Guidelines for Shop Inspection -Welding
1) MOST COMMONLY USED INSPECTION TECHNIQUES
What an Inspector looks for…
Welding inspection is based on visual inspection and other Non-Destructive Test (NDT) methods. They are used to find indications which have to be interpreted according to the inspection procedure for that particular job. Indications are inspection lingo for possible defects. These discontinuities (a fancier name for indications) have to be evaluated with reference to the acceptance criteria for that particular job. After comparison to the criteria are they considered acceptable or rejectable. These Non-Destructive Inspection (NDI) methods require special training and accumulation of experience. It is also important that both the inspector and the welder understand the flaw terminology and the welding terms and definitions.
Visual Inspection (VT)
The importance of visual inspection is often over looked. A visual test (VT) will provide a wealth of information about a weld. Many weld defects such as porosity, cracks, incomplete fusion, inclusions, overlap, edge melt, and incomplete penetration can be observed with just a simple visual exam. A weld that passes a visual exam has a much higher probability of passing further Non-Destructive Evaluation (NDE) methods.
Radiographic weld inspection is performed by pointing a radiographic source (an x-ray tube or a radioactive isotope) to the part of the weld to be inspected and by exposing for a
predetermined time a radiographic film to the radiation on the opposite side of the source tip or tube. The resulting film contains information on the internal features of the weld.
Variations in film density allow the film interpreter to accept or reject the weld based on comparison to specific hole or wire sizes in or on a penetrometer. These hole or wire sizes represent the largest acceptable defect size in a weld. Any indication that is larger than the acceptable wire or hole size is cause for rejection. All the relevant parameters including accept or reject are then recorded on an X-Ray Technique Sheet. The technique sheet and the processed film are usually turned over to the customer at the completion of the job.
Ultrasonic weld inspection is based on the fact that high frequency sound waves out of the range of human hearing can propagate in different materials, and be reflected by internal interfaces and opposite wall surfaces. These waves are generated by piezoelectric transducers of different sizes and frequencies which transform electrical vibrations into mechanical vibrations and vice-versa. These transducers are selected to match the thickness, type, temperature, and configuration of the material to be tested.
Signal reflections are evaluated on a computer screen, and by making reference to standard reflectors (normally flat bottom holes carefully machined on specimens of the same material) of given shape and size, the qualified inspector can conclude that if an echo is present where it should not be and if its reflection is larger than that of comparison, then there is an indication that must be evaluated. Additional techniques may be required to determine acceptance or rejection. Ultrasonic testing is capable of detecting thin interfaces normal to the line of propagation of the wave (that X-Rays cannot detect) so that both testing methods complement each other. Ultrasonic testing is becoming one of the most widely used methods of non- destructive testing. Its primary purpose is to detect and characterize internal discontinuities.
UT can also measure thickness, detect surface discontinuities, and define bond characteristics.
Magnetic Particle (MT)
Magnetic Particle weld inspection is used for locating surface or near surface discontinuities in ferro-magnetic material. It is based on the principals that magnetic lines of force will be distorted by a change in material continuity thereby creating a magnetic field or flux leak.
Magnetic particles will assemble at the point of the leak. The particles are made visible by adding a colored dye to the dry powder method, and by using a black light with the wet method. The inspector has to interpret the indications formed by these particles.
The most common equipment for this process is either an AC or DC yoke, or a DC prod. The AC yoke method will only detect surface discontinuities while the DC prod method may detect sub surface discontinuities to 3/16″ deep. The powder or liquid is applied after magnetizing the material. The surface is then visually checked by the inspector to find suspicious areas of particle concentrations.
Third-Party Inspection for Storage Tank-Plate Preparation
Third-Party inspectors perform visual and dimensional check to guarantee consistence with WPS and specifications. Where determined, weld bevels are analyzed by MT or PT in the wake of grinding/machining.
Edges and weld bevels should be clean, dry and free from surface deformities, laminations, cracks, voids, scores, and so on.
