Effect of Boiler Operating Practice on Circumferential Crack Growth in Weld Overlays

REPORT SUMMARY The gas metal arc welding GMAW process, used to mitigate fireside corrosion on waterwalls due to Low NOx Burner operation in pulverized coal units, results in residual str

Effect of Boiler Operating Practice on Circumferential

Crack Growth in Weld Overlays

Finite Element Modeling of Circumferential Cracking

1014248

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Effect of Boiler Operating Practice on Circumferential

Crack Growth in Weld Overlays

Finite Element Modeling of Circumferential Cracking

1014248 Technical Update, August 2007

EPRI Project Managers

A Facchiano

S Cardoso

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC (EPRI) NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

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ORGANIZATION(S) THAT PREPARED THIS DOCUMENT

Welding Services, Inc

Electric Power Research Institute

This is an EPRI Technical Update report A Technical Update report is intended as an informal report of continuing research, a meeting, or a topical study It is not a final EPRI technical report

CITATIONS

This document was prepared by

Welding Services, Inc

2225 Skyland Court

Norcross, GA 30071

Principal Investigator

T Scandroli

Wate T Bakker, Consultant

9011 Village View Drive

Effect of Boiler Operating Practice on Circumferential Crack Growth in Weld Overlays Finite Element Modeling of Circumferential Cracking; EPRI, Palo Alto, CA: 2007 1014248

REPORT SUMMARY

The gas metal arc welding (GMAW) process, used to mitigate fireside corrosion on waterwalls due to Low NOx Burner operation in pulverized coal units, results in residual stresses in weld overlays that interact with stresses that occur during boiler operations This project used finite element (FE) analysis to model the residual stresses in weld overlays made of several alloys applied to waterwall tubes and to assess how thermal stress cycles during periodic sootblowing may affect crack growth in these components

Background

During the heating and cooling cycles that occur during the welding of boiler components, thermal strains in the metal and the base metal regions near the weld eventually produce a residual stress distribution in the product These stresses interact with the stresses that occur during boiler operation and may affect crack initiation and growth

Objectives

• To model the residual stresses resulting from GMAW welding of weld overlays in waterwall tubes

• To assess the effect of thermal stress cycles caused by periodic sootblowing on crack growth

in weld overlayed tubes

Approach

The project team developed a generalized plane strain FE model to simulate the weld overlay process as performed by gas metal arc welding (GMAW) on several alloys The team used Abaqus FE code to determine the thermal and mechanical stress conditions that would occur because of the periodic shedding of slag that results from sootblowing The team assessed the effects of these stresses on fatigue crack growth for the combinations of base and weld overlay metals previously modeled

8,000-14,000 psi, depending on the weld overlay material

In the temperature range most likely experienced by weld overlays, 1000-1050°F (538-566°C), Alloy 622 is predicted to be 2 to 2.5 times more resistance to cracking than Alloy 309 This result agrees qualitatively with EPRI’s Laboratory Corrosion Fatigue Tests (EPRI reports

1009618 and 1012383) The model predicts that Alloy “Nimonic 86” should be about twice a resistant to cracking as Alloy 622 Unfortunately this alloy is not available as weld wire suitable for GMAW welding at this time

EPRI Perspective

The foundation of this work was the analysis of the residual stress state after the welding

process This evaluation is important for a successful study of the stress conditions during

operation of a boiler In-service stresses such as the cyclic thermal stresses caused by

sootblowing are critical because they can either assist or prevent the initiation and/or propagation

of cracks under such mechanisms as stress-corrosion cracking or thermal fatigue

ABSTRACT

The thermo mechanical modeling work presented in this report indicates that there are

considerable residual tensile stresses in weld overlays or waterwalls prior to service Modeling of service conditions assuming thermal cycles due to slag build up and removal by sootblowing indicates that there will be some stress relief due to operating temperature, but that considerable tensile stresses remain The model predicts that thermal stress cycles caused by slagging and de-slagging are high enough to cause crack propagation in most commonly used weld overlay materials, especially if surface defects, possibly caused by corrosion, are present The model predicts that the thermo-mechanical crack resistance of Alloy 622 is 2 to 2.5 times higher than that of Alloy 309 at 1000-1050°F (538-566°C)

3 THERMAL CYCLING SIMULATION 3-1

Surface Temperature Excursions 3-1 In-Service Stress State 3-5

4 THERMAL FATIGUE CRACK GROWTH ASESSMENT 4-1

FE Formulation 4-1 Stress Intensity Determination 4-2 Flaw Growth Analysis 4-8 Results and Discussion 4-8

5 MODEL REFINEMENTS AND SENSITIVITY STUDIES 5-1

Model Refinements 5-1 Sensitivity Studies 5-2

6 DISCUSSION AND CONCLUSION 6-1

Discussion 6-1 Conclusions 6-2

7 REFERENCES 7-1

1

INTRODUCTION

The physics of a welding process are very complex The welding arc produces very high

temperatures, instantaneously melting the material in its vicinity The heat input is defined by welding parameters that combine with heat losses, are functions of the type of materials used, geometry and size of the workpiece and fixturing; they determine the fusion-zone size and the intensity of thermal gradients during cooling

