b c
University of West Attica, Department of Mechanical Engineering, 250 Thivon & Petrou Ralli Ave., 12244 Egaleo, Greece Stassinopoulos-Uddeholm Steel Trading S.A, 20 Athinon Str., 18540 Piraeus, Greece Technical University of Cluj-Napoca, Department of Materials Science and Engineering, 103-105 Muncii Ave., 400641 Cluj-Napoca, Romania
A R T IC LE I N F O
ABS TRA CT
Keywords: Lancing system Cutting tool AISI D2 steel Cold tooling Filter pipe machine
Failure of a lancing system was investigated. A local company produces bridge slot screen filter pipes. Specific cutting tools, part of the lancing system, brake down repeatedly. The parts were manufactured from AISI D2 steel. A systematic investigation consisting of on site inspection, visual examination, dimensional verification, hardness measurements and chemical analysis was carried out. Samples were selected and prepared for destructive testing. The fracture surfaces and the cross section of the tools were analyzed by light and SEM microscopy. The tools failed due to high shear stresses, unevenly developed on the surface of the parts. Their breakage is mainly attributed to poor design. The cutting-forming system should be redesigned and some aspects regarding the machine use and maintenance should be specified. Additionally, alternatives regarding the material selection of the tools are provided in order to ensure efficiency and to improve operational reliability.
1. Introduction Cold tooling involves various manufacturing processes, usually performed at room temperature. During cold working, tools are exposed to high stresses. Consequently, materials with proper hardness, toughness and shock resistance are critical. The cold working tool steels form a particular group of relatively expensive materials, developed by the industry to meet specific requirements. These steels cover one of the highest tonnages among materials used for tool manufacturing in general (i.e. cutting tools, forming dies, gauges, collets, etc.).The material selection remains the primary challenge for any designer engineer since the suitability of the steel will assure better tool performance; it extends tools' working life and ensures a profitable production due to reduced unit costs [1]. AISI D2 is a representative cold working steel, particularly popular in metal cutting industry. The specific material is a very frequent choice for cutting tools where a high wear, abrasive resistance, high strength and good toughness are of critical importance [2].The last decades, D2 series has been widely investigated in terms of improved productivity, reduction of energy consumption and economic feasibility. In this context, a sustained effort has been made to consolidate its mechanical properties. Attaining anticipated mechanical properties depend on alloying the steel with carbide-forming elements. AISI D2 is a high carbon high alloyed steel with tungsten, manganese, chromium and molybdenum. In order to achieve high tooling performance, the pieces are subjected to conventional treatments of quenching and tempering [3]. Caution is necessary during heat treatment. After specific manufacturing processes (i.e. forging), heat treatment could produce anisotropic dimensional change [4]. Heat treatments affect the microstructure and wear resistance; this aspect has been investigated and the influence of tempering conditions on tools structure has been in detail
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Corresponding author. E-mail address: [email protected] (C. Medrea).
https://doi.org/10.1016/j.engfailanal.2019.06.030 Received 9 January 2019; Received in revised form 9 June 2019 Available online 24 June 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
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described [5]. After tempering, steel microstructure consists of a variety of carbides (chromium, molybdenum and tungsten carbides) dispersed in a matrix of tempered martensite. It has been shown that primary carbides, independently of their distribution; reduce the fracture toughness of tools [6]. In some situations, the hot-deformation process of ingot leads to undesired, oversized chromium carbides that negatively affect the post-treatment results [7]. It is not only the dimensions of the carbides that have an impact on the mechanical properties; their distribution is also important. It mainly affects the abrasive wear. Investigations of the carbides' behavior were carried out in order to improve the workability [8]. A cold deformation before the final heat treatment was proposed since it can positively affect the microstructure evolution during hardening [9,10]. On the other hand, highly hardened microstructure could result in making the steel brittle [11]. In order to diminish this possibility, a combination of several sub-zero treatments was suggested, including conventional hardening and tempering treatments [12,13]. The cryogenic treatments facilitate the transformation of retained austenite to martensite, relieve the internal stresses and refine the precipitates of carbides [14,15]. An efficient heat treatment will unfortunately result in a difficult-to-machine piece. Furthermore, in order to fulfill ever-increasing requirements, tools need high accuracy and surface quality. Grinding and polishing are applied to bring the hardened steel parts to their final dimensions as well as to improve surface quality. Evaluation of the machinability to turning for pieces with different hardness, is correlated with the flank wear, the roughness surface and tool life [16–18]. Various new techniques were implemented and the final manufacturing process was optimized by grinding [19], turning [20–23], end milling [24–26], or electrical discharge machining [27–29]. D2 steel exhibit a good wear resistance, yet the increasing demand for the processing of ultra-high strength steels puts the negative effect on the surface properties of the relevant tools on the spot light; the exposure of tools to severe conditions resulted in diminished performance. The working life of D2 components for specific applications has been significantly increased by the incorporation of several superficial thermochemical treatments. Diffusion-based procedures have been successfully implemented to enhance wear and corrosion resistance on the pieces surface [30,31]. Specific studies showed that the formation of surface layer made of refined grains and nitrides uniformly dispersed in the structure improved the surface hardening significantly [32,33]. In other cases, a nitriding process was used as a pretreatment to a deposition process [34–36]. In some applications, D2 was used as material for deposition [37]. In others, it was used for coating by friction on the surface of softer steels, improving the component's productivity [38]. The aim to increase tools' lifetime opened up opportunities for new technologies. The wear life of tools could be significantly improved by dual-coating [39] or by the addition of specific titanium interlayer [40]. The deposition of nanocrystalline TiC coating on AISI D2 steel substrate resulted in a hard layer with strong coating-to-substrate bonding suitable for applications in corrosive and harsh environments [41]. Friction stir processing of a D2 tool steel layer fabricated by laser cladding from powder of D2 steel led to improved structural properties [42,43]. Any surface treatment procedure rises surface hardness; unfortunately, in detriment of the toughness. In order to alleviate this downside, further microstructure and properties variations were promoted by chemical modification. To meet increasingly restricted requirements, some manufacturers deliver the steel adjusting its typical chemical composition [44–46]. The addition of small percentages of niobium improved the ductility and impact toughness [47] and minor addition of cerium and lanthanum substantially refined the chromium carbides and improved their distribution [48]. By adding small amounts of rare-earth elements, the morphology of the eutectic chromium carbides in the eutectic carbide morphological type was changed from lamellar to globular [49]. The continuous development of operational characteristics of D2 tools steel was sustained by the analysis of damages reported in several applications. Relative research conducted on the failure of tools made of AISI D2 steel underlined the importance of correct design [50,51], appropriate material selection [52], proper material processing [53], suitable manufacturing techniques [54,55], correct heat treatments [56], and proper use and maintenance [57]. This paper refers to the failure analysis of specific cutting tools manufactured from AISI D2 steel. The tools are part of a cuttingforming system (lancing) used in a bridge slot filter pipe machine. They break down repeatedly, often after processing only a small number of pieces, thus interrupting the production process. The determination of the causes of the particular failure and the relevant suggestions can offer practical solutions to the company, resulting in less downtime and lower tooling costs and contributed to enhance component productivity.
