ML12157A650

1501 - E118 - Welding Technology and Codes - 05 - Weldability

ML12157A650
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Weldability Module 5

Module 5 - Weldability Module 5 - Weldability 5A - Weld Defect Types 5A.1 - Solidification and Liquation Cracking 5A.2 - Solid-State Cracking 5A.3 - Hydrogen-Induced Cracking 5A.4 - Fatigue and Fracture 5B - Corrosion 5C - Fractography 5D - Case Studies 5-2

Module 5 - Weldability Module 5 Learning Objectives Definition of metallurgical and geometric defects in welds Basic understanding of different forms of weld cracking Differentiating different types of cracking Basic fatigue and fracture principles Different forms of corrosion Corrosion cracking associated with welds Basic aspects of failure analysis 5-3

Weld Defect Types Module 5A

Module 5 - Weldability Weld Defects Types of Weld Defects Fabrication-related Associated with primary fabrication or repair Can be controlled by combination of metallurgical and welding process factors Use of appropriate inspection techniques is critical Service-related Occur upon exposure to service environment Generally mechanically or environmentally induced May result from remnant weld defects or metallurgical phenomena associated with the weld thermal cycle Inspection and design issues are important to control defect formation and monitor propagation 5-5

Module 5 - Weldability Weld Defects Fabrication-Related Defects Process control Lack-of-fusion (LOF)

Weld undercut Excessive overbead or drop through Lack of penetration (LOP) or incomplete penetration Slag inclusions Porosity, voids Craters, melt-through, spatter, arc-strikes, underfill Sugaring Oxidation of root pass Cracks Other Metallurgical anomalies (e.g., local softening or embrittlement)

Geometric defects (design or fitup); e.g., distortion 5-6

Module 5 - Weldability Weld Defects Lack of Fusion Inability to wet the surfaces of the weld joint area completely leaving behind voids Reduced load-carrying capacity Detection Radiography and ultrasonic inspection Common Causes Improper process parameters Liquid metal too cold to fuse to the base material Improper welder technique No weaving Poor access Bad joint design Material composition Viscous flow 5-7

Module 5 - Weldability Weld Defects Weld Undercut Not efficient tie up of weld metal and base metal region Leads to premature fatigue failure Detection Visual and dye penetrant inspection Common Causes Improper welding technique Concave weld profile Improper weave technique Undercut Intrusion Improper process parameters Excessive current 5-8

Module 5 - Weldability Weld Defects Over-Bead or Drop-Through Over-bead (A) happens during A capping pass Drop-through (B) happens during root pass Detection Nondestructive X-ray, visual and dye penetrant inspection Destructive Optical metallographic methods Causes Inadequate welder skill Procedure restrictions Joint geometry restrictions B

5-9

Module 5 - Weldability Weld Defects Incomplete Penetration Weld pool does not penetrate the whole plate thickness Detection Nondestructive Visual, dye penetrant inspection, radiography and ultrasonic inspection Destructive Optical metallographic methods Causes Inadequate welder skill Procedure restrictions Shallow Penetration Joint geometry restrictions Material chemistry changes Deep Penetration 5-10

Module 5 - Weldability Weld Defects Entrapped Slag Inclusions This occurs in flux-shielded processes (e.g. SMAW, SAW, and FCAW)

Detection Radiography and ultrasonic inspection Causes Improper welding technique Incomplete removal of the slag from previous bead Poor joint access Inefficient partitioning of inclusions Ref: Mitra and Eager, 1999 from molten metal Liquid Solid 5-11

Module 5 - Weldability Weld Defects Porosity Gas Porosity Molten metals always dissolve more gases than solids so during molten metal solidifies the inability to outgas leads to gas porosity Shrinkage porosity Liquid to solid transition in metals leads to shrinkage creating voids and the inability to fill the voids leads to shrinkage porosity Detection Visual, dye penetrant inspection, radiography and ultrasonic inspection Causes Improper welding technique Improper process parameters Shielding gas, welding conditions, etc.

Material composition 5-12

Module 5 - Weldability Weld Defects Porosity Materials - Process interactions are complex during welding For similar processing conditions, the change in filler composition leads to a dramatic difference in porosity Laser Surface Alloying for hard and corrosion resistance coatings 5-13

Module 5 - Weldability Weld Defects Oxidation (Sugaring)

Oxidation at high temperatures removal elements like Cr from the alloy which may lead to preferential corrosion and/or creep failure Detection Visual Causes Improper welding technique Incomplete shielding of the root pass Poor joint access Courtesy: CAPSTONE Project by F. Augestine, J.

Hurst and J. Rule, OSU, 2009 5-14

Module 5 - Weldability Weld Defects Local Hardening/Softening Thermal cycles and material composition may change the local mechanical properties Multi-pass pipeline hardness distributions Blue: soft Red: hard Detection Destructive hardness testing Causes Material composition Process parameters 5-15

Module 5 - Weldability Weld Defects Dimensional Defects: Distortion Distortion happens due to localized plastic flow and resulting imbalance of internal stresses Main factors are geometry and processing conditions with secondary factors including material microstructure This topic is beyond the scope of the present module

Reference:

Masubuchi, 1980 5-16

Module 5 - Weldability Weld Defects Fabrication-Related Defects - Cracks Hot cracking Weld solidification HAZ liquation Weld metal liquation Warm cracking Ductility dip Reheat/PWHT Strain-age Liquid metal embrittlement (LME)

Cold cracking Hydrogen-induced cracking Delayed cracking This module will discuss these defects in more detail 5-17

Module 5 - Weldability Weld Defects Service-Related Damage Mechanisms Hydrogen-induced Environmentally-induced (e.g., corrosion)

Fatigue Stress-rupture Creep and creep-fatigue Corrosion-fatigue Mechanical overload This module will discuss these defects briefly due to the wide variety and complexity of different service conditions 5-18

Solidification and Liquation Cracking Module 5A.1

Module 5 - Weldability Solidification and Liquation Cracking Fabrication-Related Defects - Metallurgical Hot cracking Weld solidification HAZ liquation Weld metal liquation 5-20

Module 5 - Weldability Solidification and Liquation Cracking Weld Solidification Cracking Two essential elements Susceptible microstructure Large solidification temperature range Liquid films present along solidification grain boundaries Restraint Shrinkage resulting from solidification Thermal contraction Imposed (external) forces 5-21

Module 5 - Weldability Solidification and Liquation Cracking Solidification Cracking Temperature Range TL TS Temperature Ductility BTR or Strain Strain Rate A Strain Rate B Strain Rate C SCTR-A SCTR-B 5-22

Module 5 - Weldability Solidification and Liquation Cracking Relative and Maximum Potency Factors for Iron-Based Binary Systems Demonstration of Phase Diagram 5-23

Module 5 - Weldability Solidification and Liquation Cracking Factors Influencing Weld Solidification Cracking Circular Patch Test Composition Alloying elements Impurity elements Nature of grain boundary liquid films Volume fraction Wetting characteristics Weld pool geometry Weld bead geometry Restraint Restraints usually accentuate the solidification cracking, as a result, such geometries are used for evaluating cracking resistance of alloys 5-24

Module 5 - Weldability Solidification and Liquation Cracking Composition Alloying elements Effect on solidification behavior (e.g. austenite versus ferrite solidification in steels)

Partitioning during solidification may promote eutectic formation (e.g.

