Welding Procedures and the Fundamentals of Welding

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Understanding Welding Procedures in Modern Engineering

As welding becomes a modern engineering technology, it requires that the various elements involved be identified in a standardized way. This is accomplished by writing welding procedures, which represent a "manner of doing" or "the detailed elements (with prescribed values or range of values) of a process or method used to produce a specific result." The AWS definition for a welding procedure is "the detailed methods and practices including all joint welding procedures involved in the production of a weldment." The joint welding procedure mentioned includes "the materials, detailed methods and practices employed in the welding of a particular joint."

A welding procedure serves as a comprehensive record of all different elements, variables, and factors involved in producing a specific weld or weldment. Welding procedures should be written whenever it becomes necessary to maintain dimensions by controlling distortion, reduce residual or locked-up stresses, minimize detrimental metallurgical changes, consistently build a weldment the same way, or comply with certain specifications and codes.

Essential Components of Welding Procedure Specifications

Welding procedures must be tested or qualified and communicated to those who need to know, including the designer, the welding inspector, the welding supervisor, and the welder. When welding codes or high-quality work is involved, this can become a welding procedure specification, which lists in detail the various factors or variables involved. Different codes and specifications have somewhat different requirements for welding procedures, but generally a welding procedure consists of three parts: a detailed written explanation of how the weld is to be made, a drawing or sketch showing the weld joint design and the conditions for making each pass or bead, and a record of the test results of the resulting weld.

If the weld meets the requirements of the code or specification and if the written procedure is properly executed and signed, it becomes a qualified welding procedure. The variables involved in most specifications are considered to be essential variables. In some codes, the term nonessential variables may also be used. Essential variables are those factors which must be recorded and if they are changed in any way, the procedure must be retested and requalified. Nonessential variables are usually of less importance and may be changed within prescribed limits without requiring procedure requalification.

Critical Essential Welding Variables

Essential variables involved in welding procedures usually include the welding process and its variation, the method of applying the process, the base metal type, specification, or composition, the base metal geometry (normally thickness), the base metal need for preheat or postheat, the welding position, the filler metal and other materials consumed in making the weld, the weld joint (that is, the joint type and the weld), electrical or operational parameters involved, and welding technique.

Some specifications also include nonessential variables, which are usually the travel progression (uphill or downhill), the size of the electrode or filler wire, certain details of the weld joint design, the use and type of weld backing, and the polarity of the welding current. The procedure write-up must include each of the listed variables and describe in detail how it is to be done. The second portion of the welding procedure is the joint detail sketch and table or schedule of welding conditions.

Welding Qualification Testing and Standards

Tests are performed to determine if the weld made to the procedure specification meets certain standards as established by the code or specification. If the destructive tests meet the minimum requirements, the procedure then becomes a qualified procedure specification. The writing, testing, and qualifying procedures become quite involved and are different for different specifications.

In certain codes, welding procedures are prequalified. By using data provided in the code, individual qualified procedure specifications are not required for the standard joints on common base materials using the shielded metal arc welding process. The factors included in a procedure should be considered when approaching any new welding job. Through knowledge and experience, establish the optimum factors or variables to make the best and most economical weld on the material to be welded and in the position that must be welded.

Welding procedures take on added significance based on the quality requirements that can be involved. When exact reproducibility and perfect quality are required, the procedures will become much more technical with added requirements, particularly in testing. Tests will become more complex to determine that the weld joint has the necessary properties to withstand the service for which the weld is designed.

Procedures are written to produce the highest-quality weld required for the service involved, but at the least possible cost and to provide weld consistency. It may be necessary to try different processes, different joint details, and so on, to arrive at the lowest-cost weld which will satisfy the service requirements of the weldment.

The Physics and Chemistry of Welding Fundamentals

Welding follows all of the physical laws of nature, and a good understanding of physics and chemistry will help you better understand how welds are made. The science of sound is important to welding since one welding process and one weld nondestructive examination technique is based on the use of sound. Sound is transmitted through most materials including metals, gases, and liquids, but it will not pass through a vacuum. Sound is an alternating type of energy based on vibrations, which are regions of compaction and rarefaction.

The science of light also involves welding. The laser beam welding process utilizes light energy at very high concentrations to create heat sufficient to cause melting, which can be used for welding or cutting. Light is a by-product of the arc welding processes. Light is given off by the arc and by heated electrodes and base metals.

The science of friction also involves welding. Here we are interested in dynamic friction, better known as sliding friction. This is the force between two moving bodies, and if sufficient force is available, heat will be generated. This is the basis for the friction-welding process.

Several chemical definitions relate to welding. One is known as burning or oxidation. This takes place when any substance combines with oxygen, usually at high temperatures. An example of this is the combining of acetylene with oxygen. This produces carbon dioxide plus water plus a large amount of heat. We use the heat produced by the burning of acetylene in the flame of the oxyacetylene torch to make welds. In all oxidation reactions, heat is given off. Oxidation can occur very slowly, as in the case of rusting. If iron is exposed to oxygen at high temperature, rapid oxidation or burning will occur with the liberation of more heat. Rapid oxidation or burning does not occur until the kindling temperature of the material is reached. In the case of a liquid, this term is called the flash point. Oxidation is very important in welding operations since oxygen of the air is usually present as well as heat.