PSI - Issue 68

Mahmoud Khedr et al. / Procedia Structural Integrity 68 (2025) 1017–1023 Mahmoud Khedr / Structural Integrity Procedia 00 (2025) 000–000

1019

3

Table 1. Chemical composition of the experimental BMsand utilized filler metals in wt. %.

Elements (weight %) C Si Mn

Materials

Cr

Ni

Mo

Cu

V

N

Nb

Fe

LCS

0.053 0.04 0.37 0.025 0.03 0.005 0.09 0.001 -

0.001 Bal.

MMn-SS NiCr-SS

0.098 0.15 9.57 14.60 1.26 0.031 1.43 0.06 0.20 0.043 Bal. 0.025 0.42 1.12 18.70 7.80 0.001 0.014 0.12 0.10 0.051 Bal.

ER309MoL 0.01 0.40 1.50 22.00 14.80 2.50 0.12 -

-

-

Bal.

Heat input (kJ/mm) = η × I × s V × 1000

(1)

Where η represents the arc efficiency of the GTAW process, with a value of 0.6 according to BS EN 1011-1. I denotes the welding current in amperes (A), V signifies the arc voltage in volts (V), and s represents the welding speed in mm/s.

Table 2: GTAW process parameters of welded joints.

Current (A)

Voltage (V)

Welding speed mm/sec

Total Heat input kJ/mm

Specimen

Pass

Root Cup Root Cup

50 95 50 95

12.5 12.5 12.5 12.5

1.1

LCS/MMn-SS

0.697

2

0.9 2.2

LCS/NiCr-SS

0.741

The metallography preparation followed standard procedures. beginning with mechanical grinding using SiC papers. Subsequently, the ground surfaces underwent polishing using Al 2 O 3 suspension with a grain size of 0.5 µm. Finally, the specimens were chemically etched: Nital (5 mL HNO 3 + 100 mL ethanol 99 %) was used for the LCS sides, while an electrolytic oxalic acid etchant (10 g of oxalic acid and 100 mL distilled water at 6 V for 1 minute) was applied for the FZ, following the ASTM E407-07 standard. The microstructures of the weldments were examined using an optical microscope. δ-ferrite volume fraction was assessed via a ferrite scope. Hardness testing was conducted according to the Vickers Hardness (HV) scale using a hardness tester at room temperature, featuring a diamond indenter under a load of 10 kg for a penetration dwell time of 15 seconds. Tensile specimens were cut parallel to the rolling direction of the sheets and perpendicular to the welding pool direction. Three specimens per heat input were machined from the weld pads using electrical discharge machining according to the ASTM E8/E8M standard, featuring a gauge length of 50 mm. Uniaxial tensile testing was conducted using a hydraulic universal testing machine at a quasi-static strain rate of 10 -3 s -1 at room temperature. Additionally, scanning electron microscopy (SEM) was employed to scrutinize the details of the fractured surfaces post-tensile testing, aiming to analyze the mode of fracture. Figure 1 shows the microstructures of the BMs: LCS, MMn-SS, and NiCr-SS, which were observed via optical microscopy. It is noted that the microstructure of LCS consists of a ferritic matrix, light phase, including pearlite phase, dark, as shown in Figure 1 (a). The microstructures of both stainless steels, MM-SS and NiCr-SS, are fully austenitic structures, see Figures 1 (b and c). The microstructural characteristics of the weldments in the butt joints LCS/MMn-SS and LCS/NiCr-SS are shown in Figure 2. It is observed that the FZs of the specimens display a dendritic structure consisting of an austenitic matrix and a ferrite phase. The fraction of δ-ferrite promoted in the FZs of LCS/MMn–SS and LCS/NiCr-SS were 9.1 and 9.2%, respectively. It is worth mentioning that the filler ER309MoL bearing a high content of Mo, 2.5 wt.%, was used in welding joints to induce a certain fraction of ferrite in the microstructure of the weldment which avoids the formation of solidification cracks, as well as enhancement of ductility, toughness, and corrosion resistance. 3. Results and discussion 3.1. Microstructure Analysis