Effects of Complex Oxides on Charpy Impact Properties of Heat Affected Zones of Two API X70 Linepipe Steels

Abstract

Fig. 11. SEM micrographs of the cross-sectional area beneath the cleavage fracture surface of the Charpy impact specimens fractured at Ϫ20°C for the (a) S0-30, (b) S1-30, (c) S1-50, and (d) S1-100 steel HAZs, showing the crack propagation path. Fractured surfaces were coated by Ni.enhancing the HAZ toughness. By using the oxide metallurgy in which AFs are nucleated inhomogeneously inside austenite grains by forming fine complex oxides in linepipe steels as in the present study, linepipe steels with excellent Charpy impact properties in the HAZ can be fabricated. This oxide metallurgy technology can be applied to grain refinement by inhomogeneous nucleation when a high level of deformation cannot be imposed due to the limited rolling reduction ratio as in the case of thick high-strength steel plates. It can also be applied to the development of high-strength steel plates with excellent welding properties by controlling the HAZ microstructures. ConclusionsIn the present study, complex oxides were formed inside API X70 linepipe steels, and effects of these oxides on HAZ microstructures and Charpy impact properties were investigated.(1) Oxides present in API X70 linepipe steels were sized about 2 mm, and had characteristics of complex oxides composed of various elements. The S1 steel containing more Al, Ti, and Mg showed higher volume fraction of oxides than the S0 steel.(2) The HAZ microstructures of the S0 steel simulated with the heat input of 30 kJ/cm were composed of bainite, but the S1 steel HAZ contained 94 vol% of AF, together with a small amount of bainite. Thus, the absorbed energy of the S1 steel HAZ at Ϫ20°C and 0°C was five to eight times higher than that of the S0 steel HAZ.(3) When the weld simulation test was conducted with increased heat input of 50 kJ/cm and 100 kJ/cm, the absorbed energies at Ϫ20°C and 0°C of the S1 steel HAZs decreased down because of the increased amount of bainite as the cooling rate became slower.(4) In the S1 steel HAZs in which the AGS was relatively large (91 mm) and a number of complex oxides were contained, complex oxides worked as AF nucleation sites, thereby promoting the active formation of AF in the HAZ and improving Charpy impact properties. AcknowledgementThis work was supported by the National Research Lab-

Effects of Oxides on Tensile and Charpy Impact Properties and Fracture Toughness in Heat Affected Zones of Oxide-Containing API X80 Linepipe Steels

“…[8][9][10][11] Oxides cannot play a role sufficiently as crack or void initiation sites, because they are surrounded by tough AFs. [16] Thus, the reason why the fracture toughness increases with increasing no. of oxides is associated with the volume fraction of AF formed around oxides.…”

“…Particularly in the quasi-static fracture toughness measurement, oxides can deteriorate the fracture toughness as they work as initiation sites of cracks or voids, while they act as nucleation sites for AF and beneficially enhance the Charpy impact energy. [16] Therefore, systematic studies on mechanisms of toughness improvement in relation with microstructural modification including formation of oxides and AF under dynamic or quasi-static loading condition in a notched body or a cracked body are essentially needed.…”

Challenges and developments in pipeline weldability and mechanical properties

“…2,11,28,44 It is easy to form coarse grain microstructure and MA constituent in HAZ after such ultrahigh heat input welding. Since grain coarsening and the MA constituent present in HAZ structure are the main reasons for toughness deterioration in the weld area, a fine microstructure and reduced formation of MA will lead to improve the HAZ toughness in X70 and X80 pipes.…”

“…Thus, the possibilities of improving the toughness of HAZ and WM with increasing strength of pipeline have received extensive investigations. [42][43][44][45][46][47] Heat affected zone For a typical X80 pipeline, the resulting microstructure of the commercial alloy is one containing ferrite and bainite (see Fig. 1).…”

Effect of Mg Addition on the Microstructure and Properties of a Heat-Affected Zone in Submerged Arc Welding of an Al-Killed Low Carbon Steel

“…In recent years, numerous studies have been conducted both in China and globally focusing on the role of Mg treatment in the steel welding process. The typical related works are listed and summarized in Table 1 [ 4 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ]. As can be seen from this table, currently, the influence of Mg treatment on steel welding performance is commonly analyzed by welding thermal simulation experiments and Mg-bearing inclusions are mainly MgO, MgS, and Ti–Mg oxides.…”