Al- and Ni-base alloys)

Impurity elements Partitioning of impurities (e.g., S, P, and B in steels) significantly depresses terminal solidification temperature These elements often enhance the wetting characteristics of the terminal liquid at the SGB 5-25

Module 5 - Weldability Solidification and Liquation Cracking Grain Boundary Liquid Films Cracking is associated with liquid films along grain 1200 boundaries. 1 TEMPERATURE_CELSIUS 1000 2 The nature of these liquid films is controlled by: 800 LIQUID Volume fraction of liquid 600 Grain boundary area Wetting characteristics 400 200 0

0 0.2 0.4 0.6 0.8 1.0 MOLE_FRACTION CU 5-26

Module 5 - Weldability Solidification and Liquation Cracking Percent Eutectic Liquid versus Cracking Susceptibility 1 2 3 Cracking Susceptibility Curve in Eutectic System Cracking Susceptibility Fraction Eutectic Fraction Eutectic Composition 5-27

Module 5 - Weldability Solidification and Liquation Cracking Weld Pool Geometry Also Affects Solidification Cracking Teardrop Shape Elliptical Shape 5-28

Module 5 - Weldability Solidification and Liquation Cracking Weld Bead Geometry Large, Concave Small, Convex 5-29

Module 5 - Weldability Solidification and Liquation Cracking Restraint Intrinsic Base metal and weld metal strength Material thickness Joint design Extrinsic Fixturing Applied stress 5-30

Module 5 - Weldability Solidification and Liquation Cracking Identifying Weld Solidification Cracks Where?

Along solidification grain boundaries, occasionally along solidification sub-grain boundaries When?

During final stages of solidification How?

Metallography Fractography - exhibit distinct dendritic fracture morphology 5-31

Module 5 - Weldability Solidification and Liquation Cracking Preventing Weld Solidification Cracks Composition Solidification control (BCC vs. FCC)

Impurity content Liquid film formation Process control Heat input Bead shape Restraint Intrinsic These plots show the Extrinsic stress required to induce solidification cracking in IN939 alloys as a function of raw material source, Zr and B concentrations; Very Purity Raw Material Reduces the Cracking Tendency 5-32

Module 5 - Weldability Solidification and Liquation Cracking Fabrication-Related Defects - Metallurgical Hot cracking Weld solidification HAZ liquation Weld metal liquation HAZ liquation requires the presence of a liquid, or liquid film Associated with grain boundaries Two types HAZ/PMZ liquation cracking Weld Metal (WM) liquation cracking 5-33

Module 5 - Weldability Solidification and Liquation Cracking Liquation Mechanisms Penetration mechanism Localized melting Grain boundary migration Penetration of boundary by liquid Segregation mechanism Segregation of impurity and solute to grain boundaries Gibbsian segregation Grain boundary sweeping Pipeline diffusion 5-34

Module 5 - Weldability Solidification and Liquation Cracking Penetration Mechanism Three elements are required Local liquation phenomenon Segregation melting (BM or WM)

Constituent melting (e.g. eutectic)

Constitutional liquation Grain boundary motion and intersection Penetration and wetting of grain boundary 5-35

Module 5 - Weldability Solidification and Liquation Cracking Penetration Mechanism 5-36

Module 5 - Weldability Solidification and Liquation Cracking Localized Melting in the PMZ Incipient melting Grain boundaries are high energy sites Melting at temperatures a few degrees below bulk solidus Solute/impurity banding Local, periodic variations in composition Residual from thermo-mechanical processing Constitutional liquation Courtesy: S. Kou, Welding Metallurgy Book, Wiley Interscience 5-37

Module 5 - Weldability Solidification and Liquation Cracking The Constitutional Liquation Mechanism Proposed by Savage, et al in the 1960s Reaction between a constituent particle and the matrix Local melting at the particle/matrix interface Note: The particle does not melt, but rather reacts with the matrix prior to the onset of liquation 5-38

Module 5 - Weldability Solidification and Liquation Cracking Constitutional Liquation Liquid Solidus AxBy + L e f g T3

+L Temperature c d Te T2 b

Solvus + AxBy a

T1 A C0 CA AxBy Composition, %B 5-39

Module 5 - Weldability Solidification and Liquation Cracking Constitutional Liquation Mechanism T1 T2 Reaction AxBy Zone AxBy b

C0 5-40

Module 5 - Weldability Solidification and Liquation Cracking Constitutional Liquation Mechanism Te T3 Reaction Zone Melted Melted region AxBy AxBy region d g f

c e

C0 5-41

Module 5 - Weldability Solidification and Liquation Cracking Constitutional Liquation of NbC 5 µm Alloy 907 5-42

Module 5 - Weldability Solidification and Liquation Cracking PMZ Grain Boundary Wetting 20 µm 5 µm Alloy 907 5-43

Module 5 - Weldability Solidification and Liquation Cracking Constitutional Liquation Alloy System Susceptible Alloys Constituent Alloy 718 NbC Ni-base Waspaloy TiC Hastelloy X TiC Type 347 NbC Stainless Steels A-286 TiC Alloy 800 TiC Structural Steels HY-130 TiS 5-44

Module 5 - Weldability Solidification and Liquation Cracking Segregation Mechanism This mechanism is used to explain HAZ liquation in alloys that do not undergo liquation in discrete locations. It requires:

Diffusion of solute and/or impurity elements to grain boundaries Segregation-induced melting of the grain boundary Grain boundary wetting 5-45

Module 5 - Weldability Solidification and Liquation Cracking Effect of Segregation on Grain Boundary Melting Segregation of solute/impurities to grain boundaries depresses the local melting point Temperature gradient has a strong effect on the extent of melting 5-46

Module 5 - Weldability Solidification and Liquation Cracking Grain Boundary Sweeping Thermally-induced grain boundary motion in HAZ High affinity of some elements for grain boundaries Impurities - S, P, B Solutes - Ti, Si Elements swept up and move with boundary 5-47

Module 5 - Weldability Solidification and Liquation Cracking Pipeline Diffusion Solute segregation in fusion zone along solidification grain boundaries (SGBs)

Grain boundary pipeline due to epitaxy Rapid grain boundary diffusion 5-48

Module 5 - Weldability Solidification and Liquation Cracking Weld Metal Liquation Cracking Often referred to in literature as microfissuring Restricted to reheated weld metal (multipass welds)

Not associated with constitutional liquation Locations Solidification grain boundaries (SGBs) due to segregation during initial solidification (Case 3)

Migrated grain boundaries (MGBs) due to a segregation or penetration mechanism Most often observed in single phase, austenitic weld metal 5-49

Module 5 - Weldability Solidification and Liquation Cracking Weld Metal Liquation Cracking Molten Pool Weld Solidification Cracks Fusion Boundary Weld Metal Liquation Cracks 5-50

Module 5 - Weldability Solidification and Liquation Cracking Variables which Influence HAZ/WM Liquation Cracking Microstructure Grain size Phases and constituents Heat treatment Composition Alloying elements (Ti, Nb, etc.)

Impurities Welding conditions Heat input Filler metal 5-51

Module 5 - Weldability Solidification and Liquation Cracking Effect of Grain Size on HAZ Liquation Cracking Grain boundary liquid films High Less liquid film Cracking Susceptibility Impurity coverage with finer grain size Low Liquid film strength Impurity Strain localization Average Grain Diameter Grain Boundary Area 5-52

Module 5 - Weldability Solidification and Liquation Cracking Susceptible Alloy Systems Generally fully austenitic (FCC) microstructure Austenitic stainless steels Types 347 and 321 Type 310 Nickel-base alloys High-strength steels 5-53

Module 5 - Weldability Solidification and Liquation Cracking Identifying HAZ/PMZ Liquation Cracks Where?

Grain boundaries Close proximity to fusion boundary May be continuous across fusion boundary When?

On-cooling in region subject to liquation How?

Always intergranular Fracture surface may be decorated with liquid films 5-54

Module 5 - Weldability Solidification and Liquation Cracking Identifying WM Liquation Cracks Where?

Reheated weld metal Solidification grain boundaries and/or migrated grain boundaries When?

On-heating or on-cooling in regions heated above liquation temperature How?

Always intergranular Smooth or dendritic fracture surface 5-55

Module 5 - Weldability Solidification and Liquation Cracking Preventing Liquation Cracking Microstructure control Minimize grain size Introduce second phases (ferrite)

Control boundary mis-orientation Composition Reduce impurities Avoid local melting and constitutional liquation Restraint Weld in solution annealed condition Multi-bead techniques Design and fixturing 5-56

Module 5 - Weldability Solidification and Liquation Cracking How Do We Evaluate Weldability Under Controlled Conditions?

