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Welding lectures 14 16

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Suraj Aggarwal
Suraj Aggarwal

This document discusses various weld defects including geometric defects like misalignment and metallurgical defects like cracks, porosity, and embrittlement reactions. It explains the causes and remedies for different types of defects such as angular distortion, longitudinal distortion, incomplete fusion, cracks, and porosity. The document also covers residual stresses during welding, gas dissolution and solid solution hardening, hydrogen effects including hydrogen embrittlement and cracking, and common weld testing methods like tension tests, bend tests, hardness tests, and non-destructive tests including visual inspection, liquid penetrant, magnetic particle, x-ray and ultrasonic testing.

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1 11 Weld Defects 03 Nov 2014, Monday 8.30 am-9.30 am Welding Lecture – 14
Weld Defects 2 • Geometric defects • Metallurgical defects
1. Residual stresses • Residual stresses (internal stresses) are stresses that would exist in a body after removing all external loads • Normally due to non uniform temperature change during welding • Weld metal and adjacent base metal are restrained by the areas further away from the weld metal due to expansion and contraction 3
4 Residual stresses
Changes in temperature and stresses during welding 5 Welddirection
• A-A: Zero temperature and stress distribution • B-B: Small compressive in the weld zone and small tensile in the base metal at B-B during melting of the weld metal. • C-C: Developing of tensile stress in the weld centre and compressive in the area further away at C-C during cooling. • D-D: Further contraction of the weld metal producing higher tensile stress in the weld centre and compressive in the base metal at D-D. 6 Changes in temperature and stresses during welding
Effect of temperature and time on stress relief of steel welds 7
2. Distortion 8 Weld distortion is due to solidification shrinkage and thermal contraction of the weld metal during welding
Angular distortion 9 • Upward angular distortion usually occurs when the weld is made from the top of the workpiece alone. • The weld tends to be wider at the top than the bottom, causing more solidification shrinkage and thermal contraction.
Remedies for angular distortion • Reducing volume of weld metal • Using double-V joint and alternate welding • Placing welds around neutral axis • Controlling weld distortion 10
Remedies for angular distortion • Presetting: By compensating the amount of distortion to occur in welding. • Elastic pre-springing can reduce angular changes after restraint is removed. • Preheating and post weld treatment 11
Longitudinal distortion Heating and cooling cycles along the joint during welding build up a cumulative effect of longitudinal bowing
Remedies for Longitudinal distortion Sequences for welding short lengths of a joint to reduce longitudinal bowing • Welding short lengths on a planned or random distribution are used to controlled this problem • Mechanical methods: straightening press, jacks, clamps • Thermal methods : local heating to relieve stresses (using torches)
Defects & Discontinuity • Defect: A flaw or flaws that by nature or accumulated effect render a part or product unable to meet minimum applicable acceptance standards • Defect: The term designates rejectability • Discontinuity: An interruption of the typical structure of a material, such as a lack of homogeneity in its mechanical, metallurgical, or physical characteristics. • A discontinuity is not necessarily a defect 14
Weld Defects & Discontinuities 15
Weld defects 16
17 Mis-alignment • Amount a joint is out of alignment at the root • Cause: Transition thickness, carelessness • Prevention-workmanship • Repair- Grinding
Undercut • A groove cut at the toe of the weld and left unfilled • Cause-Electrode angle, high amperage, long arc length, rust • Prevention-set machine on scrap metal, clean metal before welding 18
Insufficient fill • The weld surface is below the adjacent surfaces of the base metal 19
• Concavity or convexity of fillet weld which exceeds the allowable limit • Cause: Amperage & travel speed • Prevention- Proper parameters & techniques • Repair- Grind 20 Excessive Concavity & Convexity
Overlap • Face of the weld extends beyond the toe of the weld 21
Imperfect shapes 22
Burn through • Undesirable open hole – completely melted through base metal • May or maynot left open • Cause- excessive heat input • Prevention-reduce heat input by adjusting parameters • Repair- Filling 23
24 Incomplete fusion 1.Lack of fusion between weld metal & base metal 2.Lack of fusion between multiple layers of weld metal
Incomplete fusion • Weld metal does not form a cohesive bond with the base metal • Cause- low amperage, steep electrode angles, fast ravel speed, lack of preheat etc. • Prevention- eliminate potential causes • Repair- remove & reweld 25
Cracks 26 • Longitudinal • Transverse • Throat • Toe • Root • Underbead & HAZ
Metallurgical defects 27
28 Gas metal reactions • Metals react with almost any gas except the noble or inert gases • Gases, including N2, O2 & H2, dissolve in liquids, including molten metals. • Gas molecules (or atoms or ions) occupy the many rather large spaces between atoms of the metal in liquid form
29 Gas Dissolution and Solublllty In Molten Metal • The amount of N2, O2 & H2 that can dissolve in molten metal almost always increases with increasing temp. of the liquid • Also a function of the partial pressure of the gas species above the liquid. • This is expressed by Sievert’s law:
30 Sievert’s law • k is the equilibrium constant, • [gas] is the concentration as weight percent (wt%) of a particular gas in the molten metal, and • Pgas2 is the partial pressure of the particular gas in diatomic molecular form. • This relationship, known as Sievert’s law, applies to all diatomic gases, including N2, O2 and H2
31 Gas Dissolution and Solublllty In Molten Metal • Once dissolved in molten metal, gases like N2, O2 and H2 can lead to one or more of several things: – they can remain in solution to cause hardening; – they can remain in solution and stabilize a particular phase – they can be rejected from the melt upon solidification (presuming solubility decreases, as it often does) to produce porosity – they can lead to formation of brittle compounds
Gas Dissolution & Solid solution hardening 32 Representation of a Dislocation Stopped by an Interstitial Atom • Nitrogen and oxygen are potent solid solution strengtheners or hardeners to most metals, whether those metals are ferrous or nonferrous. • Nitrogen and oxygen have this effect because they go into solution by occupying interstitial sites between atoms of the host or solvent.
