African Fusion June 2019

Cold repair of 625 HSS

original Inconel cladding to the medium carbon steel to be about 0.6 mm, reflecting a 0.9 mm weld bead penetration depth into the Inconel cladding for the first bead that was deposited. This distance increases to 1.1 mm in subsequent weld beads. This is attributed to subsequent passes being positionedon the toe of the previousweld beads. A small degree of spheroid- isation was observed in the medium carbon steel adjacent to the Inconel-carbon steel interface. This spheroidisation is at- tributed to a temperature field very close to or exceeding the A c1 temperature in this region. If the material experienced a temperature above the A c1 temperature but did not experience a phase transformation, it can be concluded that the delayed transformation is as a result of superheating. Hardness tests In order to confirm that no new HAZ has developed that is not detectable through visual examination, the material was tested to the requirements of NACE MR0175, which requires a maximum hardness in the HAZ 22 HRC or 250 HV. A micro- Vickers hardness test was used in order to sample the HAZ as close as possible to the Inconel-carbon steel interface. All samples were tested in the carbon steel base material, HAZ and in the Inconel cladding. Figure 8 graphically repre- sents the hardness profiles through the cladding and into the medium carbon steel of the material that had been repair welded and thematerial that had not been repair welded. The hardness in the approximate region 0.5 mm on either side of the fusion line, both in the Inconel and in the medium carbon steel, show the hardness values for the repair weldedmaterial deviated from the sample representing the original cladding. This canmost likely be attributed to the slight spheroidisation effect in the HAZ of the repair weld (Figure 6 and Figure 7). About 0.75 mm from the fusion line of the repair weld, still no significant variation in hardness could be observed. Corrosion testing The sample was subjected to an ASTM G48 corrosion test at 40 °C for 24 h. Typical acceptance criteria for this test as per NORSOK M-601 stipulates that a maximum corrosion rate represented by a mass loss of no more than 4.0 g.m-2 within 24 hours of exposure is an acceptable result [1]. The samples experienced a maximummass loss of 0.92 g.m-2. Final remarks The final part that was to be repaired was the sealing surface of a circular forging with a diameter of 1.35 m and a length of 0.25 m. This equates to 1.06 m 2 of surface area that was clad. Due to the level of automationandeaseofweldingexperienced

Figure 6: Optical micrograph of the fusion line with no cold repair applied (100× magnification).

Figure 7: Optical micrograph of the fusion line below the cold repair (100× magnification). Metallography In order to verify that little enough energy was supplied to prevent any unwanted microstructural transformations, the sample was mounted, polished and etched to observe the HAZ. As can be seen in Figure 6 and Figure 7, the depth and morphology of the HAZ is similar below the cold repaired area and below the original cladding. No new HAZ was observed after the cold repair. The average remaining clad thickness observed on the sample that was sectioned was approximately 1.7 mm. Figure 5 is a stereographic image of the cold repair, which was selectively etched to reveal the microstructure of the Inconel cladding. This shows the distance between the fusion line of the cold repair weld bead and the fusion line of the

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June 2019

AFRICAN FUSION