Some laboratory tests for evaluating weldability:

Varestraint Cracking Test (Linear and Spot)

SigmaJig Gleeble Thermomechanical Simulation Due to the limited scope, we will see how we do the SigmaJig test with a case study However, the general framework is same, temperature, stress and evaluate the material sensitivity 5-57

Module 5 - Weldability Solidification and Liquation Cracking SigmaJig Test Perform welding (similar conditions) with different applied stress on similar thickness sample Identify the critical stress at which you initiate cracking If the stress to induce cracking is higher, then the alloys are resistant to weld solidification cracking Courtesy: ORNL 5-58

Module 5 - Weldability Solidification and Liquation Cracking An Example of Weld Crack Monitoring Let us see the crack dynamics 5-59

Module 5 - Weldability Solidification and Liquation Cracking Sub-Solidus Crack Formation in a Stressed Weldment was Observed Welding Direction Weld cracks in M738 (~IN738) was transverse in nature The cracking is attributed to the presence of low-melting eutectics and the presence of longitudinal stress 5-60

Module 5 - Weldability Solidification and Liquation Cracking Let Us See the Real-Time Movie of Cracking What is the significance of this test for nuclear/energy construction applications?

5-61

Module 5 - Weldability Solidification and Liquation Cracking Threshold Cracking Stress in IN939 Linked to Minor Element Concentrations (B, Zr & S)

The above results lead to tightly controlled IN939 alloy composition for better weldability E. P. George, S. S. Babu, S. A. David, and B. B. Seth, Proceedings of the BALTICA V conference in Helsinki, 2001) 5-62

Solid-State Cracking Module 5A.2

Module 5 - Weldability Solid-State Cracking Fabrication-Related Defects - Metallurgical Warm cracking Ductility dip Reheat/PWHT Strain-age Liquid metal embrittlement (LME) (Cu contamination) 5-64

Module 5 - Weldability Solid-State Cracking Ductility-Dip Cracking Severe loss in ductility below the solidus temperature May occur on-heating or on-cooling Observed in both weld metal and base metal HAZ Always intergranular Austenitic (FCC) microstructure 5-65

Module 5 - Weldability Solid-State Cracking Ductility-Dip Cracking Susceptible materials Austenitic stainless steels Fully austenitic base metals and filler metals High purity grades Ni-base alloys Solid-solution strengthened Multipass weld metals Characteristics Along solidification grain boundaries and migrated grain boundaries in the weld metal Associated with boundary mobility and large grain size 5-66

Module 5 - Weldability Solid-State Cracking Reheat Cracking Low alloy steels and some stainless steels are susceptible Occurs during PWHT and stress relieving Associated with austenite or prior austenite grain boundaries May occur during cladding of structural steels 5-67

Module 5 - Weldability Solid-State Cracking Conditions for Reheat Cracking During welding In low alloy steels, transformation to austenite Dissolution of alloy carbides Segregation of impurity elements During reheating Reprecipitation of alloy carbides Relaxation of stresses 5-68

Module 5 - Weldability Solid-State Cracking Susceptible Materials Low alloy steels with secondary carbide formers A508, A517, A533 Cr-Mo or Cr-Mo-V steels Austenitic stainless steels (Type 347)

Other alloys with strong precipitation reactions Alloying Elements (from Haure and Bocquet)

G = 10C + Cr + 3.3Mo +

8.1V for G<2, steel is resistant Impurity Elements (from Brear and King)

I = 0.2Cu + 0.44S + 1.0P +

1.8As + 1.9Sn + 2.7Sb 5-69

Module 5 - Weldability Solid-State Cracking Mechanism for Reheat Cracking On-heating transformation to austenite Alloy carbides dissolve Austenite grain growth Segregation of impurities to grain boundaries (optional)

Upon reheating to PWHT temperature Strong intragranular precipitation response Stress relaxation occurs simultaneously Strain localization at prior austenite grain boundaries Failure at, or near, grain boundaries 5-70

Module 5 - Weldability Solid-State Cracking Mechanism for Reheat Cracking Transformation to austenite, grain Re-precipitation of strengthening precipitates, growth, impurity segregation relaxation of residual stresses, cracking along prior austenite grain boundaries Weld thermal cycle PWHT cycle A3 Temperature Time 5-71

Module 5 - Weldability Solid-State Cracking Identifying Reheat Cracking Metallography Occurs in true HAZ in close proximity to the fusion boundary Peak temperatures above A3 Intergranular along prior austenite grain boundaries Fractography Smooth IG fracture at low PWHT temperatures or with high impurity levels Ductile IG fracture at higher PWHT temperature

(>500 ºC) or with low impurity levels Courtesy: S. Kou, Welding Metallurgy 5-72

Module 5 - Weldability Solid-State Cracking Preventing Reheat Cracking Select steels that do not contain secondary carbide formers (Cr, Mo, V)

Reduce impurities, particularly S, P, Cu, As, Sb, Sn Reduce residual stress levels and subsequent stress relaxation during PWHT Eliminate stress concentrations near the fusion boundary (grinding, peening)

Control weld heat input Use buttering technique 5-73

Module 5 - Weldability Solid-State Cracking Reheat Cracking in Austenitic Stainless Steels Occurs in alloys containing secondary carbide formers (Nb, Ti)

Observed in Type 347 WM and BM HAZ Associated with precipitation of NbC during heating to stress relief temperature or during service Ductile IG fracture mode Susceptibility reduced by step heat treatment 5-74

Module 5 - Weldability Solid-State Cracking C-curve Cracking Susceptibility in Type 347 From W. Lin and J.C. Lippold 5-75

Module 5 - Weldability Solid-State Cracking Strain-Age Cracking Associated with Ni-base superalloys (gamma-prime strengthened) and precipitation-strengthened steels Occurs during PWHT or during welding Cracking along austenite or prior austenite grain boundaries Three factors combine to promote cracking Intragranular strengthening Impurity segregation Grain boundary embrittlement 5-76

Module 5 - Weldability Solid-State Cracking Mechanism for Strain-Age Cracking Strengthening precipitates are solutionized in the HAZ during welding Liquation occurs along PMZ grain boundaries (optional)

During PWHT, rapid reprecipitation of strengthening precipitates intragranularly Potential embrittlement of grain boundaries Strain accumulation at grain boundaries Relaxation of residual stress Thermal contraction stress Local stress due to precipitation Intergranular fracture 5-77

Module 5 - Weldability Solid-State Cracking Mechanism Precipitates are solutionized and grain growth occurs in the HAZ during welding During reheating Intragranular precipitation Relief of residual stresses Localization of strain at the grain boundaries From ASM Handbook, Vol.6 5-78

Module 5 - Weldability Solid-State Cracking Effect of Ti and Al 7 wrought Weight percent Aluminum IN 713C cast 6 René Mar-M-247 IN 100 108 5 Astroloy Udimet 700 Susceptible 4

Udimet 500 René 80 3

Udimet 710 René 41 2

IN 939 Resistant 1 Waspaloy IN 718 IN X-750 0

0 1 2 3 4 5 6 7

Weight percent Titanium 5-79

Module 5 - Weldability Solid-State Cracking Effect of Precipitation Select resistant material (low Ti + Al)

Heat rapidly during PWHT to avoid nose of precipitation curve Heat and hold below nose of curve to reduce residual stress Design issues From ASM Handbook, Vol. 6 5-80

Module 5 - Weldability Solid-State Cracking Identifying Strain-Age Cracking Metallography Close proximity to the fusion boundary Intergranular Fractography Smooth or ductile intergranular Possible presence of liquid films in PMZ 5-81

Module 5 - Weldability Solid-State Cracking Preventing Strain-Age Cracking Select alloys with sluggish precipitation reactions Alloy 718 Minimize residual stresses Avoid/minimize PMZ formation Alter PWHT thermal cycle for heating to solutionizing temperature Heat rapidly to avoid nose of precipitation curve Use intermediate hold at temperature below the nose to relieve residual stress 5-82