2. Experimental details Historical data concerning tools manufacturing, processing, and operation, was collected, regarding the specific machine [58]. Three representative fragments of the broken tools were received and examined. Visual inspection and macroscopic examination were performed and a complete photographic file was created. The pieces were cleaned by means of sandblasting in a Guyson-type machine in order to allow detailed observation. Dimensional measurements were performed using Standard Gage micrometer, and a 3D design was rendered. Rockwell hardness measurements were carried out on the tools' surface and core using an Alpha Duromatic tester. The pieces were segmented in a Struers Labotom-3 machine. The parts containing the fracture surfaces were further cut and representative samples were selected and prepared for destructive tests. Chemical analysis was performed on a Thermo Scientific ARL 3460 optical emission spectroscope and the tools' steel was characterized. The microstructure was analyzed under an optical microscope Leitz Metallovert. The fracture surfaces were examined in detail by means of scanning electron microscopy. A microscope JEOL-JSM5600 LV was used. Prior to the SEM inspection, the samples were cleaned using an ultrasonic method.
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Fig. 1. General aspect of a bridge slot screen filter pipes.
3. Results and discussion 3.1. Hardware characteristics A local company produces bridge slot screen filter pipes used in drilling industry. The pipes support the soil to prevent the collapse and alteration of the diameter of the drilling hole. Louvers in the shape of bridge slots, 1–3 mm wide are formed on the entire circumference, along their full length (Fig. 1). The company designed and manufactured a specific lancing machine (Fig. 2). Pipes are inserted from beneath and are lifted hydraulically up to the binding point. One side of the tube is fastened to the machine through a cam lock and the other side is tightly kept in place on the cylindrical forming base. Τhe cam lock is able to move axially and rotationally in relation to the tube. Henceforth, axial orientation, axial or linear motion refers to the direction parallel to the axis of the tube. Vertical motion refers to the motion or direction perpendicular to the axis of the tube. After fastening, the cutting-forming assembly is inserted into the tube. It is comprised of a cylindrical base and a set of lancing tools (cutting-forming) as shown on Fig. 3. The assembly performs consecutive, periodically repeating, cuts and subsequently forms louvers of specific profile on surface of the tubes. The base has straight, inclined slots on the outer circumference, running its entire length (Fig. 3a).The motion of the lancing tools is determined by a pair of spacers, located at each edge of the slots. The lancing parts have flattened head, wider than the rest of the tool. That difference in width acts as a mean to fasten and constrict the part while it is driven to position with the cylindrical base. A pair of spacers, one on each side of the base, constricts the axial movement of the lance inside the linear gap, providing stability. The level of the track, at the bottom of the slot where the cutting tools sit on the die, forms an angle in relation to the tube axis so that they can move vertically as the die moves linearly. As the base moves along to a shaft, the cutting tools protrude from the die radially to the inner cross-section of the tube, forming the bridge slots on the tube's surface. The motion of the protruding cutting parts is hydraulically powered. The procedure is CNC assisted. The cross-sectional profile of the louvers determines the type of spacers to be used (Fig. 4a). At the end of their movement, the cutting tools interlock at forming cavities located on the outer forming die, beyond the external pipe surface (Fig. 4b). After that, the tube is pushed to the next position and the process is repeated. The process subjects the cutting-forming system to increased compressive stress. As the lances come out of the base, at the end of their track, they do not always sit evenly to the bottom of the slot, according to supervising personnel. This causes uneven compressive stresses to the tool. The steadying and centering among the lancing tool, the base and the shaft, in relation to the external
Fig. 2. The lancing machine: (a) General aspect, (b) During manufacturing process. 239
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Fig. 3. General aspect of the cylindrical base; (a) Top view, (b,c,d) Consecutive positions of the lance representing the vertical motion-sinking and ascending- while the die moves axially.
