EFFECT OF CADMIUM COATING ON THE FATIGUE STRENGTH OF AISI 4140 STEEL USED IN OIL AND GAS COMPANY

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1 Proceedings of the 7th International Conference on Mechanics and Materials in Design Albufeira/Portugal June Editors J.F. Silva Gomes and S.A. Meguid. Publ. INEGI/FEUP (2017) PAPER REF: 6815 EFFECT OF CADMIUM COATING ON THE FATIGUE STRENGTH OF AISI 4140 STEEL USED IN OIL AND GAS COMPANY Jefferson R.M. Santos 1(*), Herman J.C. Voorwald 2 1 PhD Student (UNESP), São Paulo State University, Guaratinguetá, Brazil 2 Materials Department (UNESP), São Paulo State University, Guaratinguetá, Brazil (*) jffrodrigo@yahoo.com.br ABSTRACT The influence of electrodeposited coating of cadmium on AISI 4140 steel has been studied at room temperature in axial fatigue. Axial fatigue testing was performed using studs at base metal and cadmium coated conditions. Oscillating stresses with maximum stresses (σ max ) up to 780 MPa, stress ratio R=0.1 and frequency 20 Hz have been applied. For a given σ max, the lifetime decrease for threaded samples and even more when cadmium coating is applied. The reason is explained by a stress concentration induced at the thread, shown herein by Finite Element Method, be intensified by crack propagation at electrodeposited cadmium. Scanning Electron Microscope was used to show the fracture surfaces for both cadmium coated and base metal studs where is possible to identify many cracks at the coated surface. Keywords: fatigue, AISI 4140 steel, cadmium coating, oil and gas. INTRODUCTION The Oil and Gas equipment use a large quantity of studs to resist internal pressure up to 22,500 psi and any fail could be catastrophic for environment, people and company. To withstand the load due to pressure and avoid leakage the studs are pre-loaded by torque application on nuts to create tensile stress around 362 MPa that could reach 600 MPa during working or testing condition. For equipment used in sea water, oxidation is also an issue and to prevent studs from corrosion, electrodeposited coating of cadmium is used and therefore, fatigue lifetime will be decreased in comparison to base metal due to the induced crack at coated surface and also due to the hydrogen embrittlement possibility, induced by the electrodeposited process coating. Studs used on shop floor for testing purpose is essential to quantify the number of cycles under worst condition to avoid catastrophic fail as already happen in the past, causing injury and death. In this paper the dependence of axial fatigue life was analyzed for studs made of AISI 4140 steel in lab environment considering cadmium coating and base metal in the range of cycles. MATERIALS AND METHODS The studs used in the tests are made of AISI 4140 steel produce by Electric Arc Furnace and the chemical composition is shown on Table 1. The thread was manufactured by rolling process that creates a compressive stress on the surface, increasing the fatigue lifetime in comparison to machined threads. After manufactured, the specimens were submitted to quenching (900 C for 3 hours, cooling in oil to 81 C) and double tempering (620 C for

2 Topic-C: Fatigue and Fracture Mechanics hours, cooling in calm air), reaching 860 MPa yield strength, 978 MPa ultimate stress and 202 GPa elasticity modulus. For the coated specimens, 13µm of cadmium was applied and dehydrogenation treatment was performed (200 C for 8 hours) to avoid hydrogen embrittlement. Table 1 - Chemical composition of AISI 4140 steel (wt. %) C Mn P S Si Cr Mo V B Fe /1.00 <0.035 <0, / / /0.25 < Base To perform the test was used the device shown on Fig. 1, where the sleeves were fixed to the testing rig. To quantify the stress concentration, a finite element analysis was performed considering an axisymmetric finite element mesh, as shown on Fig. 2 and friction 0.13 was considered at contact pairs, reflecting the real system where no grease or oil was used NUT STUD UNC 1/4-20 TPI SLEEVE Fig. 1 - Testing Device (dimensions in millimeter, except thread) Fig. 2 - Axisymmetric Finite Element Mesh -512-

