Study of the Effect of Boundary Conditions on Residual Stresses in Welding Using Element Birth and Element Movement Techniques

The structure in which the welding process is performed highly affects the residual stresses generated in the welding. This effect is simulated by choosing the appropriate boundary conditions in modeling the welding process. The major parameters of the boundary conditions are the method by which the base metal is being fixed and the amount of heat being applied through the torch. Other parameters may include the coefficients of thermal heat loss from the plate which may simulate the media in which the welding is taking place. In modeling the welding process, 2D forms of approximation were developed in analyzing most of the models of such problem. 3D models analyzing the welding process were developed in limited applications due to its high computation time and cost. With the development of new finite element tools, namely the element movement technique developed by the authors, full 3D analysis of the welding process is becoming in hand. In the present work, three different boundary conditions shall be modeled companng their effect on the welding. These boundary conditions shall be applied to two models of the welding process: one using the element birth technique and the other using the element movement technique showing the similarity in their responses verifying the effectiveness of the latter being accomplished in a shorter time.

Fitness-for-Service and Remaining Useful Life Assessment of a Steam Drum

A steam drum in a typical power plant has experienced in-service cracking. Nondestructive examinations (NDE) were performed and a small sample was collected from the drum to evaluate the extent of the cracking that had occurred in the drum shell. Fitness-for-service and remaining useful life analyses of the drum were performed based on the NDE results and operating conditions. In this paper, the fitness-for-service analyses of the steam drum are described. The analysis procedure, material property determination, stress analysis, limiting flaw size evaluation, and remaining useful life evaluation for the drum are discussed. Recommendations for appropriate action are also presented.

External Weld Overlays for Repair of Pressure Components

External weld metal deposit overlays have been successfully implemented in industry as both temporary and permanent repair for the restoration of thinning or degraded steel piping. Pressure components systems suffer from numerous degradation mechanisms, including microbiologically influenced corrosion (MIC), erosion-corrosion damage (EC), fatigue, and general corrosion. The magnitude of the damage induced in the component determines whether a weld overlay repair can be successfully applied to restore the component’s integrity. This paper addresses the use of weld overlays for repair of pressure components degraded by wall thinning due to corrosion, erosion-corrosion, MIC and other mechanisms.

Three Turnaround Heat Treating Studies

Three vessel modifications requiring heat treatment were analyzed prior to and during a planned turnaround at a refinery. One was a thick nozzle that required weld build up. This nozzle had been in hydrogen service and required bake-out to reduce the potential for cracking during the weld build up. Finite element analysis was used to study the thermal stresses involved in the bake-out. Another heat treatment studied was a PWHT of a nozzle replacement. The heat treatment band and temperature were varied with location in order to minimize cost and reduction in remaining strength of the vessel. Again, FEA was used to provide insight into the thermal stress profiles during heat treatment. The fmal heat treatment study was for inserting a new nozzle in a 1-1/4Cr-1/2Mo reactor. While this material would ordinarily require PWHT, the alteration was proposed to be installed without PWHT. Though accepted by the Jurisdiction, this nozzle installation was ultimately cancelled.

Analysis of Non-Uniformly Degraded Pipe Sections

The management of flow assisted corrosion (FAC) has been a part of the maintenance of piping in nuclear power plants for more than 15 years. Programs have been set up to identify vulnerable locations, perform inspections, characterize the degraded configurations, and evaluate the structural integrity of the degraded sections. The section of the pipe is repaired or replaced if the structural integrity cannot be established for the projected degraded section at the next outage. During the past 15 years, significant improvements have been made to every aspect of the program including structural integrity evaluation. Simplified methods and rules are established in ASME Section XI code and in several code cases for verifying structural integrity. The evaluation of structural integrity is performed during the plant outage prior to a decision for repair or replacement. Any improvement in structural integrity evaluation to extend the life of a component by one additional operating cycle can help in performance of repair/replacement of component in a planned manner. Simplified methods and rules provided in the code can be easily used for analysis of pipe sections with degraded area with uniform wall thickness and for non-uniformly degraded sections, provided the degraded portions are modeled with uniform wall thickness equal to the lowest thickness of the section. The representation of a non-uniformly degraded section in this manner is necessarily conservative. The purpose of this paper is to develop methodology to analyze the non-uniformly degraded sections subjected to pressure and moment loading by modeling it in a manner that accounts for the non-uniform cross-section. The formulation developed here is more realistic than the code methodology and is still conservative. The results are presented in form of charts comparing the limit moment capacity of the degraded sections calculated by the formulation in this paper with that using ASME code formulation. The paper concludes that the proposed formulation can be used to supplement the ASME Code method to extend the remaining life of FAC degraded components.

