UNIVERSIDADE TÉCNICA DE LISBOA INSTITUTO SUPERIOR TÉCNICO FATIGUE BEHAVIOR OF DP600 STEEL PROCESSED BY WATER JET

Transcription

1 UNIVERSIDADE TÉCNICA DE LISBOA INSTITUTO SUPERIOR TÉCNICO FATIGUE BEHAVIOR OF DP600 STEEL PROCESSED BY WATER JET Adão José Kulazi (Graduate) Abstract of Master Theses in Mechanical Engineering Jury President: Prof. Nuno Manuel Mendes Maia Supervisor: Prof. Carlos Augusto Gomes de Moura Branco Co-supervisor: Prof. Virgínia Isabel Monteiro Nabais Infante Vowel: Prof. Edgar Luís Caramelo Gomes OCTOBER 2007

2 Abstract The industrial sectors as the automobile and the aeronautics have high losses associates to the life of mechanical components when this is not fulfilled due to premature rupture, occurring of abrupt form, contrary to the phenomenon s associates to the consuming who, general rule signal the loss of efficiency of continuous form. The main objective of the work consists of getting curves of project to the fatigue for fine plates in dual-phase steel (DP-600) in function of metallography, residual stresses and resultant superficial defects of the processes of cut for milling and water jet. The residual stresses of compression are beneficial for the fatigue strength even so are difficult in this in case that to quantify its influence due necessarily the existence of other factors that affect to the fatigue. The results show that depending on the conditions of cut to using the milling cut in such a way it can introduce residual stresses of tension or compression or still to cause modifications of the initial microstructure of the material, what won t happen with the cut for water jet. The obtained superficial finishing in the specimen in study for the two processes very leaves to desire and still they can be improved with the simple adjustment of the cut parameters what it would be of capital importance in the behavior to the fatigue of the specimen. In the analysis to the fatigue due to the strain hardening provoked for the milling relatively bigger fatigue strength in the specimen observes itself processed by milling. Key-words: Dual-phase steel, milling, water jet cut, fatigue, residual stress 1 Introduction The improvement of the fatigue strength in high cycle is of the interest in general of the industry of the sector metallomechanical. The industrial sectors as the automobile and the aeronautics have high losses associates to the life of mechanical components when this is not fulfilled due to premature rupture, occurring of abrupt form, contrarily to the phenomenon s associates to the consuming who, general rule signal the loss of efficiency of continuous form. We can cite as example, the serious accident of an aircraft of aloha Airlines in 1988 in the Hawaii when this if made the landing track lost part of its fuselage as consequence of the rupture for fatigue throughout a junction as if it concluded in the skill carried through in the aircraft [1]. Nowadays, the rupture for fatigue is a great concern in the projects of engineering in the whole world, the annual cost that the fatigue of materials imposes on the economy of the United States of America is about 3% of its Gross Domestic Product [2]. What of certain form it motivated the development of diverse materials and processes of attainment of the same ones with the purpose to minimize the element of surprise that normally happens when a material it breaches for fatigue. Dual-phase is steel that present one high yield strength, ductility and tensile strength (had to the high coefficient of strain hardening). It has a consisting microstructure of dispersed martensite in the ferrite matrix. They receive the 1

3 name from dual-phase for basically presenting the two well defined phases [3]. The Dual-Phase steel had appeared in the decade of 1970 and are composites, basically, for phases ferrite and martensite. They can be compared with HSLA steel (High Strength Low Alloy) with the advantage to have superior conformabilities and an excellent superficial finishing, due to absence of the phenomenon of discontinuous draining. The Dual-Phase steel constitute a promising commercial subject, especially in the industry automobile, since they allow to the substitution of components with many advantages, related with its resistance mechanics, life in fatigue, increase of ductility and reduction of the fragile rupture, when compared with the steel categories as, Resistant HSLA, Carbon Steel and Extreme Steel [4]. The main objective of the work consists of getting curves of project to the fatigue for fine plates in steel Dual Phase in function of the cut process, metallography, residual stresses and resultant superficial defects of the cut processes. To get a more convenient database for the cut surfaces that above contain the indicated 0 variable. In order to make possible the definition of the more convenient conditions of cut to get the maximum fatigue strength of the broken material being taken off of the advantageous conditions of the high fatigue strength of the material. 2 MATERIALS AND METHODS 2.1 Material The used material in this study is DP- 600 steel, that is a steel of average resistance, and is composed, basically, for phases ferrite and martensite, the plate was obtained by the rolling process the cold and has a thickness of 1.5 mm, for the extraction of the specimen (to see figure 2.1) of the plate the processes of cut for water jet and milling had been used. In table 2.1 it is had chemical composition of the main elements that constitute steel DP-600 and were obtained of the certificate of the material that was supplied by the manufacturer of the plate. Table 2.1 Chemical composition of the main elements of DP600 steel Chemical composition Steel % C % Si % Mn % P % S % N % Cr % Ni % Cu % Mo % Al % Nb DP In table 2.2 one has the mechanical properties DP-600 steel also obtained of the certificate of the supplier. Table 2.2 Mechanical properties DP-600 steel Mechanical Properties Steel σ 0.2 [MPa] σ R [MPa] ε R [%] DP

