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Failure analysis of a continuous press component in MDF production plant
EFA-03072; No of Pages 8 Engineering Failure Analysis xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of a continuous press component in MDF production plant A. Lanzutti a,⁎, F. Andreatta a, A. Raffaelli a, M. Magnan a, L. Zuliani b, M. Fantoni b, L. Fedrizzi a a b
University of Udine, Polytechnic Department of Engineering and Architecture, via delle scienze 208, Udine, Italy Fantoni spa, Zona Industriale Rivoli, 33010 Osoppo (UD), Italy
a r t i c l e
i n f o
Article history: Received 27 September 2016 Received in revised form 17 March 2017 Accepted 20 March 2017 Available online xxxx Keywords: Heat treatment Brittle fracture Failure analysis Intergranular fracture
a b s t r a c t This work presents the results of a failure analysis performed on a component of a continuous press that is a part of a process plant used to produce MDF (Medium Density Fibreboard) panels. This plant is composed of 2 counter rotating belts that are supported, internally, by small steel rollers, connected to each other by small axes. The failure occurred prematurely on rollers. These components undergo to static and dynamic stresses below the yield strength of material. Low alloyed steel is employed for the production of the rollers and the components are carburized on the external surface. The rollers present cracks on the external surface, that is in contact with the wood panels, already after few hours in service. Other components did not undergo cracking even for very long service times (about 5000 h). Both rollers that failed in service and rollers without cracks were characterized by optical and scanning electron microscopy in order to evaluate the microstructure of the components. In particular, the microstructure of the rollers was investigated on the surface and in the core of the rollers. Vickers micro-hardness measurements were employed for the characterization of the mechanical properties of the rollers. Successively, a cross section of a broken roller was characterized by scanning electron microscopy (SEM) On the same sample, after microstructural characterization, a micro-hardness profile was acquired from external surface to the core in order to evaluate possible over carburizing or oxidation phenomena. The results obtained from both the microstructural and mechanical characterization of the failed rollers indicate that the possible causes of the in service failure are related to quenching cracks or fatigue. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction The present work is a failure analysis of a structural component installed in a continuous press in the production line of MDF panels (Medium Density Fibreboard). The importance of the MDF production is worldwide known and the production plants are located in several countries [1]. In MDF panels wood fibers are bounded in with a suitable matrix (typically urea-formaldehyde resins) by the combined action of pressure and heat transfer. In order to obtain the wood fibers, the wood logs are reduced into chips that are further processed with washing systems. The wood chips are then digested for a few minutes in a steam pressure vessel at about 8.0 bar and then transformed into fibers in an Asplund-type refiner. The resin and the additives are normally added in at the refiner's discharge section in a complex multiphase and highly turbulent flow. The material then enters in a pneumatic flash dryer where moisture is extracted in order to reach a ⁎ Corresponding author. E-mail address: [email protected] (A. Lanzutti).
http://dx.doi.org/10.1016/j.engfailanal.2017.03.016 1350-6307/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: A. Lanzutti, et al., Failure analysis of a continuous press component in MDF production plant, Engineering Failure Analysis (2017), http://dx.doi.org/10.1016/j.engfailanal.2017.03.016
2
A. Lanzutti et al. / Engineering Failure Analysis xxx (2017) xxx–xxx
Fig. 1. Image of the continuous press plant and schematic representation of the process. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
final content of about 8–12%. The MDF material enters (MDF green) and exits continuously in the continuous press thanks to a double steel-belt conveyor and to a bed of rolling rods or small chains that act as a linear roller bearing. A schematic representation of the operation scheme of the continuous press, is shown in Fig. 1. The press is composed of two counter rotating belts moved by 2 drums. Internally, the belts are supported by a continuous carpet of small rollers that are joined together by a flexible chassis to form a web (Fig. 2). The MDF green (natural fibers + resin), during the production, is inserted in between these carpets and at the same time squeezed and heated up to 200 °C in order to bake the green and form the final panel. The function of the rollers is to impress, through a hydraulic piston, the necessary pressure to deform the belt and compress the MDF green (5–6 MPa). Out of the hot press the MDF panel is then cut with a flying saw and then cooled and stacked for further processing (e.g. sanding, cut-to-size, lamination, ecc.). The failed components considered in this experience are the rollers supporting the continuous carpet (green circles Fig. 1 and Fig. 2). These components are subjected to both static/dynamic and hertzian loads, impressed by the interaction between the MDF green and the press. Moreover, thermomechanical stresses due to the baking process of the MDF material might be also present. At the same time the components are exposed to an atmosphere composed by hot air with some traces of ammonia, water vapor and hydrogen. These gases are produced by the baking process and derive mainly from the curing process of the resin matrix. The rollers failed after few hours of service life compared to an infinite design life (the rollers are anyway substituted every 5 years). The main problems related to the failure of these small components is that they can snug in between the drums and the belts leading also to their premature failure. These components are similar, in the way they work, to rollers of cylindrical roller bearings and are produced in low alloyed steel (18NiCrMo7) that have been externally gas carburized in order to increase the surface hardness of the material. There was no specific minimum or maximum hardness requirements from the customer nor a minimum/ maximum thickness for the case hardened layer. The rollers bearings usually fail because of misalignments and fatigue induced also by the inclusion state of the material [2–6]. The rollers used in bearings are usually produced in high C steel and are installed in quenched and tempered condition, without any surface treatment. In this work, instead, the rollers are in low alloyed steel that has been carburized. The causes of failure in the case of case hardened steels are well listed and described in literature [7–8]. The main causes can be related to the carburizing process or to the heat treatment process. The carburizing process could lead to the material embrittlement because of intergranular (IG) precipitation of cementite or by the IG oxidation of material that can cause local microstructural modification. The crack nucleation morphology in this case usually is intergranular [9]. The causes of failure related to quench cracks are many and might affect components used in several applications [10–13]. The quench cracks are produced by local overloads generated by the overlay of both thermal and transformational stresses. Some steels are more prone to produce this kind of cracks during quenching or during the time between quenching and tempering than others [14]. The causes of failure usually are related to the presence of these cracks, not detected previously in the installation of the component, that act
Fig. 2. Image of the chassis that contains the failed rollers.