Fig. 4. 3D representation of the lancing assembly; (a)Relative position of a single lance while puncturing the tube, (b) The lance and the formed louver rests inside the forming cavity on the outer forming die.
forming die where the cutting tools interlock during the procedure, is accomplished through the tube itself. The correct angularity of the cutting parts in relation to the tube is essential to avoid unilateral stress. The variation of the cross section of the tube along the length makes things worse, causing shifts of the die during the movement. The result is an uneven shear stress on the cutting parts. 3.2. Historical data recording The lances are 63 mm wide, 10 mm thick (on the top) and 50,5 mm high. The flattened base is 15 mm thick. Dimensional measurements confirmed that tools were manufactured to predicted dimensions. The selection of AISI D2 tool steel was a designer's decision. The components were manufactured “in house”, at the machine shop of the end user. Hardening and tempering of the cutting-forming tools took place at a heat treatment workshop. The workplace was inspected in situ. It was confirmed that the machine operates under unsuitable working conditions. The lack of a cooling system and a waste removal system (vacuum system) was noted. According to surveillance and maintenance operators of the machine, in some cases, during the operation, as the lancing parts exit the die and form the louvers on the tube, they do not interlock uniformly on the forming cavities of the outer die. As a consequence, tools are locally stressed, facing uneven wear. According to maintenance reports, the rate of fracture for the cutting parts of the specific machine appears to be much higher than expected. The predicted working life of the system outputs five hundred working pieces. Instead, the reported life span of the tools is much lower, varying from one to two hundred pieces. The repeated fracture of cutting parts interrupts the production process. 3.3. Visual inspection Three representative specimens were examined (Fig. 5). All of them were brittle fractured on the top. An extended area of oxidation was obvious on the surface of the tool. It is mainly attributed to improper storage of the parts after failure occurred. There is a consistency in the way the examined pieces broke and they fractured at approximately the same position. The surfaces are bright and sparkling presenting a typical transgranular fracture mode as the tool had been hardened and tempered. There is relatively no plastic deformation observed to the fractured area concluding that the fracture is brittle (Fig. 6a,b,c). The working hypothesis is that the cracks were initiated from the lateral cylindrical front (arrow I on Fig. 6a) and developed instantly on the
Fig. 5. Macrograph of the failed pieces as received; (a) Front side, (b) Lateral, cylindrical side. 240
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Fig. 6. Macroscopic analysis of the first piece (P1); (a) General aspect of the fracture surface, (b) Front side, (c) Secondary crack on the lateral surface on lateral side, (d) Detail near to the fracture surface showing area with plastic deformation, (e) Abrasion on the lateral surface, (f) Scratches on the left side.
circumference around the tool. Near the center of the fractured surface, randomly oriented stress lines are observed (arrow III on Fig. 6a). This is the final detachment area of the fragments. Secondary cracks originated from the fracture surface (arrow II on Fig. 6a) and propagated vertically on the lateral surface (arrow on Fig.6b and arrow I on Fig. 6c). In very limited areas, plastic deformation preceded the total fracture more likely due to exerted shear stress (arrow on Fig. 6d). The front of the head bears signs of impact and wear (right side of the Fig. 6d). In addition, there are strain signs to the site of attachment on the slot of the internal base. This is a result of wide operational tolerance (arrow II on Fig. 6c and arrow on Fig. 6e). On the left side, on the lateral cylindrical surface there are findings of extensive wear (arrow on Fig.6f). On the same orientation, there are also impact and wear signs (lighted side of the tool, Fig. 6f). 3.4. Chemical composition The chemical composition of the three pieces and the typical analysis of the specific tool steel grade [59] are shown in Table 1. The chemical composition of the failed tools is in accordance with specifications for the material selected. AISI D2 is an “industry standard” in terms of cost and material performance; it can meet usual requirements and remains one of the cheapest solutions. 3.5. Samples preparation In order to carry out destructive testing, the failed piece P3 was cut into three parts. The part containing the fracture surfaces was cut further and samples were selected and prepared accordingly for destructive testing. Finally, eleven (ll) samples were derived. Fig. 7 shows the samples selection of the third piece (P3). Samples A7 and A4 were subjected to scanning electron microscopy. B3 was used for performing the chemical analysis. Samples B2, A3, A5 and A6 were subjected to optical metallographic microscopy and microhardness testing. In order the selected samples to be studied simultaneously these four samples were mounted together. The sides were chosen so that the surface under examination Table 1 Chemical composition of the steel compared with the typical chemical composition determined by the standards [59]. Elements [wt%]
Measured content Fragment 1 Measured content Fragment 2 Measured content Fragment 3 Required content AISI D2
C
Si
Mn
Cr
V
Mo
1.53 1.54 1.55 1.4–1.6
0.25 0.25 0.25 0.1–0.6
0.26 0.26 0.26 0.2–0.6
10.98 10.92 10.94 11.0–13.0
0.78 0.78 0.79 0.5–1.1
0.69 0.68 0.68 0.7–1.2
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Fig. 7. Schematic overview of sample selection of the third piece (P3); (a) First cutting, (b) Sample selection, (c) Samples prepared for optical and SEM analysis.
