effect of cadmium coating on the fatigue strength of …

Proceedings of the 7th International Conference on Mechanics and Materials in Design

Albufeira/Portugal 11-15 June 2017. Editors J.F. Silva Gomes and S.A. Meguid.

Publ. INEGI/FEUP (2017)

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PAPER REF: 6815

EFFECT OF CADMIUM COATING ON THE FATIGUE STRENGTH

OF AISI 4140 STEEL USED IN OIL AND GAS COMPANY

Jefferson R.M. Santos1(*)

, Herman J.C. Voorwald2

1PhD Student (UNESP), São Paulo State University, Guaratinguetá, Brazil

2Materials Department (UNESP), São Paulo State University, Guaratinguetá, Brazil

(*)Email: [email protected]

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 (σmax) up

to 780 MPa, stress ratio R=0.1 and frequency 20 Hz have been applied. For a given σmax, 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 104 - 10

6 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 3

Topic-C: Fatigue and Fracture Mechanics

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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

0.38 0.75/1.00 <0.035 <0,04 0.15/0.35 0.8/1.10 0.15/0.25 <0.05 0.0005 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.

Fig. 1 - Testing Device (dimensions in millimeter, except thread)

Fig. 2 - Axisymmetric Finite Element Mesh

STUD

UNC 1/4-20 TPISLEEVENUT

76.2

27.0 27.0


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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


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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)


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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


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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.

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effect of cadmium coating on the fatigue strength of …

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