4 Figure Geometry of the specimen (quoted in mm) 2.2 Methods The assays had been carried through for tensions ratio of R = 0.1, with a frequency of application of loads of 8 Hz and with the gradual increase of the load (5 to 30%) in relation to the load of yield strength of the material to the traction for the different specimen. The measures had been carried through in the ISQ (Institute of Weld and Quality), through the technique of sen 2 Ψ. The parameters most excellent of these analyses had been: The incident radiation (Kα), the peak of analyzed diffraction (157 Faith), the angular variation 2θ (151 < 2θ < 160 ), the measure time (45 minutes for analyzed point, what it means 90 minutes for window), seven distributed angles ψ of 5, 10, 20, 25, 30, 35 and 40, with the use of a vanadium filter. For the study of the influence of the processes of cut in the DP600 steel microstructure, specimen, one had been removed of the central region of two obtained for the process of cut for water jet and the other obtained by milling and had been prepared in the inlaying press the hot one, after the inlaying the surfaces of the samples they had been sandpapered and after that polishing. One followed the chemical attacks with the following reagents: 2% nitric acid during fifteen seconds and after washed in water and alcohol followed an attack with the watery solution of metabisulphite of sodium. Also chemical attack with the picric acid reagent was carried through. The comments had been carried through with the metallographic microscope with a connected photographic machine that allows getting the images observed through the microscope. For analysis of finite elements in this study element SOLID95 of commercial software ANSYS was used as already it was related previously. The distribution of tensions in ¼ of the specimen was evaluated, that is, of the edge of cut until the center of the specimen. In a first phase loads of traction in elastic regimen had been applied to evaluate the distribution of tensions the half one of the specimen throughout its width in the absence of the residual stresses. 3

5 3 RESULTS In figure 3.1 we have the comparison of average curves S-N of the specimen processed by milling and water jet with abrasive Stresses Amplitude Sa (R=0.1) [MPa] 100 Water jet Average curve of the water jet Milling Average curve of the milling Number of Cycles Figure Comparison of curves S-N A figure 3.1 sample that stops one same number of cycles, the specimen processed by water jet assayed comparatively supports a lesser amplitude of tensions to the specimen processed for milling. E this must to a large extent had to the bad superficial finishing of the surfaces processed for water jet [5][6][7], and of the strain hardening tax in the surface of cut of the specimen processed by milling, what it increases the fatigue strength and minimizes of certain forms the effect of the bad superficial finishing of these [6][8]. The linear relation that exist between log (σ) and log (N) of the form: logn log A m log (3.1) Where: S σa, m is the declivity of the straight line and loga the commanded in the origin. To a describe S-N curve of project the equation (3.1) generally is represented in the form: m N A (3.2a) a 1 m A a (3.2b) N For the study cases we have the following results: S-N Curve for the test tubes processed by water jet (see Figure 3.1), a N (3.3) The equation (3.3) can be express in the logarithmic form for, log a logn (3.4) S-N Curve for the specimen processed for milling (see Figure 3.1), 4