would be a cross section that contains the fracture area. Those sides were the upper side of B2 and the right sides of A3, A5 and A6, following the orientation of Fig. 7b. The examined surfaces are shown in Fig. 7c. 3.6. Hardness measurements Hardness was checked to different areas on the surface (on the circumference of each tool) and on the core of the failed tools. The measured hardness on the surface was 59–61 HRC (Fig. 8a) and 60–61 HRC in the core (Fig. 8b). That demonstrates that components have been hardened uniformly. The third tool was an exception. Hardness on the lateral parts of the side of the head varies from 45 HRC (point 1 on Fig. 8c) to 61 HRC (point 2 on Fig. 8c) on the left side and between 48 HRC (point 2 on Fig. 8d) and 56 HRC (point 4 on Fig. 8d) on the right. The deviation in hardness within such short distances could be a result of the existence of secondary cracks on the subsurface of the cutting parts. During manufacturing, the tools are subjected to heat treatment in order to achieve the optimum blend of behavior characteristics such as hardness and toughness determining their in-service performance. Steel makers suggest the hardness for cutting, forming and punching tools to vary between 56 and 60 HRC [60]. The examined pieces were hardened and tempered to the specified hardness. The heat treatment was in accordance with the specified standards. 3.7. Microscopic examination Under optical microscope, the steel displays the expected microstructure consisted of relatively large carbides distributed in a matrix of tempered martensite (Fig. 9). The high carbon-high chromium contents are prone in creating large chromium carbides, which are present in bands during material processing (hot rolling). A fraction of them remained in structure as primary carbides while fine secondary carbides precipitated during tempering. Microstructure is typical, with no extensive agglomeration or banding [60]. The pieces were total fractured along to the main direction of the applied stresses and perpendicular to the carbide orientation (Fig. 10a). In this case, the dominant stresses applied are compressive. Conventionally produced hot rolled steel has higher mechanical properties along the rolling direction than it does vertically. It is therefore safe to assume that the optimum mechanical properties were exploited, as far as the working direction of the tool is concerned. Secondary cracks were initiated from the fracture surface, developed at a plane of 45o angle in relation to the cross section. This is the direction where the maximum shear stress is exerted during compression (Figs. 10b and 11b). A closer look on the fracture network area revealed that carbides were broken due to buckling provoked by high compression loading (Fig. 11c). The surfaces which examined under electron microscope (SEM) were fractured by a combined mode. It is a combination of cleavage fracture planes and dimples which are characteristic of intergranular fracture (Fig. 11a). A transgranular fracture is depicted while in some areas ductile tearing is observed. This is a quasi-cleavage fracture [61]. There is a transgranular propagation of the
Fig. 8. Topology of the hardness measurements carried out on third piece; (a) The surface of the same fragment, (b) The core of the tool, (c) The left side surface of the head of the lance, (d) the right side surface of the head of the lance. 242
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Fig. 9. Sample B2: Steel microstructure under optical microscope.
Fig. 10. Sample A5: Images showing steel microstructure on the cross section of the fracture surface; (a) Carbides lay in bands perpendicular to fracture surface, (b) Secondary crack developed under compressive stresses, (c) Fragmented carbide particles as result of buckling.
Fig. 11. Sample A7 SEM micrograph showing: (a) Quasi-cleavage fracture on the surface, (b) Secondary crack area featuring a brittle intergranular expansion, (c) Transgranular fracture of carbides during crack propagation.
crack, through the carbides (upper left side of Fig. 11a,c).
4. Conclusions The lances failed in a quasi-cleavage fracture mode due to compressive and shear stresses, unevenly distributed on the surface of the cutting tools. Wear is a secondary failure mechanism that is present during the lancing procedure. The selected material is one of the most frequently used steel grades for the specific application. The tools were manufactured according to the specifications and they were properly heat treated, resulting in predicted and adequate hardness as well as microstructure. The machine is custom made and any modifications were not implemented after its first use. Consequently, it is mandatory to be 243
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some improvements. The failure of the examined pieces is the macroscopic evidence mainly attributed to its insufficient design. When the lances come out of the base, at the end of their track, they do not always sit evenly to the bottom of the base slot. Similarly, the lancing parts exit the die and form the louvers on the tube; they do not interlock uniformly on the forming cavities of the outer die. These two facts provoke impact and compressive stresses as well as wear to the front of the tools, causing their repeated fractures. The correction of angularity of the cutting parts on relation to the tube as well as to the misalignment during the cuttingforming process is essential in order to avoid unilateral stresses which can be reduced by decreasing the operational tolerance of the lancing procedure. Some aspects regarding the procedure, use and maintenance of the machine should be improved. As far as the modification of the procedure is concerned, a number of operational steps and additional systems are proposed. An initial formation process, a cooling and a vacuum debris removal system as well as appropriate lubrication are mandatory. The dimensional dispersion of the cross section of the tubes is pivotal. During lancing, the tools do not always acquire the appropriate position, vertically to the main axis or tangentially to the internal circumference of the tube. This causes local over-stress and vibrations. A step for the initial formation of the tubes in process, so that they present a constant cross-sectional profile, could be a valuable addition. This broaching step will achieve a minor reformation of the tubes. The cross section of each tube will be constant throughout its whole length. This will offer better alignment of the tube and the internal base in relation to the outer forming parts and will eliminate the random positioning of the moving parts of the machine. Thus, the uneven shear stress on the cutting-forming parts causing their total breakage will be reduced. Apart from that, appropriate lubrication with an oxidation resistant lubricant would diminish wear during processing. The addition of a cooling system should be considered. The cooling medium would prevent the cutting tools from overheating. Further, a vacuum system, put in place for the removal of chips and milling debris, would reduce the additional wear on the moving parts during operation. As a closure, material selection could be debated. The selected material is definitely adequate for numerous cold working applications. Nevertheless, it is not the only option available regarding the wear findings during the failure analysis procedure. A number of alternatives could potentially increase efficiency. From the material properties point of view, in the case of a cuttingforming system, the most significant properties are compressive strength, toughness and wear resistance. Electroslag remelted cold working steel grades [62–64] or powder metallurgy products [65] could be an economic solution, not because of their price but because of the expected increase of the tools' working lifetime. Further, cryogenic heat treatment sequence could offer increased fracture resistance and toughness for the same hardness. Acknowledgements The authors gratefully acknowledge Dr. Fragkos-Livanios Leonidas from Electrical Department, University of West Attica for useful discussions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.engfailanal.2019.06.030. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of a lancing tool used in a bridge slot filter pipe machine
T
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D.G. Papageorgioua,b, G. Manolasa, C. Medreaa, , I. Chicinaşc a b c
University of West Attica, Department of Mechanical Engineering, 250 Thivon & Petrou Ralli Ave., 12244 Egaleo, Greece Stassinopoulos-Uddeholm Steel Trading S.A, 20 Athinon Str., 18540 Piraeus, Greece Technical University of Cluj-Napoca, Department of Materials Science and Engineering, 103-105 Muncii Ave., 400641 Cluj-Napoca, Romania
A R T IC LE I N F O
ABS TRA CT
Keywords: Lancing system Cutting tool AISI D2 steel Cold tooling Filter pipe machine
Failure of a lancing system was investigated. A local company produces bridge slot screen filter pipes. Specific cutting tools, part of the lancing system, brake down repeatedly. The parts were manufactured from AISI D2 steel. A systematic investigation consisting of on site inspection, visual examination, dimensional verification, hardness measurements and chemical analysis was carried out. Samples were selected and prepared for destructive testing. The fracture surfaces and the cross section of the tools were analyzed by light and SEM microscopy. The tools failed due to high shear stresses, unevenly developed on the surface of the parts. Their breakage is mainly attributed to poor design. The cutting-forming system should be redesigned and some aspects regarding the machine use and maintenance should be specified. Additionally, alternatives regarding the material selection of the tools are provided in order to ensure efficiency and to improve operational reliability.
