FUNDAMENTALS OF PROFESSIONAL WELDING

PIPE WELDING

Welding is the simplest and easiest way to join sections of pipe. The need for complicated joint designs and special threading equipment is eliminated. Welded pipe has reduced flow restrictions compared to me-chanical connections and the overall installation costs are less. The most popular method for welding pipe is the shielded metal-arc process; however, gas shielded arc methods have made big inroads as a result of new advances in welding technology.

Pipe welding has become recognized as a profession in itself. Even though many of the skills are comparable to other types of welding, pipe welders develop skills that are unique only to pipe welding. Because of the hazardous materials that most pipelines carry, pipe welders are required to pass specific tests before they can be certified.

In the following paragraphs, pipe welding positions, pipe welding procedures, definitions, and related information are discussed.

PIPE WELDING POSITIONS

You may recall from and earlier lesson that there are four positions used in pipe welding. They are known as the

• horizontal rolled position (1G)
• horizontal fixed position (5G)
• pipe inclined fixed (6G)
• vertical position (2G).

Remember: these terms refer to the position of the pipe and not to the weld

PIPE WELDING PROCEDURES

Welds that you cannot make in a single pass should be made in interlocked multiple layers, not less than one layer for each 1/8 inch of pipe thickness. Deposit each layer with a weaving or oscillating motion. To prevent entrapping slag in the weld metal, you should clean each layer thoroughly before depositing the next layer.

Butt joints are commonly used between pipes and between pipes and welded fittings. They are also used for butt welding of flanges and welding stubs. In making a butt joint, place two pieces of pipe end to end, align them, and then weld them. (See fig. 7-42.) When the wall thickness of the pipe is 3/4 inch or less, you can use either the single V or single U type of butt joint; however, when the wall thickness is more than 3/4 inch, only the single U type should be used.

Fillet welds are used for welding slip-on and threaded flanges to pipe. Depending on the flange and type of service, fillet welds may be required on both sides of the flange or in combination with a bevel weld (fig. 7-43). Fillet welds are also used in welding screw or socket couplings to pipe, using a single fillet weld (fig. 7-42). Sometimes flanges require alignment. Figure 7-44 shows one type of flange square and its use in vertical and horizontal alignment.

Another form of fillet weld used in pipe fitting is a seal weld A seal weld is used primarily to obtain tight-ness and prevent leakage. Seal welds should not be considered as adding strength to the joint.

JOINT PREPARATION AND FIT-UP

You must carefully prepare pipe joints for welding if you want good results. Clean the weld edges or surfaces of all loose scale, slag, rust, paint, oil, and other foreign matter. Ensure that the joint surfaces are smooth and uniform. Remove the slag from flame-cut edges; however, it is not necessary to remove the temper color.

When you prepare joints for welding, remember that bevels must be cut accurately. Bevels can be made by machining, grinding, or using a gas cutting torch. In fieldwork, the welding operator usually must make the bevel cuts with a gas torch. When you are beveling, cut away as little metal as possible to allow for complete fusion and penetration. Proper beveling reduces the amount of filler metal required which, in turn, reduces time and expense. In addition, it also means less strain in the weld and a better job of design and welding.

Align the piping before welding and maintain it in alignment during the welding operation. The maximum alignment tolerance is 20 percent of the pipe thickness. To ensure proper initial alignment, you should use clamps or jigs as holding devices. Apiece of angle iron makes a good jig for a small-diameter pipe (fig. 7-45), while a section of channel or I-beam is more suitable for larger diameter pipe.

TACK WELDING When welding material solidly, you may use tack welds to hold it in place temporarily. Tack welding is one of the most important steps in pipe welding or any other type of welding. The number of tack welds required depends upon the diameter of the pipe. For ½-inch pipe, you need two tacks; place them directly opposite each other. As a rule, four tacks are adequate for standard size of pipe. The size of a tack weld is determined by the wall thickness of the pipe. Be sure that a tack weld is not more than twice the pipe thickness in length or two thirds of the pipe thickness in depth. Tack welds should be the same quality as the final weld. Ensure that the tack welds have good fusion and are thoroughly cleaned before proceeding with the weld.