6 a N (3.5) The equation (3.5) express in the logarithmic form is, log a logn (3.6) The figure 3.2 presents the distribution of the residual stresses throughout ½ of width of the specimen in the direction of rolling (longitudinal direction) of the plate and that it also corresponds to the direction of request the fatigue of the specimen. Residual stress in the longitudinal direction (rolling direction) Residual stress Sx [MPa] /2 of the width of the specimen [mm] Water jet F6 Water jet F4 Milling F2 Milling F14 Milling F15 Figure Distribution of the residual stress in the longitudinal direction Although terms a small difference in the distribution of residual stresses of compression in the relatively superior surface in the specimen processed by cut for water jet as shows figure 3.2, except in the specimen F2 processed for milling that presents a distribution of residual stress of compression sufficiently raised, because this specimen was processed in condition of cut different. The specimen processed by milling presents fatigue strength bigger than the specimen processed by water jet (to see figure 3.1). This must it the small difference in the distribution of the residual stress of compression in the surface between the two processes of cut in the majority of the specimen in study and the existence of other competing factors that increase or reduce the life to the fatigue in the materials processed by water jet or milling, what it becomes difficult individually to quantify with bigger severity the influence of the residual stress in the life the fatigue of this specimen [9][10]. The Figures the 3.3 and 3.4 in accordance with references [11][12], they respectively show to the microstructures of the surfaces of cut of the processed DP600 steel for jet of water and milling, the same ones are constituted of martensite (darker), pearlite (black) dispersed in a Ferrite matrix (clearer). 5

7 Figure 3.3 Microstructure of cut surface Figure 3.4 Microstructure of cut surface Processed by water jet, after the attack with processed by milling, after the attack with 2% nitric acid and Na 2 S 2 O 5, 400x 2% nitric acid and Na 2 S 2 O 5, 400x Although the rolling process the initial microstructure of the steel, presents the grains with formats and sizes little differentiated, as it can be seen in figures 3.5 and 3.6. They are observed, enters the ferrite grains, small pearlite regions. Figure Microstructure of the edge of cut of the specimen in the perpendicular surface to the surface of cut of the specimen, picric acid attack, 200x 6

8 Figure Microstructure of the central zone of the specimen in the perpendicular surface to the surface of cut of the specimen, picric acid attack, 100x Observing figures 3.3 the 3.6, notice that a great difference in the ratios of the existing phases in the microstructure of the cut surfaces and in the perpendicular surface to the surface of cut of the two processes does not exist. The cut for milling did not modify of significant form the microstructure of the steel in the cut surface, but it increased its resistance relatively mechanics, through the mechanisms of hardening for plastic deformation, from there its superior fatigue strength comparatively to the specimen processed for water jet as it shows figure 3.1. Figures 3.7 and 3.8 show the surface of cut of the specimen processed by milling and water jet. Figure 3.7 Cut surface processed by Milling Figure 3.8 Cut surface processed by water jet Since in the absence of significant internal defects, the ruptures start as if it knows, in the surface of the part or specimen where the fiction will be gone nuclear [5][7]. In figure 3.7 it is easy to identify in the cut surface the marks of the tool, that is, the direction of the marks of the tool with an orientation or periodic profile since the ridges have definite directions. In this case the ridges 7

9 introduce concentration of tensions and are the preferential places for the beginning of the fiction. This type of texture is predominant in the cut for milling and the choice of the type of tool and parameters of cut to use can define a greater or minor depth of the ridges in the material to cut, thus guaranteeing one better superficial finishing, what it provides a greater life to the fatigue. In figure 3.8 already it is more difficult to identify an only type of texture or defects, since, the cut surface presents random irregularities, such as holes, depressions, fictions and resultant risks of the random ticket of abrasives of cut of coarse granulometry in the material in the height of the processing of the cut, what it provided a bad superficial finishing in the specimen. The effect of the speed of translation and the type of used abrasive is important for the quality of the superficial finishing. Because a reduction of the translation speed originates a surface with better finishing whereas an abrasive of bigger hardness and minor diameter (granulometry) also originates one better quality of the cut surface [13][14]. Another parameter that has a considerable effect in the superficial finishing is the number of passages. With a bigger number of passages one better quality of finishing is associated [13][14]. Figures 3.9 and 3.10 present the comparison of the distribution of the tension in a specimen milling subject to a traction axial static request in the elastic linear regimen without and with inclusion of the distribution of the residual stresses determined experimentally. Figure Von Mises stresses distribution in the specimen F2 processed for milling with and without distribution of included residual stresses 8

