Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
of welding consumables, these should be specified according to a recognized
national or international code which defines their composition limits and
performance rather than by trade names. (In some cases, the user may need
to add specific limitations on performance and composition.)
The method of measurement of electrical parameters should be specified
[e.g. root mean squared (RMS) or mean] and, in some cases, it may be
necessary to specify the type of instrument and the measuring technique
[200] (see also Section 10.3 below).
The tolerances on procedure variables should be specified with due regard
to the equipment capabilities and limitations. For example, new equipment
will be calibrated in accordance with a manufacturing standard, but the
tolerances allowed are often quite wide; for most equipment, values of ±10%
of those indicated are permissible and repeatability of settings between
equipment cannot be guaranteed. Calibration of existing equipment may also
be difficult, for example in the case of simple MMA equipment which often
has poorly defined markings on controls which are subject to wear. [201] In
GMAW, equipment meters, where fitted, often become damaged and deteriorate
in the normal welding environment and cannot be relied on for calibration
purposes.
A further source of calibration error is the variation in output of conventional
welding equipment with mains input swings which may be up to ±10%;
however, it is common for more modern electronic welding power sources to
incorporate output stabilization which offers greater protection against
uncontrollable input voltage fluctuations. In addition it is usually possible to
obtain improved accuracy and repeatability from these electronic power sources
as discussed in Chapter 3.
In view of the problems listed above, it is necessary to check equipment
which is intended for use on critical welding procedures with some external
calibration device which itself has a known accuracy. The appropriate level
of calibration will be determined by the application and a two-tier system
has been proposed [202] and this is the subject of proposed codes of practice.
This system consists of the following grades:
Grade 1. This is the standard grade of calibration accuracy as required by the
power source design standards.
Grade 2. This higher or ‘precision’ grade is intended for applications requiring
greater precision, such as nuclear industry joints, mechanized, orbital and
robotic welding systems.
Target requirements for each of these grades are shown in Table 10.3.
It is not only the welding equipment which requires calibration; in the
case of MMA welding, where electrode temperature control may be very
important, it is necessary to calibrate the electrode storage ovens and, when
preheat and post-heat treatment are involved, the heating equipment and the