SPACERS In addition to tack welds, spacers sometimes are required to maintain proper joint alignment. Spacers are accurately machined pieces of metal that conform to the dimensions of the joint design used. Spacers are sometimes referred to as chill rings or backing rings, and they serve a number of purposes. They provide a means for maintaining the specified root opening, provide a con-venient location for tack welds, and aid in the pipe alignment. In addition, spacers can prevent weld spatter and the formation of slag or icicles inside the pipe.

ELECTRODE SELECTION
Select the electrode that is best suited for the posi-tion and type of welding to be done. For the root pass of a multilayer weld, you need an electrode large enough, yet not exceeding 3/16 inch, that ensures complete fusion and penetration without undercutting and slag inclusions.

Make certain the welding current is within the range recommended by the manufacturers of the welding machines and electrodes.

WEATHER CONDITIONS
Do not assign a welder to a job under any of the following conditions listed below unless the welder and the work area are properly protected:

• When the atmospheric temperature is less than 0°F
• When the surfaces are wet
• When rain or snow is falling, or moisture is condensing on the weld surfaces
• During periods of high wind

At temperatures between 0°F and 32°F, heat the weld area within 3 inches of the joint with a torch to a temperature warm to the hand before beginning to weld.

FUNDAMENTALS OF PROFESSIONAL WELDING

VERTICAL-POSITION WELDING

A “vertical weld” is defined as a weld that is applied to a vertical surface or one that is inclined 45 degrees or less (fig. 7-35). Erecting structures, such as buildings, pontoons, tanks, and pipelines, require welding in this position. Welding on a vertical surface is much more difficult than welding in the flat or horizontal position due to the force of gravity. Gravity pulls the molten metal down. To counteract this force, you should use fast-freeze or fill-freeze electrodes.

Vertical welding is done in either an upward or downward position. The terms used for the direction of welding are vertical up or vertical down. Vertical down welding is suited for welding light gauge metal because the penetration is shallow and diminishes the possibility of burning through the metal. Furthermore, vertical down welding is faster which is very important in pro-duction work.

Current Settings and Electrode Movement
In vertical arc welding, the current settings should be less than those used for the same electrode in the flat position. Another difference is that the current used for welding upward on a vertical plate is slightly higher than the current used for welding downward on the same plate.

To produce good welds, you must maintain the proper angle between the electrode and the base metal. In welding upward, you should hold the electrode at 90 degrees to the vertical, as shown in figure 7-36, view A. When weaving is necessary, oscillate the electrode, as shown in figure 7-36, view B.

In vertical down welding, incline the outer end of the electrode downward about 15 degrees from the horizontal while keeping the arc pointing upward toward the deposited molten metal (figure 7-36, view C). When vertical down welding requires a weave bead, you should oscillate the electrode, as shown in figure 7-36, view D.

Joint Type
Vertical welding is used on most types of joints. The types of joints you will most often use it on are tee joints, lap joints, and butt joints.

When making fillet welds in either tee or lap joints in the vertical position, hold the electrode at 90 degrees to the plates or not more than 15 degrees off the horizontal for proper molten metal control. Keep the arc short to obtain good fusion and penetration.

TEE JOINTS.— To weld tee joints in the vertical position, start the joint at the bottom and weld upward. Move the electrode in a triangular weaving motion, as shown in figure 7-37, view A. A slight pause in the weave, at the points indicated, improves the sidewall penetration and provides good fusion at the root of the joint.

When the weld metal overheats, you should quickly shift the electrode away from the crater without breaking the arc, as shown in figure 7-37, view B. This permits the molten metal to solidify without running downward. Return the electrode immediately to the crater of the weld in order to maintain the desired size of the weld.

When more than one pass is necessary to make a tee weld, you may use either of the weaving motions shown in figure 7-37, views C and D. A slight pause at the end of the weave will ensure fusion without undercutting the edges of the plates.

