Whatever the standard of quality, all welds should be inspected, even if the inspection involves no more than the welder glancing back over his work after running a bead. Testing may also be required, even in relatively rough weldments-such as leak-testing a container for liquids.
Inspection determines whether the prescribe standard of quality has been met. This function maybe the responsibility of the welding supervisor, foreman, a special employee of the company doing the welding, or a representative of the purchasing organization. The formal welding inspector may have a variety of duties. These may begin with interpretation of drawing and specification and follow each step to the analysis of test results. His operations are both productive and nonproductive-depending on where they are applied.
Inspection after the job is finished as a policing action. Rather then a productive function. Important as it is to assure quality, it is a burden added to the overall production cost. No amount of after the job inspection will improve the weld, it merely tells what is acceptable and what must be reworked or rejected.
Inspection as the job progresses is a different matter. It detects errors in practice and defects while correction is feasible. It prevents minor defects from piling up into major defects and leading to ultimate rejection. Inspection while weld quality is in the making and can be controlled may justifiably be looked upon as productive phase of cost, rather than an overburden.
Any program for assuring weld quality should, therefore, emphasize productive inspection and attempt to minimize the nonproductive type. This should be the guiding philosophy, even though its implementation may fall short. In most cases, such a philosophy means that visual inspection will be the main method of ascertaining quality, since it is the one method that can be applied routinely while the job is in progress.
Radiographic inspection is one of the most widely used techniques for showing the presence and nature of macroscopic defects and other discontinuities in the interior of welds. This test method is based on the ability of X-rays and gamma rays to penetrate metal and other opaque materials and produce an image on sensitized film or on a fluorescent screen. The term “X-ray quality,” widely used to imply high quality in welds, arises from the inspection method.
X-rays, which have a wave length 1/10,000 that of visible light, are produced by high-voltage generators. The depth to which the opaque material can be penetrated depends on the power of the generator. Portable units rated up to 2,000 kilovolts are available and used in weld inspection. The higher power machines, operating between 1,000 and 2,000 kilovolts, will penetrate from five to nine inches of steel.
Gamma rays are produced by the atomic disintegration of radioisotopes. They are similar to X-rays, except the wavelength is usually shorter than those X-rays produced with lower-voltage equipment. X-rays generated at 1,000 kilovolts or higher, for all practical purposes, appear to be identical to the gamma rays produced by radioisotopes. While the sort wave lengths of gamma rays allow penetration to considerable depth, exposure times required to get an interpretable picture are usually many times longer than with X-rays, because of the lower intensity of the radiation. The level of radiation is directly proportional to the amount of the radioisotope used.
When X-rays or gamma rays are directed at a section of a weldment, not all of the radiation gets through the metal and to the film placed behind it. Some is absorbed, and the denser or thicker the metal, the greater the absorption. Should there be a cavity in the weld interior, such as a blow-hole or internal crack, the radiation will have less metal to pass thorough than in sound weld. Consequently, there will be less absorption in the defective area and a greater amount of radiation striking he film. After development, the defect will show up in its shape and size as a black or darker area on the film.
The image picture is called a radiograph. Radiographs made with X-rays are referred to as exographs, those made with gamma rays, as gammagraphs.
Radiographic equipment require careful following of instructions for their proper use. Person responsible for radiographic inspection should make themselves thoroughly familiar with the equipment, reading carefully the manufacturer‘s literature and giving due attention to recommended usage, details of operation, applicable films, and safety precautions. An understanding of principles of X-rays generation, radioisotope decay and film sensitization and development should be acquired by the professional inspector or radiographic technician and will be helpful in his work.
In the shop or field use of radiographic equipment, the parameters affecting the reliability and interpretative value of the image are sharpness and contrast. Radiographic methods should give a ”sensitivity” of at least 2%. The term sensitivity is used to denote the least percentage of difference in weld thickness that can be detected visually on a radiograph. The ability of an observer to detect such a thickness difference depends on the sharpness of outline of the image and its contras with the background. Thus, if a fine crack is to be observable as a crack, its image must be outline enough to give a recognizable form and must so contrast with the background that no question arises as to the presence of a discontinuity.