33 Solid Solution Hardening and Phase Stabillzatlon • As small as the atoms of these gases are, they are still too large to fit into interstices without causing fairly substantial distortion of bonds and storing of energy. • As a result, they increase strength by resisting the motion of dislocations by the repulsion between the strain field they produce and the strain field of dislocations trying to move in response to an applied stress. • The effectiveness of nitrogen as a strengthening addition is comparable to carbon in iron. • Unfortunately, increased strength and hardness comes at the expense of ductility and toughness
34 Porosity Formation • Beyond some limit (the solubility limit), every molten metal oust as every liquid) will be unable to dissolve any more of a particular gas. • Furthermore, that solubility limit in the molten metal usually decreases with decreasing temperature, until at the melting point, upon solidification, the solubility drops precipitously.
Porosity Formation 35 Solidification Solubility decreases Bubble formation Buoyant bubbles attempt to move in the weld pool according to 1. the convection – 2. buoyancy (or gravity) force, 3. a surface tension gradient force, 4. an electromagnetic force, or an 5. impinging or plasma friction force Bubbles may/not escape
36 Porosity → Problems • First, it indicates that shielding was less than adequate, and that unwanted gas-metal reactions are occurring. • Second, pores can easily act as stress risers, thereby promoting brittle (over ductile) fracture and aggravating susceptibility to cyclic loading (fatigue). – The fact that a pore can act to arrest a propagating crack by blunting it, and, thereby, reducing the stress at its tip, is not justification for accepting porosity. Unless every pore is intentionally introduced and controlled in size and location (which is absurd!), • Third, Porosity indicates that the process is not under proper control
Porosity • Single pore • Uniformly scattered • Cluster • Linear • Piping 37
Embrittlement Reactions 38 N2, O2, H2 Nitrides Examples 1. Fe2N in Fe based alloys which reduce ductility and impact toughness. 2. AlN in Al containing steels → Hardness Oxides Examples 1. Fe2O3, 2. Al2O3, 3. SiO2 Hydrides Examples 1. TiH2 causing embrittlement
39 Embrittlement Reactions • Dissolved gases can chemically react with molten metal to form compounds that are almost always (1) undesirable (since they are nonmetallic) and (2) inherently brittle. • N2 can form nitrides → Eg. In Fe based alloys to form acicular (needle-like) and crack-like Fe2N, which reduce ductility and impact toughness. • Nitrides also form in aluminum-containing steels to form extremely hard, but less brittle, aluminum nitride, which is the basis for nitriding steels (about 4 wt% of added Al) to improve their resistance to certain kinds of wear. • O2 can form oxides, as it does in Fe-based alloys, usually with silicon to form nonmetallic silicate inclusions, and in aluminum based alloys to form aluminum oxide inclusions. • H2 can form hydrides, and does in titanium to cause severe embrittlement.
40 Hydrogen Effects • Hydrogen is one of four or five elements (H, C, N, B, and, possibly, O) with a sufficiently small atomic diameter to dissolve interstitially in most metals. • The introduction of hydrogen into construction steels has three major deleterious effects: – (1) Hydrogen embrittlement, – (2) Hydrogen porosity, and – (3) Hydrogen cracking
41 The solubility of hydrogen in iron as a function of temperature.
42 Solubility of H2 in Aluminium
43 Hydrogen Cracking
Hydrogen Cracking 44 Pre-existing defects, cracks Localized regions of biaxial or tri- axial stress concentration Residual or applied stresses Hydrogen diffuses preferentially to these sites because of the lattice expansion that exists As the local H2 concentration increases, the cohesive energy and strength of the lattice decrease
45 Hydrogen Cracking • Preferred models involve the presence of pre-existing defect sites in the material, including small cracks or discontinuities caused by minor phase particles or inclusions. • In the presence of residual or applied stresses, such sites may develop highly localized regions of biaxial or tri-axial stress concentration. • Hydrogen diffuses preferentially to these sites because of the lattice expansion that exists. • As the local hydrogen concentration increases, the cohesive energy and strength of the lattice decrease. • When the cohesive strength falls below the local intensified stress level, spontaneous fracture occurs. Additional hydrogen then evolves in the crack volume, and the process is repeated.
46 4646 Welding Lecture - 15 Weld Quality & Testing 10 Nov 2014, Monday 8.30 am -9.30 am
47 Weld Assessment
48 1. Tensile – ductility, elongation, tensile strength 2. Bending- ductility, elongation, tensile strength 3. Impact - Toughness 4. Hardness – Wear, abrasion 5. Fatigue 6. Cracking - Reeves test Destructive tests
49 Tension Tests: Specimen
Tension Tests • Transverse weld test specimen 50 • Longitudinal weld test specimen
51 Tension Tests • All weld metal test specimen
52 Fillet weld shear Tests
53 Tension shear Tests for resistance welds
Testing of Spot Welds (a) Tension shear test for spot welds. (b) Cross-tension test (c) Twist test. (d) Peel test
55 Bend Tests for corner, butt, lap, T-joints Corner Butt Lap T-Joint
56 Bend Tests-Types • Shows Physical condition of the weld • Determine welds efficiency • Tensile strength • Ductility • Fusion and penetration
Bend test (a) Wrap-around bend test method for thin samples (b) Three-point bending of welded specimens
58 Bend Tests-Root bend test
59 Bend Tests-Face bend test
60 • Three Sides Are Welded With Known Compatible Electrodes. • The front edge is welded with the test electrode. • If incompatible it will crack. Cracking- Reeves test
61 Hardness test
62 • Visual Inspection (VT) • Magnetic Particle Inspection (MT) • Liquid (Dye) Penetrant Inspection (PT) • X-Ray inspection (RT) • Ultrasonic testing (UT) • Air or water pressure testing (LT) Non-destructive inspection of welds
63 Visual Inspection (VT) • Visual is the most common inspection method • VT reveals spatter, excessive buildup, incomplete slag removal, cracks, heat distortion, undercutting, & poor penetration • Typical tools for VT consist of Fillet gauges Magnifying glasses, Flashlights, & Tape measures or calipers.
