X80 UOE Pipe With Excellent HAZ Toughness

Abstract

New X80 UOE pipe manufacturing technology has been developed to attain excellent CTOD properties in HAZ. In the new steel pipe, the HAZ microstructure near the weld fusion line is refined by utilizing the strong suppression of austenite grain growth as well as the formation of intragranular ferrite (IGF). The IGF grows radially from oxides in the same way as in Ti-O steel. The suppression of austenite grain growth is due to the pinning effect by fine particles including ultra fine oxides. As these oxides dispersed in the steels are chemically stable even near the fusion line, the microstructure can be refined. In addition, it is also necessary to suppress the formation of coarse grain boundary ferrite (GBF) near the weld fusion line in order to obtain excellent CTOD properties. So, it was found that the increase of boron addition in weld metal is effective to suppress the formation of coarse GBF near the weld fusion line because boron atoms diffuse into prior austenite grain boundaries near the fusion line from the molten pool during welding. This paper describes new technology for improving CTOD properties in HAZ. The X80 UOE pipe manufactured on a large scale and exhibited excellent CTOD properties at −30°.

Challenges and developments in pipeline weldability and mechanical properties

“…5,41 On the other hand, the microstructure of the HAZ changes from martensite to lower bainite, upper bainite and then to ferrite and pearlite, as the heat input at welding increases, or the rate of cooling decreases. 41,48,49 Especially, the low temperature toughness will deteriorate when the microstructure consists of hard martensite-austenite (MA) islands. Figure 4 shows that the reduction in toughness is dependent on the volume fraction of MA present.…”

“…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. 35,41,48 Three main methods have been advanced to increase the HAZ toughness, which are summarised in Table 3. The first method used titanium as TiN has been utilised in pipeline steels to improve the HAZ toughness.…”

“…6a). 3,35,48 Under these circumstances, a new technology has been developed where fine TiO particles dispersed in steel are utilised (TiO steel). 3,[52][53][54] In a TiO steel, TiO particles existing inside a c grain serve as nuclei of intragranular ferrite (IGF).…”

“…6b). On the basis of this, an innovative HTUFF technology has been developed by Terada et al 48 In the steel based on HTUFF, the coarsening of c grains near the FL is suppressed by pinning particles of oxides, and the IGF forms inside them. As a consequence, the microstructure of the HAZ is made remarkably fine (see Fig.…”

“…[64][65][66] It is well known that Ni and Mo in the WM play important roles in microstructural control. 67,68 However, there is no general agreement regarding the optimum 6 Schematic of HAZ microstructure control in three kind steels 3,48 7 Correlation between FATT and cooling time for coarse grained HAZ microstructures of pipeline steels 40 72 that the best impact toughness occurred at ,0?5 wt-%Mo in a controlled manner with respect to Mn. Thuvander et al 73 showed that significant amounts of Mo offered excellent properties in WMs in a highly alloyed steel.…”