Your SlideShare is downloading. ×
Module 9
WELD AND BASE METAL DISCONTINUITIES. Discontinuity is described as any
interruption in the uniform nature of an i...
cracks. Delayed or under bead cracks resulting from entrapped hydrogen would also be
categorized as cold cracks. The crack...
sharp end conditions. Incomplete fusion also has slag inclusion associated with it. In fact
the presence of slag may preve...
159/184--POROSITY—It is cavity type discontinuity formed by gas entrapment
during solidification. Since this is normally s...
preset which can reduce size and strength of a fillet weld. The excessive convexity
causes problem due to sharp notches at...
are large and take planar shape they are referred as laminations. Massive form of
laminations arises from a condition refe...
dissimilar magnetic properties can also produce this deflection. Detection of such
        missed joint is very difficult ...
Module 9
Upcoming SlideShare
Loading in...5
×

Module 9

1,347

Published on

1 Comment
1 Like
Statistics
Notes
No Downloads
Views
Total Views
1,347
On Slideshare
0
From Embeds
0
Number of Embeds
1
Actions
Shares
0
Downloads
55
Comments
1
Likes
1
Embeds 0
No embeds

No notes for slide

Transcript of "Module 9"

  1. 1. Module 9 WELD AND BASE METAL DISCONTINUITIES. Discontinuity is described as any interruption in the uniform nature of an item. It is some feature which introduces an irregularity in an otherwise uniform structure. A defect is a specific discontinuity which can impair the suitability of that structure for the intended use. Hence defect is a discontinuity which occurs in an amount great enough to render the item unsuitable for its intended use/ service based on the criteria in the applicable code. When the size or concentration of a discontinuity exceeds its acceptable limit of the discontinuity it is deemed as defect. It implies that the defect is reject able and requires some action or treatment to bring it to its acceptable limit to a particular code. 154/184 -- Configuration of Discontinuity 1. Linear, 2. Non linear. Linear discontinuities exhibit lengths which are much greater than width. Non linear discontinuity have length and width dimensions essentially same. The discontinuity perpendicular the direction of applied load/ stress represents a far more critical situation than a non linear type as it can more likely propagate and cause failure. Sharper the end of discontinuity more critical is the defect because it is more likely to propagate. If a linear discontinuity have sharp end conditions lying transverse to the applied force/ stress it becomes most detrimental situation so far as ability of the member to carry an applied load is concerned. CRACKS, INCOMPLETE FUSION, INCOMPLETE JOINT PENETRATION, SLAG INCLUSION AND POROSITY is the order of discontinuity from sharpest. If at both ends of crack holes are drilled the propagation of crack can be stopped as the sharp ends are rounded off sufficiently by the radius of the drilled hole to reduce stress concentration to the point material can with stand applied load without further crack propagation. If a wire is having a notch made by a sharp chisel it can be broken by bending 2 or 3 times as notch represents a significant stress concentration of the applied bending load. For a structure which must with stand fatigue loading, surface should be free of discontinuities providing sharp notches, abrupt change in contour or geometry should be avoided. LIST OF DEFECTS 1. CRACK, 2. INCOMPLETE FUSION, 3. INCOMPLETE JOINT PENETRATION, 4.INCLUSION 5. SLAG INCLUSION 6. TUNGSTEN INCLUSION 7. POROSITY 8. UNDERCUT 9. UNDERFILL 10. OVERLAP 11. CONVEXITY 12 WELD REINFORCEMENT 13. ARC STRIKE 14. SPATTER 15. LAMINATION 16. LAMELLAR TEAR 17. SEAM/LAP 18 DIMENSIONAL. CRACKS -- Crack is generally linear and exhibits very sharp end conditions. The cracks have tendency for it to grow or propagate if stress is applied. Cracks are initiated when load or stress applied exceeds its tensile strength . Overload conditions cause cracks. Stress can occur during welding, or immediately afterwards or when load is applied. Even if applied load does not cross the load carrying capacity/ability of the member, the presence of notch, or stress riser could cause localized stress at the tip of the stress riser to exceed the tensile strength of material. In such case cracking can occur at these stress concentration points. Cracks can be hot or cold cracks. Hot crack occurs when metal solidifies at some elevated temperature. The propagation is considered inter granular, i.e. cracks occur between individual grains. Cold cracks occur after the metal has cooled to ambient temperature. Cracks resulting from service conditions would be considered cold