64 • While welding – The rate the electrode melts – The way the weld metal flows – Sound of the arc – The light given of • After welding – Under cut – Lack of root fusion – Any pin holes from gas or slag – Amount of spatter – Dimensions of weld Visual Inspection (VT)
65 Visual Inspection (VT) • Fillet gauges measure – The “Legs” of the weld – Convexity • (weld rounded outward) – Concavity • (weld rounded inward) – Flatness
66 1. Surface cracks 2. Incomplete root penetration 3. Undercut 4. Underfill on face, groove, or fillet (concave) 5. Underfill of root (suck back) 6. Excessive face reinforcement, groove, or fillet (convex) 7. Excessive root reinforcement 8. Overlap 9. Misalignment 10.Arc strikes 11.Excessive spatter 12.Warpage (distortion) 13.Base metal defects Visual Inspection-Check list of defects
Liquid (Dye) Penetrant Inspection (PT) 67 1. The liquid penetrant (normally red) is applied on the surface containing cracks. 2. Waiting for the liquid penetrates into the cracks. 3. Clean off the excess liquid from the surface, but some liquid still remains in the cracks. 4. Developer (chalk emulsion) is applied to enhance the visible indication of cracks.
68 Liquid (Dye) Penetrant Inspection (PT) • Liquid penetrant inspection uses colored or fluorescent dye to check for surface flaws. • Surface defects: PT will not show sub- surface flaws. • Any material: PT can be used on both metallic and non metallic surfaces such as ceramic, glass, plastic, and metal. • PT dose not require the part to be Magnetized
69 Magnetic Particle Inspection (MT) • Magnetic Particle Inspection (commonly referred to as Magnaflux testing) is only effective at checking for flaws located at or near the surface. • MT uses a metallic powder or liquid along with strong magnetic field probes to locate flaws. (Particles will align along voids) • MT can only be used on materials that can be magnetized
70 Magnetic Particle Inspection (MT) • Sensitivity is maximum if the magnetic field lines are perpendicular to the crack/ defect orientation
Magnetic particle inspection Since defects may occur in various and unknown directions, each part is normally magnetized in two directions at right angles to each other. Magnetic Field Direction Longitudinal (along the axis) Transverse (perpendicular the axis)
Radiographic inspection (RT) 72 • Interior defects (porosity, cracks, voids) can be examined by using X- ray or gamma ray, which can penetrate through materials and its intensity depends on materials thickness and density. • Provide a permanent film record which is easy to interpret. • Slow and expensive, however this method is positive to determine defect size. • RT inspections can reveal flaws deep within a component
73 Ultrasonic testing (UT) • Ultrasonic testing (UT) is a method of determining the size and location of discontinuities within a component using high frequency sound waves. • Sound waves are sent through a transducer into the material and the shift in time require for their return or echo is plotted. • Ultrasonic waves will not travel through air therefore flaws will alter the echo pattern
Ultrasonic testing (UT) 74 There are two ways of using ultrasonic waves for welded joint inspection
75 • The location of the defect in the weld can be calculated • Can be used to test all kinds of metals and materials, complex weldments → widely used. Method and applications (UT)
76 Air or water pressure testing (LT) • Pressure testing or leak testing can be performed with either gasses or liquids. • Voids that allow gasses or liquids to escape from the component can be classified as gross (large) or fine leaks. • Extremely small gas leaks measured in PPM (parts per million) require a “Mass Spectrometer” to Sniff for tracer gases • Used in spacecrafts, pipelines, ships etc.