Predictions of Residual Stresses in the Mechanical Roll Expansion of HX Tubes Into TEMA Grooves

The mechanical roll expansion of heat exchanger tubes into tubesheets containing TEMA grooves, which are thought to aid in the mechanical integrity of the tube-to-tubesheet (TTT)joint, has for many years provided an acceptable means of completing a TTT joint. Inherent with the intentional roll expansion of the tube is the creation of a tensile residual stress field within the tube that is greatest in the transition region between the expanded and unexpanded zones of the tube. An additional complicating factor in the tube-to-tubesheet joint design is the choice of utilizing a seal weld or a “full strength” weld at the tube end in conjunction with a level of roll expansion quantified by the degree of tube wall reduction. This paper presents the results of an initial study of the mechanical roll expansion of 1 inch diameter tubes into a typical TEMA-R designed tubesheet, utilizing two grooves in the tubesheet hole. Two combinations of tube and tubesheet materials are studied that include duplex stainless steel tubes and tubesheet, while the second combination includes type 321 tubes roll expanded into a 2-1/4 Cr-1 Mo tubesheet, clad with 321 SS overlay. The predicted residual stress fields are calculated by the finite element method and employ a simplified two dimension nonlinear axisymmetric model.

Evaluation of Short Term Creep-Rupture Strength of High Temperature P122 Boiler Material

The complex thermal-mechanical loading of power-generating plant components usually comprises of creep, high-cycle and low-cycle fatigue which are thermally induced by start-ups, load changes and shut-downs, producing instationary temperature gradients and hence creating strain as well as stress fields. In order to select the correct materials for these hostile environmental conditions, it is vitally important to understand the behaviour of mechanical properties such as creep rupture properties of these materials. This paper reports the results of standard creep rupture tests of P122 (HCM12A or 12Cr-1.8W-1.5Cu) high temperature boiler material. P122 is one of the latest developed materials for high temperature environments, which has the potential to be successful in hostile environments. The tests were conducted at temperatures ranging from 550°C to 700°C at 50°C intervals with stress levels ranging from 80–400 MPa using a locally made creep rupture testing machine. The results are found to have stable creep-rupture strength at short term creep stage for over 800-hours at elevated temperatures. Creep life prediction from Larson-Miller relationship was also carried out and the accuracy of life prediction is demonstrated. Moreover, the fracture mode assessments strongly revealed a typical ductile transgranular fracture mode with dimples and voids.

Tensile Properties of Boiler Materials for FAD Based Failure Analysis

Failure assessment of components with crack-like flaws are performed using the Failure Assessment Diagram (FAD) approach. Both fracture toughness and tensile properties of the component material are required for the analysis. The temperature dependence for the tensile properties of yield strength, flow stress, and ratio of yield strength to ultimate tensile strength were studied. The yield/tensile ratio is used to estimate the strain hardening exponent. The materials include carbon steel, 1/2Cr to 9Cr-1Mo steels, 1CrMoV rotor material, and 422 stainless steel bolting material. The tensile data was from the NRIM data compilation. Least squares regression was performed for log of the tensile property as the dependent variable and a sixth order Chebyshev polynomials in temperature. In general, the fits were good based on comparisons of the observed and calculated, specifically the mean at each temperature, the standard deviation, and the R2 value. The distribution function was shown to be normal. Values for the regression coefficients and standard deviation in log Y are tabulated.

Safety Assessment of the Junction Between a Thick Pressure Vessel Shell and a Thinner Hemispherical Head

A junction of shell to hemispherical head whose thickness is half of that of the shell has been developed and safely used in the People’s Republic of China in the past ten years. Great achievements of mesomechanics in ductile fracture have been made in P. R. China. The load when local shear band prongs the ligament in the front of crack tip is regarded as the limit load and the direction of the maximum plastic strain along a constant radius around the crack tip might be considered as the direction of shear fracture. Based on these concepts and small-deformation elastic-plastic analysis with finite element method, safety of the junction under operating pressure, 31.4MPa, is assessed and the possible failure modes of it are forecasted. The reasonability of the design of the junction is also considered.

Corrosion Fatigue Failure Analysis and Life Prediction

A metallurgical failure analysis and life prediction was performed for an economizer tube. The tube failed after approximately nine years of service. The failure was a pin hole leak, elliptically shaped with the long axis in the circumferential direction. On the inside surface of the tube, there were several circumferential cracks and numerous oxygen pits on the top half. The cracks were transgranular and initiated at pits. The failure mechanism is corrosion fatigue and is believed to be due to a cyclic applied bending moment. Life predictions were performed using two crack geometries and zero-tension loading cycle. The geometries were a thumbnail shaped ID crack with an a/c of 0.2 and a 360° ID cracked cylinder. A parametric approach was used with two initial crack depths based on the measured pit depths and three remote stresses centered around the minimum yield strength of the SA-178-A tube material. The failure criteria was the reference stress equal to the flow stress. For the thumbnail crack geometry at a remote load of 179 MPa (equal to minimum specified yield strength), the calculated lives were 15,960 cycles for an initial crack depth of 5% and 3,450 cycles for an initial crack depth of 10%. The cyclic lives of the 360° crack geometry were approximately half of those for the thumbnail crack geometry. The slope of the log life-log stress curve was approximately −5.8.