  2. 2. cracks. Delayed or under bead cracks resulting from entrapped hydrogen would also be categorized as cold cracks. The cracks can be longitudinal or transverse cracks. The cracks can be designated by location of weld i. e. Throat, Root, Toe, Crater, Under bead, HAZ, and base metal cracks. Throat cracks are said so as it extends through the weld along the weld throat, or shortest path through weld cross section. They are longitudinal cracks and generally considered hot cracks. It can be observed visually on weld face centerline crack is other name for this. Thin root pass and concave fillet welds could result in a throat crack. ROOT CRACKS.—These are usually longitudinal, however they may propagate in either weld metal or base metal. They are generally related to the existence of shrinkage stresses from welding and hence they are considered as hot cracks. They occur when the joints are improperly fitted or prepared. Large root opening may result in stress concentration which produces root cracks. TOE CRACKS—These are base metal cracks and propagate from toes of welds. Weld reinforcement or convexity may provide a stress riser at the weld toes. This combined with a less ductile micro structure in the heat affected zone increases the susceptibility of the weldment to toe crack. These are generally cold cracks. The stress causing occurrence of toe crack could be the result of either transverse shrinkage stresses of welding, some applied service stresses or both, Toe cracks occurring in service are due to fatigue loading of welded components. CRATER CRACKS—These occur at termination point of individual weld pass. If due to wrong welding technique of arc termination when molten weld puddle does not get completely filled, the result can be crater. The presence of this thinned area combined with shrinkage stresses from welding may cause crater cracks. When there is a array of crater cracks they are commonly referred as start cracks. These occur during solidification hence hot cracks. These cracks can also result from the use of filler metals having flow characteristics which produce concave profiles when solidified. Stainless steel electrodes ending with 16 (E 308-16, E309-16) have Titanium type coating which will produce flat or slightly concave weld provide. When these electrodes are used craters must be filled properly to prevent craters cracks. UNDER BEAD CRACKS-- It is located in HAZ instead of weld metal. It will lie directly adjacent to the weld fusion line in HAZ In cross sectional view under bead cracks will often appear to run directly parallel to the fusion line of a weld. These are typically sub surface and difficult to detect. Some times these may propagate to the surface also which allows for their detection during visual examination. UB cracks do not propagate until many hours after welding. Some times these are called delayed cracks as it is likely to be noticed some times after 48 to 72 hours of weld cooling down to ambient temperature, especially high strength steel like ASTM A 514 need to be inspected after 72 hours of welding. UB cracks result from the presence of hydrogen in the weld zone. Hydrogen is added either from filler metal, base metal, surrounding atmosphere or organic surface contamination. Under hot condition metal can absorb great deal of this atomic or nascent hydrogen. H. However when solidified metal has much less capacity to absorb hydrogen. If the surrounding metal does not exhibit sufficient ductility the internal pressure created by trapped hydrogen molecules can result in under bead cracking. INCOMPLETE FUSION—It is a weld discontinuity in which fusion did not occur between weld metal and fusion faces and or adjoining weld beads. It is linear and has
  3. 3. sharp end conditions. Incomplete fusion also has slag inclusion associated with it. In fact the presence of slag may prevent fusion from occurring. It is not an internal weld flaw only. It can occur at the surface also. Cold lap is also other non standard term for IF. The most common cause for this discontinuity is the improper manipulation of electrode by the welder. In GTAW welder must concentrate on directing the arc at every location of weld joint where fusion is required. In a small weld groove if it is insufficient groove angle then IF can occur. Further extreme contamination including mill scale and tenacious oxide layer could also prevent complete fusion. 