Advanced Welding Processes: Technologies and Process Control

188
Advanced welding processes
welding parameters, or a more formal approach may be adopted, in which
the suitability of the welding procedure is tested and documented.
The traditional method of control by means of formal welding procedures
depends on:
∑ establishing and proving satisfactory welding parameters by procedure
trials and testing;
∑ maintaining the same parameters in production;
∑ monitoring by means of final inspection and non-destructive examination
(NDE);
∑ correction of errors by repair and rework.
It is assumed that, if all the process inputs remain fixed, a satisfactory repeatable
output, in terms of weld quality, will be obtained.
Any input errors or disturbance in the process that cause deterioration of
quality may not be noticed until final inspection. This may be considered to
be an open-loop system since the quality of the output is not used directly to
control the process; the control loop is closed by manual intervention to
correct any errors, but this is often carried out after the weld has been
completed and the only means of correction is repair.
Full qualification and approval of procedures in this way is a costly and
time-consuming process and is usually only justified when specific joint
quality requirements must be achieved. However, in many cases it is essential
to use these techniques to achieve an adequate measure of control and satisfy
quality assurance requirements. 3
10.3 Monitoring
Measurement of welding parameters and calibration of equipment is essential
when any method of control is to be applied. The requirements for formal
welding procedure control have been discussed above, but the availability of
suitable monitoring methods is also a prerequisite of any automatic control
system. Considerable improvements in the methods of monitoring welding
processes have been made and some of the techniques available will be
discussed below.
10.3.1 Welding parameters and measuring techniques
The techniques will be discussed under the following headings:
3 These techniques may need to be implemented to satisfy quality systems standards such
as the ISO3834, ISO 9000 EN 29000 series or specific fabrication standards e.g. for
pipelines, pressure vessels and bridges.