77 Summary of NDT of welds Method Defects detected Advantages Disadvantages Visual Inspection •Inaccuracies in size, shape, • Surface cracks, porosity, •Undercuts, overlaps •Easy to apply at any stage, •Low cost in capital & labour •No permanent record •Only surface defects Dye penetrant • Surface cracks missed by VT •Easy to apply • Low cost in capital & labour • Only surface cracks Magnetic particle Surface & subsurface flaws • Low cost, portable • only magnetic materials • Interior cracks cannot be detected
78 Summary of NDT of welds Method Defects detected Advantages Disadvantages Radiography •Porosity •Slag •Cracks •Lack of fusion • Reproducible, Permanent record • Deep flaws • Expensive • Safety • Skill Ultrasonics • All subsurface defects • Sensitive • Portable equipment • Complex • Skill Pressure testing Fine through holes/pores • high sensitivity, very fine holes can be detected • Only for closed parts/vessels • Complex set up
79 7979 Welding Lecture - 16 Weld metallurgy, Weldability, etc. 11 Nov 2014, Tuesday 10.30 am -12.30 pm 79
Pure Material Growth Modes • Mechanics and kinetics of solidification → Growth modes or growth morphology • The prevailing thermal gradient affects solidification or growth modes, – a positive temperature gradient ahead of the advancing solid - liquid interface (where temperature increases with distance into the liquid) – or a negative temperature gradient ahead of the advancing solid - liquid interface (where temperature decreases with distance into the liquid) 80
Growth Modes-Planar growth • Growth of the pure solid (i.e., solidification) can progress only as fast a heat can be conducted away from the liquid through the newly formed solid. • Thus, a positive temperature gradient into the liquid of a pure material causes a planar solid interface to be present during solidification. • Planar growth for a positive temperature gradient • Dendritic growth for a negative temperature gradient into the liquid. – The liquid is supercooled throughout its volume, possibly by enhanced cooling from the surface 81
82 Pure Material Growth Modes- Planar growth
Growth Modes- Dendritic growth • Any protuberance that forms on the solid- liquid interface can grow directionally or dendritically, in the direction of the maximum temperature gradient. • Protuberance forms as a result of atoms in the liquid attaching to the crystal lattice of the solid • The latent heat of solidification will be liberated to the super cooled liquid 83
84 Growth Modes- Dendritic growth
Growth (solidification) rate - (R) • Growth of the solid will occur parallel to the maximum temperature gradient (essentially normal to the moving solid-liquid interface at the trailing edge of the weld pool), • The rate of growth R of the solid can be written as R = v Cos φ • Where v is the welding velocity and φ is the angle between the maximum temperature gradient in the liquid (i.e., the average growth direction) and the welding velocity. 85
Growth rate-Elliptical weld pool An elliptical weld pool showing the relationship of rate of growth (R), welding velocity (v), and growth direction (φ). 86 R = v Cos φ
• For an elliptical weld pool, φ varies from 90 ° at the sides of the pool to 0° at the weld center line. • Thus, the rate of growth of solid varies from 0 at the sides of the weld pool to a maximum of v (from v Cos 0, where Cos 0 = 1) at the weld center line. • Thus in an elliptical weld pool, the maximum rate of growth, and, consequently, the maximum rate of evolution of latent heat, occurs at the centerline of the weld at the tail of the weld pool. 87 Growth rate-Elliptical weld pool
• At some critical velocity, the rate of evolution of latent heat at the centerline of the trailing edge of the weld pool exceeds the ability of the system to dissipate it. • When this happens, the rate of growth (solidification) parallel to the welding direction decreases and the weld pool assumes a teardrop shape • The change in weld pool shape, from an ellipse to a teardrop increases the minimum value of φ from 0 ° to some value (say φmin) that is directly proportional to the welding velocity • This in turn, reduces the value of the maximum growth rate to R = v Cos φmin . 88 Growth rate & tear drop formation
Growth rate & tear drop formation 89 Value of the maximum growth rate R = v Cos φmin .
• As the welding velocity is further increased, φmin becomes larger and larger, – the weld pool becomes longer and narrower, – grains continue to converge on one another along the center line of the weld. • At some critical velocity, (which is inversely proportional to the thermal conductivity of the material being welded), a "neck" may appear at the weld center line. • Along the length designated A-B, on both sides of the neck, φ = 90°, and the transverse growth rate falls to 0. • This situation is also unstable, so the neck tends to freeze off, isolating molten regions that develop shrinkage cavities at their centers. 90 Growth rate & tear drop formation
Tear drop-neck formation- shrinkage cracks 91 Along the length designated A-B, on both sides of the neck, φ = 90°, and the transverse growth rate falls to 0.
92 Weldability • Capacity of a material to be welded under the imposed fabrication conditions → into a specific suitably designed structure →and to perform satisfactorily in the intended service • Some base metals or alloys may exhibit good weldability under some conditions, but poor weldability under other conditions 92
93 Weldability • Most important factor is base metal chemical composition • Weldability depends also on the – process, – operating parameters (especially, net linear heat input), – procedures, – Environment (e.g. presence of hydrogen from any form of water or hydrocarbon), 93
94 Weldability • Composition can determine inherent weldability, with – some alloys being inherently weldable, – others being inherently difficult to weld, – and still others being essentially unweldable. • For those materials that are inherently difficult to weld, special attention must be given to the conditions under which welds are to be made • For those few materials (or assemblies) that are unweldable, an alternative method of joining must be sought. 94
95 Weldability tests • Direct tests (actual tests) and Indirect (or simulated) tests. • Direct weldability tests – actual sample of the weld metal or – entire weld zone made in the intended service material, – replicating process, process parameters, and operating conditions, as well as base material conditions, geometry and dimensions, and restraint as closely as possible. • Indirect weldability tests, – utilize basic metallurgical principles to examine welding parameters on a sample – infer the effects of those welding variables on the actual final product. – By simulating the weld thermal cycle so as to create a simulated weld zone, at least in terms of microstructure. 95
Finger test-Crack susceptibility • Involve depositing a weld bead across tightly compressed bars, simulating transverse base metal cracks by the gaps between the bars • Different degrees of severity of weld induced strain can be developed by varying the width of the fingers (more strain for more narrow fingers) • Performance is based on the percentage of the bead width that contains cracks 9696
Houldcroft HotCrack Susceptibility Test • Developed to evaluate cracking tendency of sheet materials to GTAW with or without filler • The tests are made by depositing a weld bead along the length of the specimen, complete penetration being required • Performance is based on crack length 9797
Welding: Areas of recent research interest • Welding automation • Welding of advanced materials/Alloys composites • Clean welding processes • Joining of nano-materials/ nanocomposites
Joining of nano-wires via soldering (a)Assembly of individual nanoobjects into a desired pattern using a nanomanipulator probe. (b)Placement of a sacrificial nanowire in contact with the nanostructure to be welded. (c) Nanowelding the nanoobjects together by an electrical signal (d)Completed nanoweld
Joining of nano-wires via soldering • The nanosolder is deposited by heating a tiny metal wire in contact with the materials to be joined • The solder wire melts and flows onto the join. • The welding can be watched in real-time inside an electron microscope, allowing the choice of exactly where, and how much, nanosolder is deposited.

