Presentation TranscriptWELD DISCONTINUTIES : WELD DISCONTINUTIES By YADAV VISHAL FULCHAND INSPECTION ENGINEER MANGALORE REFINERY AND PETROCHEMICALS LIMITED

WHAT IS DISCONTINUTITY? : WHAT IS DISCONTINUTITY? AWS D 1.1-96 An interruption of the typical structure of a material such as lack of homogeneity in its mechanical ,or physical characteristics. A discontinuity is an objective lack of material, an interruption in the physical consistence of a part. Examples are cracks, seams, laps, porosity or inclusions. It may or may not be considered a defect depending if its presence endangers or not the integrity, the usefulness and the serviceability of the structure.

WHAT IS WELDING DEFECT? : WHAT IS WELDING DEFECT? AWS D 1.1-96 A discontinuity or discontinuities that by a nature or accumulated defect (e.g. total crack length) render a part or a product unable to meet the minimum acceptance standard or specifications They are excessive conditions, outside the acceptance limits, which risks to compromise the stability or the functionality of the welded structure. They are also called rejectable discontinuities. This means that the same type of discontinuity of a lesser degree, might be considered harmless and acceptable.

Slide 4: This means DISCONTUITY is not a defect if it is acceptable within the limitation of applicable codes and standards. Whereas DEFECT is definitely discontinuity and defines rejectability of a part or product. Defects are otherwise called as Flaw or Imperfections that are to be repaired so that the material is made more homogeneous as assumed in the design

Influence of Discontinuities : : Influence of Discontinuities : They act as STRESS RAISERS. They reduce the cross sectional area and thereby INCREASES STRESS CONCENTRATION around them. They provide a leak path. They are potential site for CRACK initiation. They act as potential site for CORROSION & EROSION.

Characteristics of Discontinuities: : Characteristics of Discontinuities: Shape. Acuity, sharpness or notch effect. Orientation with respect to principal working stress and residual stress. Location with respect to weld.

Production without Defects helps in: : Production without Defects helps in: Save work time Material repair cost Decrease in Productivity

Significance of defects depends on: : Significance of defects depends on: Microstructure in which defect occurs. Mechanical properties of the material. Type of loading (static, cyclic or shock) The environment (corrosive or non corrosive) Section thickness Type and size of defect Stress pattern local to the defect.

Classification of Discontinuities: : Classification of Discontinuities: Broadly classified into: Planner defects or Two-dimensional defects e.g. crack, lack of fusion ,lack of penetration, lamination etc. Voluminar defects or Three-dimensional defects e.g. Porosity, Inclusions etc.

Classification based on welding process and procedure related: : Classification based on welding process and procedure related: GEMOMETRIC DISCONTINUTIES Misalignment, Undercut, Convexity & Concavity, Excessive reinforcement, Incomplete reinforcement, Lack of Penetration, Lack of fusion, Burn through, Overlap, Surface irregularity etc. OTHERS Arc strikes, Slag inclusions, Tungsten inclusions, Spatter, Arc craters, Oxide films etc.

Classification based on Metallurgical: : Classification based on Metallurgical: Cracks or fissures Hot crack, cold crack, Lamellar tear etc. Porosity Spherical, Elongated worm holes etc.

POROSITY : POROSITY It is a cavity type discontinuity (void) formed by gas entrapment during solidification. Gases present in weld environment H2,N2,O2,CO,CO2 etc from flux covered electrodes. These gases trapped during solidification and cause porosity

Porosity : Porosity

Classification of Porosity: : Classification of Porosity: According to its shape, size and location: Uniformly scattered Cause: Faulty material or poor technique used thought the weld. Clustered (localized) porosity. Cause: Improper termination/initiation of weld.

Classification of Porosity: : Classification of Porosity: Linear (aligned) porosity Porosity occur in a repetition pattern along the weld usually associated with root or inter pass in connection with LOP/LOF. CAUSE: Chemical reaction at weld joint boundary,which produces unwanted gas.This can be avoided by preparing the joint properly.

Slide 16: Wormhole (Elongated porosity) Elongated or tubular cavities caused by continued entrapment of gases at the solidifying interface e.g. piping porosity, herringbone, solidification porosity. It extend from weld root towards surface & is also evidence of the presence of surface contaminations.