1. Introduction Cold tooling involves various manufacturing processes, usually performed at room temperature. During cold working, tools are exposed to high stresses. Consequently, materials with proper hardness, toughness and shock resistance are critical. The cold working tool steels form a particular group of relatively expensive materials, developed by the industry to meet specific requirements. These steels cover one of the highest tonnages among materials used for tool manufacturing in general (i.e. cutting tools, forming dies, gauges, collets, etc.).The material selection remains the primary challenge for any designer engineer since the suitability of the steel will assure better tool performance; it extends tools' working life and ensures a profitable production due to reduced unit costs [1]. AISI D2 is a representative cold working steel, particularly popular in metal cutting industry. The specific material is a very frequent choice for cutting tools where a high wear, abrasive resistance, high strength and good toughness are of critical importance [2].The last decades, D2 series has been widely investigated in terms of improved productivity, reduction of energy consumption and economic feasibility. In this context, a sustained effort has been made to consolidate its mechanical properties. Attaining anticipated mechanical properties depend on alloying the steel with carbide-forming elements. AISI D2 is a high carbon high alloyed steel with tungsten, manganese, chromium and molybdenum. In order to achieve high tooling performance, the pieces are subjected to conventional treatments of quenching and tempering [3]. Caution is necessary during heat treatment. After specific manufacturing processes (i.e. forging), heat treatment could produce anisotropic dimensional change [4]. Heat treatments affect the microstructure and wear resistance; this aspect has been investigated and the influence of tempering conditions on tools structure has been in detail
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Corresponding author. E-mail address: [email protected] (C. Medrea).
https://doi.org/10.1016/j.engfailanal.2019.06.030 Received 9 January 2019; Received in revised form 9 June 2019 Available online 24 June 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
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described [5]. After tempering, steel microstructure consists of a variety of carbides (chromium, molybdenum and tungsten carbides) dispersed in a matrix of tempered martensite. It has been shown that primary carbides, independently of their distribution; reduce the fracture toughness of tools [6]. In some situations, the hot-deformation process of ingot leads to undesired, oversized chromium carbides that negatively affect the post-treatment results [7]. It is not only the dimensions of the carbides that have an impact on the mechanical properties; their distribution is also important. It mainly affects the abrasive wear. Investigations of the carbides' behavior were carried out in order to improve the workability [8]. A cold deformation before the final heat treatment was proposed since it can positively affect the microstructure evolution during hardening [9,10]. On the other hand, highly hardened microstructure could result in making the steel brittle [11]. In order to diminish this possibility, a combination of several sub-zero treatments was suggested, including conventional hardening and tempering treatments [12,13]. The cryogenic treatments facilitate the transformation of retained austenite to martensite, relieve the internal stresses and refine the precipitates of carbides [14,15]. An efficient heat treatment will unfortunately result in a difficult-to-machine piece. Furthermore, in order to fulfill ever-increasing requirements, tools need high accuracy and surface quality. Grinding and polishing are applied to bring the hardened steel parts to their final dimensions as well as to improve surface quality. Evaluation of the machinability to turning for pieces with different hardness, is correlated with the flank wear, the roughness surface and tool life [16–18]. Various new techniques were implemented and the final manufacturing process was optimized by grinding [19], turning [20–23], end milling [24–26], or electrical discharge machining [27–29]. D2 steel exhibit a good wear resistance, yet the increasing demand for the processing of ultra-high strength steels puts the negative effect on the surface properties of the relevant tools on the spot light; the exposure of tools to severe conditions resulted in diminished performance. The working life of D2 components for specific applications has been significantly increased by the incorporation of several superficial thermochemical treatments. Diffusion-based procedures have been successfully implemented to enhance wear and corrosion resistance on the pieces surface [30,31]. Specific studies showed that the formation of surface layer made of refined grains and nitrides uniformly dispersed in the structure improved the surface hardening significantly [32,33]. In other cases, a nitriding process was used as a pretreatment to a deposition process [34–36]. In some applications, D2 was used as material for deposition [37]. In others, it was used for coating by friction on the surface of softer steels, improving the component's productivity [38]. The aim to increase tools' lifetime opened up opportunities for new technologies. The wear life of tools could be significantly improved by dual-coating [39] or by the addition of specific titanium interlayer [40]. The deposition of nanocrystalline TiC coating on AISI D2 steel substrate resulted in a hard layer with strong coating-to-substrate bonding suitable for applications in corrosive and harsh environments [41]. Friction stir processing of a D2 tool steel layer fabricated by laser cladding from powder of D2 steel led to improved structural properties [42,43]. Any surface treatment procedure rises surface hardness; unfortunately, in detriment of the toughness. In order to alleviate this downside, further microstructure and properties variations were promoted by chemical modification. To meet increasingly restricted requirements, some manufacturers deliver the steel adjusting its typical chemical composition [44–46]. The addition of small percentages of niobium improved the ductility and impact toughness [47] and minor addition of cerium and lanthanum substantially refined the chromium carbides and improved their distribution [48]. By adding small amounts of rare-earth elements, the morphology of the eutectic chromium carbides in the eutectic carbide morphological type was changed from lamellar to globular [49]. The continuous development of operational characteristics of D2 tools steel was sustained by the analysis of damages reported in several applications. Relative research conducted on the failure of tools made of AISI D2 steel underlined the importance of correct design [50,51], appropriate material selection [52], proper material processing [53], suitable manufacturing techniques [54,55], correct heat treatments [56], and proper use and maintenance [57]. This paper refers to the failure analysis of specific cutting tools manufactured from AISI D2 steel. The tools are part of a cuttingforming system (lancing) used in a bridge slot filter pipe machine. They break down repeatedly, often after processing only a small number of pieces, thus interrupting the production process. The determination of the causes of the particular failure and the relevant suggestions can offer practical solutions to the company, resulting in less downtime and lower tooling costs and contributed to enhance component productivity.