These are foundations for rejection except if reasonable/satisfactory remedial action can be made. Welding repairs to bevels are not allowed, without endorsement from the engineering. Inspectors will need to report all excessive and/or serious defects immediately.
Plates subject to forming, rolling or dishing may require extra heat treatment and NDE to meet code requirements.
Leak testing for weld inspection is done on containers and piping systems built to hold a liquid or a gas. The tank or piping system is usually pressurized above its design operating pressure and held at that pressure for a specific amount of time. The usual test mediums are air, gas(usually nitrogen), or water. These tests are performed mostly on new construction and are part of the ASME code.
2) THE VISUAL INSPECTION OF WELDS
Defects and Causes classified by Welding Process
STICK WELDING (SMAW)
This term is used to describe the failure of the filler and base metal to fuse together at the root of the joint. Bridging occurs in groove welds when the deposited metal and base metal are not fused at the root of the joint. The frequent cause of incomplete penetration is a joint design which is not suitable for the welding process or the conditions of construction. When the groove is welded from one side only, incomplete penetration is likely to result under the following conditions.
- The root face dimension is too big even though the root opening is adequate.
- The root opening is too small.
- The included angle of a V-groove is too small.
- The electrode is too large.
- The rate of travel is too high.
- The welding current is too low.
LACK OF FUSION
Lack of fusion is the failure of a welding process to fuse together layers of weld metal or weld metal and base metal. The weld metal just rolls over the plate surfaces. This is generally referred to as overlap. Lack of fusion is caused by the following conditions:
- Failure to raise to the melting point the temperature of the base metal or the previously deposited weld metal.
- Improper fluxing, which fails to dissolve the oxide and other foreign material from the surfaces to which the deposited metal must fuse.
- Dirty plate surfaces.
- Improper electrode size or type.
- Wrong current adjustment.
Undercutting is the burning away of the base metal at the toe of the weld. Undercutting may be caused by the following conditions:
- Current adjustment that is too high.
- Arc gap that is too long.
- Failure to fill up the crater completely with weld metal.
- Travel speed too fast.
Slag inclusions are elongated or globular pockets of metallic oxides and other solids compounds. They produce porosity in the weld metal. In arc welding, slag inclusions are generally made up of electrode coating materials or fluxes. In multilayer welding operations, failure to remove the slag between the layers causes slag inclusions. Most slag inclusion can be prevented by:
- Preparing the groove and weld properly before each bead is deposited.
- Removing all slag.
- Making sure that the slag rises to the surface of the weld pool.
- Taking care to avoid leaving any contours such as a high crown which will be difficult to penetrate fully with the arc.
- Avoiding travel speed that is too slow.
- Avoiding current that is too low.
Porosity is the presence of pockets which do not contain any solid material. They differ from slag inclusions in that the pockets contain gas rather than a solid. The gases forming the voids are derived from:
- Gas released by cooling weld because of its reduced solubility temperature drops.
- Gases formed by the chemical reactions in the weld.
- Porosity is best prevented by avoiding:
- Overheating and undercutting of the weld metal.
- Too high a current setting.
- Too long an arc.
- The weld should be of consistent width throughout. The two edges should form straight parallel lines.
- The face of the weld should be slightly convex with a reinforcement of not more than 1/16 in. (1.6 mm) above the plate surface. The convexity should be even along the entire length of the weld. It should not be high in one place and low in another.
- The face of the weld should have fine, evenly spaced ripples. It should be free of excessive spatter, scale, and pitting.
- The edges of the weld should be free of undercut or overlap.
- Starts and stops should blend together so that it is difficult where they have taken place.
- The crater at the end of the weld should be filled and show no holes, or cracks.
- If the joint is a butt joint, check the back side for complete penetration through the root of the joint. A slight bead should form on the back side.
- The root penetration and fusion of lap and T-joints can be checked by putting pressure on the upper plate until it is bent double. If the weld has not penetrated through the root, the plate will crack open at the joint as it is being bent. If it breaks, observe the extent of the penetration and fusion at the root. It will probably be lacking in fusion and penetration.