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Welding lectures 14 16

  • 1. 1 11 Weld Defects 03 Nov 2014, Monday 8.30 am-9.30 am Welding Lecture – 14
  • 2. Weld Defects 2 • Geometric defects • Metallurgical defects
  • 3. 1. Residual stresses • Residual stresses (internal stresses) are stresses that would exist in a body after removing all external loads • Normally due to non uniform temperature change during welding • Weld metal and adjacent base metal are restrained by the areas further away from the weld metal due to expansion and contraction 3
  • 5. Changes in temperature and stresses during welding 5 Welddirection
  • 6. • A-A: Zero temperature and stress distribution • B-B: Small compressive in the weld zone and small tensile in the base metal at B-B during melting of the weld metal. • C-C: Developing of tensile stress in the weld centre and compressive in the area further away at C-C during cooling. • D-D: Further contraction of the weld metal producing higher tensile stress in the weld centre and compressive in the base metal at D-D. 6 Changes in temperature and stresses during welding
  • 7. Effect of temperature and time on stress relief of steel welds 7
  • 8. 2. Distortion 8 Weld distortion is due to solidification shrinkage and thermal contraction of the weld metal during welding
  • 9. Angular distortion 9 • Upward angular distortion usually occurs when the weld is made from the top of the workpiece alone. • The weld tends to be wider at the top than the bottom, causing more solidification shrinkage and thermal contraction.
  • 10. Remedies for angular distortion • Reducing volume of weld metal • Using double-V joint and alternate welding • Placing welds around neutral axis • Controlling weld distortion 10
  • 11. Remedies for angular distortion • Presetting: By compensating the amount of distortion to occur in welding. • Elastic pre-springing can reduce angular changes after restraint is removed. • Preheating and post weld treatment 11
  • 12. Longitudinal distortion Heating and cooling cycles along the joint during welding build up a cumulative effect of longitudinal bowing
  • 13. Remedies for Longitudinal distortion Sequences for welding short lengths of a joint to reduce longitudinal bowing • Welding short lengths on a planned or random distribution are used to controlled this problem • Mechanical methods: straightening press, jacks, clamps • Thermal methods : local heating to relieve stresses (using torches)
  • 14. Defects & Discontinuity • Defect: A flaw or flaws that by nature or accumulated effect render a part or product unable to meet minimum applicable acceptance standards • Defect: The term designates rejectability • Discontinuity: An interruption of the typical structure of a material, such as a lack of homogeneity in its mechanical, metallurgical, or physical characteristics. • A discontinuity is not necessarily a defect 14
  • 15. Weld Defects & Discontinuities 15
  • 17. 17 Mis-alignment • Amount a joint is out of alignment at the root • Cause: Transition thickness, carelessness • Prevention-workmanship • Repair- Grinding
  • 18. Undercut • A groove cut at the toe of the weld and left unfilled • Cause-Electrode angle, high amperage, long arc length, rust • Prevention-set machine on scrap metal, clean metal before welding 18
  • 19. Insufficient fill • The weld surface is below the adjacent surfaces of the base metal 19
  • 20. • Concavity or convexity of fillet weld which exceeds the allowable limit • Cause: Amperage & travel speed • Prevention- Proper parameters & techniques • Repair- Grind 20 Excessive Concavity & Convexity
  • 21. Overlap • Face of the weld extends beyond the toe of the weld 21
  • 23. Burn through • Undesirable open hole – completely melted through base metal • May or maynot left open • Cause- excessive heat input • Prevention-reduce heat input by adjusting parameters • Repair- Filling 23
  • 24. 24 Incomplete fusion 1.Lack of fusion between weld metal & base metal 2.Lack of fusion between multiple layers of weld metal
  • 25. Incomplete fusion • Weld metal does not form a cohesive bond with the base metal • Cause- low amperage, steep electrode angles, fast ravel speed, lack of preheat etc. • Prevention- eliminate potential causes • Repair- remove & reweld 25
  • 26. Cracks 26 • Longitudinal • Transverse • Throat • Toe • Root • Underbead & HAZ
  • 28. 28 Gas metal reactions • Metals react with almost any gas except the noble or inert gases • Gases, including N2, O2 & H2, dissolve in liquids, including molten metals. • Gas molecules (or atoms or ions) occupy the many rather large spaces between atoms of the metal in liquid form
  • 29. 29 Gas Dissolution and Solublllty In Molten Metal • The amount of N2, O2 & H2 that can dissolve in molten metal almost always increases with increasing temp. of the liquid • Also a function of the partial pressure of the gas species above the liquid. • This is expressed by Sievert’s law:
  • 30. 30 Sievert’s law • k is the equilibrium constant, • [gas] is the concentration as weight percent (wt%) of a particular gas in the molten metal, and • Pgas2 is the partial pressure of the particular gas in diatomic molecular form. • This relationship, known as Sievert’s law, applies to all diatomic gases, including N2, O2 and H2
  • 31. 31 Gas Dissolution and Solublllty In Molten Metal • Once dissolved in molten metal, gases like N2, O2 and H2 can lead to one or more of several things: – they can remain in solution to cause hardening; – they can remain in solution and stabilize a particular phase – they can be rejected from the melt upon solidification (presuming solubility decreases, as it often does) to produce porosity – they can lead to formation of brittle compounds
  • 32. Gas Dissolution & Solid solution hardening 32 Representation of a Dislocation Stopped by an Interstitial Atom • Nitrogen and oxygen are potent solid solution strengtheners or hardeners to most metals, whether those metals are ferrous or nonferrous. • Nitrogen and oxygen have this effect because they go into solution by occupying interstitial sites between atoms of the host or solvent.