Factors promoting porosity: : Factors promoting porosity: Improper coating on the electrode. Longer arcs. Faster arc travel speeds. Too low and too high arc currents. Incorrect welding technique (stringer beads are more apt than moderately weaved beads. Unclean job surface. Improper base metal composition (e.g. high S & C content will lead to porosity)

POROSITY REMDIES: : 

POROSITY REMDIES: Use low hydrogen welding process, filler metals high in deoxidizers for excessive H2,N2 atmosphere. Use preheat or increase heat input to combat high solidification rate. Clean joint surfaces and adjacent surfaces. For reducing excessive moisture use recommended procedure for baking and storing electrodes

POROSITY : POROSITY

Radiograph:Porosity : Radiograph:Porosity

Radiograph:Cluster porosity : Radiograph:Cluster porosity

POROSITY SIGNIFICANCE : POROSITY SIGNIFICANCE Less than 3 % by volume has an insignificant effect on static tensile yield strength Ductility is affected more – Higher the yield of the weld more is the effect Hydrogen Pore may act as a crack initiator in the weld. When the surface is ground the effect of surface pores became significant In fillet welds, stress concentration effects of weld toes, starts and stops were significant. Internal porosity does not affect the fatigue strength Surface porosity indicates the loss of process control during welding

CRACKS : CRACKS A CRACK is a fissure produced in a metal by tearing action. TYPES OF CRACKES Hot crack/Solidification crack Cold crack. Lamellar Tearing.

CRACKS : CRACKS

CRACKS : CRACKS

Slide 26: TYPES OF CRACKING

Cracks further classified into: : Cracks further classified into: Direction with respect to longitudinal axis of the weld. Longitudinal crack:Those lying in a direction parallel to the longitudinal weld axis. Transverse crack: Those lying in a direction perpendicular to the longitudinal weld axis.

Cracks classification: : Cracks classification: With respect to their locations: Throat crack: Root Crack: Toe Crack: Crater Crack. HAZ crack. Base metal crack.

Hot Cracks : Hot Cracks Cracks formed immediately after welding. Also called centerline or solidification crack.

Slide 30: 10 mm Hot Crack

Hot Cracking Causes: : Hot Cracking Causes: Imposition of excessive strains in restrained welds Parent metal composition with excess sulphur, Phosphorus Base metal contamination Inadequate filling of crater ( crater cracking)

Hot Cracking Remedies: : Hot Cracking Remedies: Plan welding procedures which induce less thermal strain Use low sulphur, phosphorus containing base metal for welding Avoid contamination of base metal

COLD CRACK : COLD CRACK Cold cracking is caused due to metal brittleness combined with a tensile stress exceeding the fracture stress. Brittleness is the result of (a) Solidification of low melting constituents. (b) Phase changes (e.g. formation of martensite) during cooling.

Cold crack: : Cold crack: They are also called as UNDERBEAD CRACK or DELAYED CRACK. Generally occurs in HAZ They lie in directly adjacent to weld bead in HAZ area. Subsurface and difficult to detect. However propagate to surface and can be seen visually.

Cold crack : Cold crack Underbead crack occurs due to presence of Hydrogen in weld pool. Hydrogen comes from: Base metal,welding electrodes, organic surface contamination,surrounding atmosphere This hydrogen may be absorbed by HAZ as H+ ions when the metal is in hot condition. When solidified the metal has less capacity for the hydrogen and move though metal structure to grain boundary. At this instance individual H+ ions combines & form H2 .This gases forms of hydrogen require more volume and large enough to move though metal structure.

Factors promoting Cold crack : Factors promoting Cold crack Joint restraint and high thermal severity. Hydrogen in the weld metal. Presence of impurities. Embrittlement of HAZ in low alloy steels. Weld of insufficient sectional area. High welding speeds and low current density. Cold cracking occurs in both weld metal and HAZ.

COLD CRACK : COLD CRACK

COLD CRACK : COLD CRACK

COLD CRAKING REMEDIES: : COLD CRAKING REMEDIES: Welding consumables must be hydrogen controlled Bake electrodes as recommended by electrode manufacturer Preheat as per Welding Procedure Specification Heat input to be maintained even for tack welding

Lamellar Tearing : Lamellar Tearing Fracture separation in heavy weldments, found within or just beneath HAZ of thick plates which were not adequately refined by the steel mill. Due to severe restraint on the joint. Poor ductility Presence of non-metallic inclusions running parallel to the base metal surface. Plate opens up as though it were made of stacked sheets or lamellae so called Lamellar tearing. T and corner joints are more susceptible to lamellar tearing than others.