GAS METAL-ARC WELDING (GMAW) WITH SOLID-CORE WIRE
Lack of Penetration
Lack of penetration or fusion in the root area. This poor penetration is the result of too little heat corrected by:
- Increasing the wire-feed speed and reducing the stickout distance.
- Making sure that the fit-up is correct.
- Reducing the speed of travel.
- Using proper welding techniques such as correct lead angle and making sure that both toes of the bead fuse to the base metal
Excessive penetration usually causes burn through. It is the result of too much heat in the weld area. This can be corrected by:
- Reducing the wire size.
- Reducing the wire-feed speed and increasing the speed of travel.
- Making sure that the root opening and root face are correct.
- Increasing the stickout distance during welding and weaving the gun
Whiskers are short lengths of electrode wire sticking through the weld on the root side of the joint. They are caused by pushing the electrode wire past the leading edge of the weld pool. Whiskers can be prevented by:
- Reducing the wire-feed speed and the speed of travel.
- Increasing the stickout distance and weaving the gun
Voids are sometimes referred to as wagon tracks because of their resemblance to ruts in a dirt road. They may be continued along both sides of the weld deposit. They are found in multipass welding. Voids can be prevented by:
- Avoiding a large contoured crown and undercut.
- Making sure that all edges are filled in.
- On succeeding passes , using slightly higher arc voltage and increasing travel speed
Lack of Fusion
Lack of fusion, also referred to as cold lap, is largely the result of improper torch handling, low heat, and higher speed travel. It is important that the arc be directed at the leading edge of the puddle. To prevent this defect, give careful consideration to the following:
- Direct the arc so that it covers all areas of the joint. The arc, not the puddle, should do the fusing.
- Keep the electrode at the leading edge of the puddle.
- Reduce the size of the puddle as necessary by reducing either the travel speed or wire feed speed
- Check current values carefully
The most common defect in welds produced by any welding process is porosity. Porosity that exists on the face of the weld is readily detected, but porosity in the weld metal below the surface must be determined by x-ray or other testing methods. The causes of most porosity are:
- Contamination by the atmosphere and other materials such as oil, dirt, rust, and paint.
- Changes in the physical qualities of the filler wire due to excessive current.
- Entrapment of the gas evolved during weld metal solidification.
- Loss of shielding gas because of too fast travel.
- Shielding gas flow rate too low, not providing full protection.
- Shielding gas flow rate too high, drawing air into the arc area.
- Wrong type of shielding gas being used.
- Gas shield blown away by wind or drafts.
- Defects in the gas system.
- Improper welding technique, excessive stickout, improper torch angle, and too fast removal of the gun and the shielding gas at the end of the weld.
Spatter is made up of very fine particles of metal on the plate surface adjoining the weld area. It is usually caused by high current, a long arc, an irregular and unstable arc, improper shielding gas, or a clogged nozzle
Irregular Weld Shape
Irregular welds include those that are too wide or too narrow, those that have an excessively convex or concave surface, and those that have coarse, irregular ripples. Such characteristics may be caused by poor torch manipulation, a speed of travel that is too slow, current that is too high or low, improper arc voltage, improper stickout, or improper shielding gas.
Undercutting is a cutting away of the base material along the edge of the weld. It may be present in the cover pass weld bead or in multipass welding. This condition is usually the result of high current, high voltage, excessive travel speed, low wire-feed speed, poor torch technique, improper gas shielding or the wrong filler wire. To correct undercutting, move the gun from side to side in the joint. Hesitate at each side before returning to the opposite side.
GAS METAL-ARC WELDING (GMAW) WITH FLUX-CORED WIRE
Burn-through may be caused by the following:
- Current too high.
- Excessive gap between plates.
- Travel speed too s1ow.
- Bevel angle too large.
- Nose too small.
- Wire size too small.
