Effects of Mo, Cr, and V Additions on Tensile and Charpy Impact Properties of API X80 Pipeline Steels

Abstract

In this study, four API X80 pipeline steels were fabricated by varying Mo, Cr, and V additions, and their microstructures and crystallographic orientations were analyzed to investigate the effects of their alloying compositions on tensile properties and Charpy impact properties. Because additions of Mo and V promoted the formation of fine acicular ferrite (AF) and granular bainite (GB) while prohibiting the formation of coarse GB, they increased the strength and upper-shelf energy (USE) and decreased the energy transition temperature (ETT). The addition of Cr promoted the formation of coarse GB and hard secondary phases, thereby leading to an increased effective grain size, ETT, and strength, and a decreased USE. The addition of V resulted in a higher strength, a higher USE, a smaller effective grain size, and a lower ETT, because it promoted the formation of fine and homogeneous of AF and GB. The steel that contains 0.3 wt pct Mo and 0.06 wt pct V without Cr had the highest USE and the lowest ETT, because its microstructure was composed of fine AF and GB while its maintained excellent tensile properties.

Evaluation of Microstructure and Mechanical Properties of Seamless Steel Pipes API 5L Type Obtained by Different Processes of Heat Treatments

“…It can be observed that the quenched and tempered steel (X70Q) presented the best performance, i.e., a 50% increase in fracture toughness compared to the normalizing steel (X42N) and an increase of almost 80% in relation to the hot rolled steel (X42R). This fact is again due to its microstructure [13][14][15][16][17][18][19][23][24][25][26][27][28][29][30][31][32][33][34][43][44][45][46][47][48][49][50][51][52][53] , consisting of tempered martensite, bainite and precipitated carbides.…”

“…The quenched/tempered steel showed a matrix consisting of tempered martensite, bainite and a fine distribution of precipitated carbides (cementite particles), as shown in Figure 1(c). As the steel studied has vanadium in its chemical composition, it should expect also the precipitation of vanadium carbides in all microstructures 28,29 . Table 2 shows basic tensile mechanical properties obtained in this study for the three API steels.…”

“…Another important point to be considered is the presence of vanadium (0.02%V) in the chemical composition of the steel, an element added mainly for precipitation strengthening and microstructure refinement. The presence of this unique microalloyed element allows the reduction of the carbon content, ensuring improved weldability and fracture toughness 28,29 . Microstructural analysis showed that hot-rolled and normalized steels had a microstructure consisting of polygonal ferrite and pearlite with a slight banding -ASTM 36 degree of banding between 0.13 and 0.28, a common occurrence in hot-rolled, low alloy steels 41,42 .…”

Microstructure and Mechanical Properties in Hot-Rolled Extra High-Yield-Strength Steel Plates for Offshore Structure and Shipbuilding

“…The achievement of deformed nonrecrystallized austenite by finish rolling is critically important for enhancing the transformation of acicular ferrite (AF) [22] during AcC, which offers the best combination of strength and toughness. [23,24] An important characteristic of recrystallization is the resulting grain size; grain refinement from pass to pass occurs as long as complete recrystallization is possible. Revealing the recrystallized grain size for the present low carbon steels proved to be a challenging task; estimates of the grain sizes indicated that they are similar to those observed in steels with higher carbon and alloy levels.…”

“…Han et al [23,24] found that the grain-boundary misorientation in coarse high-temperature transformed bainite is low, and hence, the effective grain size is large; this condition decreases the low-temperature toughness in the thermomechanically processed highstrength steels. The fraction of fine-grained AF microstructure is high in the present study, leading to decreased effective grain size and improved low-temperature toughness.…”