LAP JOINTS.— To make welds on lap joints in the vertical position, you should move the electrode in a triangular weaving motion, as shown in figure 7-37, view E. Use the same procedure, as outlined above for the tee joint, except direct the electrode more toward the vertical plate marked “G.” Hold the arc short, and pause slightly at the surface of plate G. Try not to undercut either of the plates or to allow the molten metal to overlap at the edges of the weave.

Lap joints on heavier plate may require more than one bead. If it does, clean the initial bead thoroughly and place all subsequent beads as shown in figure 7-37, view F. The precautions to ensure good fusion and uniform weld deposits that was previously outlined for tee joints also apply to lap joints.

BUTT JOINTS.— Prepare the plates used in vertical welding identically to those prepared for welding in the flat position. To obtain good fusion and penetration with no undercutting, you should hold a short arc and the motion of the arc should be carefully controlled.

Butt joints on beveled plates 1/4 inch thick can be welded in one pass by using a triangular weave motion, as shown in figure 7-38, view A.

Welds made on 1/2-inch plate or heavier should be done in several passes, as shown in figure 7-38, view B. Deposit the last pass with a semicircular weaving motion with a slight “whip-up” and pause of the electrode at the edge of the bead. This produces a good cover pass with no undercutting. Welds made on plates with a backup strip should be done in the same manner.

E-7018 Electrode Welding Technique
The previously described vertical welding techniques generally cover all types of electrodes; however, you should modify the procedure slightly when using E-7018 electrodes.

When vertical down welding, you should drag the electrode lightly using a very short arc. Refrain from using a long arc since the weld depends on the molten slag for shielding. Small weaves and stringer beads are preferred to wide weave passes. Use higher amperage with ac than with dc. Point the electrode straight into the joint and tip it forward only a few degrees in the direction of travel.

On vertical up welding, a triangular weave motion produces the best results. Do not use a whipping motion or remove the electrode from the molten puddle. Point the electrode straight into the joint and slightly upward in order to allow the arc force to help control the puddle.

Adjust the amperage in the lower level of the recommended range.

WELDING GAUGE

AWS Weld Gauge GAL-1

Automatic Weld Size Weld Gauge For Accurate Calibration of Butt Fillet Type Welds The AWS Weld Gauge is for accurate calibration of butt and fillet type welds. The redesigned gauge is pocket size and easy to operate. Weld convexity and concavity sizes have automatically been predetermined in accordance with AWS DI.I para 3.6. Check Reinforcement of Butt Weld Check The Throat Of Fillet Weld Check Leg Size Of Fillet Weld Bridgecam Gauge GAL-2 The NEW Bridge Cam Gauge simple to use and very versatile. Measures: • Angle of preparation 0° to 60° • Fillet weld throat size • Excess weld cap size • Depth of undercut • Depth of pitting • Fillet weld length & Misalignment (high-low) Stainless steel, small, lightweight - fit in top pocket, either inches or millimeters Adjustable Fillet Weld Gauge $65.94 GAL-5 Measure any 1/8" to 1" fillet weld to 1/32" accuracy with this one simple to use gauge. Plus measure weld throat thickness to 1/16". Made from stainless steel and weighs only 1 I/2 ozs and fits in your top pocket Available Options: Measurement:  Inch/ EnglishMetric HI-LO Weld Gauge GAL-6 Welding Hi-Lo gauge. Measures internal misalignment, Fit-up gap, Bevel on Pipe and Plate end preparation, Crown height, and Pipe and Plate wall thickness. Available Options: Measurement: Inch/ EnglishMetric

Radiographic testing

Radiographic Testing (RT), or industrial radiography, is a nondestructive testing (NDT) method of inspecting materials for hidden flaws by using the ability of short wavelength electromagnetic radiation (high energy photons) to penetrate various materials.

Either an X-ray machine or a radioactive source (Ir-192, Co-60, or in rare cases Cs-137) can be used as a source of photons. Neutron radiographic testing (NR) is a variant of radiographic testing which uses neutrons instead of photons to penetrate materials. This can see very different things from X-rays, because neutrons can pass with ease through lead and steel but are stopped by plastics, water and oils.