Lamellar tearing : Lamellar tearing

Radiograph showing Crack : Radiograph showing Crack

CRACKS SIGNIFICANCE : CRACKS SIGNIFICANCE The most detrimental defect in welding They act as stress raisers for crack growth and not acceptable No fabrication code permits cracks

Incomplete joint penetration: : Incomplete joint penetration: Penetration is the distance from the base metal top surface to the maximum extent of the weld nugget.

Incomplete penetration : Incomplete penetration

Excessive Penetration : Excessive Penetration

Causes of Incomplete joint penetration: : Causes of Incomplete joint penetration: Improper Joints( U joint gives better than J butt joint) Too large root face. Root gap too small. Less arc current. Faster arc travel speed. Too large electrode diameter. Misalignment.

Radiograph:Incomplete penetration : Radiograph:Incomplete penetration

Remedies of Incomplete Penetration: : Remedies of Incomplete Penetration: Use proper joint geometry. Increase root opening or Use small electrode in root. Follow correct WPS (Welding Procedure Specification).

Lack of fusion : Lack of fusion “Weld discontinuity in which fusion did not occur between weld metal and fusion faces joining or adjoining weld beads.”

Lack of Fusion(LOF) : Lack of Fusion(LOF)

Factors promoting LOF: : 

Factors promoting LOF: Improper manipulation of welding electrode. Weld joint design. Improper heat input. Surface contamination which leads slag formation prevents fusion.

LOF REMEDIES: : LOF REMEDIES: Minimum heat input to be maintained Correct Electrode angle Avoid molten pool flooding the arc Proper cleaning of oxides slag

Radiograph:Lack of Fusion : Radiograph:Lack of Fusion

INCLUSIONS: : INCLUSIONS: Inclusion is entrapped foreign solid material,such as slag,flux,tungsten or oxide. Associated with Lack of Fusion(LOF). Only occurs when any welding technique uses Flux shielding. Due to Improper manipulation of welding electrode and improper cleaning between multi run .

Radiograph:Slag Inclusion : Radiograph:Slag Inclusion

INCLUSIONS : INCLUSIONS Tungsten inclusions are almost associated with GTAW or TIG welding process. Tungsten electrode makes the contact with molten weld puddle and remain in the final weld if not remove by grinding. Welding current is more than recommended.

REMEDIES FOR INCLUSION : REMEDIES FOR INCLUSION Better inter-pass cleaning Remove heavy convex beads Do not use electrodes with damaged coating

Radiograph:Tungsten inclusion : Radiograph:Tungsten inclusion

UNDERCUT : UNDERCUT Undercut is a surface discontinuity which occurs in the base metal directly adjacent to the weld. It is a condition in which the base metal has been melted away during the welding operation and there was insufficient filler metal deposited to adequately fill the resulting depression. This result is a linear groove in the base metal which may have a relatively sharp configuration. Cause:Improper welding technique, Too high welding heat

Undercut : Undercut

Radiograph: UNDERCUT : Radiograph: UNDERCUT

Radiograph: UNDERCUT : Radiograph: UNDERCUT

Underfill : 

Underfill Underfill A condition in which the weld face or root surface extends below the adjacent surface of the base metal. Underfill at the root weld of Pipes are termed as INTERNAL CONCAVITY or SUCK BACK It can be cause by excessive heating and melting of root pass during deposition of second pass. Cause: Excessive travel speed

Underfill : Underfill

Radiograph:Internal concavity : Radiograph:Internal concavity

Overlap : Overlap Overlap is described as the protrusions of weld metal beyond the weld toe or weld root. Also called as rollover Cause:Improper welding technique like slow arc travel speed.

Overlap : Overlap

Misalignment : Misalignment Offset or mismatch occurs where two pieces welded together are not properly aligned.

Inadequate weld reinforcement : Inadequate weld reinforcement

Excessive weld reinforcement : Excessive weld reinforcement

Spatter : Spatter AWS A 3.0 describes spatter as “metal particles expelled during fusion welding that do not form a part of weld. Cause: High welding current which can cause excessive turbulence in the weld zone. Its presence gives difficulty in UT,MT,PT Premature failure of applied coatings.

Spatter : Spatter

Arc Strikes : Arc Strikes Arc strikes result when the arc is initiated on the base metal surface away from the weld joint,there is a localized area of the base metal surface which melts and rapidly cooled. In high alloy steel it can create localized HAZ which leads to crack initiation. Cause: Improper welding technique