Module 5 - Weldability Solid-State Cracking Copper Contamination Cracking Austenitic steels or ferritic THERMO-CALC (2009.04.01:17.18) :CU FE DATABASE:TCBIN steels that are austenitic P=1E5, N=1; above 1000ºC 1600 LIQUID Occurs in HAZ remote 1400 TEMPERATURE_CELSIUS from fusion boundary 1200 LIQUID+FCC_A1 Liquid Cu penetration 1000 along austenite grain FCC_A1+FCC_A1#2 boundaries 800 No heat-to-heat variation 600 in susceptibility BCC_A2+FCC_A1#2 400 200 0

0 0.2 0.4 0.6 0.8 1.0 MOLE_FRACTION CU 5-83

Module 5 - Weldability Solid-State Cracking Mechanism for Copper Contamination Cracking Copper abraded onto surface of workpiece Heating above melting point of copper (1083ºC) or copper alloy Liquid copper penetrates the grain boundary via a liquid metal embrittlement (LME) mechanism Grain boundary embrittlement and cracking with threshold level of restraint 5-84

Module 5 - Weldability Solid-State Cracking Identifying Copper Contamination Cracking Metallography Intergranular failure in HAZ remote from the fusion boundary Presence of copper along grain boundary in as-polished condition Fractography Smooth intergranular fracture Cu can be detected using SEM/EDS 5-85

Module 5 - Weldability Solid-State Cracking Preventing Copper Contamination Cracking Eliminate source of copper Fixturing, shielding gas delivery systems, or other sources that allow Cu to be abraded onto the material prior to welding Cu added as an alloying element is not a potential source Alternate materials Alloys that are ferritic at elevated temperatures (e.g., ferritic stainless steels)

Ni-base austenitic alloys 5-86

Module 5 - Weldability Solid-State Cracking How Do We Test These Weldability in Controlled Conditions?

Some laboratory tests for evaluating weldability Varestraint Cracking Test (Linear and Spot)

SigmaJig Strain-to-Fracture Test Gleeble Thermomechanical Simulation Due to the limited scope, we will see how we do the Gleeble thermomechanical simulation However, the general framework is same, temperature, stress and evaluate the material sensitivity 5-87

Module 5 - Weldability Solid-State Cracking Thermomechanical Simulation On heating & on cooling tests are used to evaluate the solid-state cracking tendency Ductility dip cracking 5-88

Module 5 - Weldability Solid-State Cracking Typical Results This data is from CMSX4 nickel base superalloy 5-89

Hydrogen-Induced Cracking Module 5A.3

Module 5 - Weldability Hydrogen-Induced Cracking Hydrogen-Induced Cracking Occurs in a wide range of materials if sufficient hydrogen is present Most common in structural steels and other alloys that are primarily ferritic at ambient temperature Associated with the diffusion and accumulation of hydrogen in the microstructure May occur immediately following welding or after an incubation, or delay, time A unified mechanism still does not exist 5-91

Module 5 - Weldability Hydrogen-Induced Cracking Conditions for Hydrogen-Induced Cracking Threshold level of hydrogen Susceptible microstructure Tensile restraint Ambient or near ambient temperature If one of these conditions can be eliminated, hydrogen cracking in welds will be avoided 5-92

Module 5 - Weldability Hydrogen-Induced Cracking Hydrogen in Welds Threshold amount is difficult to define Sources of hydrogen Base and/or filler metal Moisture in fluxes and coatings (SMAW)

Organic contamination (oil, grease, paint, etc.)

Shielding gas Condensation (dew point)

Measurement Diffusible Total Hydrogen trapping 5-93

Module 5 - Weldability Hydrogen-Induced Cracking Effect of Microstructure Microstructure control very effective in eliminating HIC Wide range of microstructures possible as function of Composition Cooling rate from above A3 PWHT A hard martensitic microstructure in steel 5-94

Module 5 - Weldability Hydrogen-Induced Cracking Continuous Cooling Transformation (CCT)

Diagram for a Plain Carbon Steel Softer microstructure 5-95

Module 5 - Weldability Hydrogen-Induced Cracking Continuous Cooling Transformation (CCT)

Diagram for a Low Alloy Steel Harder microstructure 5-96

Module 5 - Weldability Hydrogen-Induced Cracking Relative Susceptibility of Microstructures to HIC Microstructure Susceptibility Twinned martensite Highest Martensite Bainite + Martensite Bainite Tempered Martensite Pearlite Acicular ferrite Austenite Lowest 5-97

Module 5 - Weldability Hydrogen-Induced Cracking Restraint Induces high tensile stress Difficult to quantify Combination of applied and residual Effect of stress concentrations Cracks Geometric Slag inclusions 5-98

Module 5 - Weldability Hydrogen-Induced Cracking Temperature Above 150ºC Hydrogen-free hydrogen is very samples mobile Below -100ºC mobility Notch Tensile Strength is low Hydrogen trapping effects Hydrogen-bearing samples

-100 0 100 200 Temperature, oC adapted from Threadgill 5-99

Module 5 - Weldability Hydrogen-Induced Cracking Identifying Hydrogen-Induced Cracking Metallography Weld metal or HAZ May be intergranular or transgranular Initiation at stress concentration Associated with transformed region of weldment Fractography Intergranular fracture normally flat or micro-ductility Transgranular Cleavage or quasi-cleavage Ductile dimples 5-100

Module 5 - Weldability Hydrogen-Induced Cracking Preventing Hydrogen-Induced Cracking Hydrogen Restraint Low H practice Avoid stress concentrations Cleaning prior to welding Reduce residual stress Shielding gas Control weld contour Preheat/interpass control Peening of weld toes Microstructure Temperature Avoid martensitic structures Preheat/interpass control Acicular ferrite has best combination Cooling rate control of strength and resistance to HIC Hydrogen diffusion Minimize impurity segregation Microstructure 5-101

Module 5 - Weldability Hydrogen-Induced Cracking AWS Method From AWS D1.1-2000, Appendix XI CE = C + (Mn + Si)/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 5-102

Module 5 - Weldability Hydrogen-Induced Cracking AWS Hardness Control Method 5-103

Module 5 - Weldability Hydrogen-Induced Cracking AWS Hardness Control Method - Cooling Rate From AWS D1.1-2000, Annex XI 5-104

Module 5 - Weldability Hydrogen-Induced Cracking AWS Hardness Control Method - Cooling Rate From AWS D1.1-2000, Annex XI 5-105

Module 5 - Weldability Hydrogen-Induced Cracking AWS Method - Determining Susceptibility Index 5-106

Module 5 - Weldability Hydrogen-Induced Cracking AWS Method - Selection of Preheat Temperature 5-107

Module 5 - Weldability Hydrogen-Induced Cracking By Controlling the Diffusible Hydrogen Concentration AWS Classification such as E7018-H4R H4 means electrode will deposit diffusible hydrogen average not to exceed 4 mL of hydrogen per 100g of deposited weld metal R means electrode is resistant to hydrogen pick-up Increased exposure limits Relevant: ASME Section II-Part C, SFA-5.1 5-108

Fatigue and Fracture Module 5A.4

Module 5 - Weldability Fatigue and Fracture Fracture and Fatigue of Weldments Welds are often associated with structural failure Catastrophic, brittle fracture Overload Fatigue Presence of brittle microstructures Low toughness Low ductility Stress concentration Defects Stress risers Residual stress effects 5-110

Module 5 - Weldability Fatigue and Fracture What Causes Brittle Fracture?