Since the amount of radiation emerging from the opposite side of the material can be detected and measured, variations in this amount (or intensity) of radiation are used to determine thickness or composition of material. Penetrating radiations are those restricted to that part of the electromagnetic spectrum of wavelength less than about 10 nanometres.

Inspection of welds

The beam of radiation must be directed to the middle of the section under examination and must be normal to the material surface at that point, except in special techniques where known defects are best revealed by a different alignment of the beam. The length of weld under examination for each exposure shall be such that the thickness of the material at the diagnostic extremities, measured in the direction of the incident beam, does not exceed the actual thickness at that point by more than 6%. The specimen to be inspected is placed between the source of radiation and the detecting device, usually the film in a light tight holder or cassette, and the radiation is allowed to penetrate the part for the required length of time to be adequately recorded.

The result is a two-dimensional projection of the part onto the film, producing a latent image of varying densities according to the amount of radiation reaching each area. It is known as a radiograph, as distinct from a photograph produced by light. Because film is cumulative in its response (the exposure increasing as it absorbs more radiation), relatively weak radiation can be detected by prolonging the exposure until the film can record an image that will be visible after development. The radiograph is examined as a negative, without printing as a positive as in photography. This is because, in printing, some of the detail is always lost and no useful purpose is served.

Before commencing a radiographic examination, it is always advisable to examine the component with one's own eyes, to eliminate any possible external defects. If the surface of a weld is too irregular, it may be desirable to grind it to obtain a smooth finish, but this is likely to be limited to those cases in which the surface irregularities (which will be visible on the radiograph) may make detecting internal defects difficult.

After this visual examination, the operator will have a clear idea of the possibilities of access to the two faces of the weld, which is important both for the setting up of the equipment and for the choice of the most appropriate technique.

Defects such as delaminations and planar cracks are difficult to detect using radiography, which is why penetrants are often used to enhance the contrast in the detection of such defects. Penetrants used include silver nitrate, zinc iodide, chloroform and diiodomethane. Choice of the penetrant is determined by the ease with which it can penetrate the cracks and also with which it can be removed. Diiodomethane has the advantages of high opacity, ease of penetration, and ease of removal because it evaporates relatively quickly. However, it can cause skin burns.

Safety

Industrial radiography appears to have one of the worst safety profiles of the radiation professions, possibly because there are many operators using strong gamma sources (> 2 Ci) in remote sites with little supervision when compared with workers within the nuclear industry or within hospitals.

Standards

International Organization for Standardization (ISO)

• ISO 4993, Steel and iron castings - Radiographic inspection
• ISO 5579, Non-destructive testing - Radiographic examination of metallic materials by X- and gamma-rays - Basic rules
• ISO 10675-1, Non-destructive testing of welds - Acceptance levels for radiographic testing - Part 1: Steel, nickel, titanium and their alloys
• ISO 11699-1, Non-destructive testing - Industrial radiographic films - Part 1: Classification of film systems for industrial radiography
• ISO 11699-2, Non-destructive testing - Industrial radiographic films - Part 2: Control of film processing by means of reference values
• ISO 14096-1, Non-destructive testing - Qualification of radiographic film digitisation systems - Part 1: Definitions, quantitative measurements of image quality parameters, standard reference film and qualitative control
• ISO 14096-2, Non-destructive testing - Qualification of radiographic film digitisation systems - Part 2: Minimum requirements
• ISO 17636, Non-destructive testing of welds - Radiographic testing of fusion-welded joints
• ISO 19232, Non-destructive testing - Image quality of radiographs

European Committee for Standardization (CEN)