Please cite this article as: A. Lanzutti, et al., Failure analysis of a continuous press component in MDF production plant, Engineering Failure Analysis (2017), http://dx.doi.org/10.1016/j.engfailanal.2017.03.016
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as local stress raisers, during operation, leading to a certain failure on components subjected to fatigue stresses. Quench cracks are rare on case hardened steels due to the existence of high compressive stresses near the hardened layer [7]. Some authors explained the formation of quench cracks as an H induced failure of the component [15,16]. The intergranular oxidation effect on material properties has been studied by many authors [7–9,16–18]. This phenomenon reduces the fatigue performance of the components because of the production of brittle/less resistant phases close to the oxidized region (grain boundary). The fracture surface also in this case is intergranular. The same effect arises in the case of the presence of cementite layers at grain boundaries [7–9,17,18]. In this case the grain boundaries are embrittled because of a slight overcarburizing, usually not visible by light microscope analysis, that reduces the fatigue properties of material leading to an intergranular crack nucleation. The intergranular fracture surface depth usually corresponds more or less to the case hardened layer. This explains most of the fatigue fractures occurred in carburized materials. The aim of this work is to find the causes of failure in the rollers used to bear the stainless steel belt. 2. Material and Methods The chassis that contains the rollers has been extracted from the continuous press (Fig. 2). The failed rollers have been then extracted from chassis and have been substituted with new and undamaged rollers. One failed roller has been used for mechanical and microstructural characterization and the obtained results have been compared to the results obtained on one roller that has survived to 5 years of service life. A representative image a failed roller is shown in Fig. 3. The failed specimen has undergone to a preliminary stereoscope analysis of the damaged areas in order to detect macro morphologies of interest for the failure analysis. One of the detected cracks on the failed roller has been opened and analyzed by means of stereoscope to determine some macro features of the fracture surface. The fracture surface has been analyzed by SEM in order to determine the fracture morphology and the possible crack nucleation site. The specimen, before the SEM analysis, has been degreased. Both the failed and the unfailed rollers have been cross sectioned, using a refrigerated grinding wheel, and embedded in epoxy resin. The embedded specimens have undergone a metallographic preparation (grinding + polishing) and then an acid etching (Nital 2 for 30 s) in order to evidence the microstructure of the case hardened layer. The etched specimens have been analyzed by means of light microscope in proximity of both internal surface (side in contact to the chassis shaft) and external surface (side in contact to belt) of the roller (Fig. 4). SEM + EDXS analyses have also been performed on the cross section of the specimens, with a particular attention to the material in contact to the belt (external surface), in order to determine possible clues that could be linked to the failure process of the roller. Micro-hardness profiles have been acquired on the specimens in cross section in order to determine and foresee the mechanical properties of material. The test is also important to determine the correctness of the heat treatment. In particular the micro-hardness profiles (Vickers hardness with an applied load of 0,1 Kg and a step between each measurement of 0,1 mm) have been acquired from carburized surface to the core of material, as indicated in Fig. 4. The obtained results have been discussed and compared in order to determine the causes of failure for these components. The comparison has been performed mainly between the microstructure and the mechanical properties of both failed and not failed rollers. The chemical composition of this component has not been analyzed because of the small dimensions of the roller. 3. Results 3.1. Visual inspection and stereoscope analyses The rollers have been extracted from the chassis and the failed ones have been analyzed by means of several instruments in order to determine the causes that generated the malfunction of the continuous press. These analyses are used to individuate some macro defects on the roller. The failed roller is shown in Fig. 3. The failed roller, by visual inspection, presents a region of detached material located on lateral surface. This region is localized on one lateral side of the small cylinder. The component has been then analyzed by stereoscope in order to determine some possible macro-defects that can give some indications of the possible causes of failure. This analysis (Fig. 5) shows that these defective areas are localized on one lateral side of the roller, confirming the visual inspection results.
Fig. 3. Specimen failed during the service life.
Please cite this article as: A. Lanzutti, et al., Failure analysis of a continuous press component in MDF production plant, Engineering Failure Analysis (2017), http://dx.doi.org/10.1016/j.engfailanal.2017.03.016
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Fig. 4. Schematic representation of the analysis performed on cross section.