157/184—In the radio graph it appears as a dark density lines which are generally straighter than the images of either cracks or elongated slag. The lateral position of these indications on the film will be a hint as to their actual path. In a single V groove weld incomplete fusion near the root will appear near the weld center line where as IF closer to the face shall appear as an image positioned closer to the weld toe. 158/184--INCOMPLETE JOINT PREPARATION—It is only with the groove weld that the IJP is noticed. It is a condition where weld metal does not join the base metal in the joint where complete penetration is essential as per specs. Its location is always adjacent to the root. Few codes place limits on amount and degree of IJP permissible. It can be caused by Improper technique, improper joint configuration or excessive contamination. Radiograph shall show a typically dark straight line. INCLUSIONS--It is an entrapped foreign solid material such as slag, flux, tungsten or oxide, it can be metallic or non metallic. While generally slag inclusion is totally contained within weld cross section some times slag inclusion is observed at the surface of weld as well. Slag inclusion can occur between weld and base metal or between individual weld beads. Density of slag is usually much less than that of the metal hence slag inclusion appear on radiograph as dark indications. When the density of slag and base metal is same it is difficult to find out slag inclusion by RT. Tungsten inclusion are always associated with GTAW. If current applied is more than recommended the electrode may get decomposed and trapped in molten metal . Reasons for tungsten inclusion. 1. Contact of filler metal with hot tip of electrode. 2. Contamination of electrode by spatter. 3. Extension of electrodes beyond their normal distance from the collet, resulting in overheating of electrode. 4. Inadequate tightening of collet. 5. Inadequate shielding gas flow rate or excessive wind drafts resulting in oxidation of the electrode tip. 6. Use of improper shielding gas. 7. Defects such as splits or cracks in the electrode. 8. Use of excessive current for a given size of electrode . 9. Improper grinding of the electrode 10. Use of too small an electrode. The tungsten having higher density it shows as light area in radiography.
  4. 4. 159/184--POROSITY—It is cavity type discontinuity formed by gas entrapment during solidification. Since this is normally spherical shape it is less harmful. Types- Uniformly scattered cavity, Cluster porosity, Linear porosity and piping porosity. Some porosity are not spherical but elongated, it is referred as worm hole porosity. This occurs when gas is trapped between molten metal and solidified slag, This can occur when depth of granular flux used for SAW is excessive. When this occurs weight of the flux may be too great to permit the gas to escape properly. Porosity is normally caused by presence of contaminants or moisture in weld zone which forms gas due to heat. The contaminants can come from electrode, base metal, shielding gas or surrounding atmosphere. Welding with long arc in SMAW and low hydrogen electrode can cause porosity. High travel speed with SAW can cause piping porosity. When porosity is noticed it indicates that welding operation is not conducted properly. Porosity is shown as well defined dark region on radiograph due to significant loss of material density. It will appear round indication except in case of piping porosity which will have tail associated with it. 160/184—UNDERCUT It is a surface discontinuity, it occurs on base metal directly adjacent to weld. Here the base metal gets melted away during welding and there is insufficient filler metal deposited to adequately fill the resulting depression, hence there is a linear groove. Since it is a surface condition it is detrimental for structure with fatigue loading. For groove weld the under cut may occur at either face or root surface of the weld. It is normally the result of improper welding technique, especially when the weld travel speed is excessive, there may not be enough filler metal deposited to fill depression caused by melting of base metal adjacent to weld. It can also happen when welding heat is very high causing excessive melting of base metal or when electrode manipulation is incorrect. 161/184-- UNDERFILL-- This also is a surface discontinuity, this results in a loss of material cross section .This occurs in the weld metal of a groove weld. This results when there is no enough filler metal deposited to adequately fill the weld joint. This can occur at face as well as root of the weld joint. Under fill at the weld root of pipe weld is referred as internal concavity or suck back. It can be caused by excessive heating and melting of root pass during deposition of second pass. Primary cause of under fill is poor or incorrect technique used by welder. Excessive travel speed also does not allow sufficient filler metal to be melted and deposited to fill weld zone. 161/184—OVERLAP This also is a surface discontinuity. This is caused by improper welding techniques. It is protrusion of weld metal beyond weld toe or weld root. It gives an appearance as if weld metal has over flown and is laying on adjacent base metal surface. This can occur at face and root of the weld. Since it can cause sharp notch at surface of weldment it is a critical discontinuity. If amount of overlap is great enough it can hide a crack propagating from stress riser. If welding travel speed is too low and filler metal will be in excess of required. 