Three factors contribute to brittle fracture Material toughness Crack, or flaw, size Stress level 5-111

Module 5 - Weldability Fatigue and Fracture Material Toughness Austenitic stainless steels Low and medium strength steels Fracture Energy High strength steels and Ti-alloys High strength Al-alloys Temperature 5-112

Module 5 - Weldability Fatigue and Fracture Fracture Toughness 2a Increasing material toughness Stress, KC of tougher steel f

0 KC a0 af Flaw Size, 2a Adapted from Barsom and Rolfe 5-113

Module 5 - Weldability Fatigue and Fracture Application of Fracture Mechanics to Engineering Materials

High strength High strength More ductile Ductile material material in material in material in plane with spread of plane strain plane stress stress or plane plasticity strain Linear elastic behavior - LEFM Elastic-plastic behavior - EPFM Sketch adapted from Barsom and Rolfe 5-114

Module 5 - Weldability Fatigue and Fracture Fatigue Cracking Repetitive, or cyclic, application of load Three stages Initiation Propagation Failure Effect of stress intensity 5-115

Module 5 - Weldability Fatigue and Fracture S-N Curve Poor fatigue Good fatigue Stress Range resistance resistance Fatigue limit Number of Cycles 5-116

Module 5 - Weldability Fatigue and Fracture Initiation versus Propagation Propagation Component Stress Range Total Life Initiation Component Number of Cycles, log scale 5-117

Module 5 - Weldability Fatigue and Fracture S-N Curve for Welded Joint From ASM Handbook, Volume 19 5-118

Module 5 - Weldability Fatigue and Fracture Effect of Fluctuating Stress on Crack Growth 1 > 2 > 3 Fatigue Crack Length 1 2 3 Number of Cycles 5-119

Module 5 - Weldability Fatigue and Fracture Effect of Material on Crack Growth Rate Three regions Steady state crack growth rate defined by Paris Law da/dN = C(K)m Increasing crack growth rate with stress intensity Little effect of material type in steady state crack growth region Initiation and final failure regions more influenced by material type, strength, and environment From ASM Handbook, Volume 19 5-120

Module 5 - Weldability Fatigue and Fracture Effect of Microstructure on Crack Growth Rate Little effect of microstructure on steady state crack growth Base metal, weld metal, and HAZ fall within scatter band for mild steels and matching weld metals Similar behavior observed in other alloy systems From Barsom and Rolfe 5-121

Module 5 - Weldability Fatigue and Fracture Identifying Fatigue Cracks Initiation at flaws or stress concentration points Weld toe (slag intrusions, undercut, etc.)

Lack of penetration Fabrication cracks (weld metal or HAZ)

Visual and metallography Little macroscopic deformation Cracks tend to be straight Usually transgranular Fractography Macroscopically flat Beach marks and river lines Finely spaced striations related to da/dN 5-122

Module 5 - Weldability Fatigue and Fracture Fatigue Fracture Surface Features Initiation Initiation Beach marks River lines Overload region Failure in rotating shaft Failure initiating at corner 5-123

Corrosion Module 5B

Module 5 - Weldability Corrosion Eight Forms of Corrosion General Galvanic Crevice Pitting Intergranular Selective leaching Erosion corrosion Stress-assisted 5-125

Module 5 - Weldability Corrosion General Corrosion General, or uniform, surface attack Function of composition Nature of oxide Continuous or porous Adherence Stability Influence of welding Thermal damage Local changes in composition Residual stress 5-126

Module 5 - Weldability Corrosion Galvanic Corrosion Chemical potential difference between dissimilar metals Galvanic couple between dissimilar metals Anode - active metal has lower potential Cathode - noble metal has higher potential Net current flow from anode to cathode Effect of dissimilar base and/or weld metals 5-127

Module 5 - Weldability Corrosion Galvanic Series for Commercial Metals in Seawater Platinum Gold Titanium Silver Hastelloy C 18-8 stainless steel (passivated) Cathodic (noble)

Copper, monel, and cupronickels Nickel and Inconels (active) Anodic (active)

Tin Lead 18-8 stainless steel (active)

Steel and iron Aluminum alloy 2024 Aluminum alloy 1100 Zinc Magnesium and Mg-alloys Adapted from Fontana and Greene 5-128

Module 5 - Weldability Corrosion Solution Potential versus Location Anode Anode (Good)

(OK)

Cathode Cathode (Bad)

Weld Metal HAZ Base Metal Distance 5-129

Module 5 - Weldability Corrosion Crevice Corrosion Localized corrosion at mechanical discontinuity or crevice Weld-induced crevices Slag intrusions or entrapment Lack-of-fusion or penetration defects Cracks 5-130

Module 5 - Weldability Corrosion Pitting Corrosion Localized attack Small pit or pinhole at surface Grows in direction of gravity Strong effect of composition Pit Microstructure effects Surface Subsurface Attack 5-131

Module 5 - Weldability Corrosion Pitting Resistance Strong function of Mo content 90

Measured in terms of critical 80 254SMO pitting temperature (CPT)

AL-6X Minimum temperature at which pits form Critical Pitting Temperature, ºC 70 60 Welds have lower CPT than base metal 50 AL-6X 904L 20Cb3 34L

40 904L 254SMO 317L 30 316L 34L 317L 20 20Cb3 316L

0 1 2 3 4 5 6 7 Molybdenum Content, wt%

5-132

Module 5 - Weldability Corrosion Pitting Corrosion - Effect of Alloying Elements Element Effect on Pitting Resistance Chromium Increases Nickel Increases Molybdenum Increases Tungsten Increases Silicon Decreases, except with Mo Titanium and Niobium Decreases resistance in FeCl3 Sulfur Decreases Carbon Decreases, especially when sensitized Nitrogen Increase Pitting Resistance Equivalent (PRE) = Cr + 3.3(Mo + 0.5W) + 16N 5-133

Module 5 - Weldability Corrosion Selective Leaching Referred to as de-alloying Loss of an alloying element Dezincification in brass alloys 5-134

Module 5 - Weldability Corrosion Erosion Corrosion Similar to general corrosion Accelerated by relative motion and impingement of the corrosive medium Softer metals most susceptible Welds may be more susceptible than base metals due to softened regions 5-135

Module 5 - Weldability Corrosion Intergranular Corrosion Localized attack at, or adjacent to, grain boundaries Associated with Impurity segregation Enrichment/depletion of alloying elements Formation of intermetallics Second phases Galvanic contribution Attack may be quite rapid 5-136

Module 5 - Weldability Corrosion Sensitization Mechanism Grain boundary attack in Type 304 5-137

Module 5 - Weldability Corrosion Carbide Precipitation 5-138

Module 5 - Weldability Corrosion Knifeline Attack Associated with stabilized grades of stainless steel (321 and 347)

Dissolution of NbC or TiC adjacent to fusion boundary Formation of Cr-rich carbide during cooling Sensitization of boundary in very narrow region Carbide Dissolution Type 347 or 321 HAZ 5-139

Module 5 - Weldability Corrosion Low Temperature Sensitization Cr-carbide formation in service 10-20 years Service temperature below 300°C (572°F)

Results in IGA or IGSCC 5-140

Module 5 - Weldability Corrosion Avoiding Intergranular Attack in Stainless Steels Composition control Low-carbon (L-grade) alloys Stabilized alloys (additions of Nb and Ti)

Microstructure control Use annealed base metals (cold work accelerates precipitation)

Solution heat treat after welding and cool rapidly Welding process/procedure Low heat input Low or no preheat and interpass Accelerated cooling 5-141

Module 5 - Weldability Corrosion Stress-Corrosion Cracking Localized cracking resulting from combination of Tensile stress Corrosive environment Variables Environment (concentration)

Temperature Material composition Stress level Microstructure Transgranular or intergranular 5-142

Module 5 - Weldability Corrosion Systems Susceptible to SCC Alloy or Alloy System Environment Aluminum alloys NaCl solutions, seawater Copper alloys Ammonia vapors and solutions Gold alloys FeCl3 solutions, acetic acid-salt solutions Inconel Caustic soda solutions Lead Lead acetate solutions Magnesium alloys Distilled water Monel Fused caustic soda, hydrofluoric acid Nickel Fused caustic soda Carbon and Low Alloy Steels Multiple Stainless Steel Multiple, including seawater and H2S Titanium alloys Fuming nitric acid, seawater, N2O4 5-143

Module 5 - Weldability Corrosion SCC in Stainless Steels and Nickel Alloys Time to Failure (hours) 1000 Cracking 100 No cracking 10 Tested in boiling Fe3Cl 1

