Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 133 (2015 ) 726 735 6th Fatigue Design conference, Fatigue Design 2015 Challenges in the fatigue assessment of large components from forged or cast iron Matthias Decker a*, Marion Eiber a, Steffen Rödling a IABG mbh, Einsteinstrasse 20, 85521 Ottobrunn, Germany Abstract Hydroelectric, thermal or wind power plants contain large power transmitting components which are often produced from forged or cast iron. These components set high requirements to efficient and reliable methods for design, validation and quality assurance. Due to their large dimensions and the high forces and moments acting during operation, full scale fatigue tests with the actual components are usually not possible with the number of specimen required for a reliable verification of safety against failure. Hence, usually a theoretical fatigue assessment based on the results of finite element calculations and material parameters is performed. The great uncertainties about material parameters are covered by safety factors that reduce the allowable stresses. Since this reduction is normally applied for the whole component, the material tends to be over specified for regions that are not highly stressed. In this paper, important aspects will be shown that have to be taken into account when dealing with large cast or forged power transmitting components. A method for obtaining a set of valuable and reliable local material parameters for a safe fatigue assessment is presented that facilitates a better material utilization and also takes into account the need for precise criteria for quality assurance in series production. A special focus is set on the importance of metallographic investigations along the process of specimen testing and quality control. 2015 Published The Authors. by Elsevier Published Ltd. by This Elsevier is an open Ltd. access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of CETIM. Peer-review under responsibility of CETIM Keywords: fatigue, strength, casting, forging, quality assurance * Corresponding author. Tel.: +49-89-6088-2479; fax: +49-89-6088-3170. E-mail address: decker@iabg.de 1877-7058 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of CETIM doi:10.1016/j.proeng.2015.12.654
Matthias Decker et al. / Procedia Engineering 133 ( 2015 ) 726 735 727 1. Introduction An efficient and reliable method for fatigue assessment is critical for the economic design, validation and operation of power plants. E.g. wind turbines are heading for larger dimensions continuously showing an increasing rate of failure with increasing nominal power [1]. Roughly 36% of the failures of wind turbines are caused by defects of components. 63% of the failures lead to 6 days of down time in average [2]. Due to limited access, the reliability of off shore wind power plants is especially critical and of vital interest for the operating company. Besides the necessary assurance of reliable operation, reduction of the costs for development and production is of increasing interest for both producing and operating companies. Increasing competition on the market makes efficient methods for design, validation, production and quality control more and more important. This paper is based on investigations presented in [3] and further work of the authors. It shows aspects of the fatigue assessment in the process of design, validation and quality control that - from the authors point of view - are not yet sufficiently described in standards used for the fatigue assessment like [4] or [5], where not all relevant influences on large components from forged or cast iron are treated in an way that facilitates an optimized use of material and reliability. The topic is discussed for components in the drivetrain of wind turbines as a current example. However, the procedures may be transferred to other applications with similar requirements. 2. Fatigue assessment of components for wind turbines Fatigue design and proof out of components of wind turbines put special requirements on the methods and data used. This paper focusses on components of the drive train of wind turbines made from forged and cast iron. Here, large mechanical power must be transmitted with low rotational speeds, yielding large forces and moments. Because of the large size of the components and large forces to be applied, a full scale fatigue test (FST) cannot be carried out in many cases. If a FST is carried out, it is usually limited to one test, hence a statistic evaluation and extrapolation cannot be carried out. Therefore, the FST can only be used as a final integration test, after a thorough analytical or numerical evaluation based on reliable material properties has been carried out. Making use of the potential of modern finite element analyses, fatigue assessment based on local stresses or strains has developed the chosen method over a broad range of applications. Already in 1977 it was stated in [6] that for an estimation of the cycles until crack initiation based on local stresses or strains, several factors like surface condition, stress gradient, residual stresses and local material strength have to be taken into account. E. g. [4] covers some of these effects using influence factors for technological size effect and surface influences like roughness and hardening depth. The technological size factor is applied on the material properties for the whole volume of the component to be analyzed not taking into account the changes within the microstructure over a cross section of the component that may be observed in large components made from forged or cast iron. [5] gives some additional factors describing parameters of production and requirements like the avoidance of shrink holes and non-metallic inclusions without a clear specification and description of how these should be taken into consideration in the fatigue assessment. For a safe fatigue evaluation, a conservative approach is taken in these guidelines, assuming the worst condition of material parameters over the whole volume of the component. Rotary bending and torsion being the dominant loads in mayor components of the drive train of e.g. wind turbines causes significantly higher stresses near the surface than near the center of e.g. a rotor shaft. Notches causing a stress gradient increase this situation. Hence, only a very small portion of the overall volume of the component will be loaded with the highest stress level and only in these areas the material properties must be appropriate to withstand these stresses. If the material properties required at the hot spots are specified for the whole component, large effort in production and quality assurance may be necessary to guarantee material properties at locations where they are not really needed. In order to facilitate optimized material usage with a reduced effort in production and higher safety against failure, this paper shows some significant influences on local material properties that should be considered in fatigue assessment. Also, the integration of metallographic and failure analyses over the whole process from specimen testing up to quality assurance in series production is described.