161/184 – CONVEXITY—This applies to fillet weld only. The amount of weld built up on the face of fillet weld beyond what would be considered flush. Within certain limits convexity is not damaging it is desirable as it ensures that concavity is not
  5. 5. preset which can reduce size and strength of a fillet weld. The excessive convexity causes problem due to sharp notches at weld toes. These could produce stress risers weakening the structure in case of fatigue loading. Excessive convexity can be corrected by depositing additional weld metal at weld toes to provide smoother transition between weld and base metal. It is a result of slow speed of welding travel, manipulation of electrode is incorrect, and the excessive weld metal is deposited which does not wet the base properly. The contamination on base metal surface or the use of shielding gases which do not adequately clear away these contaminations can also result into undesirable weld profile. 162/184—WELD REINFORCEMENT—It describes the excessive weld metal present in the groove weld. Face reinforcement and root reinforcement can occur when weld is deposited from one side only. For a weld joint welded from one side only. For a weld joint welded from both sides this is called face reinforcement. This can cause sharp notches. As the reinforcement angle increases (Caused by an increase in the amount of weld reinforcement) there is a significant decrease in fatigue resistance of weld joint. It must be noted that reducing the amount of weld reinforcement does not really improve the situation, only after performing blend grinding to increase the weld reinforcement angle and increase the notch radius the situation improves. Grinding to remove the top of weld reinforcement does nothing to decrease the sharpness of the notches at the weld toes. ARC STRIKES-- It is very detrimental base metal discontinuity especially for low alloy steel and high strength steels. It result when arc is initiated on the base metal surface away from joint either intentionally or accidentally. It is localized area of base which is melted and then rapidly cooled due to massive heat sink in surrounding base metal. On high strength steel this can produce localized HAZ, which may contain Martensite , if this hard brittle microstructure is produced the tendency for cracking can be great. Improper connection of the work clamp to the work can also result in production of arc strike. While using prod type magnetic particle testing method which uses on conduction of electricity through parts to produce magnetic field the possibility exists for arc strike. SPATTER – These are metal particles expelled during fusing welding, that do not form part of weld, while spatter does not cause great concern, however large globules or spatter may have sufficient heat to cause a localized heat affected zone on base metal like arc strike. The presence of spatter on the base metal surface could provide a local stress riser. During NDE testing spatter can either prevent performance of a valid test or produce irrelevant indications. Spatter can cause problem in UT, MT, PT etc. Spatter can cause premature failure of applied coating. High welding current can cause spatter, short circuiting and globular transfer GMAW tend to produce more spatter than the use of spray transfer. The use of argon mixture will reduce the amount of spatter compared to the amount produced when pure CO2 shielding gas is used for GMAW and FCAW. Spatter can be reduced by applying anti spatter compounds to adjacent area. LAMINATIONS – This is a base metal flaw. This results from the presence of non metallic inclusions which occur in steel when it is being produced. These inclusions are generally oxides, produced when steel is in molten stage. During subsequent rolling processes these inclusions get elongated to form stringers. If these stringers