• EN 444, Non-destructive testing; general principles for the radiographic examination of metallic materials using X-rays and gamma-rays
• EN 462-2, Non-destructive testing - image quality of radiographs - Part 2: image quality indicators (step/hole type) - determination of image quality value
• EN 462-3, Non-destructive testing - Image quality of radiogrammes - Part 3: Image quality classes for ferrous metals
• EN 462-4, Non-destructive testing - Image quality of radiographs - Part 4: Experimental evaluation of image quality values and image quality tables
• EN 462-5, Non-destructive testing - Image quality of radiographs - Part 5: Image quality of indicators (duplex wire type), determination of image unsharpness value
• EN 584-1, Non-destructive testing - Industrial radiographic film - Part 1: Classification of film systems for industrial radiography
• EN 584-2, Non-destructive testing - Industrial radiographic film - Part 2: Control of film processing by means of reference values
• EN 1330-3, Non-destructive testing - Terminology - Part 3: Terms used in industrial radiographic testing
• EN 1435, Non-destructive testing of welds - Radiographic testing of welded joints
• EN 2002-21, Aerospace series - Metallic materials; test methods - Part 21: Radiographic testing of castings
• EN 10246-10, Non-destructive testing of steel tubes - Part 10: Radiographic testing of the weld seam of automatic fusion arc welded steel tubes for the detection of imperfections
• EN 12517-1, Non-destructive testing of welds - Part 1: Evaluation of welded joints in steel, nickel, titanium and their alloys by radiography - Acceptance levels
• EN 12517-2, Non-destructive testing of welds - Part 2: Evaluation of welded joints in aluminium and its alloys by radiography - Acceptance levels
• EN 12679, Non-destructive testing - Determination of the size of industrial radiographic sources - Radiographic method
• EN 12681, Founding - Radiographic examination
• EN 13068, Non-destructive testing - Radioscopic testing
• EN 14096, Non-destructive testing - Qualification of radiographic film digitisation systems
• EN 14784-1, Non-destructive testing - Industrial computed radiography with storage phosphor imaging plates - Part 1: Classification of systems
• EN 14584-2, Non-destructive testing - Industrial computed radiography with storage phosphor imaging plates - Part 2: General principles for testing of metallic materials using X-rays and gamma rays

ASTM International (ASTM)

• ASTM E 94, Standard Guide for Radiographic Examination
• ASTM E 155, Standard Reference Radiographs for Inspection of Aluminum and Magnesium Castings
• ASTM E 592, Standard Guide to Obtainable ASTM Equivalent Penetrameter Sensitivity for Radiography of Steel Plates 1/4 to 2 in. [6 to 51 mm] Thick with X Rays and 1 to 6 in. [25 to 152 mm] Thick with Cobalt-60
• ASTM E 747, Standard Practice for Design, Manufacture and Material Grouping Classification of Wire Image Quality Indicators (IQI) Used for Radiology
• ASTM E 801, Standard Practice for Controlling Quality of Radiological Examination of Electronic Devices
• ASTM E 1030, Standard Test Method for Radiographic Examination of Metallic Castings
• ASTM E 1032, Standard Test Method for Radiographic Examination of Weldments
• ASTM 1161, Standard Practice for Radiologic Examination of Semiconductors and Electronic Components
• ASTM E 1648, Standard Reference Radiographs for Examination of Aluminum Fusion Welds
• ASTM E 1735, Standard Test Method for Determining Relative Image Quality of Industrial Radiographic Film Exposed to X-Radiation from 4 to 25 MeV
• ASTM E 1815, Standard Test Method for Classification of Film Systems for Industrial Radiography
• ASTM E 1817, Standard Practice for Controlling Quality of Radiological Examination by Using Representative Quality Indicators (RQIs)
• ASTM E 2104, Standard Practice for Radiographic Examination of Advanced Aero and Turbine Materials and Components

Ultrasonic testing

In ultrasonic testing (UT), very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz and occasionally up to 50 MHz are launched into materials to detect internal flaws or to characterize materials. The technique is also commonly used to determine the thickness of the test object, for example, to monitor pipework corrosion.

Ultrasonic testing is often performed on steel and other metals and alloys, though it can also be used on concrete, wood and composites, albeit with less resolution. It is a form of non-destructive testing used in many industries including aerospace, automotive and other transportation sectors.

