Assuring Weld Quality
In production welding, the term "weld quality" is relative. The application determines what is good or bad. Generally, any weld is a good weld if it meets appearance requirements and will continue indefinitely to do the job for which it is intended. But a weld can be "too good". This is the case when a high degree of quality has been obtained at excessive production cost and the customer is getting no discernible value from the added expenditure. Insisting on any method of inspection, x-ray for example, if it serves no function is illogical as well as wasteful.
The first step, then, in assuring weld quality is to ascertain the degree required by the application. A standard should be established based on service needs. Engineering performance will be the main consideration in arriving at the standard, but appearance may also be important. A safety factor must, of necessity, be built into the standard, but it should be reasonable. Once the standard has been set, it is the responsibility of everyone concerned with the job to see that it is followed.
On the low side, the predetermined standard of quality should never be compromised. On the high side, there is no objection to extra quality, providing it has been obtained at no penalty in cost. If tests repeatedly show that the welds are exhibiting a degree of quality far greater than required by the standard, a cost reduction through modification of weldment design or procedures may be possible.
Frequently, the standards are preset by prevailing specifications or engineering and legal codes. Sometimes such standards are ultraconservative, but when they apply, they must be honored. The engineer can do his company or the customer a service by pointing out unrealistic specifications and the opportunities for cost savings, but the specifications must be adhered to rigidly until revised.
After the quality standard has been established, the most important step toward its achievement is the selection of the best process and procedures. By giving attention to the five Ps, weld quality will come about almost automatically, reducing subsequent inspection to a routine checking and policing activity. The five Ps are:
- Process Selection--The process must be right for the job.
- Preparation--The joint configuration must be right and compatible with the welding process.
- Procedures--To assure uniform results the procedures must be spelled out in detail and followed religiously during welding.
- Pretesting--By full-scale mockups or simulated specimens the process and procedures are proved to give the desired standard of quality.
- Personnel--Qualified people must be assigned to the job.
In view of the various automatic and semiautomatic processes available, process selection imposes a challenging decision. Of all the processes, manual welding is the most versatile, but economic considerations necessitate its being ruled out in favor of a partially or fully mechanized process wherever such is applicable. Process selection is tied up integrally with the need of the joint. Some joints require Fast-fill predominantly; others, Fast-follow. The most important needs in still others may be deep penetration or Fast-freeze. Each process has its advantages and limitations, and each introduces problems affecting joint preparation, welding procedures, and operator training. The process that gives the correct balance between the needs of the joint in terms of fill, follow, freeze, and penetration is likely to be the one that gives optimum weld quality.
Manual, submerged arc, flux-cored arc and gas-metal arc welding--except short-circuiting transfer--enjoy a prequalified status on certain specific joint configurations. Most are subject to specific thickness limitations as discussed in the AWS Structural Welding Code D1.1. Deviations from these prequalified joints may be accomplished by running qualification tests since practically all codes state that "other welding processes and procedures may be used, provided the contractor qualified them in accordance with the prescribed requirements."
Joint preparations are standardized and specified by applicable codes. The decision of the joint preparation is made by the designer rather than the fabricator, with the latter usually given the choice of what process and procedure to use to make the weld with the prescribed preparation. Acceptable butt-joint preparations are nothing more than a compromise between the included angle of bevel and the root spacing dimension. A large included angle will permit a smaller root spacing and, conversely, a small included angle requires a larger root spacing.
These and other factors are taken into consideration in prequalified joints. The joint detail influences process selection, electrode size, and welding position. The joint preparation must be correct before welding is started, not only to meet specifications, but to give assurance of weld quality.
Reliable welding procedures are developed through first-hand experience. They should be completely detailed in advance of production work. A full-scale mockup of the joint, using the same type steel sizes, and shapes that will be used on the job, should be made to test the procedures if such is possible. If a full-scale mockup is infeasible, a simulated setup should be used to produce specimens that can be destructively and nondestructively tested. By trial and error, all the procedure details for making an acceptable weld can be determined.
Pretesting production-size or simulated production specimens also tests the process as well as the procedures. Once it has been ascertained that the procedures and the process give the desired quality, such test specimens can also be used as a final check on the qualifications of the operator.
The closer the conditions of test approach conditions of service, the more meaningful the results. The ideal would be service life tests under slightly exaggerated conditions. Since this is usually impractical, simulated service tests are the next best choice. Specimen assemblies may be subjected to radiographic, ultrasonic, or other nondestructive inspection procedures to evaluate the weld quality. Or they may be proof-tested or submitted to destructive tests to determine ultimate limits.
By proper use of process-procedure qualification--backed up by convincing test evidence--final inspection takes on the nature of a quality control activity. This is desirable; the intent should be to make the welding so deadly precise in giving the desired quality that all subsequent inspection is for the detection of the unexpected and unexplained, rather than the explainable defects.