The cracks, detected by stereoscope analysis, have been opened by means of refrigerated abrasive wheel in order to expose the fracture surface. These analyses are necessary in order to determine the possible causes of failure of material trough the investigation of the fracture surface morphology. The fracture surface, analyzed by stereoscope, (Fig. 6a) presents two different morphologies: a wide region of granular morphology (grey spot on Fig. 6a), followed by a fibrous propagation region (white spot on Fig. 6a), internal to the roller. The crack nucleation is located on external surface of the roller, probably in the carburized area. 3.2. SEM analyses The SEM analysis has aimed to determine the fracture surface morphology and these information could give an additional clue to determine the causes of failure for this component. The analysis (Fig. 6b) confirms the presence of an intergranular fracture surface on the region observed by stereoscope in the grey spot of Fig. 6a. In this region, the fracture surface, analyzed by SEM, is partially worn and some deposits hard to remove by degreasing process are present. The SEM analysis of propagation area (white spot of Fig. 6a), instead, has evidenced the presence of a completely worn surface composed by fibrous macromorphology that indicates a material degradation linked to a probable fatigue process. In order to better evidence the origin of these cracks, a SEM analysis, near to a crack, on cross section of the specimen has been performed. As it is possible to observe in Fig. 7, the crack presents a polygonal morphology. This indicates that the crack probably started in intergranular mode, as evidenced by previous SEM analysis of fracture surface. Close to the crack are also present some polygonal morphologies, situated just underneath the treated surface (zoomed window in Fig. 7). The EDXS results in this area (Table 1) indicate that in this small regions the material is oxidized in proximity of grain boundaries (Intergranular oxidation). 3.3. Microstrctural characterization The microstructural characterization has been performed in order to determine possible causes that could origin a weak close to the grain boundaries, that are the regions of material where the crack started. In addition, these results could also give an indication on correctness of both surface treatment and the consequent heat treatment performed on material. The microstructure of the roller (Fig. 8) is composed by high C tempered martensite in the outer part and medium C tempered martensite in the internal surface of the component. This microstructure is linked to the carburizing diffusive process that led to
Fig. 5. Stereoscope images of the failed roll: a) side view and b) top view of the damaged area.
Please cite this article as: A. Lanzutti, et al., Failure analysis of a continuous press component in MDF production plant, Engineering Failure Analysis (2017), http://dx.doi.org/10.1016/j.engfailanal.2017.03.016
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Fig. 6. Images of the fracture surface: a) stereoscope analysis, b) SEM analysis of the fracture surface in proximity of grey circle and c) SEM analysis of fracture surface in proximity of the white circle.
Fig. 7. SEM + EDXS analysis in cross section of the rolls in proximity of the crack and zoomed area in the region of IG paths.
Please cite this article as: A. Lanzutti, et al., Failure analysis of a continuous press component in MDF production plant, Engineering Failure Analysis (2017), http://dx.doi.org/10.1016/j.engfailanal.2017.03.016
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Table 1 Semi-quantitative results on areas analyzed by EDXS in Fig. 7 (wt%). Spectrum
O
Si
Cr
Fe
Ni
Sp.1 Sp.2 Bulk
5.3 8.2
0.7 2.1
0.4
Bal. Bal. Bal.
1.5 1.6 1.7
0.4
local enrichment of C on the surface of the roller. This justifies also the difference of microstructure between the core and the external surface of the component. No differences have been evidenced during the microstructural characterization of the component not failed during the service life compared to the failed one. In addition, the microstructural characterization didn't show any evidence of intergranular oxidation (phase change because of the local depletion of C in proximity of grain boundaries). 3.4. Microhardness profiles The microhardness profiles have been extracted in order to determine the mechanical properties of both external and internal surface of the component. This also could give an indication on the possible correctness of both carburizing treatment and the bulk heat treatment of material. The microhardness profiles acquired on both failed and not failed rollers are shown in Fig. 9. As indicated by these results, the carburized layer has a thickness of about 0,7 mm for each analyzed sample. The maximum hardness is localized at surface for both the analyzed specimens and has a maximum value of 570 HV0,1, for the failed roller, and 530 HV0,1, for the not failed component. The failed specimen presents highest hardness values in the carburized layer compared to the not failed roller, for the whole thickness of the hardened layer. The core of material for both the analyzed rollers has a similar hardness value. 4. Discussion The failure of the roller is an important fact in order to guarantee the correct working service of the press and is also important to avoid the breaking of the stainless steel belt. Determine the causes of failure of a small cheap component means solve problems that can impair both the quality of the produced MDF and the productivity of the production plant. The experimental results, in this work, have evidenced several differences between the quality of material that has survived during the service life compared to the material that has not survived.
Fig. 8. microstructural characterization of both failed and not failed specimens: a) failed specimen in proximity of surface; b) failed specimen in proximity of the core, c) not failed specimen in proximity of the surface, not failed specimen in proximity of the core.
Please cite this article as: A. Lanzutti, et al., Failure analysis of a continuous press component in MDF production plant, Engineering Failure Analysis (2017), http://dx.doi.org/10.1016/j.engfailanal.2017.03.016
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Fig. 9. micro hardness profile acquired on both failed and not failed specimens from surface to the bulk of material.