An example of Ultrasonic Testing (UT) on blade roots of a V2500 IAE aircraft engine.
Step 1: The UT probe is placed on the root of the blades to be inspected with the help of a special borescope tool (video probe).
Step 2: Instrument settings are input.
Step 3: The probe is scanned over the blade root. In this case, an indication (peak in the data) through the red line (or gate) indicates a good blade; an indication to the left of that range indicates a crack.

How it works
In ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is passed over the object being inspected. The transducer is typically separated from the test object by a couplant (such as oil) or by water, as in immersion testing.

There are two methods of receiving the ultrasound waveform, reflection and attenuation. In reflection (or pulse-echo) mode, the transducer performs both the sending and the receiving of the pulsed waves as the "sound" is reflected back to the device. Reflected ultrasound comes from an interface, such as the back wall of the object or from an imperfection within the object. The diagnostic machine displays these results in the form of a signal with an amplitude representing the intensity of the reflection and the distance, representing the arrival time of the reflection. In attenuation (or through-transmission) mode, a transmitter sends ultrasound through one surface, and a separate receiver detects the amount that has reached it on another surface after traveling through the medium. Imperfections or other conditions in the space between the transmitter and receiver reduce the amount of sound transmitted, thus revealing their presence. Using the couplant increases the efficiency of the process by reducing the losses in the ultrasonic wave energy due to separation between the surfaces.

At a construction site, a technician tests a pipeline weld for defects using an ultrasonic phased array instrument. The scanner, which consists of a frame with magnetic wheels, holds the probe in contact with the pipe by a spring. The wet area is the ultrasonic couplant that allows the sound to pass into the pipe wall.

 

Features

 

Advantages

1. High penetrating power, which allows the detection of flaws deep in the part.
2. High sensitivity, permitting the detection of extremely small flaws.
3. Only one surface need be accessible.
4. Greater accuracy than other nondestructive methods in determining the depth of internal flaws and the thickness of parts with parallel surfaces.
5. Some capability of estimating the size, orientation, shape and nature of defects.
6. Nonhazardous to operations or to nearby personnel and has no effect on equipment and materials in the vicinity.
7. Capable of portable or highly automated operation.

Disadvantages

1. Manual operation requires careful attention by experienced technicians
2. Extensive technical knowledge is required for the development of inspection procedures.
3. Parts that are rough, irregular in shape, very small or thin, or not homogeneous are difficult to inspect.
4. Surface must be prepared by cleaning and removing loose scale, paint, etc., although paint that is properly bonded to a surface need not be removed.
5. Couplants are needed to provide effective transfer of ultrasonic wave energy between transducers and parts being inspected unless a non-contact technique is used. Non-contact techniques include Laser and Electro Magnetic Acoustic Transducers (EMAT).
6. Inspected items must be water resistant, when using water based couplants that do not contain rust inhibitors.

Non-destructive testing of a swing shaft showing spline cracking

 

Standards

 

International Organization for Standardization (ISO)

• ISO 7963, Non-destructive testing - Ultrasonic testing - Specification for calibration block No. 2
• ISO/DIS 11666, Non-destructive testing of welds - Ultrasonic testing of welded joints - Acceptance levels
• ISO/DIS 17640, Non-destructive testing of welds - Ultrasonic testing of welded joints
• ISO 22825, Non-destructive testing of welds - Ultrasonic testing - Testing of welds in austenitic steels and nickel-based alloys

European Committee for Standardization (CEN)

• EN 583, Non-destructive testing - Ultrasonic examination
• EN 1330-4, Non destructive testing - Terminology - Part 4: Terms used in ultrasonic testing
• EN 1712, Non-destructive testing of welds - Ultrasonic testing of welded joints - Acceptance levels
• EN 1713, Non-destructive testing of welds - Ultrasonic testing - Characterization of indications in welds
• EN 1714, Non-destructive testing of welds - Ultrasonic testing of welded joints
• EN 12223, Non-destructive testing - Ultrasonic examination - Specification for calibration block No. 1
• EN 12668-1, Non-destructive testing - Characterization and verification of ultrasonic examination equipment - Part 1: Instruments
• EN 12668-2, Non-destructive testing - Characterization and verification of ultrasonic examination equipment - Part 2: Probes
• EN 12668-3, Non-destructive testing - Characterization and verification of ultrasonic examination equipment - Part 3: Combined equipment
• EN 12680, Founding - Ultrasonic examination
• EN 14127, Non-destructive testing - Ultrasonic thickness measurement