0 20 40 60 80 Nickel (wt%)

5-144

Module 5 - Weldability Corrosion Transgranular SCC in Type 316 Tubesheet 309L Filler Carbon Steel Metal 5-145

Module 5 - Weldability Corrosion Avoiding SCC in Welded Structures Alloy selection Substitute ferritic or duplex alloy for austenitic stainless steels Use high-Ni alloys Avoid sensitization Eliminate stress concentrations Reduce residual stresses Environmental control or isolation 5-146

Module 5 - Weldability Corrosion Heat Tint and Sugaring If stainless steel surfaces are heated to moderately high temperatures in air during welding or grinding, a chromium oxide heat tint develops Heat tints are thicker than Chromium oxides films and are very visible Color depends on thickness, with the thickest oxides appearing black Chromium content of the metal is reduced Lower corrosion resistance Heat tints should be removed as well as the underlying layer with reduced carbon content Figure showing decrease in heat tint as a function of oxygen exposure. AWS D18.1 1999 5-147

Module 5 - Weldability Corrosion Primary Water Stress Corrosion Cracking (PWSCC)

PWSCC is a form of stress corrosion cracking unique to primary cooling water containment in nuclear power plants Operating experience has shown that Ni-base Alloy 600 and Filler Metal 82 are susceptible to PWSCC Minimum Cr content of 25 wt% required to avoid PWSCC Use of Structural Weld Overlay (SWOL) and Pre-emptive Weld Overlay (PWOL) approaches to avoid PWSCC 5-148

Module 5 - Weldability Corrosion Structural Weld Overlay (SWOL) and Pre-emptive Weld Overlay (PWOL)

Safe-end welds used to attach stainless steel piping to steel nozzle FM82 dissimilar weld Type 308L between nozzle and SS IN52M Overlay Buffer Layer casting or forging IN52/52M overlay SA-508, Class 2 Cast Type 316 Pipe, Type 316 contains ~30 wt% Cr Structural support and Type 316 Cladding FM82 Weld and Type 308L corrosion resistance Butter against PWSCC Courtesy J.C. Lippold SS buffer layer required in some cases to prevent cracking during welding 5-149

Module 5 - Weldability Corrosion Microbiologically-Induced Corrosion (MIC)

MIC attack in Type 308 SMA weld 5-150

Fractography Module 5C

Module 5 - Weldability Fractography Fractography Outline Introduction - description of fractography and its development Scanning Electron Microscope - overview of SEM components and operating principles Fracture morphologies - fracture paths and principal fracture modes Fractography of defects in welds - hot cracking, warm cracking, cold cracking 5-152

Module 5 - Weldability Fractography Fractography Term coined in 1944 by Carl A. Zapffe Definition: The study of fracture surfaces for the purpose of relating the topographical features to the causes and/or basic mechanisms of fracture.

An important tool for fracture or failure analysis and understanding of material properties.

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Module 5 - Weldability Fractography History of Fractography Fracture surfaces analyzed since the Bronze Age First written description of fracture surface to estimate metal quality in 1540s In 1722 de Reaumur used engravings to reproduce fracture morphologies and classified 7 fracture types 5-154

Module 5 - Weldability Fractography History of Fractography In 1800s metallography caused decline in fractography In 1940s fractography experienced a rebirth with the development of light fractography The electron microscope began to be used in the 1950s to look at fracture surfaces ushering in modern fractography SEM - bulk sample analysis TEM - fracture surface replicas 5-155

Module 5 - Weldability Fractography Fracture Morphologies Fracture Paths Transgranular Intergranular Interphase Fracture Modes Dimple rupture Cleavage Fatigue Decohesive rupture 5-159

Module 5 - Weldability Fractography Fracture Paths Transgranular Fracture passes through the grain Intragranular Intergranular Fracture follows path along grain boundaries Alloy 718 Courtesy Seth Norton 5-160

Module 5 - Weldability Fractography Dimple Rupture Mode Occurs when overload is the primary cause of failure Microvoid Coalescence Nucleation Local strain concentrations Second phase particles Inclusions Grain boundaries Dislocation pileups Coalesce to form a continuous network of cuplike dimples 5-161

Module 5 - Weldability Fractography Ductile Rupture: Microvoid Coalescence From ASM Handbook, Fractography, 1992 5-162

Module 5 - Weldability Fractography State of Stress Mode I Tension Mode II and Mode III Shear From Barsom, et al, Fracture & Fatigue Control in Structures, 1987 5-163

Module 5 - Weldability Fractography Effect of State of Stress on Dimple Direction Mode I - tear Dimples oriented in the same direction Mode II and III - shear Dimples oriented in opposing directions From ASM Handbook, Fractography, 1992 5-164

Module 5 - Weldability Fractography Cleavage Fracture Mode Low energy fracture along crystallographic planes called cleavage planes Fracture is typically smooth and featureless Associated with brittle failure Failure often initiates at flaw with loads below design levels 5-165

Module 5 - Weldability Fractography Cleavage Initiation and Propagation Initiate on many parallel planes Continue uninterrupted through tilt boundaries Re-initiate at twist boundaries From ASM Handbook, Fractography, 1992 5-166

Module 5 - Weldability Fractography Cleavage Fracture Ferritic Stainless Steel Weld Metal Courtesy John Lippold 5-167

Module 5 - Weldability Fractography Fatigue Fracture Mode Fracture that is a result of repetitive or cyclic loading Three stages Stage I : Initiation Follows crystallographic planes Faceted, resembles cleavage High cycle, low stress Stage II Generally transgranular Fatigue striations related to da/dN Stage III Static fracture: dimple rupture or cleavage takes over 5-168

Module 5 - Weldability Fractography Fatigue Cracks Aluminum Alloy 7050-T7651 Commercially Pure Titanium From ASM Handbook, Fractography, 1992 From ASM Handbook, Fractography, 1992 5-169

Module 5 - Weldability Fractography Fractography Weld Defects Hot Cracking Solidification cracking Liquation cracking Warm Cracking Ductility dip cracking Strain age cracking Reheat cracking Cold Cracking Hydrogen embrittlement 5-170

Module 5 - Weldability Fractography Solidification Cracking Along solidification grain boundaries Evidence of liquid films - smooth surfaces Two morphologies Type D : Deep dendritic Cracking near the liquidus and bulk solidus Egg crate appearance Type F : Shallow dendritic or Flat Cracking between the bulk solidus and true solidus 5-171

Module 5 - Weldability Fractography Solidification Cracking of a Nickel Base Superalloy IN939 Alloy Courtesy E. P. George et al 5-172

Module 5 - Weldability Fractography Solid-Solid Bridging During Solidification Uddeholm NU744LN Courtesy David Nelson 5-173

Module 5 - Weldability Fractography Liquation Cracking HAZ and Weld Metal Liquation Intergranular Evidence of liquid films Thin liquid layer: clearly intergranular Thicker liquid layer: more irregular and dendritic appearance 5-174

Module 5 - Weldability Fractography HAZ Liquation Cracking of Duplex Stainless Steel A B Ferralium Alloy 255 Courtesy David Nelson 5-175

Module 5 - Weldability Fractography Constitutional Liquation Waspaloy Courtesy Ming Qian 5-176

Module 5 - Weldability Fractography Grain Boundary Liquation Waspaloy Courtesy Ming Qian 5-177

Module 5 - Weldability Fractography Ductility Dip Cracking Macroscopic crack appearance Short and relatively straight Flat crack surfaces Intergranular along migrated grain boundaries in weld metal Transition of surface as temperature increases Low temperature: flat with ductile dimples Mid and high temperatures : wavy with fine ruggedness and minor ductile dimples Extreme high temperatures: reverts back to flat or smooth with ductile dimples 5-178

Module 5 - Weldability Fractography DDC Fracture Appearance Macroscopically Flat Microscopic Wavy (700°C) features (950°C) 310 stainless steel Courtesy Nathan Nissley 5-179

Module 5 - Weldability Fractography DDC Fracture Appearance Macroscopic Flat Increased Waviness at high Appearance (1100°C) temperatures (1100°C) 310 Stainless Steel Courtesy Nathan Nissley 5-180

