Microstructural Changes and Impact Toughness of Fill Pass in X80 Steel Weld Metal

Abstract

Multi-pass welding is used in high-pressure and thick-walled pipes in natural gas and oil pipelines. When a welding layer of a welded joint is subjected to different welding thermal cycles, its microstructure and properties change, thereby affecting the overall welding performance. In this study, the temperature and microstructural variations of the fill pass 2 (FP2) in the entire welding process were investigated by combining the thermal cycle with the cascade welding method. The original FP2 and FP2 after double thermal cycles had the worse deformation ability by tensile test. The toughness of FP2 improved after a single thermal cycle, decreased after double thermal cycles, and improved again after triple thermal cycles. The content of martensite–austenite (M–A) constituents and the average grain size of FP2 in the cascade samples were inversely proportional to FP2 toughness. Massive M–A constituents and their unique distribution at the inter-critical temperature were harmful to weld metal toughness. Controlling the size and fraction of M–A constituents can improve weld metal toughness.

Influence of rotation frequency and rotation diameter on mechanical properties and microstructure of weld metal produced by MCAW-RE

“…The metallographic examination revealed a microstructure where the presence of primary ferrite (PF), acicular ferrite (AF), and ferrite with second phase (FS) are observed. These results are in accordance with the available literature for weld metals of the class 700 MPa [16,38,[42][43][44][45][46][47][48]. In addition, the comparison of the columnar and reheated regions (Fig.…”

“…12). In addition, it is important to mention that both factors contribute to the refined microstructure with higher amount of acicular ferrite than the expected for similar weld metals [44,46,48,65] obtained in this work. Finally, a significant influence of the MA constituents (Fig.…”

“…Finally, a significant influence of the MA constituents (Fig. 14) on the mechanical properties is not expected, because they are not necklacing the grain boundaries [48,58,59,66].…”

Investigation into the Impact Toughness of API 5L X80 Steel Weldments and its Relationship with Safe Welding Procedures

“…In addition, PWHT did not promote any significant changes (<4%) to the tensile strength (Table 3). The weld metal presented a refined microstructure composed of acicular ferrite (AF), primary ferrite (PF) and ferrite with a second phase, as expected for 700 MPa class steels [53][54][55][56][57][58][59][60][61][62][63][64] . This microstructure provided Charpy-V results much greater than the minimum required 45 , both in the as welded and heat treated conditions, as shown in Figure 7.…”