ON FACTORS INFLUENCING FATIGUE PROCESS IN STEEL 316L USED IN HYDROGEN ENERGY TECHNOLOGIES

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1 Journal Donka of Chemical Angelova, Technology Rozina Yordanova, and Metallurgy, Tsvetelina 49, Lazarova 1, 2014, ON FACTORS INFLUENCING FATIGUE PROCESS IN STEEL 316L USED IN HYDROGEN ENERGY TECHNOLOGIES Donka Angelova, Rozina Yordanova, Tsvetelina Lazarova Department of Physical Metallurgy and Thermal Equipment University of Chemical Technology and Metallurgy 8 Kl. Ohridski, 1756 Sofia, Bulgaria donkaangelova@abv.bg Received 30 July 2013 Accepted 05 November 2013 ABSTRACT Investigations of fatigue in steels exposed to hydrogen media is extremely important problem. In this work, an austenitic stainless steel ASTM 316L resistant to hydrogen destructive influence is examined. The experiments presented have used hydrogen charged and uncharged specimens and were carried out under rotating bending and tension-compression fatigue in three different laboratories: at The University of Chemical Technology and Metallurgy, Sofia, Bulgaria; at Sandia National Laboratory, California and The University of Tufts, Medford, Massachusetts, USA; The Institute Hydrogenous at Kyushu University, Japan. The results are presented in Wöhler curves complemented by Short fatigue crack length Number of cycles curves and Frequency - Lifetimes plots, and compared respectively. Keywords: fatigue, hydrogen fatigue, stainless steel, Wöhler curve, short fatigue crack. INTRODUCTION Worldwide there are many investigations on one of most attractive alternative energy technologies, the hydrogen technology including hydrogen produce, and hydrogen storage and infrastructure. The main experimental results of these studies are summarized in the scheme shown in Fig. 1, which presents the main hydrogen technology applications. The scheme has been called Hydrogen Economy of the Future and its author is the well known Prof. Murakami, the Director of the Japanese Institute for hydrogen-economy research and development, HYDROGENIUS, at Kyushu University, Japan [1]. Based on the research related to application of hydrogen technology various types of fuel cells for electricity produce are constructed. In the recent years an intense development of automotive fuel cell technology led to usage of certain number of hydrogen vehicles in most big cities across the world. There are still many questions to be answered about the influence of hydrogen on metal and alloy s performance when those materials are used for hydrogen storage tanks and manufacturing of pipes and different members of elements for hydrogen transportation. The most frequently used alloys are steels, and especially some austenitic stainless steels. It is known that hydrogen affects the microstructure of steels changing the crystal lattice, which affects their mechanical properties and fatigue life. The phenomenon is known as hydrogen embrittlement, and plays a fundamental role in corrosion fatigue [2-5]. Classical theoretical studies of hydrogen influence on metals and alloys Of the many investigations on hydrogen embrittlement mechanism and corresponding models two basic models will be represented. Hydrogen-enhanced decohesion. The decohesion 29

2 Journal of Chemical Technology and Metallurgy, 49, 1, 2014 Fig. 1. Hydrogen Economy of the Future. model is one of the oldest models used to represent the change of properties as a result of atomic hydrogen. It was described first in 1941 by Zapffe and Sims [6]. It is based on the increased solubility of hydrogen in a tensile stress field, for example at the tip of a crack or in areas under internal tensile stresses or in the tensile field of edge dislocations (Fig. 2). The increased solubility of hydrogen in such a stress field results in decreased atom binding forces of metal lattice. The impact of stress leads to a premature brittle fracture along the grain boundaries (intergranular cleavage) or through the grains (transgranular cleavage). Hydrogen-enhanced localized plasticity. One of most recent models is so-called HELP (Hydrogen Enhanced Local Plasticity) model [6]. It presents the HELP mechanism described through dislocation movement initiated by external stresses. Hydrogen eases dislocation movement by shielding the dislocation stress field; the effect of some other lattice defects is taken into consideration too. The local drop of yield stress due to hydrogen causes local dislocation movement at low levels of shearing stress (Fig. 3). Sliding localization occurs, leading to a micro crack caused by the formation and merging of micro pores. Once the crack leaves the area of reduced yield stress, it stops propagating any further. The study of fatigue in steels exposed to hydrogen reveals the complicated nature of this research and turns it into important and topical issue. Our paper presents the results of research on fatigue behavior of steel 316L at different fatigue conditions, which will be used for future comparative analysis and characterization of fatigue behavior of the steel in hydrogen environment. Fig. 2. Schematic decohesion model. Fig. 3. Schematic HELP model. 30