Module 5 - Weldability Fractography Strain Age Cracking Precipitation-strengthened, Ni-base alloys Intergranular Fracture surface varies with grain boundary orientation Microductility Flat Alloy 718 Courtesy Seth Norton 5-181

Module 5 - Weldability Fractography Strain Age Cracking: Grain Orientation Effect Alloy 718 Courtesy Seth Norton 5-182

Module 5 - Weldability Fractography Reheat Cracking Low alloy steels and stabilized stainless steels Reheat cracking mechanism On heating Carbide dissolution Impurity segregation to grain boundaries On reheating Intragranular precipitation of carbides Simultaneous stress relaxation Intergranular failure Fracture surface varies with grain boundary orientation Microductility Flat 5-183

Module 5 - Weldability Fractography Reheat Cracking From Nawrocki, et al, The Weldability of a Modified 2.25 Cr-Mo Steel (HCM2S), 1998 5-184

Module 5 - Weldability Fractography Hydrogen Assisted Cracking No single characteristic fracture surface for Hydrogen Assisted Cracking (HAC)

Three fracture morphologies Microvoid Coalescence (MVC)

Quasi-cleavage (QC)

Intergranular Fracture (IG)

Crack morphology is a function of Stress intensity factor at crack tip Concentration of hydrogen at the crack tip Material characteristics 5-185

Module 5 - Weldability Fractography HAC Fracture Morphologies From C.D. Beachem, A New Model for Hydrogen-Assisted Cracking (Hydrogen Embrittlement), 1972 5-186

Module 5 - Weldability Fractography HAC Microvoid Coalescence Weld Metal Courtesy Matt Johnson 5-187

Module 5 - Weldability Fractography HAC Quasi-Cleavage E9010-G Weld Metal Courtesy Matt Johnson 5-188

Module 5 - Weldability Fractography HAC Quasi-Cleavage E71T-1 Weld Metal Courtesy Matt Johnson 5-189

Module 5 - Weldability Fractography HAC Intergranular HSLA-100 Steel Base Metal HAZ Courtesy Matt Johnson 5-190

Weldability Case Studies Module 5D

Module 5 - Weldability Weldability Case Study 1 Case Study 1: Hydrogen Assisted Cracking Alexander Kielland Disaster

Background

Nb - microalloyed fine ferrite grain steel 6 mm fillet weld on a non-load bearing flange initiated the failure Sea temperature was 6°C Some of the findings reported in the book by Easterling This is available on the Carmen web site Please study the notes carefully Web Pages http://www.exponent.com/kielland_platform/

http://en.wikipedia.org/wiki/Alexander_Kielland_(Platform) 5-192

Module 5 - Weldability Weldability Case Study 1 One of the braces fractured suddenly!

Failure Analyses was done 5-193

Module 5 - Weldability Weldability Case Study 1 What do we learn from the failed part?

Unzipping cracks were NOT associated with girth welds!

5-194

Module 5 - Weldability Weldability Case Study 1 Fracture analyses showed interesting features Fatigue cracks were present already!

5-195

Module 5 - Weldability Weldability Case Study 1 Flange welds: Lack of penetration!

How about the fillet welds?

5-196

Module 5 - Weldability Weldability Case Study 1 Fillet welds: Showed many problems!

Bead shape concave Lamellar tearing!

flange Undercuts!

Where did the fatigue crack start?

5-197

Module 5 - Weldability Weldability Case Study 1 Micro-cracks near the fillet welds initiated the crack!

Paint was observed inside the crack Means the cracking started as soon as the welds were done!

What are the specifications of these steels?

5-198

Module 5 - Weldability Weldability Case Study 1 Microstructural and Mechanical Property Requirements 5-199

Module 5 - Weldability Weldability Case Study 1 Failure Analysis - Hardness Testing Results 5-200

Module 5 - Weldability Weldability Case Study 1 Summary of Failure Analysis Failure initially by HIC, fatigue and then overloading!

What started the fatigue cracks?

Butt weld of sonar flange plate contained toe and root cracks; butt welds are poor Secondary cracks between crossover between butt and fillet weld Possible reasons:

Quality of the fillet welds - not good uneven profile Lamellar tearing in the flange plate and not in the brace plate Cracks around fillet welds and main welds Cold cracks were found close to the initiation point These formed during construction itself!

Flange steels are not as good as brace steels Stress concentration at the hole was high!

5-201

Module 5 - Weldability Weldability Case Study 2 Case study 2: Cracking in Heavy Section Welds for Nuclear Application Crack Mitigation during Buttering and Cladding of a Low Alloy Steel Pipe Yu-Ping Yang, Suresh Babu and Suhas Vaze Jeffrey Kikel and David Dewees Edison Welding Institute Babcock and Wilcox yyang@ewi.org jmkikel@babcock.com 8th International Conference on Trends in Welding Research Session 12 - Physical Processes in Weldiing II June 3, 2008 5-202

Module 5 - Weldability Weldability Case Study 2 Outline

Background

Experiment study Finite element modeling Crack mitigation Summary 5-203

Module 5 - Weldability Weldability Case Study 2 Cracks in the Low-Alloy Steel Pipe during Cladding and Buttering Both Solidification and ductility dip cracking are found in the shaded region More cracks near the top (OD of pipe)

Cracks also observed in the buttering region Cladding and Buttering deposited using Hot WIRE GTAW using Inconel Filler Metal 82 5-204

Module 5 - Weldability Weldability Case Study 2 A Mockup Design 3 layers Buttering A mockup was Cladding designed to investigate the cracking problem 17 layers to fill up The mockup Cladding Buttering Outer bead Inner bead 5-205

Module 5 - Weldability Weldability Case Study 2 Weld Parameters and Sequences Cladding Outer Bead Overlap Inner bead Welding Current 3 a ab b Weld: 300 A 2 a ab b Voltage: 13 V Buttering Layer 1 a ab b 17 b ab a Traveling Speed, 6.5 16 b ab a IPM Steel 15 b ab a 14 b ab a Buttering Clading Layer 13 b ab a Welding Current 12 b ab a 11 b ab a Weld: 300 A 10 b ab a Voltage: 16 V 9 b ab a 8 b ab a Traveling Speed, 6.5 IPM Steel 5-206

Module 5 - Weldability Weldability Case Study 2 A Clad and Buttered Pipe start stop 270º 280º 355º 0º 180º 275º 265º C 90º A 260º B Non-cladding: 262.5º~277.5º 5-207

Module 5 - Weldability Weldability Case Study 2 Etched Microstructures Region 1 Region 3 High cracking tendency Region 2 Region 1 Region 2 Reduced cracking tendency Region 3 Buttered region where cracks are observed Region 4 Region 4 Mixed region 5-208

Module 5 - Weldability Weldability Case Study 2 Summary of Experimental Study Experimental results show that the cracks are mainly located in the outer bead region of cladding near the pipe OD surface buttering region near the end of the pipe Finite element modeling was conducted to find the reason for the cracks 5-209

Module 5 - Weldability Weldability Case Study 2 Modeling Process Model Validation Predict fusion zone shape and size and compare with the experimental data Predict temperature-time profiles and compare with the experimental data Model Prediction Predict thermo-mechanical strain during solidification and during the following-on pass deposition 5-210

Module 5 - Weldability Weldability Case Study 2 Modeling Approach Build a finite element model based on the mockup design and weld cross sections Thermal analysis and thermo-mechanical analysis performed with EWI weld FEA software by inputting:

Temperature dependent thermal-physical and mechanical material properties Cladding and buttering specifications Preheating, interpass temperature, welding parameters, welding sequences, and torch weaving Welding fixture 5-211

Module 5 - Weldability Weldability Case Study 2 Finite Element Model Three locations were modeled 3

2 3

1 2

Non-Modeling area 1

5-212

Module 5 - Weldability Weldability Case Study 2 Fusion-Zone Comparison between Prediction and Experiment - Inner Bead of Cladding 5-213

Module 5 - Weldability Weldability Case Study 2 Fusion-Zone Comparison between Prediction and Experiment - Outer Bead of Cladding Bead 2 is a normal weld pool shape 5-214

Module 5 - Weldability Weldability Case Study 2 Temperature Profile TC1 Outer bead TC2 C Inner bead Thermocouple TC3 Locations Thermocouple histories 800 1000 700 TC3 900 Experiment Experiment TC1 800 Prediction 600 TC2 Prediction Temperature (C) Temperature (C) 700 TC2 500 600 400 500 400 300 TC1 300 200 TC3 200 100 100 Inner Bead 0 Outer Bead 0

0 50 100 150 200 1400 1450 1500 1550 1600 Time (Sec.) Time (Sec.)