2. Experimental details Historical data concerning tools manufacturing, processing, and operation, was collected, regarding the specific machine [58]. Three representative fragments of the broken tools were received and examined. Visual inspection and macroscopic examination were performed and a complete photographic file was created. The pieces were cleaned by means of sandblasting in a Guyson-type machine in order to allow detailed observation. Dimensional measurements were performed using Standard Gage micrometer, and a 3D design was rendered. Rockwell hardness measurements were carried out on the tools' surface and core using an Alpha Duromatic tester. The pieces were segmented in a Struers Labotom-3 machine. The parts containing the fracture surfaces were further cut and representative samples were selected and prepared for destructive tests. Chemical analysis was performed on a Thermo Scientific ARL 3460 optical emission spectroscope and the tools' steel was characterized. The microstructure was analyzed under an optical microscope Leitz Metallovert. The fracture surfaces were examined in detail by means of scanning electron microscopy. A microscope JEOL-JSM5600 LV was used. Prior to the SEM inspection, the samples were cleaned using an ultrasonic method.
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Fig. 1. General aspect of a bridge slot screen filter pipes.
3. Results and discussion 3.1. Hardware characteristics A local company produces bridge slot screen filter pipes used in drilling industry. The pipes support the soil to prevent the collapse and alteration of the diameter of the drilling hole. Louvers in the shape of bridge slots, 1–3 mm wide are formed on the entire circumference, along their full length (Fig. 1). The company designed and manufactured a specific lancing machine (Fig. 2). Pipes are inserted from beneath and are lifted hydraulically up to the binding point. One side of the tube is fastened to the machine through a cam lock and the other side is tightly kept in place on the cylindrical forming base. Τhe cam lock is able to move axially and rotationally in relation to the tube. Henceforth, axial orientation, axial or linear motion refers to the direction parallel to the axis of the tube. Vertical motion refers to the motion or direction perpendicular to the axis of the tube. After fastening, the cutting-forming assembly is inserted into the tube. It is comprised of a cylindrical base and a set of lancing tools (cutting-forming) as shown on Fig. 3. The assembly performs consecutive, periodically repeating, cuts and subsequently forms louvers of specific profile on surface of the tubes. The base has straight, inclined slots on the outer circumference, running its entire length (Fig. 3a).The motion of the lancing tools is determined by a pair of spacers, located at each edge of the slots. The lancing parts have flattened head, wider than the rest of the tool. That difference in width acts as a mean to fasten and constrict the part while it is driven to position with the cylindrical base. A pair of spacers, one on each side of the base, constricts the axial movement of the lance inside the linear gap, providing stability. The level of the track, at the bottom of the slot where the cutting tools sit on the die, forms an angle in relation to the tube axis so that they can move vertically as the die moves linearly. As the base moves along to a shaft, the cutting tools protrude from the die radially to the inner cross-section of the tube, forming the bridge slots on the tube's surface. The motion of the protruding cutting parts is hydraulically powered. The procedure is CNC assisted. The cross-sectional profile of the louvers determines the type of spacers to be used (Fig. 4a). At the end of their movement, the cutting tools interlock at forming cavities located on the outer forming die, beyond the external pipe surface (Fig. 4b). After that, the tube is pushed to the next position and the process is repeated. The process subjects the cutting-forming system to increased compressive stress. As the lances come out of the base, at the end of their track, they do not always sit evenly to the bottom of the slot, according to supervising personnel. This causes uneven compressive stresses to the tool. The steadying and centering among the lancing tool, the base and the shaft, in relation to the external
Fig. 2. The lancing machine: (a) General aspect, (b) During manufacturing process. 239
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Fig. 3. General aspect of the cylindrical base; (a) Top view, (b,c,d) Consecutive positions of the lance representing the vertical motion-sinking and ascending- while the die moves axially.
