PSI - Issue 60

Mantosh Mandal et al. / Procedia Structural Integrity 60 (2024) 510–516 Mantosh Mandal/ StructuralIntegrity Procedia 00 (2019) 000 – 000

511

2

While the in-service performance and degradation of this alloy have been actively studied [3], there is very little information on manufacturing-induced defects, especially in the public domain. One reason for this scarcity could be the classical view that clean casting conditions greatly reduce defects [4]. However, similar alloys are not only prone to external defects but also intrinsic defects arising from alloy chemistry and processing history [2, 5]. To underscore this point, a case study focusing on defects in a Ni-base superalloy is presented. This case study serves to emphasize the significance of addressing manufacturing-related challenges and to identify the recommended actions for mitigating these issues. By shedding light on specific instances of defects arising from inadequate attention to prior thermal and mechanical processing history, this study aims to raise awareness about the importance of meticulous manufacturing processes and the need for comprehensive understanding and control of material processing parameters. The product form discussed in this study is Alloy 617M seamless tube, with nominal chemical composition listed in Table 1. Seamless tube manufacturing comprises double-vacuum melting, ingot forging, machining of hollow billets, hot extrusion of these billets into extruded tubes, and cold working of the extruded tubes. At the hot extrusion stage , t he extruded tubes were found to develop transverse cracks along the curved surface, throughout the extruded length (Fig. 1). The tube dimensions at this stage were (approx.) 18 mm (wall thickness) x 114 mm (outer diameter) x 600 mm (length). The cracks were macroscopic, extended nearly 25 mm into the thickness of the billet, and were fairly regularly spaced along the length of the tube.The cracking pattern, ident ified as ‘fir - tree cracking’ [6, 7], was reproduced across several extruded tubes. The root-cause and process correlations of these fir-tree cracks were identified through physical simulation of the defects, as described in the following section. 2. Case Study

Table1: Nominal chemical composition of Ni-base alloy used for this study

Elements

Ni

Cr

Co

Mo

Fe

Al

Ti

Wt. (%)

55

22.5

13

8

0.1

0.8

0.5

3. Methods

To investigate the origin of the cracks, attempts were made to recreate the thermomechanical environment that existed during hot extrusion. To this end, cylindrical specimens of dimensions 6 mm diameter x 9 mm height were manufactured from the hollow billets used for extrusion. The specimens were extracted radially from the billet such that the cylinder axis was along the wall thickness direction. Extrusion is primarily a compressive process and can be physically simulated through uniaxial compression tests [8]. Therefore, the cylindrical specimens were uniaxially compressed (to 50%) at temperature of 1175 o C, which matches the average hot extrusion temperature. The direction of compression in the specimens was the same as that of wall thickness reduction (indirect