It can be easily seen that a radiograph could be so much off in sharpness and contras that it would fail reveal a defect. To be sure that this doesn’t happen, a gage known as penetrameter is used on the side of the weld away from the film. An ASME Boiler Code Penetrameter is a thin strip of metal with the same absorption characteristics as the weld metal. When a weld is to be X-rayed, a penetrameter with a thickness equal or less than 2% of the weld thickness is selected. A lead numeral at one end shows the thickness of weld for which the penetrameter is to be used. For reference purposes, three holes are drilled in the face of the gage, the diameters of which are multiples of the penetrameter thickness. The appearance of the image of the penetrameter on the radiographic film tells the observer whether he has the minimum of 2% sensitivity and adequate sharpness and contrast for meaningful interpretation. In the hands of skilled inspector, use of the penetrameter also gives other items of information. Sharp image delineation gives assurance that the radiographic procedure is correct. The presence of a penetrameter image is also evidence that can be presented at any time later to prove that the weld was radiographed properly.
Radiograph show a variety of weld defects. An inspector’s ability to recognize defects and identify them is largely a matter of experience. Film-handling marks and streaks, fog, and spots caused by errors in the film-developing procedure complicate identification. Surface defects will also show on the film and must be recognized.
The angle of exposure has an influence on the radiograph. An X-ray picture of the interior of a weld may be viewed on a fluorescent screen a well as on developed film. These make possible rapid, low-cost inspection, but definition is poorer.
A radiograph compresses into one plane all the indications of defects that occur throughout the weld. Thus, the radiograph tends to give an exaggerated impression of scattered types of defects, such as porosity or inclusions. Unless allowance is made for this fact, particularly on thick plates, a weld that is entirely adequate for its function could be ruled defective.
Consider the volume of weldment contains a number of distributed particles or spots of porosity. On the radiographic film, these appear to exist in a single plane-which could be interpreted as evidence of an excessive number of particles or voids in some section of the volume. When the slice of the volume, however, is cut, it is seen that it contains only three spots. Any failure due to an applied force would have to occur in some finite section. The three tiny particles or voids in this sample are unlikely to predetermine or accelerate failure. Thus, the radiographic picture of this condition is misleading.
Because the streaks throughout the member are accumulated on the radiographic film, one might gain the impression that they constitute a sizeable proportion of the material. But when a section is removed and viewed from a different direction, it is seen that the streaks take up only a minute percentage of the cross section.
Radiography is useful in the qualification of welders. The individual’s ability to produce welds conforming to specification requirements is readily determined by an examination of welds produced on test plates. The procedure is also useful in evaluating processes proposed for a particular application. When radiography is required by the specifications governing the welding, written procedures for radiography are usually included. In this way, the purchaser exercises control over the inspection and specifies exactly what is wanted to meet his requirements.
Since radiation from X-ray machines or radioisotope sources can be damaging to body tissue when the exposure is excessive, safety precautions must be taken. The American Standard Safety Code for the Industrial Use of X-ray should be consulted for this purpose.
• MAGNETIC PARTICLE INSPECTION
Magnetic-particle inspection is a method of locating and defining discontinuities in magnetic materials. It is excellent for detecting surface defects in welds, revealing discontinuities that are too fine to be seen with the naked eye. With special equipment, it can also be used to detect defects that are close to the surface.
When using this method for weld inspection, probes are usually placed on each side of the area to be inspected and a high amperage passed through the workpiece. A magnetic flux is produced at right angles the flow of current, which may be represented by circular lines of force within the workpiece. When these lines of force encounter a discontinuity, such as a longitudinal crack, they are diverted and leak through the surface out into the air, creating magnetic poles or points of attraction. A magnetic powder dusted onto the surface will thus cling to be the leakage area more tenaciously than elsewhere, forming an indication of the discontinuity.
A workpiece can also be magnetized by putting it inside a solenoid. In this case, the magnetic lines of force are longitudinal and parallel with workpiece. Transverse crack show up under this arrangement. The discontinuity must be angled against the magnetic lines of force for an indication to be developed. If the discontinuity is parallel to the lines of force, the diversion needed to break through the surface and create poles will not occur.
The magnetic-particle inspection method is much simpler to use than radiographic inspection, but has its limitation. It is applicable to ferromagnetic materials only. It cannot be used with austenitic steels. A joint between a base metal and a weld metal of different magnetic characteristics will create magnetic discontinuities, which may produce indications interpretable as unsoundness even though the joint is entirely sound. On the other hand, a true defect can be obscured by the powder clinging over the harmless magnetic discontinuity. The sensitivity of the method lessens with decrease in size of the defect. Sensitivity is less with round forms, such as gas pockets, and best with elongated forms, such as cracks.
To have external leakage, the magnetic field must be distorted sufficiently. Fine, elongated discontinuities, such as hairline cracks, seams, or inclusions, that are “strung” parallel with the magnetic field will not distort it sufficiently to develop leakage. Thus, no indication will result. By changing the direction of the field, however, the indications can be developed. To be certain that discontinuities are detected, it is advisable to apply the field from two directions, preferably at right angles to each other.