  • 33. 33 Solid Solution Hardening and Phase Stabillzatlon • As small as the atoms of these gases are, they are still too large to fit into interstices without causing fairly substantial distortion of bonds and storing of energy. • As a result, they increase strength by resisting the motion of dislocations by the repulsion between the strain field they produce and the strain field of dislocations trying to move in response to an applied stress. • The effectiveness of nitrogen as a strengthening addition is comparable to carbon in iron. • Unfortunately, increased strength and hardness comes at the expense of ductility and toughness
  • 34. 34 Porosity Formation • Beyond some limit (the solubility limit), every molten metal oust as every liquid) will be unable to dissolve any more of a particular gas. • Furthermore, that solubility limit in the molten metal usually decreases with decreasing temperature, until at the melting point, upon solidification, the solubility drops precipitously.
  • 35. Porosity Formation 35 Solidification Solubility decreases Bubble formation Buoyant bubbles attempt to move in the weld pool according to 1. the convection – 2. buoyancy (or gravity) force, 3. a surface tension gradient force, 4. an electromagnetic force, or an 5. impinging or plasma friction force Bubbles may/not escape
  • 36. 36 Porosity → Problems • First, it indicates that shielding was less than adequate, and that unwanted gas-metal reactions are occurring. • Second, pores can easily act as stress risers, thereby promoting brittle (over ductile) fracture and aggravating susceptibility to cyclic loading (fatigue). – The fact that a pore can act to arrest a propagating crack by blunting it, and, thereby, reducing the stress at its tip, is not justification for accepting porosity. Unless every pore is intentionally introduced and controlled in size and location (which is absurd!), • Third, Porosity indicates that the process is not under proper control
  • 37. Porosity • Single pore • Uniformly scattered • Cluster • Linear • Piping 37
  • 38. Embrittlement Reactions 38 N2, O2, H2 Nitrides Examples 1. Fe2N in Fe based alloys which reduce ductility and impact toughness. 2. AlN in Al containing steels → Hardness Oxides Examples 1. Fe2O3, 2. Al2O3, 3. SiO2 Hydrides Examples 1. TiH2 causing embrittlement
  • 39. 39 Embrittlement Reactions • Dissolved gases can chemically react with molten metal to form compounds that are almost always (1) undesirable (since they are nonmetallic) and (2) inherently brittle. • N2 can form nitrides → Eg. In Fe based alloys to form acicular (needle-like) and crack-like Fe2N, which reduce ductility and impact toughness. • Nitrides also form in aluminum-containing steels to form extremely hard, but less brittle, aluminum nitride, which is the basis for nitriding steels (about 4 wt% of added Al) to improve their resistance to certain kinds of wear. • O2 can form oxides, as it does in Fe-based alloys, usually with silicon to form nonmetallic silicate inclusions, and in aluminum based alloys to form aluminum oxide inclusions. • H2 can form hydrides, and does in titanium to cause severe embrittlement.
  • 40. 40 Hydrogen Effects • Hydrogen is one of four or five elements (H, C, N, B, and, possibly, O) with a sufficiently small atomic diameter to dissolve interstitially in most metals. • The introduction of hydrogen into construction steels has three major deleterious effects: – (1) Hydrogen embrittlement, – (2) Hydrogen porosity, and – (3) Hydrogen cracking
  • 41. 41 The solubility of hydrogen in iron as a function of temperature.
  • 42. 42 Solubility of H2 in Aluminium
  • 44. Hydrogen Cracking 44 Pre-existing defects, cracks Localized regions of biaxial or tri- axial stress concentration Residual or applied stresses Hydrogen diffuses preferentially to these sites because of the lattice expansion that exists As the local H2 concentration increases, the cohesive energy and strength of the lattice decrease
  • 45. 45 Hydrogen Cracking • Preferred models involve the presence of pre-existing defect sites in the material, including small cracks or discontinuities caused by minor phase particles or inclusions. • In the presence of residual or applied stresses, such sites may develop highly localized regions of biaxial or tri-axial stress concentration. • Hydrogen diffuses preferentially to these sites because of the lattice expansion that exists. • As the local hydrogen concentration increases, the cohesive energy and strength of the lattice decrease. • When the cohesive strength falls below the local intensified stress level, spontaneous fracture occurs. Additional hydrogen then evolves in the crack volume, and the process is repeated.
  • 46. 46 4646 Welding Lecture - 15 Weld Quality & Testing 10 Nov 2014, Monday 8.30 am -9.30 am
  • 48. 48 1. Tensile – ductility, elongation, tensile strength 2. Bending- ductility, elongation, tensile strength 3. Impact - Toughness 4. Hardness – Wear, abrasion 5. Fatigue 6. Cracking - Reeves test Destructive tests
  • 50. Tension Tests • Transverse weld test specimen 50 • Longitudinal weld test specimen
  • 51. 51 Tension Tests • All weld metal test specimen
  • 53. 53 Tension shear Tests for resistance welds
  • 54. Testing of Spot Welds (a) Tension shear test for spot welds. (b) Cross-tension test (c) Twist test. (d) Peel test
  • 55. 55 Bend Tests for corner, butt, lap, T-joints Corner Butt Lap T-Joint
  • 56. 56 Bend Tests-Types • Shows Physical condition of the weld • Determine welds efficiency • Tensile strength • Ductility • Fusion and penetration
  • 57. Bend test (a) Wrap-around bend test method for thin samples (b) Three-point bending of welded specimens
  • 60. 60 • Three Sides Are Welded With Known Compatible Electrodes. • The front edge is welded with the test electrode. • If incompatible it will crack. Cracking- Reeves test
  • 62. 62 • Visual Inspection (VT) • Magnetic Particle Inspection (MT) • Liquid (Dye) Penetrant Inspection (PT) • X-Ray inspection (RT) • Ultrasonic testing (UT) • Air or water pressure testing (LT) Non-destructive inspection of welds
  • 63. 63 Visual Inspection (VT) • Visual is the most common inspection method • VT reveals spatter, excessive buildup, incomplete slag removal, cracks, heat distortion, undercutting, & poor penetration • Typical tools for VT consist of Fillet gauges Magnifying glasses, Flashlights, & Tape measures or calipers.