10 Figure Von Mises stresses distribution in the specimen F4 processed for water jet with and without distribution of included residual stresses Observing figures 3,9 and 3,10, a reduction of the total distribution of tensions in the surface of the test tube in the zone in resultant study of the inclusion of the distribution of residual stresses of compression due is noticed the overlapping of the applied tension and the existing residual stress, what it implies that in the reality the request imposed to the specimen will be the difference (in the case of residual stresses of compression and requests of traction) between the applied tension and the existing residual stress in the specimen. What explanation the reason of the specimen in study alone to breach with the superior load application the load of yield strength of the material [10]. It is to point out that in the analysis for finite elements in this study the effect of the roughness in the surfaces of cut of the specimen was not considered. Depending on the degree of the roughness, this can significantly modify the results obtained in the analysis for finite elements. The consideration of this detail cannot take the catastrophic situations for the component or structure in the presence of residual stresses traction, data that the application of a tension in the elastic linear regimen can be converted into the reality in a request above of the tension of cession of the component or structure. 4 - CONCLUSIONS In the analysis to the fatigue a relatively greater resistance in the specimen observes itself comparatively obtained by milling to the specimen obtained by water jet and this difference in the fatigue strength must it the fact of the milling is provoked strain hardening in the surface of cut of the material thus increasing its fatigue strength. The presence of the residual stress of compression in the surface of the specimen benefits the same fatigue strength of, either the even so difficult to quantify with precision this influence due the existence of other factors that affect to the fatigue. If the cut for water jet does not introduce stresses in the material as it is cited in diverse literatures, then the results obtained in the measurement of the residual stresses in takes them to conclude that the cut for milling can introduce residual stresses of compression 9

11 (beneficial for fatigue) or residual stresses of traction (harmful for fatigue), depending on the parameters of cut to using. In the process of cut for water jet the microstructure does not suffer modifications to the step that in the process of cut for milling this already is not truth, since, in this process the greater or minor variation of the microstructure is widely on the cut conditions. In the cut conditions with that they had been obtained the test tubes in study we did not have significant variations in relation the initial microstructure of the material, even so has had a relative increase of the fatigue strength due to the mechanism of hardening for plastic deformation. How much to the superficial finishing, this parameter is of capital importance in the component citizens the requests of given fatigue that a bad finishing tends to speed up the nucleation of the fiction. The finishing s obtained in such a way in the milling as in the cut for water jet in this study although each process to present a characteristic surface, still can be improved, being enough for this some adjustments in the parameters of cut for milling, and the finer abrasive use and more tickets in the cut for water jet. REFERENCES [1] Electronic address: [2] N. E. Dowling, Mechanical Behaviour of Materials, Prentice Hall, 1 st Edition, New Jersey, [3] High-strength structural and high-strength low-alloy steels. : ASM, v. 1: Properties and selection: irons, steels and highperformance alloys pp [4] A. J. Abdalla, C. M. Neto, T. M. Hashimoto, W. A. Monteiro, Optimization of the Mechanical Properties of a Low-Carbon Steels by Formation of a Multiphase Microstructure, Congresso Internacional de Tecnologia Da Mobilidade, São Paulo: SAE do Brasil, [5] C.M. Branco, Mecânica dos Materiais, Fundação Calouste Gulbenkian, 4ª Edição, [6] Charles C.S. Yen, Metal Fatigue: Theory and Design, Angel F. Madayag, [7] J. A. Collins, Failure of Materials in Mechanical Design: Analysis Prediction Prevention, John Wiley & Sons, [8] Aditya Modgil, Effects of Speed Machining on Surface Topography of Titanium Alloy (Ti6Al4V), Thesis of Master of the University of Florida, [9] F.C.S. Cordovil, Desenvolvimento de um Dispositivo para Simulação Experimental de Tensões Residuais por Carregamento Mecânico, Tese de Mestrado da Universidade Federal de Santa Catarina, [10] Serope Kalpakjian, manufacturing processes for engineering materials, Addison- Wesley Publishing Company, 3 rd Edition, [11] Metals Handbook, Metallography and Microstructures, ASM Handbook Committee, Vol. 9, 9 th Edition. [12] Metals Handbook, Atlas of Microstructures of Industrial Alloy, ASM, Vol. 7, 8 th Edition, [13] M. Hashish, Experimental Studies of Cutting with Abrasive Waterjets, Flow Industries, Inc., Kent, Washington, [14] M. Hashish, Milling With Abrasive- Waterjets: A Preliminary Investigation, Flow Research Company, Kent, Washington,