728 Matthias Decker et al. / Procedia Engineering 133 ( 2015 ) 726 735 3. Fatigue assessment of components from forged iron Material utilization and reliability of components from cast iron can be optimized, if the influence of the production process on the local material properties is taken into account. The influence of surface roughness and surface hardening on the fatigue strength has been investigated extensively over the past and is described well in e.g. [7] and [8]. This is already taken into account in guidelines for fatigue assessment and will not be further discussed here even though the authors see potential for optimization here, too. Forging blanks in practical application show imperfections like segregations and non-metallic inclusions over their volume. The process of forging causes varying local deformations. Hence, the segregations in the blanks are skewed, yielding locally varying microstructure (Fig. 1). This results in different cyclic strength properties depending on the load orientation. Fig. 2 shows S-N-curves of specimen taken from a forged 42CrMo4 alloy. They were generated by testing round specimens taken out of the forging with different angles λ between the longitudinal axis of the specimen and the orientation of the segregations. The amplitude values are normalized to the fatigue limit with the segregations running across the specimen orientation (λ = 90 ). An increase of the fatigue limit up to 40% was observed for specimen taken out parallel to the orientation of the segregation (λ = 0 ). This large influence on the fatigue strength must be taken into account when assessing the fatigue behavior of forged parts based on material data generated from specimen tests. Fig. 3 shows stress-strain curves of 42CrMo4 after different forms of heat treatment. A significant influence on the material behavior is apparent. Similar effects can be observed for cyclic strength with a significant correlation to the micro hardness [9]. These effects must be taken into account in fatigue design and validation. To make use of the positive effects in a reliable way, the production parameters must be documented and observed precisely. The correlation between cyclic strength and micro hardness can be used to define acceptance criteria for quality assurance during series production. Non-metallic inclusions cannot be avoided in steel production and may cause fatigue damage under cyclic loading, especially in higher strength steels. Fig. 4 shows results from constant amplitude tests of passenger car suspension springs. Out of 25 springs tested, one failure was observed at a very low load level at which even runouts occurred. Using a scanning electron microscope for a microfractografic investigation of the fracture surface, a non-metallic inclusion was identified as root cause for the early failure. Numerous investigations in [10], [11] and other studies of the authors show these isolated failures from non-metallic inclusions in different applications. Nonmetallic inclusion may occur dispersed over the volume of any component. Depending on the size of the inclusion and the local cyclic stress amplitude, they can act like a crack. In [12], a method for secure detection of the size distribution of non-metallic inclusions and the maximum allowable stress as a function of the size of the inclusions is described (Fig. 5). Using this correlation, specific criteria for quality assurance can be defined as shown in Fig. 6. Fig. 1. Influence of the degree of deformation in the local micro structure ( [9]) Fig. 2. Influence of the specimen orientation on the cyclic strength of 42CrMo4 (λ = angle between specimen longitudinal axis and segregation orientation [9])
Matthias Decker et al. / Procedia Engineering 133 ( 2015 ) 726 735 729 Fig. 3. Influence of the heat treatment on the static strength of 42CrMo4 ( [9]) Fig. 4. Influence of non-metallic inclusions on the cyclic strength springs made from high strength steels ( [10]) Fig. 5. Influence of the size of non-metallic inclusions on the maximum local endurance limit of steels ( [12]) Fig. 6. Statistical distribution of the size of non-metallic inclusions in high strength steel from different steel manufacturers ( [11]) 4. Fatigue assessment of components from cast iron Current standards for fatigue design and assessment of cast components use a linear correlation between ultimate tensile strength or yield strength and fatigue limit ([4], [5]). This correlation can be observed comparing static and cyclic strength tests performed on specimens, normally taken from especially casted forms like Y4-bodies. Tests with specimens taken from large components out of series production however often show additional effects that have a stronger influence on the fatigue behavior than the static strength values. Thick-walled cast components can contain material imperfections such as shrink holes or large graphite spheres that may not be detected securely by non-destructive testing during quality control. While the influence on the ultimate tensile