When structures and pressurized items are fabricated
by welding, it is essential that all the welded joints are sound and have
suitable properties for their application.
Control of welding is achieved by means of Welding
Procedure Specifications (WPSs) that give detailed written instructions about
the welding conditions that must be used to ensure that welded joints have the required
properties.
Although WPSs are shop floor documents to instruct
welders, welding inspectors need to be familiar with them because they will
need to refer to WPSs when they are checking that welders are working in
accordance with the specified requirements.
Welders need to be able to understand WPSs have the
skill to make welds that are not defective and demonstrate these abilities
before being allowed to make production welds.
Qualified
Welding Procedure Specifications
It is industry practice to use qualified WPSs for
most applications.
A welding procedure is usually qualified by making a
test weld to demonstrate that the properties of the joint satisfy the
requirements specified by the application standard and the client/end user.
Demonstrating the mechanical properties of the joint
is the principal purpose of qualification tests, but showing that a defect-free
weld can be produced is also very important.
Production welds made in accordance with welding
conditions similar to those used for a test weld should have similar properties
and therefore be fit for their intended purpose.
Welding standards for procedure qualification
European and American Standards have been developed
to give comprehensive details about:
How a welded test piece must be made to demonstrate
joint properties.
How the test piece must be tested.
What welding details need to be included in a WPS.
The range of production welding allowed by a
particular qualification test weld
The principal European Standards that specify these
requirements are:
EN
ISO 15614 Specification and qualification of welding
procedures for metallic materials – Welding procedure test
The
qualification process for welding procedures
Although qualified WPSs are usually based on test
welds that have been made to demonstrate weld joint properties; welding
standards also allow qualified WPSs to be written based on other data (for some
applications).
Some alternative ways that can be used for writing
qualified WPSs for some applications are:
Qualification by adoption of a standard welding
procedure – test welds previously qualified and documented by other
manufacturers.
Qualification based on previous welding experience –
weld joints that have been repeatedly made and proved to have suitable
properties by their service record.
Procedure qualification to European Standards by
means of a test weld (and similar in ASME Section IX and AWS) requires a
sequence of actions that is typified by WPS.
A successful procedure qualification test is completed
by the production of a Welding Procedure Qualification Record (WPQR).
Relationship
between a WPQR and a WPS
Once a WPQR has been produced, the welding engineer
is able to write qualified WPSs for the various production weld joints that
need to be made.
The welding conditions that are allowed to be
written on a qualified WPS are referred to as the qualification range and this
range depends on the welding conditions used for the test piece (the as-run
details) and form part of the WPQR.
Welding conditions are referred to as welding
variables by European and American Welding Standards and are classified as either
essential or nonessential variables.
These variables can be defined as follows:
Essential
variable: A variable that has an effect on the mechanical properties
of the weldment (and if changed beyond the limits specified by the standard
will require the WPS to be re-qualified).
Non-essential
variable: A variable that must be specified on a WPS but does
not have a significant effect on the mechanical properties of the weldment (and
can be changed without need for re-qualification but will require a new WPS to
be written).
It is because essential variables can have a significant
effect on mechanical properties that they are the controlling variables that
govern the qualification range and determine what can be written in a WPS.
If a welder makes a production weld using conditions
outside the qualification range given on a particular WPS, there is danger that
the welded joint will not have the required properties and there are then two options:
Make another test weld using similar welding
conditions to those used for the affected weld and subject this to the same
tests used for the relevant WPQR to demonstrate that the properties still
satisfy specified requirements.
Remove the affected weld and re-weld the joint
strictly in accordance with the designated WPS.
Most of the welding variables that are classed as
essential are the same in both the European and American Welding Standards but
their qualification ranges may differ.
Some application standards specify their own
essential variables and it is necessary to ensure these are taken into
consideration when procedures are qualified and WPSs written.
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