During the heating and cooling cycles while welding, thermal strains occur in the metal and the base metal regions near the weld The strains produced during heating are accompanied by plastic upsetting The stresses resulting from the strains combine and react to produce internal forces, causing shrinkage of the material Depending on the shrinkage pattern, various structural deformations such as bending, buckling and rotation take place These deformations finally produce the residual stress distribution in the product

The foundation of this work was to analyze the residual stress state after the welding process; this evaluation is important for a successful study of the stress conditions during operation of a boiler The in-service stresses are a crucial factor because they can either assist or prevent the initiation and/or propagation of cracks under various mechanisms, such as stress-corrosion cracking, thermal fatigue, or a combination of these

A generalized plane strain FE model was developed to simulate the weld overlay process

performed in one layer – gas metal arc welding (GMAW) Thermal and mechanical analyses were performed using Abaqus FE code

Overview of the Welding Process

The welding process consists of a multi-pass application applied in the longitudinal direction of the waterwall panel producing a single layer The weld metal layer is made with a single

electrode by the automated GMAW process with filler material The width of the weld bead is approximately 0.375 of an inch, which indicates that a special welding technique – electrode weaving – is used to achieve the desired weld width The tube is water-cooled during welding, and preheat is not applied Table 1-1 shows the welding parameters adequate for modeling

Finite Element Analysis

Complex numerical models are required to thoroughly model the physics of the welding process

In the thermal analysis one should account for 1) conductive and convective heat transfer in the weld pool, 2) convective, radiative and evaporative heat losses at the weld pool surface, and

3) heat conduction into the surrounding solid material, as well as the conductive and convective heat transfer to the ambient through cooling substances surrounding the panel Furthermore, one needs to account for temperature-dependent material properties, the effect of latent heat of

fusion, as well as the material phase transformation effects Capturing all of these effects, results

in a complex FE model However, some effects may considerably complicate the analysis and

yet have negligible effects on the final result Therefore, one’s best judgment should be used in such cases to decide on simplifications and assumptions for establishing an effective and yet

reasonably accurate FE model

A generalized plane strain model was considered because of the longitudinal welding direction This assumption considerably simplifies the modeling effort and reduces the computational time Computations were performed using Abaqus FE code Thermal and mechanical analyses are

uncoupled and performed in two separate runs First, the thermal analysis is performed

calculating the transient temperature distributions during welding The model for the mechanical analysis is similar to the thermal one, except for the type of finite elements and the applied

boundary conditions The mechanical part relies on the thermal analysis results and calculates the stress strain distribution on the basis of the temperature gradients

2

WELDING SIMULATIONS

Heat Source Model

To simulate arc heating effects during welding, the welding arc was modeled by a heat flux

moving across an observed section of the panel The overall heat flux was calculated as

where r, φ, z are the radial, tangential, and axial coordinates, respectively, with the origin at the

material surface Constants 2a, 2b and 2c represent the characteristic arc dimensions in r, φ and z direction within which 95% of the energy is transferred The constant f1 is defined by equation (Ref 1)

Because a two-dimensional model was assumed, the dimension c does not physically exist

Therefore, adding a time component to the model simulated the longitudinal dimension The

time component is defined as the total time the electrode needs to travel over the observed

section of interest, i.e., the heating time To determine the heating time, certain mathematical

manipulations and assumptions were necessary

The use of a quasi-stationary temperature distribution equation provides the basis for

determining accurate heat flux distribution and heating time Determining a distance yp from the

weld centerline to a peak temperature enables the prediction of heat-affected zone size and

certain geometric relationships However, to calculate the width accurately the outer extremity of the heat–affected zone must be clearly identified with a specific peak temperature, which in turn

is associated with some characteristic change in structure or properties For example, for most plain carbon or low alloy steels, there is a distinct etching boundary (as observed on a polished and etched weld cross section), corresponding to a peak temperature of 1346°F (730°C)

Assuming this etching boundary defines the outer extremity, the width of the heat-affected zone can be calculated (Ref 2) The HAZ size and geometric relationships are then used to calculate

the heat flux distribution and heating time Figure 2-1 shows these geometric relationships

between the HAZ and the weld pool widths The quasi-stationary temperature distribution is

defined by equation (Ref 3)

on the waterwall panel To account for heat transfer effects due to fluid flow in the weld pool, an increase in the thermal conductivity above the melting temperature was assumed To account for heat losses, the radiative and convective heat transfer at the weld surface and the convective heat transfer to the surrounding air were modeled The heat generated by the welding process is

modeled as a Gaussian distribution of heat flux

To model the conductive heat transfer, the thermal conductivity coefficients of materials 309L SST, Alloy 622 and SA213-T11 were given as a function of temperature To account for heat transfer effects due to fluid flow in the weld pool, a linear increase in thermal conductivity by a factor of three between the melting temperature and 5432°F (3000°C) was assumed for all materials Figures 2-2 through 2-4 show the applied thermal conductivity and specific heat for 309L SST, Alloy 622, and SA213-T11