Personnel qualification--the last of the five Ps--can be evaluated in a preliminary way by the AWS Operator Qualification Test and the contractor's judgement. If a semiautomatic process is to be used, some experience with it is desirable, or the welder may require training. As mentioned previously, the welding of test specimens with the selected process and procedures will affirm the operator's capability.
Inspection determines whether the prescribed standard of quality has been met. This function may be the responsibility of the welding supervisor or 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 drawings and specifications 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 is a policing action, rather than 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 a 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.
In a sense, everyone connected with the job, as well as the formal inspector, participates in visual inspection. A conscientious worker does not knowingly pass on work in which he recognizes defects of his making. Nevertheless, it is usually desirable that someone be assigned responsibility for quality checking each operation.
In addition to good eyesight and good lighting, the tools for visual inspection are simple--a pocket rule, a weld-size gauge, a magnifying glass, and sometimes a straight edge and square for determining straightness, alignment and perpendicularity.
Visual inspection should begin before the first arc is struck. The materials should be examined to see if they meet specifications for quality, type, size, cleanliness, and freedom from defects. Foreign matter--grease, paint, oil, oxide film, heavy scale that could be detrimental to the weld should be removed. The pieces to be joined should be checked for straightness, flatness, and dimensions. Warped, bent, improperly cut or damaged pieces should be ordered for repair or rejected. Alignment and fit of parts and the fixturing should be scrutinized. Joint preparation should be checked. Often, little more than a passing glance is required in this preliminary inspection, but despite its almost casual nature such inspection can be a significant factor in weld quality.
Inspection prior to welding also includes verification that the correct process and procedures are to be employed, that the electrode type and size and the equipment settings for voltage and amperage are as specified, and that provisions are made for the required preheat or postheat.
Assuming the preliminary requirements are in good order, the most productive inspection will take place while the weldment is being fabricated. Examination of a weld bend and the end crater may reveal quality deficiencies such as cracks, inadequate penetration, and gas and slag inclusions to a competent inspector. Several types of weld defects can be recognized visually.
On simple welds, inspection of a specimen at the beginning of the operation and periodically as the work progresses may be adequate. When more than one layer of filler metal is deposited, however, it may be desirable to inspect each layer before a subsequent layer is placed.
The root pass in a multipass weld is the most critical one from the standpoint of weld soundness. It is especially susceptible to cracking, and because it tends to solidify quickly, is prone to trap gas and slag. Subsequent passes are subject to a variety of weld defect-creating conditions which result from the shape of the weld bead or change the configuration of the joint. These can be visually detected by the welder and repair cost minimized if the problem is corrected before welding progresses.
A workmanship standard, constructed for the specific purpose, can be helpful both to the welder and the inspector in visually appraising the production weld during the stages of its formation.
Visual inspection at an early stage of the fabrication will also detect underwelding and overwelding. Underwelding is in violation of specifications and cannot be tolerated. Overwelding should be of as much concern to the purchaser's inspector as to those members of the shop responsible for monitoring costs, since it is a major cause of distortion. Usually the designer has specified a weld size approaching the limit possible in good practice. The welder (perhaps wanting to make certain that the joint is strong enough, or having been criticized for making undersize welds) takes it upon himself to add, say, 1/16-inch to a 1/4-inch fillet. Since the weld metal deposited increases as the square of the size, the 1/16-inch increase in leg size increases the amount of weld metal deposited 60 percent, and has the same effect on shrinkage stress and cost.
Visual inspection after the weldment has been completed is also useful in evaluating quality, even if ultrasonic, radiographic, or other methods are to be employed. Here, as with visual inspection as welding progresses, surface flaws, such as cracks, porosity, and unfilled craters can be detected, and may be of such consequence that repairs are required or the work is rejected without use of subsequent inspection procedures. There is no point in submitting and obviously bad weld to sophisticated inspection methods.
Dimensional variations from tolerances, warpage, and faults in appearance are detected visually at this stage. The extent and continuity of the weld, its size, and the length of segments in intermittent welds can be readily measured or noted.
Welds must be cleaned of slag to make inspection for surface flaws possible. A glass with magnification of up to 10 diameters is helpful in detecting fine cracks and other defects. Shotblasting should not be used in preparing the weld for examination, since the peening action may seal fine cracks and make them invisible.
The objective of visual inspection here is not only to seek defects not permissible under the quality standard, but also to give clues to what may be amiss in the entire fabrication process. If the inspector has a sound knowledge of the welding, he can read much from what he sees. Thus, the presence of excessive porosity and slag inclusions may be a tip-off to the fact that the current is not adequate, no matter what the dial readings may be. Subsequent tests will also give clues to faults in equipment or procedures, but the information acquired through visual examination permits corrections to be made before the results from complicated tests are available.
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