(110f) A Study on the Improvement of Radiographic Test Method on 9% Ni Steel Welds for Lng Storage Tanks I

Inner tank material of LNG storage tanks has been traditionally making of 9% Ni steel plate for extremely low temperature applications. Many great efforts have also been making to develop thick 9% Ni plate and welding method which should be followed by a trend of large sized tank development. Since tanks have recently become larger and are operated under severer conditions, the material now used is at least 30mm thick and the welded joints must have mechanical properties equivalent to those of the base metal.

To meet these requirements, one of test methods is radiographic test on welding joints.

Electrode of Inconel or Hasteloy series are generally used in 9% Ni steel weld joints that can be considered as a dissimilar weld. It is similar to the weld of austenitic stainless steel and the properties of weld metal is quite differ from base metal and have a coarsening in grain size.

Therefore as a result of radiographic test, false indications from X-ray diffraction and absorption coefficient difference between base metal and weld metal. There has been controversies on whether these are flaws or not on the construction sites.

To solve this problem, we carried out the mechanical test on the welds with false indication in order to verify its integrity and developed the standard for assessment method of it by retesting with changing radiographic conditions and comparing to original radiograph. As a result of new method application, we could reduce unnecessary repairs and support the economical construction of LNG tank by means of reducing the term of works.