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 a procedure which is simply 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.”
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.
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 a procedure which is simply 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 is used to make a record of all of the different elements,
variables, and factors that are involved in producing a specific weld or weldment.
Welding procedures should be written whenever it is necessary to:
- Maintain dimensions by controlling distortion
- Reduce residual or locked up stresses
- Minimize detrimental metallurgical changes
- Consistently build a weldment the same way
- Comply with certain specifications and codes.
Welding procedures must be tested or qualified and they must be communicated
to those who need to know. This includes the designer, the welding inspector,
the welding supervisor, and last but not least, 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 a welding procedure, but in general a welding
procedure consists of three parts as follows:
- 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
- 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 and the procedure need not
Essential variables involved in the procedure usually include the following:
- 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
- Welding technique.
Some specifications also include nonessential variables and these
are usually the following:
- 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
- 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.
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 and will be covered
in detail in a later chapter.
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 in approaching
any new welding job. By means of knowledge and experience establish the
optimum factors or variables in order 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
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
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: metals, gases, liquids, etc., but it will not pass
through a vacuum. Sound is an alternating type of energy based
on vibrations, which are regions of compaction and rarification.
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.