Starting from the visual inspection, the failed roller has presented a crack from one side of cylinder that has propagated longitudinally. It has also been observed the presence of a material detachment (Fig. 3 and Fig. 5). This particular distribution of the crack could be linked to possible quench cracks [8] that have been enucleated during the cooling and have been generated from possible differential residual stresses between the two sides of the roller. The crack then propagated during the service life by hertzian fatigue [9]. The hypothesis of quench crack, as responsible of crack nucleation, has also been confirmed by the fracture surface morphology analyzed by both stereoscope and SEM in Fig. 6. The SEM analysis has evidenced the presence of an intergranular fracture surface morphology, in the crack nucleation region, that could also be linked of embittering phases that could be present at grain boundaries. Indeed it is well known in literature [7–13] that the quench cracks are originated during the cooling process and present an intergranular fracture surface morphology. The origin of these cracks could be linked to the high residual stresses present in the component during the quenching process because of the overlay of both thermal and martensitic transformation stresses. The quench cracks could be generated also by lack of tempering process or by the permanence of material for a long time in as quenched condition before the tempering process. During SEM + EDXS analyses (Fig. 7), some intergranular oxidation (IG) has also been detected on the external surface of the component, just close to the crack. The presence of IG has been confirmed by the EDXS analysis (Table 1) where it has been found a high amount of O in proximity of probably prior austenite grain boundaries. The depth of this layer is of about 10-15 μm. It is well known from literature that oxidation is generated by the scarce control of carburizing atmosphere and this phenomenon could be responsible of premature fatigue failure of the installed components, that present an intergranular fracture surface [7–19] as observed in the failed rollers (Fig. 6). These small oxidized paths induce a local decarburization of the material, very difficult to identify by means of light microscope analyses, that could reduce locally its mechanical properties by forming small areas of a phase with lower C content (Bainite, perlite or ferrite thin films) [7–9]. These phases distributed on the grain boundary could produce an intergranular crack during the cooling process because of both transformational, thermal stresses and the low mechanical properties of the grain boundary compared to the rest of the material. These phases could also decrease locally the fatigue resistance of material, increasing the probability to have the crack nucleation at the grain boundary [17]. The microstructural characterization has not evidenced any important difference between the two analyzed rollers. On the other hand, the micro hardness profiles (Fig. 9) have evidenced a lack of tempering process in the failed material. This is confirmed by the presence of a harder case hardened layer in the failed roller than the not failed roller (Fig. 9). The lack of tempering process in the high C layer could lead to the formation of cracks, similar in morphology to quench cracks [7–9].
5. Conclusions The present work has aimed to determine the causes of failure in some rollers used in continuous press that is a part of MDF production plant. The rollers, that are produced in low alloyed steel carburized on the external surface, have presented a failure linked to the low control of heat treatment process. Indeed, from the experimental results seems that the rollers have been installed with some quenching cracks. These cracks have been originated probably because of the lack of tempering process on roller. The crack nucleation has been facilitated by the presence of intergranular oxidation. The in service failure has been only linked to Please cite this article as: A. Lanzutti, et al., Failure analysis of a continuous press component in MDF production plant, Engineering Failure Analysis (2017), http://dx.doi.org/10.1016/j.engfailanal.2017.03.016
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the fatigue crack propagation of the pre-existing cracks. For these components the problem of failure is linked to the scarce quality of the material and not to other external effects (environment, human errors etc.). The failure of these components could be reduced just by an accurate control of the heat treatment process. In particular the weak points of the heat treatment process are: • Scarce control of the material and the post quenching process (lack of tempering treatment); • Scarce control of the carburizing atmosphere (presence of intergranular oxidation); • Scarce quality control of the component after the heat treatment. It is suggested to perform the following controls: • Check of surface hardness to verify both the correctness of both carburizing process and heat treatment; • Better control of the O partial pressure in the endogas atmosphere; • Perform some no destructive control on some rollers after heat treatment. It is also possible to suggest to use an alternative material that doesn't require the surface treatment as the 100Cr6 steel, that is a steel typically used to produce the bearings. If a surface treatment is desired, it is also suggested to use gas nitrided components.
