1 Full document available on EDMS Metallurgical qualification of AISI 316L TIG welded tubes Summery - Richard French Report and testing - S.Sogobba, A. Geradin, P.Deweulf CERN EN/MME

2 Overview NDT carried out at Sheffield in showed good results. No real DT readily available for these diameters/wall L tube butt weld samples were produced in late 2011 to demonstrate automatic welding process repeatability. Initial set up problems [gas purge + electrode life] once understood, enabled mass production 1036 samples tube samples were made in total 994 completed one after the other without failure. 316L weld batch 316L weld Samples 137,428,968, 2, 205, 309 were randomly selected. The only control was I chose samples made by 3 different people operating the welding system. The samples were then sent to CERN EN/MME for metallurgical testing after discussions of the best testing methods

3 Testing: Six samples have been cross-sectioned, polished and etched (etchant 13a of ASTM E407 see note (a)). Metallographic observations have been performed on a LEICA MZ16 stereomicroscope. Micro-hardness profiles across the weld bead have also been carried out on the three metallographic samples with a WOLPERT 402 MVD micro-hardness tester. Tensile tests on 3 samples have been performed on a UTS tensile test machine equipped with auto-tightening grips and a 200 kN load cell. Tested samples: Metallography: 137, 428 and 968 Tensile tests: 2, 205 and 309 NOTE (a): By virtue of the fact that Stainless Steels are highly corrosion resistant exceptionally strong acids are required to reveal their structure. ASTM E407 is a method of etching a metal sample to reveal its microstructure (observance of grain size, direction etc v adjacent grains in the tube). This microstructure can be inspected by mounting, cross sectioning, and polishing the tube, followed by etching per ASTM E407. This standard specifies what etching chemicals (acid in etchant 13a -approx 40% aqueous sodium hydroxide solution) and procedures are required to reveal the microstructure of different types of metals. Differences in microstructure are important because they can help to determine if a metal has been subjected to corrosive chemicals, is softer or harder at the surface, has been deformed, was welded properly, or has been over- heated. Microstructural results are typically reported as pictures of the microstructure along with a paragraph interpreting the meaning of the structure.

Metallographic observations 4 Metallographic observations have been performed on a LEICA DMRME microscope. Figure 1- Sample 137 a/ Original magnification: 25 x: General view of the weld b/ Original magnification: 200 x: Detail of the weld bead c/ Original magnification: 200 x: Detail HAZ (polarized light) d/ Original magnification: 200 x: Detail base metal (polarized light) Black marks seen are the fiducial indicators required for inspection purposes A – the darkening is indicative of the chemical etchant to highlight grain structure. AB CD

Metallographic observations 5 Figure 2- Sample 428 a/ Original magnification: 25 x: General view of the weld b/ Original magnification: 200 x: Detail of the weld bead c/ Original magnification: 200 x: Detail HAZ (polarized light) d/ Original magnification: 200 x: Detail base metal (polarized light) A B C D Samples are again showing fiducial indicators for inspection purposes Picture A shows the darkening of the etchant to highlight grain structures. One very important comment to note; when welding thin wall tube: What you see is what you get! Tests such as dye penetrant are negated.

Metallographic observations 6 Figure 3- Sample 968 a/ Original magnification: 25 x: General view of the weld b/ Original magnification: 200 x: Detail of the weld bead c/ Original magnification: 200 x: Detail HAZ (polarized light) d/ Original magnification: 200 x: Detail base metal (polarized light)

Micro-hardness profiles & tensile testing 7 Micro-hardness profiles across the weld bead have been carried out on the three metallographic samples with a WOLPERT 402 MVD micro-hardness tester with HV 0.1. Tensile tests on 3 samples have been performed on a UTS tensile test machine equipped with auto-tightening grips and a 200 kN load cell. Elongation has been measured with a MF25 extensometer. One pin has been inserted at each extremity of the samples to avoid crushing.

Conclusions 8 To the extent of the performed micro-optical observations neither structural nor geometrical imperfections could be detected. For the micro-hardness tests, tests have been performed according to NF EN maximum value in the weld bead is 231 HV 0.1 (sample 428). For the tensile tests, all samples broke in the weld. The average Ultimate Tensile Strength (UTS) is 588 ± 11 MPa. According to NF EN 288-3, the UTS should not be less than the minimum specified value of the parent metal. The minimum UTS value for AISI 316L pipes as stated in ASTM A312 is 485 MPa. What does this mean? Tensile test: Looking at test data from TIG butt welded AISI 316L pipes (OD = 4 mm with 0.5 mm and 0.7 mm thicknesses) used by the AMS experiment. Similar results around MPa for UTS, no extensometer could be used to determine RP0.2 (estimation according to elastic-plastic transition on the curve around 300 MPa). Elongation determined with measurement of the whole sample before and after test was about 20 %. Failure occurred either in the weld or in the HAZ (Heat Affected Zone). Results obtained for our pipes are very similar to these ones. Looking at other test data from large bore tubes and pipes – this is a virtually identical mode of failure indicative of an excellent weld. Metallographic /hardness observations/tests: Parent and weld material hardness and strength look fine eg, the weld is not too hard or brittle. The weld bead grain looks correct when compared to other 316L welds (grain size). The adjacent grains in the tube look marginally bigger, ( we assume they are not bigger through the heat generated by welding). There are enough grains though the cross section not to have potential leak path issues via grain boundaries as we have previously proved by rigorous leak and pressure testing.