  6. 6. are large and take planar shape they are referred as laminations. Massive form of laminations arises from a condition referred as PIPE. This phenomenon is developed in the upper part of steel ingot during final stage of solidification. These pipe cavities usually contain some complex oxides which if rolled out within the plate to form as laminations. ANSI/AWS standard B 1.10 Guide for Non destructive inspection of welds As per above standard de lamination is separation of a lamination under stress. Heat of welding may re melt the stringers in lamination zone immediately adjacent to weld and stringer ends may fuse or get opened up. Laminations may show up during thermal cutting. If stresses are acting on material in a direction perpendicular to lamination it will severely weaken the structure. However the lamination oriented parallel to applied stress may not cause concern. If weld prepared surface has lamination, it can help the weld crack to propagate from lamination due to stress concentration. The hydrogen accumulation can take place in these laminations. UT must be done to find out if there is any lamination. LAMELLAR TEAR – This is base metal discontinuity. It is terrace like structure in the base metal with basic orientation parallel to the rolled surface. This defect occurs when there are high stresses in the Z direction or through thickness direction. This often results from welding shrinkage. The tearing always lies within base metal, usually out side HAZ and generally parallel to weld fusion boundary. This discontinuity is directly related to actual configuration of joint. These joint configurations in which the shrinkage stresses from welding are applied in direction which tends to pull to pull the rolled materials in its Z direction or through thickness direction will be more susceptible to lamellar tearing. If material is thicker and contamination is more then it shall have greater possibility of experiencing lamellar tearing. For the onset of lamellar tearing 3 conditions must exist simultaneously-- 1. Stress in through thickness direction. 2. Susceptible joint configuration. 3. Material having high inclusion content. SEAMS AND LAPS-- This is a discontinuity related to steel making process. These are occurring to the rolled surface of the metal instead of edge. If these are cross sectional they may run parallel to the rolled surface for some distance then tail off towards that surface. Seams are described as straight line longitudinal crevices or opening that may appear on the surface of steel. Seams are caused by imperfection in ingots. Laps are the result of overfilling in the rolling mill passes that cause fins or projections which turn down as material roll through succeeding stand in the mill train. These are defects occurring during manufacturing of plates, hence inspector cannot do anything for this. These are possibly revealed using either the visual, MP, DP, or UT or eddy current testing. Discontinuities in Laser & Electron beam Welding. Since these two processes use narrow and deep weld profiles and high travel speed is utilized few problems/defects occur as below. 1. Missed joint—If the beam is deflected off the weld joint it is a missed joint. If the axis of small dia laser is not aligned with joint root or along the length of the joint. For electron beam welding even when the beam is aligned properly magnetic forces on beam can cause deflection off the joint. EBW and LBW of metals with
  7. 7. dissimilar magnetic properties can also produce this deflection. Detection of such missed joint is very difficult if it is sub surface. Other discontinuity is Void formation at the bottom of weld which is often referred as Root Porosity. These are caused by gases which form in the weld metal and cannot escape through deep weld metals. As EBW is done in vacuum porosity can easily get trapped. Voids or cold shuts also form when molten metal does not completely fill the cavity produced by the beam. Shrinkage voids and micro fissures or hot cracks can also form near the weld centerline of EBW and LBW welds. This happens as metal solidifies from both sides towards centerline. Spiking or inconsistent penetration is also a discontinuity, this occurs in partial preparation welds., some times it can occur near the root of CJP. For EBW this is caused by 1. Variation in the power density of beam. 2. By vaporization of elements during welding and by turbulent weld pool. It occurs mainly with higher power, deeper penetrating welds. In CPJ welds there can be over penetration. Here the tendency for liquid metal to be expelled from the root of the weld in the form of spatter. Spiking is accompanied by incomplete fusion along the root or sides of weld . This is caused by narrow beam, improper or inconsistent joint fit up and by fast travel speed. Defects are discontinuities which require some corrective action. Discontinuities beyond permissible limits of codes shall become defect. End of Module 9

×