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NDT Inspection of Cryogenic Pressure Vessels in AL-Mg Alloy 5083 H 111 with Thicknesses up to 160 mm
Author : PAPPONETTI Mario
Head of NDT department RINA S.p.A., GENOVA
Co-Author : BIESTRO Alberto
Quality Assurance Manager SIMIC S.p.A., Camerana-CUNEO
The welding of cryogenic pressure vessels having diameter of 4950 mm and total length of 3150 mm with thickness up to 160 mm in Al-Mg Alloy intended to an international research centre for a project on high energies, has shown since the qualification of the welding processes that same particular defects such as lack of fusion and micro porosity were very difficult to find with traditional NDT, namely RT, performed according to the usual procedures foreseen in the Codes for pressure vessels.
To overpass the problem an extensive research has been conducted with the aid of various destructive and non destructive methods in particular by means of RT,UT,PT and their combination.
The problems encountered as regards welding together with the NDT applied for the detection of defects are here reported.
Typical " Mock up " containing defects were manufactured and " had hoc " NDT procedures prepared together with suitable training for the NDT operators.
As a result, the parameters of the welding processes were optimised reducing the work of repairs particularly dangerous for possible distortion of the vessels.
During the mechanical tests performed in order to qualify the welding processes relevant to the construction of two pressure vessels in Al-Mg part of which are showed for information in fig.1, 2, 3 it has been noticed that tensile tests and bend test were having not acceptable results showing defects in the weld starting near the fusion line .A review of the results of prior NDT performed by means of RT confirmed that no defects have been detected.
2 Action taken
- A careful examination of the welding process on hand was started while in the mean time an extensive program of ndt tests was defined in order to find the reasons that could have caused the defects not to be detected.
- As regards the welding process, it was found by metallographic examination of the broken samples that the structure on hand in addition to the presence of intermetallic compound due to ageing treatment showed small lack of fusion and micro-porosity.
- In order to overpass the problem of the formation of the defects, the welding parameters and shielding gas were reviewed having in mind that due to the very high thickness involved a high cooling rate accompanied to a low heat input would probably have resulted in local side lack of fusion and inclusion of gas in the fusion zone.
- As regard the NDT problems many samples welded with various parameters were prepared in order to study the best conditions to apply when performing NDT tests. It was considered necessary to have samples defects free ( welded with the final welding parameters )and other containing the intended defects ( welded with the initial welding parameters ) fig. 4.
Fig 4: |
3 NDT Initial Program
- The samples containing defects and those defect free were inspected with the NDT procedures usually adopted for pressure vessels by means of RT,UT, and Dye penetrant by personnel duly qualified level 2 in the applicable methods. Defects were not detected.
- RT performed with the head inclined so to send a parallel xray beam to the bevel of the weld gave not rise to the detection of defects.
- Dye Penetrant inspection performed on the surface of the welded samples gave no rise to the detection of defects.
- UT inspection was considered the most suitable for the detection of the intended defects. But as said before with the conventional procedures did not give acceptable .result.
4 UT Tests
- In addition to the conventional 45- 60- 70 angle probes, 4 Mhz 8 x 9 mm dimension crystal the following were tested:
- Double crystal probes of different angles focused to the half thickness to be inspected
- Tandem system with one probe transmitting and one receiving from the same side of the welded joint
- Delta technique
- The best results were obtained using single crystal angle probes having 20 x 22 mm, 2 Mhz from the two sides of the weld and from the two faces keeping to distance to be inspected shorter as possible. The calibration in sensitivity was regulated against a 1,6 side drilled hole at mid thickness at 80% FSH plus 6 db.
- DAC curve was designed for the inspected part. on 1,6 side drilled holes at different depth form the surface;,
- All echoes on the screen greater than 20% of DAC were investigated in order to verify the presence of linear defects such side lack of fusion
- The presence of micro porosity was considered detected when on the screen small indication of grass appeared, and not present in defect free samples.
6 Confirmation of defects
- In order to verify the finding of the defects said above, a careful cross check was conducted while new welding tests were performed by changing:
considering that the greater risk to the formation of cracks in welded zone being taken into account by reducing the dimension of passes
the samples were then cut in correspondence of the indications obtained by UT examination. The micrographs performed have shown very good correspondence both as regards side lack of fusion and micro porosity.
same procedure as in point 2 above was applied to the samples prepared with known defects
The Xray examination conducted on part considered defective by UT inclining the head in order to obtain parallel beam to the bevel have given cross confirmation of side lack of fusion only in major dimensions ( in the range of 3 mm thickness ).
- pre-heat temperature
- interpass temperature
- protective gas
7 Specific training of the UT operators
- The operators intended to inspect the welds were trained against samples containing same type and dimension of the defects to be searched.
- The defects detected during the welding procedure qualification were deeply investigated in order to find their origin and possibly avoid the causes during actual production.( Fig.5)
- NDT intended to be used during actual production were studied in order to be able to detect said defects should they be still present for any reason.
- Inspection procedures suitably prepared, together with samples to be used for training of the NDT operators have shown to be of paramount importance.
- As a conclusion the pressure vessels on hand have been welded with welding processes that have not requested particular repairs thus avoiding undue distortion of plates while the inspection has been conducted with the confidence that should some defects for any reason be present they would have been detected.
Fig 5: |