- Insufficient metal hold-down or clamping.
Crown Too High or Too Low
The crown of the weld may be incorrect due to the following:
- Current too high or low.
- Voltage too high or low.
- Travel speed too high or low.
- Improper weld backing.
- Improper spacing in welds with backing.
- Workpiece not level.
Penetration Too Deep or Too Shallow
Incorrect penetration may be caused by any of the following:
- Current too high or low.
- Voltage too high or low.
- Improper gap between plates.
- Improper wire size.
- Travel speed too slow or fast.
Porosity and Gas Pockets
These defects may be the results of any of the following:
- Flux too shallow.
- Improper cleaning.
- Contaminated weld backing.
- Improper fit-up in welds with manual backing.
- Insufficient penetration in double welds.
Reinforcement Narrow and Steep-Sloped (Pointed)
Narrow and pointed reinforcements may be caused by the following:
- Insufficient width of flux.
- Voltage too low.
Mountain Range Reinforcement
If the reinforcement is ragged, the flux was too deep.
Undercutting may be caused by any of the following:
- Travel speed too high.
- Improper wire position (fillet welding).
- Improper weld backing.
- Current or voltage too high.
Voids and Cracks
These weld deficiencies may be caused by any of the following:
- Improper cooling.
- Failure to preheat.
- Improper fit-up.
- Concave reinforcement (fillet weld).
- Excessive convexity (full penetration weld).
- Practical Visual Inspection Tips
- Know the code or codes that apply to the job you are working on. They go hand in hand with the design specifications.
- Review all of the applicable weld procedures.
- Check each welder’s qualification to the welding procedure that they are currently working with.
- Talk to the welders and fitters-not at them. Most of them already hate inspectors because of past bad experiences. They are your best source of information for potential quality problems because they are the ones actually doing the job. Be just as fast to tell welders when they have it right as you are when they have it wrong.
- Always carry a flashlight, magnifier glass, and inspection mirror. You will eventually need them all.
- Always inspect each weld 100% on both sides if possible. Clearly mark any defects on the work piece.
- Reject welds professionally and not personally because you don’t like the welder or the boss.
- If you run into procedural problems or catch a blatant procedure deviation and it isn’t corrected in a timely manner, do write an NCR (Non-Conformance Report). Both the owner and design engineer have to sign it off. That relieves you of responsibility for the infraction if a failure should occur in the future. In today’s finger pointing society you need to cover your butt.
3) Welding Terms and Definitions
A weld that meets all requirements and acceptance criteria of the design and welding specifications.
The shortest distance between the weld root and face.
Metal having different properties than the original base elements.
The deflection or wandering of an electric welding arc from its normal path due to magnetism in the base metal.
A group of welding processes wherein coalescence or complete fusion is produced by heating with an electric arc.
A discontinuity on the surface of the base metal or weld consisting of pits, craters and/or re-melted metal.
The condition of weld metal after welding but prior to any thermal, mechanical, or chemical treatment.
Welding with equipment which performs the welding operation without adjustment of the controls by a welding operator.
A weld resulting from a back pass.
The removal of weld and base metal by arc gouging or grinding from the other side of a partially welded joint to assure complete fusion and penetration upon subsequent welding from that side.
The material to be formed, welded, brazed, soldered, or cut.
A weld joining two members laying in approximately the same plane.
A relatively thick layer of (> 0.04″ ) of different material applied by surface welding for the purpose of improving corrosion or heat resistance.
Fusion occurring over the entire base metal surfaces intended for welding and between all joining weld beads and layers.
A vessel made from plate having a corrosion resistant material bonded to a base of less resistant material.
Complete Joint Penetration
Joint penetration in which the weld metal completely fills the groove and is fused to the base metal throughout its thickness.
A weld that falls below the intended perpendicular plane.
A weld that exceeds the intended perpendicular plane.
The pressure used in the design of a vessel. It is recommended to design a vessel and its parts for a pressure 10% above its operating pressure.