strength may be negligible, the cyclic strength can be reduced significantly, especially for high strength cast iron. Current guidelines for fatigue evaluation like [4] and [5] try to cover these effects by using safety factors. If these factors are defined conservatively to cover the risk for high strength cast iron, they will be too severe for lower strength materials, hence leading to an inferior material utilization. Especially for thick-walled cast components under rotary bending or torsional loading, the highly stressed portion of the component is small compared with the overall volume of the component. Large portions of the material are only subjected to low stresses. Therefore, a procedure that makes use of local material properties is of high economic interest to facilitate an optimized material utilization with validated local criteria of allowable material imperfections
730 Matthias Decker et al. / Procedia Engineering 133 ( 2015 ) 726 735 for quality assurance. Fig. 7 gives an overview over such imperfections. To cover these influences, a material data base must be generated by static and cyclic testing of specimens taken from the different areas of interest of the component. A correlation between fatigue strength and microstructure can be established by metallographic analysis of the tested specimen. The number of areas to be tested within the component depends on technological and economical aspects. More areas tested facilitate a finer design to optimal material utilization on the one hand. On the other hand, the effort for specimen testing and quality control will be increased. Taking into account all aspects, an optimum procedure can be defined for individual applications. From the authors point of view, the following parameters should be evaluated: 1. Mean stress sensitivity: With a growing size of microstructural defects, the risk increases that a defect may act like a crack that follows the laws of fracture mechanics. Linear elastic fracture mechanics shows significantly higher mean stress sensitivity than observed for pure cast iron without imperfections. Cast iron with larger imperfections that act like cracks can be expected to show a mean stress sensitivity that lies between these limits and depends on the characteristics of these imperfections, as schematically displayed in Fig. 8. For a safe fatigue assessment based on local material parameters, this local mean stress sensitivity must be characterized and correlated to the respective metallographic analyses. 2. Statistical and Stress-mechanical size effect: Shape and size of imperfections can have an influence on statistical and stress-mechanical size effect. Using S-N-curves for axial compression-tension as a reference, additional S-N-curves for axial or rotary bending can give a first indication of this influence. If applicable, this should be supported by tests with notched specimen with different stress gradients. Depending on the data basis generated, different models to describe the size effects can be used as described in [13]. 3. Recent investigations of the authors show that the influence of surface roughness may be overestimated by the commonly used factors according to [7] for components of higher strength cast iron. It is assumed that this is due to material imperfections dominating the influence of surface roughness. This bears a large potential for optimization and should be investigated additionally for the individual application. Local material parameters and influence factors correlated to metallographic analyses can be used for an optimized fatigue design and assessment. Especially for large, thick-walled components from higher strength cast iron, the risk of failure can be lowered. At the same time, the reject quota in quality assurance can be lowered using local quality criteria that take into account that the highest material quality is only necessary within the highest stressed areas of the components. In [14], the fatigue behavior of a thick-walled planet carrier for a wind turbine gear made from GJS-700 was analyzed with and without pearlitizing after casting. The pearlitized carrier showed yield strength properties of GJS- 700 all over the volume of the component, while the carrier that was not pearlitized showed the yield strength of GJS-700 only in areas near the surface and values of GJS-400 within the center. However, the normalized cyclic strength was 100% close to the surface and decreased down to 56% within the center for both carriers. The pearlitizing showed no influence on the fatigue strength, only on the yield strength. Here, using local material properties gained from specimens taken out of the areas of interest of the component on the one hand can help to increase the reliability of the fatigue assessment. On the other hand, the influence of the production process on the local material properties can be analyzed. This can help to decide about the necessity of certain steps in production like heat treatment that may have a major influence on production time and costs.
