M. G. Mahmoud

This study investigates the surface cracking of an intermediate-strength Cr-Mo steel drill pipe through integrated magnetic particle inspection, chemical and mechanical characterization, and metallographic examination. Routine inspection of materials stored for approximately six months revealed faint, intermittent longitudinal cracks up to 43 cm in length and widespread pitting corrosion on pipes reported as newly purchased. Although the material’s properties largely met the specifications for Cr-Mo steel, microscopic and EDS analyses identified substantial concentrations of sulfur (0.26 - 0.65 wt.%) and chlorine (0.21-0.65 wt.%) within the corrosion products. These findings contradict the claim of unused material and indicate prior exposure to aggressive downhole drilling environments rich in chlorides and sulfides, where surface pitting and cracking had initiated before the pipes were refurbished and resold. The failure was primarily attributed to hydrogen sulfide (H₂S) stress corrosion cracking (SSC), a form of environment-assisted cracking (EAC). Alternative mechanisms including fatigue cracking, hydrogen embrittlement, and conventional stress corrosion cracking were systematically excluded based on microstructural and fractographic evidence. The predominance of longitudinal cracking was linked to propagation under circumferential principal stress (CPS), the maximum tensile stress in a pressurized pipe. Additionally, a shallow decarburized layer (~70 μm deep) near the outer surface was observed, creating a mechanically weakened region that facilitated crack initiation. The failure was ultimately driven by the combined effects of the corrosive environment (Cl⁻ and H₂S), manufacturing defects, and operational stresses.

Friction Stir Processing (FSP) was applied to as-rolled AA5083 aluminium alloy to enhance its wear resistance through microstructural refinement. Processing was conducted at a tool rotation speed of 1120 rpm and a traverse speed of 40 mm/min. The Peak processing temperature increased with the number of passes, reaching ~492°C after three passes, which, combined with severe plastic deformation, promoted dynamic recrystallization and grain refinement. Orientation maps confirmed progressive refinement in the nugget zone, with average grain size reduced from ~10 μm (one pass) to 4 μm (three passes). This refinement enhanced hardness in line with the Hall–Petch effect. Tribological testing showed that the as-rolled alloy had a wear rate of 6 × 10⁻³ g/min, which rose to 7 × 10⁻³ g/min after one FSP pass due to abrasive particle generation. With additional passes, the wear rate decreased to 5.5 × 10⁻³ g/min and 4.5 × 10⁻³ g/min, demonstrating improved resistance beyond the as-rolled condition. The enhancement is attributed to grain refinement, and higher hardness.

The present work was performed on A413.1 alloy containing 0.2–1.5 wt% rare earth metals (lanthanum or cerium), 0.05–0.15% Ti, and 0–0.02 wt% Sr. These elements were either added individually or combined. Thermal analysis, image analysis, and electron probe microanalysis were the main techniques employed in the present study. The results show that the use of the depression in the eutectic temperature as a function of alloy modification cannot be applied in the case when the alloy is treated with rare earth metals. Increasing the concentration of RE increases the solidification zone especially in Sr-modified alloys leading to poor feeding ability. This observation is more prominent in the case of Ce addition. Depending upon the amount of added Ti, two RE based intermetallics can be formed: (i) a white phase, mainly platelet-like (approximately 2.5 μm thick), that is rich in RE, Si, Cu, and Al and (ii) a second phase made up of mainly grey sludge particles (star-like) branching in different directions. The grey phase is rich in Ti with some RE (almost 20% of that in the white phase) with traces of Si and Cu. There is a strong interaction between RE and Sr leading to a reduction in the efficiency of Sr as a eutectic Si modifier causing particle demodification.