References [1] FAOSTAT-Food and Agriculture Organization of the United Nations, Forestry Database, Forestry Production and TradeAccessed 28 December 2014 http://faostat3. fao.org/faostat-gateway/go/to/download/F/FO/E 2014. [2] J. Guan, L. Wing, C. Zhang, X. Ma, Effect of non- metallic inclusions on the crack propagation in bearing steel, Tribol. Int. 104 (2017) 123–131. [3] P. RycerzA, A. Kadric Olver, Propagation of surface initiated rolling contact fatigue cracks in bearing steel, Int. J. Fatigue 97 (2017) 29–38. [4] B. Warda, A. Chudzik, Effect of ring misalignment on the fatigue life of radial cylindrical roller bearing, Int. J. Mech. Sci. 111–112 (2016) 1–11. [5] T. Bruce, E. Rounding, H. Long, R.S. Dwyer-Joyce, Characterization of white etching crack in wind turbine gearbox bearings, Wear 38–339 (2015) 164–177. [6] K. Gurumoorthy, A. Ghosh, Failure investigation of a taper roller bearing: A case study, Case Stud. Eng. Fail. Anal. 1 (2013) 110–114. [7] L.C.F. Canale, R.A. Mesquita, G.E. Totten, Failure Analysis of Heat Treated Steel Components, first ed. ASM International, Ohio, 2008. [8] G.E. Totten, Steel Heat Treatment Handbook, second ed. CRC press, New York, 2007. [9] W.T. Becker, R.J. Shipley, Failure Analysis and prevention, first ed., ASM Metal Handbook, 11, ASM International, Ohio, 2002. [10] D. Gallina, Finite element prediction of crack formation induced by quenching in a forged valve, Eng. Fail. Anal. 18 (2011) 2250–2259. [11] S.N. Lingamanaik, B.K. Chen, Microstructural and thermo-mechanical analysis of quench cracking during the production of bainitic-martensitic railway wheels, Eng. Fail. Anal. 40 (2014) 25–32. [12] N.K. Mukhopadhyay, S.K. Das, B. Ravikumar, V.R. Ranganath, S.G. Chowdhury, Premature failure of a leaf spring due to improper material processing, Eng. Fail. Anal. 4 (1997) 161–170. [13] B.R. Kumar, D.K. Bhattacharaya, S.K. Das, S.G. Chowdhury, Premature fatigue failure of a spring due to quench cracks, Eng. Fail. Anal. 7 (2000) 377–384. [14] ASM society, Heat treating, first ed., ASM Metal Handbook, 4, ASM International, Ohio, 1991. [15] X.Y. Liu, C.J. McMahon Jr., Quench cracking in steel as a case of hydrogen embrittlement, Mater. Sci. Eng. A 499 (2009) 540–541. [16] X. An, J. Cawley, W.M. Rainforth, L. Chen, Study of internal oxidation in carburized steel by glow discharge optical emission spectroscopy and scanning electron microscopy, Spectrochim. Acta A B 58 (2003) 689–698. [17] K. Genel, M. Demirkol, Effect of case depth on fatigue performance of AISI 8620 carburized steel, Int. J. Fatigue 21 (1999) 207–212. [18] Z.W. Yu, X.I. Xu, Z. Yang, Y.Y. Li, Case internal oxidation and intergranular fracture of carburized splined shaft, Eng. Fail. Anal. 22 (2012) 141–151.
Please cite this article as: A. Lanzutti, et al., Failure analysis of a continuous press component in MDF production plant, Engineering Failure Analysis (2017), http://dx.doi.org/10.1016/j.engfailanal.2017.03.016
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of a continuous press component in MDF production plant A. Lanzutti a,⁎, F. Andreatta a, A. Raffaelli a, M. Magnan a, L. Zuliani b, M. Fantoni b, L. Fedrizzi a a b
University of Udine, Polytechnic Department of Engineering and Architecture, via delle scienze 208, Udine, Italy Fantoni spa, Zona Industriale Rivoli, 33010 Osoppo (UD), Italy
a r t i c l e
i n f o
Article history: Received 27 September 2016 Received in revised form 17 March 2017 Accepted 20 March 2017 Available online xxxx Keywords: Heat treatment Brittle fracture Failure analysis Intergranular fracture
a b s t r a c t This work presents the results of a failure analysis performed on a component of a continuous press that is a part of a process plant used to produce MDF (Medium Density Fibreboard) panels. This plant is composed of 2 counter rotating belts that are supported, internally, by small steel rollers, connected to each other by small axes. The failure occurred prematurely on rollers. These components undergo to static and dynamic stresses below the yield strength of material. Low alloyed steel is employed for the production of the rollers and the components are carburized on the external surface. The rollers present cracks on the external surface, that is in contact with the wood panels, already after few hours in service. Other components did not undergo cracking even for very long service times (about 5000 h). Both rollers that failed in service and rollers without cracks were characterized by optical and scanning electron microscopy in order to evaluate the microstructure of the components. In particular, the microstructure of the rollers was investigated on the surface and in the core of the rollers. Vickers micro-hardness measurements were employed for the characterization of the mechanical properties of the rollers. Successively, a cross section of a broken roller was characterized by scanning electron microscopy (SEM) On the same sample, after microstructural characterization, a micro-hardness profile was acquired from external surface to the core in order to evaluate possible over carburizing or oxidation phenomena. The results obtained from both the microstructural and mechanical characterization of the failed rollers indicate that the possible causes of the in service failure are related to quenching cracks or fatigue. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction The present work is a failure analysis of a structural component installed in a continuous press in the production line of MDF panels (Medium Density Fibreboard). The importance of the MDF production is worldwide known and the production plants are located in several countries [1]. In MDF panels wood fibers are bounded in with a suitable matrix (typically urea-formaldehyde resins) by the combined action of pressure and heat transfer. In order to obtain the wood fibers, the wood logs are reduced into chips that are further processed with washing systems. The wood chips are then digested for a few minutes in a steam pressure vessel at about 8.0 bar and then transformed into fibers in an Asplund-type refiner. The resin and the additives are normally added in at the refiner's discharge section in a complex multiphase and highly turbulent flow. The material then enters in a pneumatic flash dryer where moisture is extracted in order to reach a ⁎ Corresponding author. E-mail address: [email protected] (A. Lanzutti).
