TPT March 2011

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March 2011 109 ›
Inspection, Testing & Quality Control
Tungsten carbide
insert for outside
tube scarfing
HSS-DMo5 saw blades
& friction circular
saw blades
CON.T.R.A.S.T.
Impeder core for
high frequency
welding of tubes
Fiberglass pipes
for impeder
core
CON.T.R.A.S.T.
30026 PORTOGRUARO - Italy - Via Umbria, 15
Tel. ++39.0421.271699 Fax ++39.0421.770571
IN recent years the use of duplex stainless
tubing, characterised by higher strength
and better stress corrosion cracking
resistance than most austenitic alloys,
has increased, thereby creating a need
for more stringent eddy current testing.
This increase is particularly found in the
onshore and offshore oil and gas industry,
chemical processing, heat exchangers for
power generation, desalination equipment
and wastewater treatment facilities.
A problem with these alloys is the
occurrence of a metallurgical condition
Detecting sigma phase in duplex stainless steel tubes
called sigma phase. This problematic
condition, which can be introduced into the
tube from a malfunction during the heating
process, reduces the carbon and chromium
content that normally provides strength and
corrosion resistance. However, the condition
can be found successfully with proper eddy
current testing techniques.
Duplex and super-duplex stainless steel
tubes contain about 50% ferrite in their
microstructure, which results in permeability
variations that present a problem for eddy
current testing because they interfere with
successful test results.
The key is to suppress
the permeability variations by
magnetically saturating the test
material using an external source.
Saturation is defined as the degree
of magnetisation produced in a
ferromagnetic material for which
the incremental permeability has
decreased substantially to unity.
Without this magnetic saturation,
permeability variations show up as
‘noise’. The end goal is to minimise
this noise in order to optimise the
signal-to-noise (S/N) ratio, the signal being
from the degree of defect established as the
rejection threshold.
One solution provided by Magnetic
Analysis Corporation for accurately
detecting sigma phase is a two channel
MultiMac ® coil eddy current tester. One
channel is used for the detection of typical
defects, while the other channel is used for
finding sigma phase. The second channel
on the MultiMac is an absolute tester using
two Varimac® comparator coils. One coil is
balanced on air and used to calibrate the
equipment with a reference standard. The
other is the actual test coil installed on a
coil platform.
The system is mounted on a dual pinch
stand test bench and consists of a saturation
platform, such as a MAC CP352, CP351,
or CP65mm for defect detection, followed
by a CP30 for sigma phase. This system
operates on duplex stainless steel tubes.
Magnetic Analysis Corporation – USA
Fax: +1 914 703 3790
Email:
Website:
Two channel Multimac for detecting sigma phase and other typical
defects of duplex stainless steel