On the relation of microstructure and impact toughness characteristics of DSAW steel of grade API X70

Abstract

A B S T R A C T In this research, mechanical characteristics of double submerged arc-welded line pipe steel of grade API X70 (70ksi yield strength) were investigated. Different experimental examinations including chemical analysis, microstructural investigation, microhardness, tensile and Charpy tests and standard fractography were carried out on different zones of test material. Impact properties of the fusion zone, the heat-affected zone (HAZ) and the base metal were measured on an instrumented Charpy rig and correlated then to their microstructural features. The experimental results showed that the base metal was the toughest, and fusion zone was the least tough region with average Charpy energy of 222 and 128 J, respectively. The latter fulfilled service requirements set by API 5L, despite its relatively low impact energy. Interestingly, the cast microstructure and presence of grain boundary phases such as proeutectoid ferrite in fusion zone confirmed its low energy characteristics. Reduction in HAZ impact energy (compared to base metal) was observed too, together with grain coarsening and the associated HAZ softening adjacent to the weld.

A Microstructural and Fractographic Study on the Failure of a Drive Shaft

“…According to the literature [8][9][10][11], the formation of proeutectoid ferrite can be minimized, or perhaps even suppressed, by heat treating at lower temperatures and for shorter times, with relatively higher cooling rates, although not as high as the ones necessary for the formation of martensite. A continuous or semicontinuous network of proeutectoid ferrite, such as the one formed in this material, favors the propagation of cracks, thus lowering the fracture toughness, i.e., leading to brittle fracture [12][13][14]. This material could be heat treated at a considerably lower temperature and cooled at a significantly higher rate, thus minimizing the presence of proeutectoid ferrite.…”

Comparative study of fracture appearance in crack tip opening angle testing of gas pipeline steels

“…According to reference [7], such high hardness regions were possibly due to upper-bainite and martensite bands across the specimen thickness. These are the direct result of thermo-mechanical controlled rolling processes; TMCR and heavy rolling schedules of API plates [14]. These hard spots can act as crack initiation sites, and increase the material tendency to cleavage tearing.…”

“…The axial elongation and load data were monitored in each experiment. The measured mechanical properties of tested steels are set out in Table 3 [8,10,[12][13][14] together with target values specified by API 5L [15].…”

Field corrosion characterization of soil corrosion of X70 pipeline steel in a red clay soil

“…Iron and its alloys are common materials which are significantly used in oil, gas and petroleum industries due to their availability and low cost [5][6][7][8][9][10][11]. X70 steel pipelines are considerable candidate due to good combination of strength and toughness, good weld ability, low crack sensitivity coefficient and low ductile to brittle transition temperature [12,13]. These advantages are enhanced due to controlled rolling and controlled cooling techniques used in original coil manufacturing [14,15] and it is widely used in the petroleum industries.…”