Matthias Decker et al. / Procedia Engineering 133 ( 2015 ) 726 735 731 Fig. 7. Material imperfections in cast iron ( [9]) Fig. 8. Haigh-diagrams for cast iron with different material imperfections (schematic diagram)
732 Matthias Decker et al. / Procedia Engineering 133 ( 2015 ) 726 735 5. Quality assurance For a reliable fatigue assessment of large components using material properties gained from specimen testing, it has to be ensured that these values are applicable for the components in series production. This correlation can be obtained by metallographic analyses that are carried out on the specimens and locally on the components within quality control in series production. To do so, a correlation must be set up between metallographic results and the corresponding mechanical material properties. Especially for large and thick-walled components, the varying parameters across the volume must be taken into account. For qualification of cast components, the microstructure of the areas of interest should be analyzed using metallographic methods based on light microscopy and image analysis. Special focus has to be set on cracks, porosities, shrink holes, dross, nodularity, size and distribution of graphite particles as well as on the structural matrix and intermetallic phases (Fig. 9). Qualitative and quantitative analyses can describe the different states and make them comparable. Based on this, precise specifications for the desired local material properties can be set up. For large and thick-walled cast components, the desired microstructure can be described using the correlation with mechanical material properties as described above. Different cooling rates over the volume of component yield different material properties. Areas close to the surface may show different graphite formations than within the center of the component (Fig. 11). Scanning electron microscopic pictures of fracture surfaces of many specimens used at the authors laboratories show various crack initiating imperfections like surface defects, shrink holes, segregations or non-metallic inclusions (Fig. 10). From a statistical evaluation of these defects, maximum allowable dimensions can be determined for the individual local stress levels based on the laws of fracture mechanics. These can be used to define acceptance criteria for quality control. [12] shows this in detail for allowable non-metallic inclusions in high strength steels. Using nondestructive testing methods, the surface and to some degree the inner volume of a component can be inspected for imperfections. The inspection for defects in the volume of large components is often difficult and cannot yet be performed as a 100% test with the accuracy needed. Complex geometries make this an even more difficult task. Often, quality specifications describe such analyses that cannot be performed in an economic way. To overcome this problem, besides a better knowledge of the existing material properties, improved methods and procedures have to be developed and integrated. Metallographic analyses are based on polished cross-sections of the material, giving a two dimensional picture of the microstructure and defects. This limited view can be enhanced using digitial x-ray analysis. Computed tomography uses mathematical algorithms to reconstruct a volume out of numerous two dimensional x-ray pictures. Imperfections, cracks or other unrequested defects can be detected and measured in three spatial directions within the range of the gained resolution. Fig. 11 shows a graphite particle as a three dimensional model where a size measurement can be carried out according to the largest dimensions detected, independent of direction. A higher degree of information depth through a spatial interpretation increases its degree of accuracy and provides the possibility of defect characterization. The limited two dimensional view of the same graphite particle like it would be obtained from metallographic investigations of a random plane trough the volume is simulated Fig.12. It is obvious that from the 2D cross section it is not possible to get a correct impression of the size and orientation of the inclusion. This shows the potential that lies within advanced methods for inspection and visualization. For the new methods to be integrated into quality standards, work has to be done to optimize and standardize the procedures of analysis and documentation.