NDT Methods for Flaw Detection during Welding

NDT Methods for Flaw Detection during Welding

Detectability of different defect types using TOFD

A collaborative research project funded by the EC under the BRITE-EURAM II "Industrial and Materials Technologies" Programme.

Partners:- Mitsui Babcock Energy Ltd. (UK)
Institut de Soudure (FR)
Isotopen Technik Dr. Sauerwein GmBH (GER)
Nordon & CIE (FR)
The University of Surrey (UK)

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A suite of PC based automated TOFD interpretion algorithms have been benchmarked through a series of demonstration trials on both 80mm thick carbon steel submerged arc welded (SAW) testpieces, and 25mm thick carbon steel tungsten inert gas (TIG) welded testpieces. The range of intentionally implanted defects, from root cracks to lack of side wall fusion, were detected with an overall accuracy of 79% on a data set of 174 defects on scans performed at 10-90% weld completion. The trials were performed at the workshops of Nordon & CIE in France and attended by Mitsui Babcock Energy (responsible for ultrasonic data acqusisition), the Institut de Soudure (responsible for manual interpretation of data and subsequent destructive testing of the testpieces), EDF (external sponsors of the work), and the University of Surrey (developers of the software).

TOFD ultrasonic scans of the TIG and SAW testpieces was obtained at various stages of weld completion, typically before and after an intentional defect was implanted, using a MicroPlus system. The scans were mostly obtained during welding, though is some cases this was made impracticakl due to the loss of couplant between the TOFD transmitter and receiver probes and the workpiece surface, and also electromagnetic and electrical switching interference from other welding sources in the workshop. The effects of welding interference on ultrasonic sensors has been discussed recently by Bastos et al (1996) though at our trials we found that the encoder signal was affected much more than the actual ultrasonic signal.

Table 1 - Automatic defect detection and false alarm ratings used in the assessment of the ultrasonic scans

After acquisition each scan was processed by the automated software to locate defect and component echoes. The effectiveness of the automatic defect identification process has been measured using the rating system shown in Table 1 for both the visibility of the defect and the level of associated false alarms. The full effectiveness of the algorithms was quantified by a comparison using full destructive testing of the testpieces, as performed by the Institut de Soudure.

Analysis of the TIG testpieces

The TIG welded testpieces contained a total of 15 defects of 6 types. The defects are abbreviated as follows:

LOFS - lack of side-wall fusion
RC - root crack
P - porosity
LOP - lack of penetration
W - weld inclusion

LOIRF - lack of inter-run fusion.
Three sets of 1m long testpieces were manufactured by Nordon, denoted sample TIG A, TIG B, and TIG C. Defects were intentionally positioned along the length of each weld at various stages of weld completion. Ultrasonic TOFD was performed at approx.: 10, 25, 50, 75, 85 and 90% weld completion.

Table 2 - Results of the automated signal processing software for the detection of defects in the TIG welded testpieces.

Analysis of the SAW testpieces

The SA welded testpieces contained a total of 15 defects of 6 types. The defects are abbreviated as follows:

LOFS - lack of side-wall fusion
RC - root crack
P - porosity
LOI - lack of root (inter-run) fusion
SI - slag inclusion
LOIRF - lack of inter-run fusion.

Three sets of 1m long testpieces were manufactured by Nordon. Defects were intentionally positioned along the length of each weld at various stages of weld completion. Ultrasonic TOFD was performed at various stages of completion for each testpiece, from: 25, 30, 40, 50, 75, 80 and 100% weld completion.





