Advanced Welding Processes: Technologies and Process Control

Monitoring and control of welding processes 187
Table 10.3 Calibration requirements
Parameter Grade 1 Grade 2
Current ±10% ±2.5%
Voltage ±10% ±5%
Slope up/down time ±5%
Pulse time ±5%
Measuring instruments ±2.5% ±1%
Wire feed speed ±2.5%
Calibration frequency 1 yr max. 0.5 yr max.
devices used for checking temperature need to be considered. In mechanized
welding processes, the travel speed and positional accuracy of the system
will need to be checked regularly.
Monitoring of procedures. Once accepted, it is essential that production
welding operations are monitored to ensure that the procedure is being followed
and that the required results are being achieved. Traditionally, testing of the
completed fabrication using non-destructive test techniques (e.g. x-ray,
ultrasonic, MPI and dye penetrant) have been used to check the weld quality
and, if necessary, defects have been removed and the joint repaired. Progressive
monitoring at an early stage of production can, however, prevent costly
rework after final inspection. Routine monitoring should at least include
checking the critical procedure variables (e.g. current, voltage, wire feed
speed, temperature, consumable treatment, travel speed). Improved control
may be achieved by use of the monitoring instrumentation described in more
detail below.
The use of portable monitoring devices which comply with traceable
standards of calibration is an effective way of ensuring that the optimum
parameters are being used; it is particularly beneficial to use these same
devices at the procedure qualification stage. In practice, accurate high-quality
analogue meters could be used for this purpose, but these tend to be less
robust than digital meters; however, digital meters must be used with care
especially if the parameter being measured is not either a constant DC or
pure sinusoidal AC waveform. Many recent monitoring and calibration devices
incorporate a computer data logger which provides permanent records of the
welding parameters and also allows the data to be transferred to a computer
for permanent storage. (These systems will be described more fully in Section
10.3.1.)
10.2.2 Summary: welding procedure control
Control of the welding may be an informal process, where the welding
engineer or the welder is left to assess the requirements and select appropriate