The maximum metal temperature expected under operating conditions.
A discontinuity or discontinuities that accumulate to render a weld or part unable to meet minimum acceptance standards or criteria of the design specifications
A weld of approximately triangular cross section joining two surfaces at right angles to each other.
Eddy Current Testing (ET)
A non-destructive test method that induces an electrical current into a test piece by an alternating magnetic field. This is accomplished by the use of different probes or coils and eddy current devices operating at frequencies that are appropriate for the material being tested.
Metal added in making a welded, brazed, or soldered joint.
Flux Cored Arc Welding. Welding process that uses a tubular weld wire with an internal flux with or without an externally supplied gas shielding.
Gas Metal Arc Cutting. Metals are cut by the heat of an arc between a continuous filler metal electrode and the work piece with externally supplied shielding gas.
Gas Metal Arc Welding. Metals are welded by the heat of an arc between a continuous filler metal electrode and the work piece with externally supplied shielding gas.
GMAW-P (pulse arc) and GMAW-S (short arc) are variations of this method.
Gas Tungsten Arc Welding. A gas shielded welding method where the heating is produced using a non-consumable tungsten electrode to transfer the arc to the work piece.
A weld made in the groove between two plates to be joined.
The end of a cylindrical shell. Types include flat, ellipsoidal, flanged and dished, conical and hemispherical
The portion of the base metal which has not been melted, but whose mechanical properties have been altered by the heat of welding or cutting.
Steel containing large percentages of elements other than carbon.
A vessel, pipe spool or piping system is completely filled with water and subjected to a test pressure which is equal to 1.5 times the design pressure.
Determination of the resistance of a material to breaking by impact. The test is measured by breaking the material by a single blow.
The ratio of the strength of a joint to the strength of the base metal, expressed in percent.
A welded joint in which two overlapping metal parts are joined by means of a fillet, plug or slot weld.
Substance that is of such a nature that a small amount of the substance is dangerous to life when absorbed by a human or animal.
Liquid Penetrant Examination (PT)
A method of non-destructive examination that uses three different chemicals to expose welding defects that are open to the surface. These chemicals are applied using an inspection procedure.
Magnetic Particle Examination (MT)
A method of detecting cracks at or near the surface in magnetic alloys using a specially designed electro-magnet and a magnetic liquid or powder.
Material Test Report (MTR)
A document on which the material manufacturer records the results of test examinations or treatments required by the material specification.
The pressure at the top of a pressure vessel at which it normally operates. It cannot exceed the maximum allowable working pressure (Stamped Pressure).
A circular weld made through a hole in one member of a joint to fuse it to another member.
A rounded or elongated cavity formed by gas entrapment during cool down or solidification.
Post weld Heat Treatment (PWHT)
Heating a weld or welded assembly to a sufficient temperature to relieve the residual stresses which are the result of welding and forming.
Heat applied to base metal prior to welding.
Procedure Qualification Record (PQR)
The written demonstration record that welds made by a specific welding procedure can meet code standards.
The process of passing radiation through a weld joint or base material to obtain a film record of its soundness.
Weld used primarily to obtain complete tightness against leakage.
Shielded Metal Arc Welding. Also known as stick welding. This process uses a powder coated welding rod placed in a holder that is arced against the base metal.
Joining a metal stud to a work piece by welding with arc, friction, resistance, or other process.
Submerged Arc Welding. A bare metal electrode(s) method of welding where the arc and molten metal are shielded by a granular fusible blanket of material on the work piece.
A weld made to hold parts of a weldment in proper alignment until final welds are made.
Ultrasonic Testing (UT)
A non-destructive inspection method using sound waves for surface or sub-surface welding or base material testing. Ultrasonic can also be used for thickness testing.
A jagged groove melted into the base metal adjacent to the toe or root of a weld and left unfilled by weld metal. Also known as edge melt.
Weld Procedure Specification. A written document providing in detail the required variables for specific application to assure repeatability by properly qualified welders and welding operators.