http://dx.doi.org/10.1016/j.engfailanal.2017.03.016 1350-6307/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: A. Lanzutti, et al., Failure analysis of a continuous press component in MDF production plant, Engineering Failure Analysis (2017), http://dx.doi.org/10.1016/j.engfailanal.2017.03.016
2
A. Lanzutti et al. / Engineering Failure Analysis xxx (2017) xxx–xxx
Fig. 1. Image of the continuous press plant and schematic representation of the process. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
final content of about 8–12%. The MDF material enters (MDF green) and exits continuously in the continuous press thanks to a double steel-belt conveyor and to a bed of rolling rods or small chains that act as a linear roller bearing. A schematic representation of the operation scheme of the continuous press, is shown in Fig. 1. The press is composed of two counter rotating belts moved by 2 drums. Internally, the belts are supported by a continuous carpet of small rollers that are joined together by a flexible chassis to form a web (Fig. 2). The MDF green (natural fibers + resin), during the production, is inserted in between these carpets and at the same time squeezed and heated up to 200 °C in order to bake the green and form the final panel. The function of the rollers is to impress, through a hydraulic piston, the necessary pressure to deform the belt and compress the MDF green (5–6 MPa). Out of the hot press the MDF panel is then cut with a flying saw and then cooled and stacked for further processing (e.g. sanding, cut-to-size, lamination, ecc.). The failed components considered in this experience are the rollers supporting the continuous carpet (green circles Fig. 1 and Fig. 2). These components are subjected to both static/dynamic and hertzian loads, impressed by the interaction between the MDF green and the press. Moreover, thermomechanical stresses due to the baking process of the MDF material might be also present. At the same time the components are exposed to an atmosphere composed by hot air with some traces of ammonia, water vapor and hydrogen. These gases are produced by the baking process and derive mainly from the curing process of the resin matrix. The rollers failed after few hours of service life compared to an infinite design life (the rollers are anyway substituted every 5 years). The main problems related to the failure of these small components is that they can snug in between the drums and the belts leading also to their premature failure. These components are similar, in the way they work, to rollers of cylindrical roller bearings and are produced in low alloyed steel (18NiCrMo7) that have been externally gas carburized in order to increase the surface hardness of the material. There was no specific minimum or maximum hardness requirements from the customer nor a minimum/ maximum thickness for the case hardened layer. The rollers bearings usually fail because of misalignments and fatigue induced also by the inclusion state of the material [2–6]. The rollers used in bearings are usually produced in high C steel and are installed in quenched and tempered condition, without any surface treatment. In this work, instead, the rollers are in low alloyed steel that has been carburized. The causes of failure in the case of case hardened steels are well listed and described in literature [7–8]. The main causes can be related to the carburizing process or to the heat treatment process. The carburizing process could lead to the material embrittlement because of intergranular (IG) precipitation of cementite or by the IG oxidation of material that can cause local microstructural modification. The crack nucleation morphology in this case usually is intergranular [9]. The causes of failure related to quench cracks are many and might affect components used in several applications [10–13]. The quench cracks are produced by local overloads generated by the overlay of both thermal and transformational stresses. Some steels are more prone to produce this kind of cracks during quenching or during the time between quenching and tempering than others [14]. The causes of failure usually are related to the presence of these cracks, not detected previously in the installation of the component, that act
Fig. 2. Image of the chassis that contains the failed rollers.
Please cite this article as: A. Lanzutti, et al., Failure analysis of a continuous press component in MDF production plant, Engineering Failure Analysis (2017), http://dx.doi.org/10.1016/j.engfailanal.2017.03.016
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as local stress raisers, during operation, leading to a certain failure on components subjected to fatigue stresses. Quench cracks are rare on case hardened steels due to the existence of high compressive stresses near the hardened layer [7]. Some authors explained the formation of quench cracks as an H induced failure of the component [15,16]. The intergranular oxidation effect on material properties has been studied by many authors [7–9,16–18]. This phenomenon reduces the fatigue performance of the components because of the production of brittle/less resistant phases close to the oxidized region (grain boundary). The fracture surface also in this case is intergranular. The same effect arises in the case of the presence of cementite layers at grain boundaries [7–9,17,18]. In this case the grain boundaries are embrittled because of a slight overcarburizing, usually not visible by light microscope analysis, that reduces the fatigue properties of material leading to an intergranular crack nucleation. The intergranular fracture surface depth usually corresponds more or less to the case hardened layer. This explains most of the fatigue fractures occurred in carburized materials. The aim of this work is to find the causes of failure in the rollers used to bear the stainless steel belt. 2. Material and Methods The chassis that contains the rollers has been extracted from the continuous press (Fig. 2). The failed rollers have been then extracted from chassis and have been substituted with new and undamaged rollers. One failed roller has been used for mechanical and microstructural characterization and the obtained results have been compared to the results obtained on one roller that has survived to 5 years of service life. A representative image a failed roller is shown in Fig. 3. The failed specimen has undergone to a preliminary stereoscope analysis of the damaged areas in order to detect macro morphologies of interest for the failure analysis. One of the detected cracks on the failed roller has been opened and analyzed by means of stereoscope to determine some macro features of the fracture surface. The fracture surface has been analyzed by SEM in order to determine the fracture morphology and the possible crack nucleation site. The specimen, before the SEM analysis, has been degreased. Both the failed and the unfailed rollers have been cross sectioned, using a refrigerated grinding wheel, and embedded in epoxy resin. The embedded specimens have undergone a metallographic preparation (grinding + polishing) and then an acid etching (Nital 2 for 30 s) in order to evidence the microstructure of the case hardened layer. The etched specimens have been analyzed by means of light microscope in proximity of both internal surface (side in contact to the chassis shaft) and external surface (side in contact to belt) of the roller (Fig. 4). SEM + EDXS analyses have also been performed on the cross section of the specimens, with a particular attention to the material in contact to the belt (external surface), in order to determine possible clues that could be linked to the failure process of the roller. Micro-hardness profiles have been acquired on the specimens in cross section in order to determine and foresee the mechanical properties of material. The test is also important to determine the correctness of the heat treatment. In particular the micro-hardness profiles (Vickers hardness with an applied load of 0,1 Kg and a step between each measurement of 0,1 mm) have been acquired from carburized surface to the core of material, as indicated in Fig. 4. The obtained results have been discussed and compared in order to determine the causes of failure for these components. The comparison has been performed mainly between the microstructure and the mechanical properties of both failed and not failed rollers. The chemical composition of this component has not been analyzed because of the small dimensions of the roller. 3. Results 3.1. Visual inspection and stereoscope analyses The rollers have been extracted from the chassis and the failed ones have been analyzed by means of several instruments in order to determine the causes that generated the malfunction of the continuous press. These analyses are used to individuate some macro defects on the roller. The failed roller is shown in Fig. 3. The failed roller, by visual inspection, presents a region of detached material located on lateral surface. This region is localized on one lateral side of the small cylinder. The component has been then analyzed by stereoscope in order to determine some possible macro-defects that can give some indications of the possible causes of failure. This analysis (Fig. 5) shows that these defective areas are localized on one lateral side of the roller, confirming the visual inspection results.