Fig. 4. 3D representation of the lancing assembly; (a)Relative position of a single lance while puncturing the tube, (b) The lance and the formed louver rests inside the forming cavity on the outer forming die.
forming die where the cutting tools interlock during the procedure, is accomplished through the tube itself. The correct angularity of the cutting parts in relation to the tube is essential to avoid unilateral stress. The variation of the cross section of the tube along the length makes things worse, causing shifts of the die during the movement. The result is an uneven shear stress on the cutting parts. 3.2. Historical data recording The lances are 63 mm wide, 10 mm thick (on the top) and 50,5 mm high. The flattened base is 15 mm thick. Dimensional measurements confirmed that tools were manufactured to predicted dimensions. The selection of AISI D2 tool steel was a designer's decision. The components were manufactured “in house”, at the machine shop of the end user. Hardening and tempering of the cutting-forming tools took place at a heat treatment workshop. The workplace was inspected in situ. It was confirmed that the machine operates under unsuitable working conditions. The lack of a cooling system and a waste removal system (vacuum system) was noted. According to surveillance and maintenance operators of the machine, in some cases, during the operation, as the lancing parts exit the die and form the louvers on the tube, they do not interlock uniformly on the forming cavities of the outer die. As a consequence, tools are locally stressed, facing uneven wear. According to maintenance reports, the rate of fracture for the cutting parts of the specific machine appears to be much higher than expected. The predicted working life of the system outputs five hundred working pieces. Instead, the reported life span of the tools is much lower, varying from one to two hundred pieces. The repeated fracture of cutting parts interrupts the production process. 3.3. Visual inspection Three representative specimens were examined (Fig. 5). All of them were brittle fractured on the top. An extended area of oxidation was obvious on the surface of the tool. It is mainly attributed to improper storage of the parts after failure occurred. There is a consistency in the way the examined pieces broke and they fractured at approximately the same position. The surfaces are bright and sparkling presenting a typical transgranular fracture mode as the tool had been hardened and tempered. There is relatively no plastic deformation observed to the fractured area concluding that the fracture is brittle (Fig. 6a,b,c). The working hypothesis is that the cracks were initiated from the lateral cylindrical front (arrow I on Fig. 6a) and developed instantly on the
Fig. 5. Macrograph of the failed pieces as received; (a) Front side, (b) Lateral, cylindrical side. 240
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Fig. 6. Macroscopic analysis of the first piece (P1); (a) General aspect of the fracture surface, (b) Front side, (c) Secondary crack on the lateral surface on lateral side, (d) Detail near to the fracture surface showing area with plastic deformation, (e) Abrasion on the lateral surface, (f) Scratches on the left side.
circumference around the tool. Near the center of the fractured surface, randomly oriented stress lines are observed (arrow III on Fig. 6a). This is the final detachment area of the fragments. Secondary cracks originated from the fracture surface (arrow II on Fig. 6a) and propagated vertically on the lateral surface (arrow on Fig.6b and arrow I on Fig. 6c). In very limited areas, plastic deformation preceded the total fracture more likely due to exerted shear stress (arrow on Fig. 6d). The front of the head bears signs of impact and wear (right side of the Fig. 6d). In addition, there are strain signs to the site of attachment on the slot of the internal base. This is a result of wide operational tolerance (arrow II on Fig. 6c and arrow on Fig. 6e). On the left side, on the lateral cylindrical surface there are findings of extensive wear (arrow on Fig.6f). On the same orientation, there are also impact and wear signs (lighted side of the tool, Fig. 6f). 3.4. Chemical composition The chemical composition of the three pieces and the typical analysis of the specific tool steel grade [59] are shown in Table 1. The chemical composition of the failed tools is in accordance with specifications for the material selected. AISI D2 is an “industry standard” in terms of cost and material performance; it can meet usual requirements and remains one of the cheapest solutions. 3.5. Samples preparation In order to carry out destructive testing, the failed piece P3 was cut into three parts. The part containing the fracture surfaces was cut further and samples were selected and prepared accordingly for destructive testing. Finally, eleven (ll) samples were derived. Fig. 7 shows the samples selection of the third piece (P3). Samples A7 and A4 were subjected to scanning electron microscopy. B3 was used for performing the chemical analysis. Samples B2, A3, A5 and A6 were subjected to optical metallographic microscopy and microhardness testing. In order the selected samples to be studied simultaneously these four samples were mounted together. The sides were chosen so that the surface under examination Table 1 Chemical composition of the steel compared with the typical chemical composition determined by the standards [59]. Elements [wt%]
Measured content Fragment 1 Measured content Fragment 2 Measured content Fragment 3 Required content AISI D2
C
Si
Mn
Cr
V
Mo
1.53 1.54 1.55 1.4–1.6
0.25 0.25 0.25 0.1–0.6
0.26 0.26 0.26 0.2–0.6
10.98 10.92 10.94 11.0–13.0
0.78 0.78 0.79 0.5–1.1
0.69 0.68 0.68 0.7–1.2
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Fig. 7. Schematic overview of sample selection of the third piece (P3); (a) First cutting, (b) Sample selection, (c) Samples prepared for optical and SEM analysis.
