TPT March 2011

I nspection , T esting & Q uality C ontrol

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. signal-to-noise (S/N) ratio, the signal being from the degree of defect established as the rejection threshold.

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

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

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.

Two channel Multimac for detecting sigma phase and other typical defects of duplex stainless steel

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

Email: info@mac-ndt.com Website: www.mac-ndt.com

CON.T.R.A.S.T.

www.contrastsale.com

info@contrastsale.com

Impeder core for high frequency welding of tubes

Fiberglass pipes for impeder core

Tungsten carbide insert for outside tube scarfing

HSS-DMo5 saw blades & friction circular saw blades

CON.T.R.A.S.T. 30026 PORTOGRUARO - Italy - Via Umbria, 15 Tel. ++39.0421.271699 Fax ++39.0421.770571

109

www.read-tpt.com

M arch 2011

›