Fig. 3. Specimen failed during the service life.
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Fig. 4. Schematic representation of the analysis performed on cross section.
The cracks, detected by stereoscope analysis, have been opened by means of refrigerated abrasive wheel in order to expose the fracture surface. These analyses are necessary in order to determine the possible causes of failure of material trough the investigation of the fracture surface morphology. The fracture surface, analyzed by stereoscope, (Fig. 6a) presents two different morphologies: a wide region of granular morphology (grey spot on Fig. 6a), followed by a fibrous propagation region (white spot on Fig. 6a), internal to the roller. The crack nucleation is located on external surface of the roller, probably in the carburized area. 3.2. SEM analyses The SEM analysis has aimed to determine the fracture surface morphology and these information could give an additional clue to determine the causes of failure for this component. The analysis (Fig. 6b) confirms the presence of an intergranular fracture surface on the region observed by stereoscope in the grey spot of Fig. 6a. In this region, the fracture surface, analyzed by SEM, is partially worn and some deposits hard to remove by degreasing process are present. The SEM analysis of propagation area (white spot of Fig. 6a), instead, has evidenced the presence of a completely worn surface composed by fibrous macromorphology that indicates a material degradation linked to a probable fatigue process. In order to better evidence the origin of these cracks, a SEM analysis, near to a crack, on cross section of the specimen has been performed. As it is possible to observe in Fig. 7, the crack presents a polygonal morphology. This indicates that the crack probably started in intergranular mode, as evidenced by previous SEM analysis of fracture surface. Close to the crack are also present some polygonal morphologies, situated just underneath the treated surface (zoomed window in Fig. 7). The EDXS results in this area (Table 1) indicate that in this small regions the material is oxidized in proximity of grain boundaries (Intergranular oxidation). 3.3. Microstrctural characterization The microstructural characterization has been performed in order to determine possible causes that could origin a weak close to the grain boundaries, that are the regions of material where the crack started. In addition, these results could also give an indication on correctness of both surface treatment and the consequent heat treatment performed on material. The microstructure of the roller (Fig. 8) is composed by high C tempered martensite in the outer part and medium C tempered martensite in the internal surface of the component. This microstructure is linked to the carburizing diffusive process that led to
Fig. 5. Stereoscope images of the failed roll: a) side view and b) top view of the damaged area.
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Fig. 6. Images of the fracture surface: a) stereoscope analysis, b) SEM analysis of the fracture surface in proximity of grey circle and c) SEM analysis of fracture surface in proximity of the white circle.
Fig. 7. SEM + EDXS analysis in cross section of the rolls in proximity of the crack and zoomed area in the region of IG paths.
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Table 1 Semi-quantitative results on areas analyzed by EDXS in Fig. 7 (wt%). Spectrum
O
Si
Cr
Fe
Ni
Sp.1 Sp.2 Bulk
5.3 8.2
0.7 2.1
0.4
Bal. Bal. Bal.
1.5 1.6 1.7
0.4
local enrichment of C on the surface of the roller. This justifies also the difference of microstructure between the core and the external surface of the component. No differences have been evidenced during the microstructural characterization of the component not failed during the service life compared to the failed one. In addition, the microstructural characterization didn't show any evidence of intergranular oxidation (phase change because of the local depletion of C in proximity of grain boundaries). 3.4. Microhardness profiles The microhardness profiles have been extracted in order to determine the mechanical properties of both external and internal surface of the component. This also could give an indication on the possible correctness of both carburizing treatment and the bulk heat treatment of material. The microhardness profiles acquired on both failed and not failed rollers are shown in Fig. 9. As indicated by these results, the carburized layer has a thickness of about 0,7 mm for each analyzed sample. The maximum hardness is localized at surface for both the analyzed specimens and has a maximum value of 570 HV0,1, for the failed roller, and 530 HV0,1, for the not failed component. The failed specimen presents highest hardness values in the carburized layer compared to the not failed roller, for the whole thickness of the hardened layer. The core of material for both the analyzed rollers has a similar hardness value. 4. Discussion The failure of the roller is an important fact in order to guarantee the correct working service of the press and is also important to avoid the breaking of the stainless steel belt. Determine the causes of failure of a small cheap component means solve problems that can impair both the quality of the produced MDF and the productivity of the production plant. The experimental results, in this work, have evidenced several differences between the quality of material that has survived during the service life compared to the material that has not survived.
