Fatigue Analysis of Clad and Unclad Thick-Wall Structures

It is common practice to clad steel components with a relatively thin layer of a stainless material to prevent corrosion economically. Little, however, has been published regarding the effect of such cladding on fatigue published regarding the effect of such cladding on fatigue life in areas of localized high stress. Large valves that are pressure-cycled often and offshore equipment, such as pressure-cycled often and offshore equipment, such as risers, tensioners, and wellhead flanges that are loaded cyclically by ocean currents and waves, must be analyzed for fatigue life during design. Unlike storage vessels, drilling and completion hardware generally has areas of relatively high stress concentrations because of abrupt section size changes, threads, grooves for seals, bolt holes, and other stress-concentrating geometries. While yielding or rupturing is a function of bulk stresses, fatigue life is a function of peak stresses, which typically are highest on the surface of an area of stress concentration. It has been determined that both the metallurgical characteristics of the cladding and the pressure/load history can be varied to enhance or diminish significantly the fatigue life of a clad steel component. The results and conclusions of this study are based on laboratory studies. Axial fatigue tests (R=0.05) were performed using a side-notched fatigue specimen that produces combined axial and bending stresses in the notched area. Specimens of AISI 4130 (dt HRc 20) were tested unclad and with the notched area clad with Inconel 625 or AISI 316L. Each set of specimens was tested both unpreloaded and preloaded to produce localized yielding at the notched surface only.The findings of this study are applicable to components subject to failure by fatigue and corrosion fatigue and sour service steel components that become locally work-hardened either in service or during overload proof testing as required by most API specifications. Introduction Fatigue failure of a homogeneous, unflawed metal occurs in two stages:nucleation of a stable crack andcrack growth until failure occurs. The nucleation portion is the result of alternating strain of a magnitude portion is the result of alternating strain of a magnitude sufficient to cause the formation and the coalescence of dislocations to form a crack. Crack growth can be predicted by fracture mechanics techniques. predicted by fracture mechanics techniques. Although fatigue curves often are plotted with alternating stress on the abscissa and cycles to failure on the ordinate, it is actually the cyclic strain that determines fatigue life. Fatigue prediction methods therefore must relate calculated stresses to cyclic strain. Stress vs. strain relationships are complex and include at least the following variables: part geometry, grain size, microconstituents, cold working coefficient, direction of forces, magnitude of forces, and strength and modulus of the material. It is seen that fatigue is associated strongly with the metallurgy of the materials being tested. Purpose of Study Purpose of Study The purpose of this study was to develop data and evaluate an analytical technique to predict fatigue life of thick-wall, clad and unclad, pressure vessels in the long- and short-cycle fatigue mode. More specifically, data were generated to simulate high-pressure wellhead equipment fabricated from quenched and tempered low alloy steels. Claddings studied were the austenitic-nickel-base Inconel 625 and iron-base AISI 316L. Both these cladding materials have substantially different metallurgical properties from those of low alloy steel. Since fatigue failures generally result from peak surface stresses, nucleation of fatigue cracks will occur in the cladding. The cladding therefore controls the fatigue life of the vessel since crack nucleation comprises the majority of the total cycles compared to crack growth. SPEJ P. 151