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Quality Cannot Be Inspected
Into A Weld
By
André A. Odermatt
President, Hobart Institute of Welding Technology
Welding,
particularly manual and semiautomatic arc welding, is a process
considered to have inherently high variability and should be
avoided in product design, if possible. However, welding is
still the most economic and efficient way to join metals
permanently and it is therefore a widely used process.
The
ASM Handbook (v.20, pg. 299) contains a table that rates
characteristics for common manufacturing processes. The word
quality is defined as an inclusive term describing fitness
for use. Included under this characteristic is surface
roughness, ability to manufacture parts to a set tolerance,
product integrity (freedom from voids, pores, cracks, and
inclusions) and ability to produce controlled properties
(microstructure control). On a scale of one to five, (1 =
poorest) welding ranks number 2.
I think this low
rating, given by ASM International, has much to do with the fact
that indeed welding is a process with many variables. Every type
of weld has certain “specification limits” with regard to
variables the weld deposit has to be within. To consistently
control these variables so each weld meets the specification
limits is not an easy task and requires many hours of training
and experience.
It is not
possible to inspect quality into a weld. After a weld is
made with one or more variables that do not meet the
specification, inspection is only a sorting operation. 100%
inspection rarely assures 100% accuracy. Therefore it is of
critical importance that each weld is at least within the
parameters of the given specification. Testing, often
destructive, is done on one or more samples before a production
run begins. A traditional view of quality used to be “meet
specifications or meet requirements.” Those specification limits
are set based on the needs and desires of the customer. If a
welder “meets specifications” on a consistent basis he is a good
welder… or is he?
Hobart
Institute of Welding Technology looks at weld quality based
not only on meeting specifications, but also on never ending
process improvement, narrowing the envelope of manual weld
process capability through systematic training and testing of
students. Besides the steady sizzling sound that a correct arc
produces, students learn to recognize that the shape of the
molten pool and the movement of the metal at the rear of the
pool serve as a guide in checking weld quality. In a correctly
made deposit, the ripples produced on the bead will be uniform
and the bead will be smooth with no overlap or undercut. To
pass the different tests as applicable for each situation HIWT
instructors consider obvious and students identify as a required
goal that they aim to achieve.
There may be
those who claim that a better-than-required quality is a waste
and drives up cost. We don’t agree. The philosophy of
never-ending improvement adopted in welding will make companies
more competitive. Labor represents about three-quarters of total
welding-related production cost. Increasing productivity
in welding (measured in parts completed per hour, or welding
defect rate, or feet welded per period of time) can
substantially decrease welding labor cost and hence help to
reduce product cost and improve the competitive position.
References:
ASM
Handbook:
Materials Selection and Design, Volume 20
HIWT
Pocket Welding Guide, 27th Edition
The National Economic Impact Of Welding,
AWS Convention, 2002.
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