Matthias Decker et al. / Procedia Engineering 133 ( 2015 ) 726 735 733 Fig. 9. Micro hardness indentions in matrix (a) and intermetallic phase (b) Fig. 10. Shrink-hole within a fracture surface of cast iron Fig. 11. ADI cast iron, graphite formation near surface (left) and in the center (right) Fig. 12. 3D µct scan of graphite formations within cast iron Fig. 13. 2D-cross section derived from 3D-scan (simulated two dimensional cross section of blue plane of Fig. 12)
734 Matthias Decker et al. / Procedia Engineering 133 ( 2015 ) 726 735 6. Conclusion and outlook Large components from forged or cast iron show different static and cyclic strength properties in different areas of the component. In order to meet both technological and economical requirements, a fatigue assessment based on finite element calculations and material data should take this into account and should compare the varying local stresses with varying local strength values. From the authors point of view, this is not represented in commonly used standards sufficiently. However, e.g. [5] allows methods deviating from the standard that make use of additional information available to the user. This opens the door for an optimized fatigue assessment that takes into account local material properties. In this paper, a method was presented that uses local material properties gathered from testing of specimens taken from different areas of interest of series production parts. The obtained local material properties are correlated with local metallographic properties. From these, local acceptance gates for quality control can be derived. This method allows an optimized material utilization taking into account the local material properties that are matched with local strength requirements coming from finite element analyses. Within large components, usually only a small portion of the overall volume is loaded with high stresses and thus requires high strength values. For the rest of the volume, lower values are sufficient for a safe operation. Using experimentally validated local strength properties opens a wide range of possibilities to optimize material utilization. The need for more expensive materials or costly production steps like heat treatment can be evaluated for the specific application. A case study successfully using these methods was presented. Metallographic analyses of the tested specimens enable a correlation of the metallographic and strength results. From this, local acceptance criteria can be defined for quality control in series production. Using these local criteria can lower the reject rates in quality assurance, because inferior material properties and larger defects can be tolerated in regions with low stresses. For highly stressed regions, the risk of failure during service operation can be minimized taking into account all parameters that may influence fatigue strength in a negative way. Local metallographic analyses should be used extensively to validate the local material properties. However, since the results are two dimensional, the depth of information that can be derived is limited. New analysis methods like 3D computed tomography scans can be used to improve the understanding of the local microstructure. Further investigation should be performed to optimize and standardize the respective procedures and to correlate three dimensional findings to the static and cyclic strength properties. References [1] B. Hahn, K. Dursewitz and K. Rohrig, Reliability of Wind Turbines, pp. 329-332, 2007. [2] E. Sensen, Schäden an Windenergieanlagen aus Sicht des Versicherers, in VDI-Jahrestagung Schadensanalyse 36, Düsseldorf, VDI, 2010, pp. 137-174. [3] M. Decker, M. Eiber und S. Rödling, Herausforderungen an Schmiede- und Gußkomponenten in Windenergieanlagen, DVM Bericht 1681, pp. 9-18, 2014. [4] FKM, FKM-Richtlinie: Rechnerischer Festigkeitsnachweis für Maschinenbauteile, 5. Ausgabe Hrsg., F. M. (FKM), Hrsg., 2003. [5] Richtlinie für die Zertifizierung von Windenergieanlagen, Hamburg: Germanischer Lloyd, 2010. [6] J. Bergmann und T. Seeger, Über neue Verfahren der Anrißlebensdauervorhersage für schwingbelastete Bauteile auf der Grundlage örtlicher Beanspruchungen, Z. Werkstofftech. 8, pp. 89-100, 1977. [7] E. Siebel und M. Gaier, Untersuchungen über den Einfluss der Oberflächenbeschaffenheit auf die Dauerschwingfestigkeit metallsicher Bauteile, VDI-Zeitschrift 98, pp. 1715-1723, 1956. [8] K.-H. Kloos, Einfluss des Oberflächenzustandes und der Probengröße auf die Schwingfestigkeitseigenschaften, VDI Bericht 268, pp. 63-76, 1976. [9] J. Fröschl, Fatigue behaviour of forged components: Technological effects and multiaxial fatigue.
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