African Fusion June 2019

Cold repair of 625 HSS

It was determined that the minimum travel speed used for the chosen electrical parameters would be 480 mm/min, equivalent to a heat input of 0.4 kJ/mm. Using these param- eters, the distance between the fusion line and the point at which the peak temperature was 735 °C was estimated at about 1.5 mm – Figure 2. Sample preparation The sample used to simulate the original Inconel cladding on which the cold repair was performed was produced using a Fronius FPA 9000Compact CladdingCell (CCC). This equipment is an automated hot wire GTAW system. The coupon dimensions were 6-inch Sch XXS with a specified outer diameter of 168.3 mm and a wall thickness of 21.95 mm. The sample length was 300 mm. Two layers of Inconel were applied to the sample in the 2G position, which resulted in a 6.0mmclad thickness. The samplewas subjected toPWHT at 635 °C for two hours beforemachining the cladding down to a remaining clad thickness of 1.5mm. The remaining clad thicknesswas verifiedusing a Fisher clad thickness probe, at between 1.4 and 1.7 mm. Repair welding methodology The welding parameters used in the cold repair were selected with the intent of producing a broad soft arc with little base metal penetration. However, due to the poor wetting charac- teristics of Inconel 625weldmetal compared to carbon steels, enough heat needed to be supplied to ensure sufficient fusion at the toes. To achieve this, the following was adjusted. • The arc voltage was increased to about 25.5 V as opposed to the usual voltage of about 22 V. • A pushing torch angle of 15 to 45° was used. • Weld bead overlap was approximately 50% in order to transfer as little heat into the base metal as possible. • The arc columnwas directed at theweld toe of the previous run to reduce the risk of lack of fusion (LoF) in this position. A working angle of 10-20° was used. • Heat input was limited to a maximum of 0.4 kJ/mm. • The shielding gas used was selected to increase the weld pool fluidity by providing more arc energy. The 1G rotating positionwas used. This helped tominimise the variation inwelding speed and allowed thewelder to focus on torch angle and arc placement. Results and discussion Qualification tests were performed in conformance to client specifications in conjunction with ASME IX requirements. The tests included: bend tests, hardness tests, macro examination, micro examination, chemical analysis and corrosion testing. During qualification, both a single layer repair as well as a multi-layer repairwas qualified. For thepurposes of this article, only the single layer repair will be discussed. Bend tests There is a very narrow operating band in which cold repairs can be implemented successfully. The upper extreme limit is where an excess of energy is supplied to thematerial such that a phase transformation is induced in the underlying medium carbon steel (see ‘Metallography’ below). The lower extreme limit is when not enough energy is supplied to the weld pool to provide complete fusion of the new Inconel weld metal to the remaining original Inconel clad layer, resulting in lack

Figure 3: A cold repair side bend sample after testing. Inconel presents at the top of the sample. of fusion type defects. In order to test for lack of fusion, the coupon was sectioned and tested using side bend tests as per ASME A370. Figure 3 shows a bend test sample after testing. No defects were observed. As per ASME IX, four bend test samples were performed. All four samples passed the tests with no indications being observed. This result indicated that there was sufficient energy provided to fuse the remaining clad material and the new weld metal.

Figure 4: HAZ of original cladding (left of the image) can be seen to be identical to that of the area that was cold repaired (right of the image).

Figure 5: Stereoscopic image of a cold repair bead fused to the remaining original cladding.

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June 2019

AFRICAN FUSION