3 Donka Angelova, Rozina Yordanova, Tsvetelina Lazarova Table 1. Chemical composition, tensile properties and Vickers hardness of Steel 316L. C Si Mn P S Cr Ni Cu N Mo R m, MPa R p0.2, MPa R p1, MPa Z, % A, % HV, MPa EXPERIMENTAL AND DISCUSSION Material and specimens. Initial studies of our team on hydrogen influence on metals are focused at obtaining of fatigue characteristics of the austenitic corrosionresistant Steel 316L, used for hydrogen infrastructure. The chemical composition and the mechanical characteristics of this steel are shown in Table 1. Fatigue test specimens of Steel 316L were machined in hour glass shape shown in Fig. 4. Experimental work and discussion. Fatigue tests were carried out at room temperature in laboratory air on a table model Rotating Bending Machine FATROBEM Its principal scheme including a corrosion testing box for environment-assisted short fatigue crack growth investigations is shown in Fig. 5. The applied fatigue loading conditions were chosen as follows: cyclic rotating-bending; testing frequency of 11 Hz; stress ratio R = 1; stress ranges of σ a = 260, 280, 300, 320, 340, 360, 380, 400, 440, 460 MPa [7]. Fig. 4. Geometry and dimensions of fatigue test specimens, in mm. Fig. 6. Wöhler curve of steel 316L under symmetrical rotating bending (own data). Fig. 5. Scheme of FATROBEM 2004: electric engine 1, driving belt 2, ball-bearing unit 3, testing box 4, specimen 5, device for loading 6, counter 7, device for circulation and aeration of corrosion agent 8. Fig. 7. R.R. Moore rotating beam fatigue testing machine. 31

4 Journal of Chemical Technology and Metallurgy, 49, 1, 2014 Table 2. Fatigue life at different stress range. σ a, [МРа] N f, [cycles] σ a, [МРа] N f, [cycles] The employed stress ranges σa and the registered corresponding fatigue lifetimes Nf are shown in Table 2 and presented in Fig. 6 as a basis for obtaining the Wöhler curve of the steel. The equation of the Wöhler curve is calculated and given in Eq.1: σ a = N f (1) The coefficients in Eq. (1) are calculated from experimental data processed by the least - squares method; the graphical presentation of the Wöhler curve is shown in Fig. 6. The results from the own experiments are compared with the results obtained by other authors using the same steel: Skipper from the University of Tufts, Medford, Massachusetts, USA [8]; Murakami from the Institute of HYDROGENIUS at Kyushu University, Japan [9-11]. Skipper examines the hydrogen influence on Steel 316L by two types of specimens: hydrogen charged and uncharged ones. The hydrogen charging of specimens is performed in the following sequence: Thermal precharging at 573K in 138 MPa hydrogen gas for more than 30 days; Storing hydrogen charged specimens in a freezer, before and after fatigue experiments for minimizing hydrogen loss; Keeping each charged specimen at room temperature for approximately an hour before fatigue testing; Measuring hydrogen content of each charged specimen by inert gas fusion at a commercial vendor. Fatigue testing was performed at room temperature on a R.R. Moore rotating beam fatigue testing machine (Fig. 7) at a frequency of 50 Hz. The stress range (σ a) in rotating beam fatigue tests is constant throughout the test. The number of cycles to failure (Nf) is determined by fracture of the specimen or when sufficient deformation precluded rotation. Murakami investigates the influence of hydrogen on Steel 316L using mainly two types of specimens: hydrogen charged and uncharged ones. The tests were carried out under the following conditions: Cathodic and gas environment hydrogen charging; Applying of special heat treatment Non-Diffusible Hydrogen Desorption Heat Treatment (NDH-HT) to some specimens for removing non-diffusible hydrogen reaching a level of 0.4 wppm; Drilling of small artificial hall with diameter and depth 100 µm into the specimens; Tension-compression fatigue testing at stress ranges 260 and 280 MPa, stress ratio R= 1, and frequencies f = , 1.5 and 5 Hz; Surface replicating of short fatigue crack growth. A comparative analysis between our own results and those obtained by Skipper and Murakami for Steel 316L can be made considering the Wohler curves shown in Fig. 8. uncharged specimen lifetimes - the a-n curve located at the far right in Fig. 8, and the single round symbol in Fig. 9. Murakami considers the fact that this non-diffusible hydrogen has not been considered in the 32