would be a cross section that contains the fracture area. Those sides were the upper side of B2 and the right sides of A3, A5 and A6, following the orientation of Fig. 7b. The examined surfaces are shown in Fig. 7c. 3.6. Hardness measurements Hardness was checked to different areas on the surface (on the circumference of each tool) and on the core of the failed tools. The measured hardness on the surface was 59–61 HRC (Fig. 8a) and 60–61 HRC in the core (Fig. 8b). That demonstrates that components have been hardened uniformly. The third tool was an exception. Hardness on the lateral parts of the side of the head varies from 45 HRC (point 1 on Fig. 8c) to 61 HRC (point 2 on Fig. 8c) on the left side and between 48 HRC (point 2 on Fig. 8d) and 56 HRC (point 4 on Fig. 8d) on the right. The deviation in hardness within such short distances could be a result of the existence of secondary cracks on the subsurface of the cutting parts. During manufacturing, the tools are subjected to heat treatment in order to achieve the optimum blend of behavior characteristics such as hardness and toughness determining their in-service performance. Steel makers suggest the hardness for cutting, forming and punching tools to vary between 56 and 60 HRC [60]. The examined pieces were hardened and tempered to the specified hardness. The heat treatment was in accordance with the specified standards. 3.7. Microscopic examination Under optical microscope, the steel displays the expected microstructure consisted of relatively large carbides distributed in a matrix of tempered martensite (Fig. 9). The high carbon-high chromium contents are prone in creating large chromium carbides, which are present in bands during material processing (hot rolling). A fraction of them remained in structure as primary carbides while fine secondary carbides precipitated during tempering. Microstructure is typical, with no extensive agglomeration or banding [60]. The pieces were total fractured along to the main direction of the applied stresses and perpendicular to the carbide orientation (Fig. 10a). In this case, the dominant stresses applied are compressive. Conventionally produced hot rolled steel has higher mechanical properties along the rolling direction than it does vertically. It is therefore safe to assume that the optimum mechanical properties were exploited, as far as the working direction of the tool is concerned. Secondary cracks were initiated from the fracture surface, developed at a plane of 45o angle in relation to the cross section. This is the direction where the maximum shear stress is exerted during compression (Figs. 10b and 11b). A closer look on the fracture network area revealed that carbides were broken due to buckling provoked by high compression loading (Fig. 11c). The surfaces which examined under electron microscope (SEM) were fractured by a combined mode. It is a combination of cleavage fracture planes and dimples which are characteristic of intergranular fracture (Fig. 11a). A transgranular fracture is depicted while in some areas ductile tearing is observed. This is a quasi-cleavage fracture [61]. There is a transgranular propagation of the
Fig. 8. Topology of the hardness measurements carried out on third piece; (a) The surface of the same fragment, (b) The core of the tool, (c) The left side surface of the head of the lance, (d) the right side surface of the head of the lance. 242
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Fig. 9. Sample B2: Steel microstructure under optical microscope.
Fig. 10. Sample A5: Images showing steel microstructure on the cross section of the fracture surface; (a) Carbides lay in bands perpendicular to fracture surface, (b) Secondary crack developed under compressive stresses, (c) Fragmented carbide particles as result of buckling.
Fig. 11. Sample A7 SEM micrograph showing: (a) Quasi-cleavage fracture on the surface, (b) Secondary crack area featuring a brittle intergranular expansion, (c) Transgranular fracture of carbides during crack propagation.
crack, through the carbides (upper left side of Fig. 11a,c).
4. Conclusions The lances failed in a quasi-cleavage fracture mode due to compressive and shear stresses, unevenly distributed on the surface of the cutting tools. Wear is a secondary failure mechanism that is present during the lancing procedure. The selected material is one of the most frequently used steel grades for the specific application. The tools were manufactured according to the specifications and they were properly heat treated, resulting in predicted and adequate hardness as well as microstructure. The machine is custom made and any modifications were not implemented after its first use. Consequently, it is mandatory to be 243
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some improvements. The failure of the examined pieces is the macroscopic evidence mainly attributed to its insufficient design. When the lances come out of the base, at the end of their track, they do not always sit evenly to the bottom of the base slot. Similarly, the lancing parts exit the die and form the louvers on the tube; they do not interlock uniformly on the forming cavities of the outer die. These two facts provoke impact and compressive stresses as well as wear to the front of the tools, causing their repeated fractures. The correction of angularity of the cutting parts on relation to the tube as well as to the misalignment during the cuttingforming process is essential in order to avoid unilateral stresses which can be reduced by decreasing the operational tolerance of the lancing procedure. Some aspects regarding the procedure, use and maintenance of the machine should be improved. As far as the modification of the procedure is concerned, a number of operational steps and additional systems are proposed. An initial formation process, a cooling and a vacuum debris removal system as well as appropriate lubrication are mandatory. The dimensional dispersion of the cross section of the tubes is pivotal. During lancing, the tools do not always acquire the appropriate position, vertically to the main axis or tangentially to the internal circumference of the tube. This causes local over-stress and vibrations. A step for the initial formation of the tubes in process, so that they present a constant cross-sectional profile, could be a valuable addition. This broaching step will achieve a minor reformation of the tubes. The cross section of each tube will be constant throughout its whole length. This will offer better alignment of the tube and the internal base in relation to the outer forming parts and will eliminate the random positioning of the moving parts of the machine. Thus, the uneven shear stress on the cutting-forming parts causing their total breakage will be reduced. Apart from that, appropriate lubrication with an oxidation resistant lubricant would diminish wear during processing. The addition of a cooling system should be considered. The cooling medium would prevent the cutting tools from overheating. Further, a vacuum system, put in place for the removal of chips and milling debris, would reduce the additional wear on the moving parts during operation. As a closure, material selection could be debated. The selected material is definitely adequate for numerous cold working applications. Nevertheless, it is not the only option available regarding the wear findings during the failure analysis procedure. A number of alternatives could potentially increase efficiency. From the material properties point of view, in the case of a cuttingforming system, the most significant properties are compressive strength, toughness and wear resistance. Electroslag remelted cold working steel grades [62–64] or powder metallurgy products [65] could be an economic solution, not because of their price but because of the expected increase of the tools' working lifetime. Further, cryogenic heat treatment sequence could offer increased fracture resistance and toughness for the same hardness. Acknowledgements The authors gratefully acknowledge Dr. Fragkos-Livanios Leonidas from Electrical Department, University of West Attica for useful discussions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.engfailanal.2019.06.030. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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