  • 64. 64 • While welding – The rate the electrode melts – The way the weld metal flows – Sound of the arc – The light given of • After welding – Under cut – Lack of root fusion – Any pin holes from gas or slag – Amount of spatter – Dimensions of weld Visual Inspection (VT)
  • 65. 65 Visual Inspection (VT) • Fillet gauges measure – The “Legs” of the weld – Convexity • (weld rounded outward) – Concavity • (weld rounded inward) – Flatness
  • 66. 66 1. Surface cracks 2. Incomplete root penetration 3. Undercut 4. Underfill on face, groove, or fillet (concave) 5. Underfill of root (suck back) 6. Excessive face reinforcement, groove, or fillet (convex) 7. Excessive root reinforcement 8. Overlap 9. Misalignment 10.Arc strikes 11.Excessive spatter 12.Warpage (distortion) 13.Base metal defects Visual Inspection-Check list of defects
  • 67. Liquid (Dye) Penetrant Inspection (PT) 67 1. The liquid penetrant (normally red) is applied on the surface containing cracks. 2. Waiting for the liquid penetrates into the cracks. 3. Clean off the excess liquid from the surface, but some liquid still remains in the cracks. 4. Developer (chalk emulsion) is applied to enhance the visible indication of cracks.
  • 68. 68 Liquid (Dye) Penetrant Inspection (PT) • Liquid penetrant inspection uses colored or fluorescent dye to check for surface flaws. • Surface defects: PT will not show sub- surface flaws. • Any material: PT can be used on both metallic and non metallic surfaces such as ceramic, glass, plastic, and metal. • PT dose not require the part to be Magnetized
  • 69. 69 Magnetic Particle Inspection (MT) • Magnetic Particle Inspection (commonly referred to as Magnaflux testing) is only effective at checking for flaws located at or near the surface. • MT uses a metallic powder or liquid along with strong magnetic field probes to locate flaws. (Particles will align along voids) • MT can only be used on materials that can be magnetized
  • 70. 70 Magnetic Particle Inspection (MT) • Sensitivity is maximum if the magnetic field lines are perpendicular to the crack/ defect orientation
  • 71. Magnetic particle inspection Since defects may occur in various and unknown directions, each part is normally magnetized in two directions at right angles to each other. Magnetic Field Direction Longitudinal (along the axis) Transverse (perpendicular the axis)
  • 72. Radiographic inspection (RT) 72 • Interior defects (porosity, cracks, voids) can be examined by using X- ray or gamma ray, which can penetrate through materials and its intensity depends on materials thickness and density. • Provide a permanent film record which is easy to interpret. • Slow and expensive, however this method is positive to determine defect size. • RT inspections can reveal flaws deep within a component
  • 73. 73 Ultrasonic testing (UT) • Ultrasonic testing (UT) is a method of determining the size and location of discontinuities within a component using high frequency sound waves. • Sound waves are sent through a transducer into the material and the shift in time require for their return or echo is plotted. • Ultrasonic waves will not travel through air therefore flaws will alter the echo pattern
  • 74. Ultrasonic testing (UT) 74 There are two ways of using ultrasonic waves for welded joint inspection
  • 75. 75 • The location of the defect in the weld can be calculated • Can be used to test all kinds of metals and materials, complex weldments → widely used. Method and applications (UT)
  • 76. 76 Air or water pressure testing (LT) • Pressure testing or leak testing can be performed with either gasses or liquids. • Voids that allow gasses or liquids to escape from the component can be classified as gross (large) or fine leaks. • Extremely small gas leaks measured in PPM (parts per million) require a “Mass Spectrometer” to Sniff for tracer gases • Used in spacecrafts, pipelines, ships etc.
  • 77. 77 Summary of NDT of welds Method Defects detected Advantages Disadvantages Visual Inspection •Inaccuracies in size, shape, • Surface cracks, porosity, •Undercuts, overlaps •Easy to apply at any stage, •Low cost in capital & labour •No permanent record •Only surface defects Dye penetrant • Surface cracks missed by VT •Easy to apply • Low cost in capital & labour • Only surface cracks Magnetic particle Surface & subsurface flaws • Low cost, portable • only magnetic materials • Interior cracks cannot be detected
  • 78. 78 Summary of NDT of welds Method Defects detected Advantages Disadvantages Radiography •Porosity •Slag •Cracks •Lack of fusion • Reproducible, Permanent record • Deep flaws • Expensive • Safety • Skill Ultrasonics • All subsurface defects • Sensitive • Portable equipment • Complex • Skill Pressure testing Fine through holes/pores • high sensitivity, very fine holes can be detected • Only for closed parts/vessels • Complex set up
  • 79. 79 7979 Welding Lecture - 16 Weld metallurgy, Weldability, etc. 11 Nov 2014, Tuesday 10.30 am -12.30 pm 79
  • 80. Pure Material Growth Modes • Mechanics and kinetics of solidification → Growth modes or growth morphology • The prevailing thermal gradient affects solidification or growth modes, – a positive temperature gradient ahead of the advancing solid - liquid interface (where temperature increases with distance into the liquid) – or a negative temperature gradient ahead of the advancing solid - liquid interface (where temperature decreases with distance into the liquid) 80
  • 81. Growth Modes-Planar growth • Growth of the pure solid (i.e., solidification) can progress only as fast a heat can be conducted away from the liquid through the newly formed solid. • Thus, a positive temperature gradient into the liquid of a pure material causes a planar solid interface to be present during solidification. • Planar growth for a positive temperature gradient • Dendritic growth for a negative temperature gradient into the liquid. – The liquid is supercooled throughout its volume, possibly by enhanced cooling from the surface 81
  • 82. 82 Pure Material Growth Modes- Planar growth