5 Donka Angelova, Rozina Yordanova, Tsvetelina Lazarova Fig. 8. Wöhler curves of steel at different schemes of fatigue loading and test frequencies. previous classical hydrogen embrittlement studies. In both cases of 260 MPA and 280 MPa (Fig. 9) the hydrogen charged and uncharged specimens of Steel 316L show almost the same lifetimes at lower frequencies (from Hz to 5 Hz) and smaller lifetimes for the hydrogen charged specimens at higher frequencies, at above 5 Hz. It means that the frequency is the most important factor of influence for alloys fatigue at hydrogen media. We should note as well that the fatigue loading Fig. 9. Influence of hydrogen on fatigue lifetimes at different test frequency. condition after 5 Hz changes from tension-compression to rotating-bending. CONCLUSIONS The alternative hydrogen-energy technology still shows many unsolved problems connected with hydrogen tank storage and hydrogen infrastructure. Now it becomes clear that hydrogen-immuned metals and alloys can be very vulnerable at some fatigue conditions. Fre- 33

6 Journal of Chemical Technology and Metallurgy, 49, 1, 2014 quency is one of the most important factors for fatigue lifetime which in combination with hydrogen media at high pressure leads to microstructure transformations diminishing life of metal members. This show that our classical notion for hydrogen influence on metals and alloys must to be revised in connection with various metal applications; on the whole more deep knowledge is needed for clarifying the studied hydrogen phenomenon in different alloys and materials. REFERENCES 1. Y. Murakami, The effect of hydrogen on fatigue properties of metals used for fuel cell system, International Journal of Fracture, 138, 1-4, 2006, N. Dowling, Mechanical Behavior of Materials, Prentice- Hall, New Jersey, USA, 1999, R. Yordanova, D. Angelova, Plastic Deformation and Fracture of Metals. Physical Basis and Technological Descriptions. Methods for Mechanical Testing, Textbook for laboratory seminars and experimental work, UCTM, Sofia, 2009, р. 284, (in Bulgarian). 4. K.J. Miller, Metal fatigue past, current and future, Twentyseventh John Player lecture, Proc. Inst. Mech. Engrs., London, D. Angelova, D. Kolarov, К. Philipov, R. Yordanova, Metal Science and Metal Forming, Handbook, Part 5, Scientific editor Prof. D. Angelova, Academic publishing house М. Drinov, Sofia, 2009, р. 348, (in Bulgarian). 6. A. Barnoush, Hydrogen embrittlement, December, 2011, p. 9-44, ( 7. Z. Todorova, D. Angelova, R. Yordanova, S. Yankova, Investigation of Fatigue Properties of Steel 316L Used in Hydrogen Economy, Scientific Proceedings, Year XX, No 1(133), June 2012, pp C. Skipper, G. Leisk, A. Saigal, D. Matson, C. San Marchi, Effect of Internal Hydrogen on Fatigue Strength of Type 316 Stainless Steel, Proceedings of International Hydrogen Conference (ASM International), July 2009, pp Y. Murakami, Metal Fatigue: Effects of Small Defects & Nonmetallic Inclusions, Elsevier, Ox., UK, Y. Murakami, T. Kanezaki, Y. Mine, Hydrogen Effect against Hydrogen Embrittlement, Metallurgical and Materials Transactions, A, Vol 41A, October 2010, pp Y. Murakami, Effects of Hydrogen on Fatigue Crack Growth of Metals, Proceedings of 17th European Conference on Fracture, September 2008, Brno, Czech Republic, pp