Blending NDT into the welding business
October 24, 2002
Leaders who oversee weld designs, materials, methods, personnel training, and manufacturing teams should promote welding performance by addressing quality and testing issues in contracts for any work. Total quality systems and weld acceptance criteria must be specific--specified for each contract--and adherence must be enforced without wavering. Acceptance criteria for welding should be agreed to in a face-to-face meeting before fabrication begins. Before a project begins, all parties should agree on the retention time for inspection and test records—including X-rays, personnel qualifications and other project data--and the contract should state the location and description of the records storage facility.
Leaders who oversee weld designs, materials, methods, personnel training, and manufacturing teams should ensure high-quality welding performance by addressing quality and testing in work contracts.
Owners must understand what is needed to manage every quality system diligently if they are to achieve overall business objectives. Total quality systems and weld acceptance criteria must be specific—specified for each contract—and adherence must be enforced without wavering.
Most welding operations have established quality control (QC) systems with written procedures, but welding acceptance criteria always must be agreed to in a face-to-face meeting before fabrication begins. Numerous standards are available, and most specify test equipment and routine calibrations.
Before a project begins, all parties must agree on the retention time for inspection and test records. These include X-rays, personnel qualifications, and other project data. The location and description of the records storage facility also should be stated in the contract. Throughout the project the welding inspector should monitor these records and storage areas to be sure they are being used.
The welder is the absolute first-line quality control agent. While the weld metal is solidifying, before the welder raises his hood, the completed weld displays the characteristics that indicate if it will meet or fall short of the contractual acceptance criteria.
Nondestructive testing (NDT) gives inspectors enhanced views of welds that show discontinuities along the entire length and width of a weld.
NDT technician qualifications are a major aspect of a QC NDT program. Typically, a program requires developing a company or shop standard and personally adhering to the criteria of American Society for Nondestructive Testing (ASNT) SNT-TC-1A. Every employer of NDT technicians must have a written practice that prescribes the technician's experience and training requirements for each NDT process, as well as the requirements for records retention.
Contract documents always should specify the NDT acceptance standards. Many types of QC programs are used for welded fabrication.
Every type of commercial welding has an applicable consensus standard. When in doubt, contact the AWS or the standards organization responsible for the welding criteria for your project to obtain accurate information on quality standards.
International Standards Organization (ISO) welding criteria are not the same as those of ASME, AWS, military standards (MIL-STD), or the API. If you do not know the difference, ask a qualified welding expert. Welding to global fabrication standards is a challenge to any QC program.
Each QC department should publish routine nonconformance reports (NCR) about welding projects entering production, projects in process, and warranty items on completed projects.
These reports are a good way to stay aware of the welding program and play a vital role in a project's profitability. Put yourself on the distribution lists for daily inspection reports, all NDT results, or welding procedure updates for a while. Such reports should be written in plain English.
Confirm that written QC procedures are available and the latest revision is being used. The QC technician should be able to direct you to the physical location of these procedures. You can learn to use the fabricator's QC system to your advantage. Make yourself available to shop and field inspectors because they can warn you about pending difficulties before the project is delayed or rework is required.
Include the services of an AWS-certified welding inspector (CWI) in your project's budget. This individual should be on site before welding begins and remain on site during the entire project, from initial material delivery through shipment of the last welded part.It is a fact that AWS-CWIs will save your project more money than they cost.
Examine your company's test equipment, specifically current calibration stickers. The file for each piece of test equipment should contain a report on routine maintenance and calibrations. The calibrations must be traceable to a national standard. Most calibrations and operational tests are performed in accordance with ASTM or an internationally accepted standard.
Quality is the systematic reduction of variation. We must get over the "goal post syndrome" and strive for consistent welding.
In football, if a kicker puts the ball between the uprights of the goal posts, points are scored. The obvious best placement of the ball is exactly in the middle of the goal posts. Even if the ball goes off center and comes near one of the posts, it still is in and scores points. However, if the ball goes just a few more inches to either side, it is out and no points are scored.
You cannot afford goal post syndrome. You can't say, "Well, the process is "in," so everything must be all right." In reality, a "just-in" process is also nearly out.
In welding, it's better to be exactly in the middle than close to the outer limits of acceptability. Problems can develop when several "just-ins" from several processes are allowed to add up, ultimately causing the overall dimensions of a work piece to be outside specifications.