3 Proceedings of the 7th International Conference on Mechanics and Materials in Design RESULTS On Fig. 3 is shown the fracture surface from a stud base metal, broken after 326,092 cycles under oscillating stress with maximum stress (σmax) 516 MPa, stress ratio R=0.1 and frequency 20 Hz. Fig. 3 - Surface fracture from a base metal stud On Fig. 4 is shown the fracture surface from a cadmium coated specimen, broken after 76,406 cycles under oscillating stress with maximum stress (σmax) 516 MPa, stress ratio R=0.1 and frequency 20 Hz. Fig. 4 - Surface fracture from a cadmium coated stud -513-

4 Topic-C: Fatigue and Fracture Mechanics On Fig. 5 is shown the S-N curve for both base metal and cadmium coated studs. Fig. 5 - S-N Curves On Fig. 6 is shown the von Mises stress for a situation where tensile stress is 516 MPa. Fig. 6 - von Mises stress (MPa) -514-

5 Proceedings of the 7th International Conference on Mechanics and Materials in Design On Fig. 7 are shown some studs made of base metal broken after testing. Fig. 7 - Tested base metal studs On Fig. 8 are shown some studs cadmium coated broken after testing. Fig. 8 - Tested cadmium coated studs -515-

6 Topic-C: Fatigue and Fracture Mechanics CONCLUSIONS On Fig. 4 was identified the influence of cadmium coating by the increased crack nucleation in comparison to the base metal stud shown on Fig. 3. On Fig. 5 are shown the S-N curves and for maximum allowable stress 600 MPa is possible to identify the fatigue lifetime decrease from about 62,000 cycles to about 57,000 cycles that represents 8% lifetime reduction when the studs are cadmium coated. For lower cycles is possible to identify the influence of the coating on fatigue lifetime decrease and probably can be neglected for static condition. On Fig. 7 and 8 are shown some broken specimens at the region predicted by finite element analysis, shown on Fig. 6, where the gray region identifies von Mises stress higher than 516 MPa, representing the effect of both tensile, bending and shear stress at contact between stud and nut. ACKNOWLEDGMENTS The authors gratefully acknowledge all support from São Paulo State University (UNESP) and the donated specimens by Onesubsea, Prec-Tech and CPF companies. REFERENCES [1]-B.Vargas-Arista, J.Teran-Guillen, J.Solis, G.García-Cerecero, M.Martínez-Madrid, Normalizing effect on fatigue crack propagation at the heat-affected zone of AISI 4140 steel shielded metal arc weldings. Mat. Res. 2013, vol.16, n.4, p [2]-Chapetti, M., Estimation of the plain high-cycle fatigue propagation resistance in steels. Mat. Res. 2002, vol.5, n.2, p [3]-Fragoudakis, R.; Karditsas, S., Savaidis, G.; Michailidis, N., The effect of heat and surface treatment on the fatigue behavior of 56SiCr7 spring steel. XVII International Colloquium on Mechanical Fatigue of Metals (ICMFM17), Procedia Engineering 74 (2014) p [4]-Marcelino, P. Nascimento, Marcelo A.S. Torres, Renato C. Souza, Herman J.C. Voorwald, Effect of shot peening pre heatment on the fatigue behavior of hard chromium on electroless nickel interlayer coated AISI 4340 aeronautical steel. Mat. Res. V.5, no.2 [5]-Miguel, I. M.; Amorim, C. E. S.; Peres, M. P.; Voorwald, H. J. C., Study of influence of zinc-nickel and cadmium electroplated coating on fatigue strength of aeronautical steels. In: Fatigue 2002, 2002, Stockholm. Proceeding of Eighth International Fatigue, v.4, p [6]-Wittenberghe, J. V.; Pauw, J., Baetes, P.; Waele, W. Wahab, M. A.; Roeck, G., Experimental determination of the fatigue life of modified threaded pipe couplings. In: Fatigue 2010, Procedia Engineering 2 (2010) p