Calculation of the convection coefficient, hf, for convection to the cooling fluid was based on the

water properties at 70°F (21°C) and a stagnant condition in the tube In addition, the tube-air

convection coefficient, ha, was calculated for the natural convection to the air of 70°F (21°C)

Radiation and convection boundary conditions are assigned for all free surfaces Radiant heat transfer was assumed at the outside tube surface The value of 0.2 was used for the emissivity coefficient

Three computational steps are required to complete one welding pass In the first step, the heat source passes across the observed section and heats/melts the panel material The weld material

is added in the second step The third step, which is required because of the generalized plane strain model assumption, simulates the cooling of the observed section before the electrode returns for the next welding pass Six welding passes are simulated to obtain a weld of sufficient length and to get a uniform region of residual stresses that are unaffected by beginning and end

of the welding process The heating time (first step) equals the time the electrode with assumed diameter needs to travel across the observed point with the actual welding speed The cooling time (third step) equals the time the electrode needs to travel the full length of one pass and returns to the next pass, with the actual traveling speed specified in Table 1-1 The final cooling takes place after the final pass is finished and lasts until the tube reaches the isothermal ambient temperature

Mechanical Analysis

An elasto-plastic constituitive law was used in the mechanical analysis along with a visco

procedure to simulate creep effects during the cooling of the observed section The mesh is identical to that used in the thermal analysis The analysis is based on temperature gradients calculated in the thermal analysis, which represents the input information, or loading

Elasto-plastic material response was assumed and is described by the constitutive, or strain, relationship The mechanical properties were obtained elsewhere (Ref 4) Figures 2-7 through 2-9 show the temperature-dependent yield and ultimate stresses for 309L SST, Alloy

stress-622 and SA213-T11 Both yield and ultimate stresses were reduced to 15 psi (0.1 MPa) at the melting temperature to simulate low strength at high temperatures Elastic modulus was also given as a function of temperature and reduced to a small value of 15 psi (0.1 MPa) at high temperatures for all materials The Poisson’s ratio of 309L SST and Alloy 622, which has the value of 0.28 at room temperature, increases linearly to 0.33 at 2500ºF (1375ºC) The Poisson’s ratio of SA213-T11 is 0.28 at room temperature and linearly increases to 0.33 at 2500ºF

(1375ºC) Figures 2-7 through 2-9 show the temperature-dependent elastic modulus and thermal coefficients of expansion for all materials Phase transformation effects due to rapid cooling on mechanical properties were not included in analysis

Residual stresses and elastic strains were calculated after the welding process was completed and the tube had cooled to ambient temperature

Figure 2-4

Thermophysical Material Properties of SA213-T11

Figure 2-5

Mechanical Properties of 309L SST

Figure 2-10

Mechanical Properties of SA213-T11

Weld Simulation – Results and Discussion

Residual Stresses – 309L SST / T11

Residual stresses and strains were evaluated after the welding process was completed and the tube had cooled to room temperature Calculated residual stresses at the tube crown and side passes were averaged and presented for longitudinal and transverse directions in Figures 2-11 and 2-12, respectively

Figures 2-11 and 2-12 show the stress distribution for the applied 309L SST Unifuse® weld overlay on a SA213-T11 tube to membrane panel A shaded cross-section plot is added to each figure to show the stress pattern The crown and side weld passes were selected to capture the average through-thickness stresses for the tube, which are shown in the shaded plots

Longitudinal residual stresses in the weld metal are tensile and lower than the yield stress The calculated average stress is between 28 ksi and 45 ksi Stresses at the weld interface are tensile (28 ksi), decreases to about a 10 ksi compressive stress at approximately two-thirds of the tube thickness, and remain constant to the inner surface – Figure 2-11

Figure 2-12 shows that the calculated transverse residual stress distribution is similar The weld metal is tensile and decreases to a compressive stress of equal magnitude within the core of the tube

In general, residual stresses in the weld overlay and at the interface of the two materials are high and tensile, which is important information for evaluating the waterwall panel with respect to potential failure mechanisms

Residual Stresses – Alloy 622 Inco / T11

Similar residual stress patterns are present for 622 Inco/ T11 Panel High tensile stresses in the weld metal overlay and at the interface are present Figures 2-13 and 2-14 shows the average through-thickness plots

Figure 2-11

Longitudinal Residual Stress – 309L SST / T11

Figure 2-12

Transverse Residual Stress – 309L SST / T11

Figure 2-13

Longitudinal Residual Stress – 622 Inco / T11

Figure 2-14

Transverse Residual Stress – 622 Inco / T11