Welds determined to be within the range of acceptability actually may indicate only that no problem has been discovered because no one has looked closely.
NDT helps to ensure product reliability, prevents accidents, saves human life, guarantees customer satisfaction, and maintains a manufacturer's good name— all of which contribute to profitability. It also aids better product design, control of manufacturing processes, reduction of manufacturing costs, and maintenance of a uniform quality level.
NDT acceptance standards are stated in the contract documents. If these standards are missing, take the initiative to ask, "What defines an acceptable part or weld? What does it look like, and when should the part be examined?"
Many welding facilities use visual methods to determine whether a weld is acceptable or unacceptable. When engineering and shop floor personnel understand NDT standards, the potential for confusion about defective welds is reduced. NDT terminology is consistent, and decisions can be made about corrective actions.
Leaders maintain records of NDT results and visual examinations on past projects. A weld reject log can provide clues if your process is drifting away from median settings into questionable regions that may result in weld rejects. A record of weld defects can help you isolate an operator, welding machine, or material handling technique that is producing unacceptable weld quality.
Which NDT method and acceptance criteria you choose depend on the inspection location, whether the surface or interior of the part will be examined, and the effects of critical defects on the part when it's put into service. Visual inspection (VT) is the oldest, most common, and least costly method of locating surface discontinuities.
To enhance visual sensitivity, use magnetic particle testing (MT) for ferromagnetic steel alloys, or liquid penetrant testing (PT) for ferromagnetic steel alloys or nonmagnetic alloys such as aluminum and austenitic stainless steels. VT, MT, and PT are inexpensive processes, and technician training is inexpensive, but the benefits can be astounding.
Eddy current testing often is used to locate discontinuities on surfaces like airplane wings, but abrupt surface variations may make signals difficult to interpret accurately.
To locate subsurface discontinuities, use radiographic testing (RT) when discontinuities are likely to be oriented so that they will produce indications on films. The depth of a discontinuity often is difficult to determine by RT, but a trained technician can get really close.
Ultrasonic tests (UT) also can locate volumetric, subsurface discontinuities. Discontinuities near or at the surface are hard to detect with UT or RT, because the burst of energy from the incoming UT beam obscures ultrasonic waves reflected close to the surface.
While fabricators apply RT and UT widely, the cost may be considered high. That decision depends on the designer's intent or the true cost of a failure when the part is in service. To reduce NDT costs, use in-process VT coupled with MT or PT to reveal discontinuities as they occur, when it is simple and inexpensive to make repairs.
Standards and tolerances are the most important factors in the manufacturing cost of any product. The economic criteria for adopting NDT are determined by the customer's need, the applicable industry standards, and the designer's intent. Complex problems may be encountered during fabrication or manufacturing if NDT is conducted to specifications that are inappropriate or unnecessarily strict. All specifications must include a tolerance because absolute perfection is unattainable.
The value of a product is zero until it attains a certain level of quality. The value of a product rises rapidly as its quality approaches perfection.
Jesse A. Grantham is president of Welding & Joining Management Group, West, 7100 N. Broadway 1C, Denver, CO 80233-2917, phone 303-451-6759, fax 303-280-4747, e-mail email@example.com, Web site www.wjmg.com. Welding & Joining Management Group, West, specializes in failure analysis and metallurgy; provides welding technology; develops NDT procedures; offers training for NDT inspectors, welders, engineers, and supervisors; and provides commercial testing and metallurgical laboratory services.
This article is adapted and updated from Grantham's conference presented at the FMA Welding Management Seminar, Oct. 5, 1998, Baltimore, ©1998 by the Fabricators & Manufacturers Association, Intl. (FMA, www.fmanet.org).
American Petroleum Institute, 1220 L St. N.W., Washington, DC 20005-4070, phone 202-682-8000, Web site www.api.org.
American Society for Nondestructive Testing, P.O. Box 28518, 1711 Arlingate Lane, Columbus, OH 43228-0518, phone 800-222-2768, fax 614-274-6899, e-mail firstname.lastname@example.org, Web site www.asnt.org.
International Organization for Standardization (ISO), 1, rue de Varembé, Case postale 56, CH-1211 Geneva 20, Switzerland, phone + 41 22 749 01 11, fax + 41 22 733 34 30, e-mail email@example.com, Web site www.iso.org.