  • 83. Growth Modes- Dendritic growth • Any protuberance that forms on the solid- liquid interface can grow directionally or dendritically, in the direction of the maximum temperature gradient. • Protuberance forms as a result of atoms in the liquid attaching to the crystal lattice of the solid • The latent heat of solidification will be liberated to the super cooled liquid 83
  • 85. Growth (solidification) rate - (R) • Growth of the solid will occur parallel to the maximum temperature gradient (essentially normal to the moving solid-liquid interface at the trailing edge of the weld pool), • The rate of growth R of the solid can be written as R = v Cos φ • Where v is the welding velocity and φ is the angle between the maximum temperature gradient in the liquid (i.e., the average growth direction) and the welding velocity. 85
  • 86. Growth rate-Elliptical weld pool An elliptical weld pool showing the relationship of rate of growth (R), welding velocity (v), and growth direction (φ). 86 R = v Cos φ
  • 87. • For an elliptical weld pool, φ varies from 90 ° at the sides of the pool to 0° at the weld center line. • Thus, the rate of growth of solid varies from 0 at the sides of the weld pool to a maximum of v (from v Cos 0, where Cos 0 = 1) at the weld center line. • Thus in an elliptical weld pool, the maximum rate of growth, and, consequently, the maximum rate of evolution of latent heat, occurs at the centerline of the weld at the tail of the weld pool. 87 Growth rate-Elliptical weld pool
  • 88. • At some critical velocity, the rate of evolution of latent heat at the centerline of the trailing edge of the weld pool exceeds the ability of the system to dissipate it. • When this happens, the rate of growth (solidification) parallel to the welding direction decreases and the weld pool assumes a teardrop shape • The change in weld pool shape, from an ellipse to a teardrop increases the minimum value of φ from 0 ° to some value (say φmin) that is directly proportional to the welding velocity • This in turn, reduces the value of the maximum growth rate to R = v Cos φmin . 88 Growth rate & tear drop formation
  • 89. Growth rate & tear drop formation 89 Value of the maximum growth rate R = v Cos φmin .
  • 90. • As the welding velocity is further increased, φmin becomes larger and larger, – the weld pool becomes longer and narrower, – grains continue to converge on one another along the center line of the weld. • At some critical velocity, (which is inversely proportional to the thermal conductivity of the material being welded), a "neck" may appear at the weld center line. • Along the length designated A-B, on both sides of the neck, φ = 90°, and the transverse growth rate falls to 0. • This situation is also unstable, so the neck tends to freeze off, isolating molten regions that develop shrinkage cavities at their centers. 90 Growth rate & tear drop formation
  • 91. Tear drop-neck formation- shrinkage cracks 91 Along the length designated A-B, on both sides of the neck, φ = 90°, and the transverse growth rate falls to 0.
  • 92. 92 Weldability • Capacity of a material to be welded under the imposed fabrication conditions → into a specific suitably designed structure →and to perform satisfactorily in the intended service • Some base metals or alloys may exhibit good weldability under some conditions, but poor weldability under other conditions 92
  • 93. 93 Weldability • Most important factor is base metal chemical composition • Weldability depends also on the – process, – operating parameters (especially, net linear heat input), – procedures, – Environment (e.g. presence of hydrogen from any form of water or hydrocarbon), 93
  • 94. 94 Weldability • Composition can determine inherent weldability, with – some alloys being inherently weldable, – others being inherently difficult to weld, – and still others being essentially unweldable. • For those materials that are inherently difficult to weld, special attention must be given to the conditions under which welds are to be made • For those few materials (or assemblies) that are unweldable, an alternative method of joining must be sought. 94
  • 95. 95 Weldability tests • Direct tests (actual tests) and Indirect (or simulated) tests. • Direct weldability tests – actual sample of the weld metal or – entire weld zone made in the intended service material, – replicating process, process parameters, and operating conditions, as well as base material conditions, geometry and dimensions, and restraint as closely as possible. • Indirect weldability tests, – utilize basic metallurgical principles to examine welding parameters on a sample – infer the effects of those welding variables on the actual final product. – By simulating the weld thermal cycle so as to create a simulated weld zone, at least in terms of microstructure. 95
  • 96. Finger test-Crack susceptibility • Involve depositing a weld bead across tightly compressed bars, simulating transverse base metal cracks by the gaps between the bars • Different degrees of severity of weld induced strain can be developed by varying the width of the fingers (more strain for more narrow fingers) • Performance is based on the percentage of the bead width that contains cracks 9696
  • 97. Houldcroft HotCrack Susceptibility Test • Developed to evaluate cracking tendency of sheet materials to GTAW with or without filler • The tests are made by depositing a weld bead along the length of the specimen, complete penetration being required • Performance is based on crack length 9797
  • 98. Welding: Areas of recent research interest • Welding automation • Welding of advanced materials/Alloys composites • Clean welding processes • Joining of nano-materials/ nanocomposites
  • 99. Joining of nano-wires via soldering (a)Assembly of individual nanoobjects into a desired pattern using a nanomanipulator probe. (b)Placement of a sacrificial nanowire in contact with the nanostructure to be welded. (c) Nanowelding the nanoobjects together by an electrical signal (d)Completed nanoweld
  • 100. Joining of nano-wires via soldering • The nanosolder is deposited by heating a tiny metal wire in contact with the materials to be joined • The solder wire melts and flows onto the join. • The welding can be watched in real-time inside an electron microscope, allowing the choice of exactly where, and how much, nanosolder is deposited.