Pieces to be inspected must be clean and dry. Wire-brushing or sand-blasting are satisfactory methods for cleaning welds. Surface roughness decreases the sensitivity, tends to distort the magnetic field, and interferes mechanically with formation of the powder pattern.
The shape, sharpness of outline, and width and height to which the particles have built up are features used for identifying discontinuities. When unusual patterns are produced, other test methods may be required to establish identity. Once a patterns has been interpreted by correlating it with the identification established by other methods, the interpretation can be applied to similar indications on other parts
Since a powder pattern results from various types of discontinuities in the magnetic field, it is easy to mistake an irrelevant indication for a defect. As noted previously, a chance in the magnetic characteristics of materials can create an irrelevant indication in the area of the interface. A change in section or a hole drilled in the part will tend to produce indications that have no significance in respect to weld soundness. These patterns are usually readily recognized for what they represent by their shapes and locations in the part. Abrupt changes in magnetic properties may occur at the edge of the heat-affected zone. The pattern will be fuzzy and diffused, running along the base metal close to the edge of the weld. It resembles the pattern caused by undercutting, a difference being that the particles are much less adherent. If a part is heat-treated or stress-relieved before subjecting to inspection, the magnetic characteristics of the heat-affected zone are restored and no indications will appear.
On the shop use of magnetic-particle inspection, judgment as to the significance of patterns can be guided by rules or standards based on experience or laboratory tests. Assuming that the inspector has ruled out the possibility of a false indication and has properly identified the indicated defect, he must then decide whether to pass or reject the part or require its repair. All defects do not affect the integrity of the part in service. Thus, slag inclusions and porosity may have no bearing on the service ability of the weld. Surface cracks revealed by magnetic-particle inspection, however, should be considered potential stress raisers or focal points for fatigue and corrosion.
The equipment for magnetic-particle inspection is relatively simple. Commercial units, portable and stationary, provide for nearly every situation where the method is applicable. The manufacturer’s instructions and recommendations for use of the equipment should be read carefully and thoroughly understood. The units may provide magnetization by direct, alternating, or rectified currents, or combinations thereof. Portable equipment, making use of electromagnets and permanent magnets, is also available.
Direct or rectified current is required for deep penetration. Alternating current magnetizes the surface only, and thus is limited to surface inspection. Full-wave, three-phase rectified current produces results comparable to those produced by battery direct current. Half-wave, single-phase rectified current gives maximum density. High amperage, low voltage is normally used in all magnetic-particle testing to limit arcing or burning on the test piece.
Magnetic powders be applied either by the dry or wet methods. Dry powder is dusted uniformly over the work with a spray gun, dusting bag, or atomizer. The finely divided magnetic particles are coated to give them greater mobility and are available in gray, black and red colors. It is desirable that the particles impinge on the surface at low velocity and with just enough residual force after impact to move them to possible sites of leakage. Excess powder is removed with a light stream of air.
In the wet method, very fine red or black particles are suspended in water or light petroleum distillate. Powders for liquid suspensions come from the manufacturer in either paste or dry form, prepared for use in water or oil baths. After the suspension has been made-in accordance with the manufacturer’s instructions-it is flowed or sprayed onto the surface to be inspected, or the piece may be dipped into the liquid. The wet method is more sensitive than the dry method since extremely fine particles may be used, this enables detection of exceedingly fine defects. Red particles improve visibility on dark surfaces. When the particles coating is a dye that fluoresces under ultraviolet light, sensitivity is further increased. Fluorescent powders are excellent for locating discontinuities in corners, key ways, splines, deep hole sand similar locations.
The techniques for creating a magnetic field in workpieces of various sizes and shapes, the sequences of operations in magnetizing and applying magnetic particles, adjustment of current to bring out desired results, the particles modes for orienting magnetic fields to produce or better delineate indications-all are important to the successful use of this inspection method. The equipment manufacturers’ literature and various ASTM and other specifications (such as ASTM E-109 ”Method for Dry Powder Magnetic-Particle Inspection” and ASTM E-138 “Method for Wet Magnetic-Particle Inspection”) should be consulted for operational details.
Magnetic-particle inspection is applied to many types of weldments in production practice. The dry-powder method is especially popular for heavy weldments. Many steel weldments in air craft manufacture are inspected by the wet method, using direct current. Airframe parts are subject to fatigue conditions, which means that surface cracks cannot be tolerated. Since these weldments are relatively thin, magnetic-particle inspection will usually detect subsurface defects as well as show up the finest surface cracks.
No comments yet.