5-215

Module 5 - Weldability Weldability Case Study 2 Maximum Temperature Comparison between Outer Bead and Inner Bead Predicted maximum temperature Experiments show that outer bead is always hotter.

Maximum temperature, C Temperature, C layer bead 1 bead 2 difference 1 765 898 133 2 548 728 180 3 438 637 199 layer TC1 Outer Bead (bead 2)

Non-modeling TC2 region TC3 Inner Bead (bead 1)

Temperature difference increasing between the outer bead and the inner bead as the layer number increases 5-216

Module 5 - Weldability Weldability Case Study 2 Displacements Comparison between Experiment and Prediction 0.04 Displacement (inch) 0.03 0-180º 0.02 0.01 0

-0.01 270º

-0.02 0 1000 2000 3000 4000 5000 180º Time (Sec.)

0º 0.04 Displacement (inch) 0.03 90-270º 0.02 90º 0.01 0

-0.01

-0.02 0 1000 2000 3000 4000 5000 Time (Sec.)

5-217

Module 5 - Weldability Weldability Case Study 2 Summary of Model Validation Predicted fusion zone has a good agreement with experimental results.

Predicted temperature-time history has a good agreement with experimental measured data.

Higher temperature was observed at the outer bead by comparing with the inner bead from both experiment and modeling.

5-218

Module 5 - Weldability Weldability Case Study 2 Crack Mitigation Studies Reduce the heat input of the outer region by 15%

Original heat input Cladding power input, 3900W Buttering power input, 4800W New heat input Cladding power input, 3315W Buttering power input, 4080W Traveling Speed 6.5 IPM Heat input reduced area 5-219

Module 5 - Weldability Weldability Case Study 2 Fusion-Zone Comparison between Two Heat Inputs Grey color - fusion zone Normal Heat Input Reduced Power input by 15%

5-220

Module 5 - Weldability Weldability Case Study 2 Temperature Comparison between Two Outer Bead Heat Inputs Bead 2 2500 2328ºC Normal heat input 2000 Temperature (C)

Cladding layer 17 Cladding layer 16 Outer bead 1500 buttering layer 1 Outer bead melting 1492ºC Inner bead 1000 buttering layer 1 Cladding layer 17 Outer bead 500 Inner bead 0

15000 15500 16000 16500 17000 17500 18000 18500 19000 buttering layer 1 Inner bead Time (Sec.) Cladding layer 17 2500 Cladding layer 16 Outer bead 2000 Temperature (C) 2032ºC Reduced heat input 1500 1477ºC melting 1000 500 Result locations 0

15000 15500 16000 16500 17000 17500 18000 18500 19000 Time (Sec.)

5-221

Module 5 - Weldability Weldability Case Study 2 Hoop Strain Comparison between Two Heat Inputs 0.03 0.02 cladding-16b1 cladding-17b1 Outer Bead 0.01 Hoop Strain 0

-0.01 Inner Bead

-0.02 cladding-16b2 cladding-17b2

-0.03 14000 15000 16000 17000 18000 19000 buttering layer 1 Time (Sec.) Inner bead Cladding layer 17 (a) Normal heat input Cladding layer 16 Outer bead 0.03 0.02 Outer Bead Hoop Strain 0.01 0

-0.01 Result locations

-0.02 Inner Bead

-0.03 14000 15000 16000 17000 18000 19000 Time (Sec.)

(b) Reduced heat input 5-222

Module 5 - Weldability Weldability Case Study 2 Summary on Heat Input Study Predicted weld pool size using the reduced heat input is smaller than that using the normal heat input.

The modeling results shows:

Thermo-mechanical strain is reduced by reducing heat input.

Reduced heat input may lead to reduced cracking tendency Production welding with reduced heat input has shown decreased susceptibility to cracking 5-223

Module 5 - Weldability Weldability Case Study 3 Case Study 3: Replacement Steam Generator Divider Plate to Channel Head Weld Separation NRC Information Notice: 2010-7 Released on April 5, 2010

Title:

welding defects in replacement steam generators Outline of the presentations Circumstances Failure Analyses Root Cause Problem Relevant to ASME Section X1; IWA-4461 5-224

Module 5 - Weldability Weldability Case Study 3

Background

The licensee, Southern California Edison (SCE), contracted Mitsubishi Heavy Industries (MHI), to manufacture four RSGs in Japan for installation at San Onofre Nuclear Generating Station (SONGS) Units 2 and 3.

MHI completed manufacturing and testing of the first two RSGs in 2008 and shipped them to SONGS Unit 2 for scheduled installation in October 2009.

MHI was scheduled to complete manufacturing and testing of the two RSGs for SONGS Unit 3 in 2009.

5-225

Module 5 - Weldability Weldability Case Study 3 Routine Visual Inspection Showed a Crack After completion of the ASME Boiler and Pressure Vessel Code (ASME Code),Section III primary and secondary side hydrostatic pressure test on the SONGS Unit 3 B RSG.

5-inch long surface flaw (crack) in the dissimilar metal weld between the divider plate, made from Alloy 690, and the channel head, made from low-alloy steel (LAS).

5-226

Module 5 - Weldability Weldability Case Study 3 Flaw was observed in regions between the low-alloy-steel and alloy 152 layer Where and when did the crack start?

5-227

Module 5 - Weldability Weldability Case Study 3 Fabrication Procedure Clad; carbon arc gouge and then butter; Surface cracks were observed after the clad removal Higher carbon content observed in the fusion zone Higher than expected hardness in the HAZ region of low-alloy steel 5-228

Module 5 - Weldability Weldability Case Study 3 Results from Mock-up Confirm Brittle Regions Is there any other effect possible?

5-229

Module 5 - Weldability Weldability Case Study 3 Hydrogen induced cracking cannot be eliminated Remember the three components for HIC cracking; stress, microstructure and hydrogen levels 5-230

Module 5 - Weldability Weldability Case Study 3 Improper Air carbon arc gouging technique may lead to carbon pick up Air Jet Carbon electrode Carbon is indeed consumed during this process. The air stream should remove carbon-rich metal from the groove to leave only minimal contamination of the sidewalls. Poor gouging technique or insufficient air flow will result in carbon pick-up!!! (See reference from TWI web page)

Let us evaluate the sensitivity of rapid cooling and carbon pick up with CCT diagrams (calculations.ewi.org) 5-231

Module 5 - Weldability Weldability Case Study 3 Effect of Carbon on Steel Phase Transformations Carbon pick up will make the steel more hardenable and will lead to cracking!

5-232

Module 5 - Weldability Weldability Case Study 3 Remedial Action - Conclusions ACAG is not specifically covered in Section III of the ASME Code; however, ASME Code,Section XI, IWA-4461 covers the qualification and use of a thermal removal process like ACAG. In addition, 10 CFR 50.55a(b)(2)(xxiii) states:

The use of provisions to eliminate the mechanical processing of thermally cut surfaces in IWA-4461.4.2 of Section XI, 2001 Edition through the latest edition and addenda incorporated by reference in paragraph (b)(2) of 10 CFR 50.55a are prohibited.

Although all specific requirements or standards were met, this event illustrates that control over all aspects of welding ASME Code Class 1, 2, and 3 components can prevent welding defects like those found in the RSGs for SONGS Unit 3 from occurring.

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