Table 3 - Results of the automated signal processing software for the detection of defects in the SAW welded testpieces.

From the manual conventional NDT of the welds it can be summarised that the defects present in the SAW testpieces are of the intentional type and are located at the intended location. However, extra unintentional defects are present and the conventional ultrasonics fails to detect several defects (detected by radiography). Table 3 shows the results of the automated signal processing software for the detection of defects in the SAW welded testpieces.

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Summary

The results of automatic processing of the high temperature TIG and SA scans acquired during welding have been presented. Overall the automated defect detection algorithms worked very well, demonstrating that weld flaw detection during welding is possible using the ultrasonic time of flight diffraction (TOFD) method. It should be noted that the parameters of the interpretation software required some 'tuning' between scans of different resolutions due to the different spatial resolution occupied by the defect and the differing signal-to-noise of the ultrasonic signal. The poorest performance was with the incomplete TIG welded specimens where the small amount of weld present causes difficulties for the defect detection algorithms.





During the automatic TOFD demonstration trials 64 scans were attempted, with 42 being processed by the signal processing algorithms. The majority of the unprocessed scans were due to either the affects of interference from other workshop welding equipment upon the encoder wheel or the poor quality of the initial TIG scans. With development work the interference problems should be overcome through the use of adequate shielding of the encoder and its cables. Examining the detectability at the various levels of weld completion, Table 4 shows the percentage of defects which can be detected at 25, 50, and 85% completion of the TIG welds. It should be recognised that there were very few TIG scans at low levels of completion which were suitable for processing. Table 5 shows the percentage of defects which can be detected at 25, 50, 80 and 100% weld completion of the SAW welds, again when compared against the interpretation of the testpieces when using conventional NDT techniques.

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Reference

Bastos, T. F., Calderon, L., Martin, J.M. and Ceres, R., "Ultrasonic sensors and arc welding - a noisy mix", Sensor Review, Vol. 16(3), pp. 26-32, 1996.

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This note has been taken from the forthcoming paper Bonser, G.R. and Lawson, S.W., Defect detection in partially complete SAW and TIG welds using the ultrasonic time of flight diffraction method, Proc of SPIE Int Symp on Nondestructive Evaluation Techniques for Aging Infrastructure and Manufacturing, San Antonio, Texas, March 1998.
For further information on this :-
Shaun Lawson

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All legal boffs please read this copyright and disclaimer notice.
Last change: January 2000

MIG Welding

Gas Metal Arc Welding (GMAW) is frequently referred to as MIG welding. MIG welding is a commonly used high deposition rate welding process. Wire is continuously fed from a spool. MIG welding is therefore referred to as a semiautomatic welding process.

MIG Welding Benefits

• All position capability
• Higher deposition rates than SMAW
• Less operator skill required
• Long welds can be made without starts and stops
• Minimal post weld cleaning is required

MIG Welding Shielding Gas

The shielding gas, forms the arc plasma, stabilizes the arc on the metal being welded, shields the arc and molten weld pool, and allows smooth transfer of metal from the weld wire to the molten weld pool. There are three primary metal transfer modes:

• Spray transfer (MP3 Audio)
• Globular transfer (MP3 Audio)
• Short circuiting transfer (MP3 Audio)

The primary shielding gasses used are:

• Argon
• Argon - 1 to 5% Oxygen
• Argon - 3 to 25% CO₂
• Argon/Helium

CO₂ is also used in its pure form in some MIG welding processes. However, in some applications the presence of CO₂ in the shielding gas may adversely affect the mechanical properties of the weld.

Common MIG Welding Concerns

We can help optimize your MIG welding process variables. Evaluate your current welding parameters and techniques. Help eliminate common welding problems and discontinuities such as those listed below:

Weld Discontinuities Undercutting Excessive melt-through Incomplete fusion Incomplete joint penetration Porosity Weld metal cracks Heat affected zone cracks

MIG Welding Problems

• Heavily oxidized weld deposit
• Irregular wire feed
• Burnback
• Porosity
• Unstable arc
• Difficult arc starting