Fig. 8. microstructural characterization of both failed and not failed specimens: a) failed specimen in proximity of surface; b) failed specimen in proximity of the core, c) not failed specimen in proximity of the surface, not failed specimen in proximity of the core.
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Fig. 9. micro hardness profile acquired on both failed and not failed specimens from surface to the bulk of material.
Starting from the visual inspection, the failed roller has presented a crack from one side of cylinder that has propagated longitudinally. It has also been observed the presence of a material detachment (Fig. 3 and Fig. 5). This particular distribution of the crack could be linked to possible quench cracks [8] that have been enucleated during the cooling and have been generated from possible differential residual stresses between the two sides of the roller. The crack then propagated during the service life by hertzian fatigue [9]. The hypothesis of quench crack, as responsible of crack nucleation, has also been confirmed by the fracture surface morphology analyzed by both stereoscope and SEM in Fig. 6. The SEM analysis has evidenced the presence of an intergranular fracture surface morphology, in the crack nucleation region, that could also be linked of embittering phases that could be present at grain boundaries. Indeed it is well known in literature [7–13] that the quench cracks are originated during the cooling process and present an intergranular fracture surface morphology. The origin of these cracks could be linked to the high residual stresses present in the component during the quenching process because of the overlay of both thermal and martensitic transformation stresses. The quench cracks could be generated also by lack of tempering process or by the permanence of material for a long time in as quenched condition before the tempering process. During SEM + EDXS analyses (Fig. 7), some intergranular oxidation (IG) has also been detected on the external surface of the component, just close to the crack. The presence of IG has been confirmed by the EDXS analysis (Table 1) where it has been found a high amount of O in proximity of probably prior austenite grain boundaries. The depth of this layer is of about 10-15 μm. It is well known from literature that oxidation is generated by the scarce control of carburizing atmosphere and this phenomenon could be responsible of premature fatigue failure of the installed components, that present an intergranular fracture surface [7–19] as observed in the failed rollers (Fig. 6). These small oxidized paths induce a local decarburization of the material, very difficult to identify by means of light microscope analyses, that could reduce locally its mechanical properties by forming small areas of a phase with lower C content (Bainite, perlite or ferrite thin films) [7–9]. These phases distributed on the grain boundary could produce an intergranular crack during the cooling process because of both transformational, thermal stresses and the low mechanical properties of the grain boundary compared to the rest of the material. These phases could also decrease locally the fatigue resistance of material, increasing the probability to have the crack nucleation at the grain boundary [17]. The microstructural characterization has not evidenced any important difference between the two analyzed rollers. On the other hand, the micro hardness profiles (Fig. 9) have evidenced a lack of tempering process in the failed material. This is confirmed by the presence of a harder case hardened layer in the failed roller than the not failed roller (Fig. 9). The lack of tempering process in the high C layer could lead to the formation of cracks, similar in morphology to quench cracks [7–9].
5. Conclusions The present work has aimed to determine the causes of failure in some rollers used in continuous press that is a part of MDF production plant. The rollers, that are produced in low alloyed steel carburized on the external surface, have presented a failure linked to the low control of heat treatment process. Indeed, from the experimental results seems that the rollers have been installed with some quenching cracks. These cracks have been originated probably because of the lack of tempering process on roller. The crack nucleation has been facilitated by the presence of intergranular oxidation. The in service failure has been only linked to Please cite this article as: A. Lanzutti, et al., Failure analysis of a continuous press component in MDF production plant, Engineering Failure Analysis (2017), http://dx.doi.org/10.1016/j.engfailanal.2017.03.016
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the fatigue crack propagation of the pre-existing cracks. For these components the problem of failure is linked to the scarce quality of the material and not to other external effects (environment, human errors etc.). The failure of these components could be reduced just by an accurate control of the heat treatment process. In particular the weak points of the heat treatment process are: • Scarce control of the material and the post quenching process (lack of tempering treatment); • Scarce control of the carburizing atmosphere (presence of intergranular oxidation); • Scarce quality control of the component after the heat treatment. It is suggested to perform the following controls: • Check of surface hardness to verify both the correctness of both carburizing process and heat treatment; • Better control of the O partial pressure in the endogas atmosphere; • Perform some no destructive control on some rollers after heat treatment. It is also possible to suggest to use an alternative material that doesn't require the surface treatment as the 100Cr6 steel, that is a steel typically used to produce the bearings. If a surface treatment is desired, it is also suggested to use gas nitrided components.
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Please cite this article as: A. Lanzutti, et al., Failure analysis of a continuous press component in MDF production plant, Engineering Failure Analysis (2017), http://dx.doi.org/10.1016/j.engfailanal.2017.03.016