以Mn-Ni-Mo为主要合金体系,研制了K65热煨弯管用高强高韧埋弧焊丝。采用该焊丝制得的直缝管焊缝金属抗拉强度达741~768 MPa,显微硬度为231~250 HV10,-40 ℃冲击功为90~185 J;直缝管焊缝经热处理后,-40 ℃冲击功为65~124 J,比直缝管焊缝出现较大幅度下降。利用OM、LePera、SEM (EBSD)及TEM观察焊缝组织,研究焊缝中Mn、Ni、Mo含量对K65热煨弯管组织转变和低温韧性的影响。结果表明：直缝管焊缝中Mn、Ni含量的增加会促进针状铁素体的形成,适当增加Mo含量,降低Mn、Ni含量能使焊缝达到最佳强韧性能;经过热处理后,焊缝中针状铁素体含量降低,上贝氏体含量增加,大尺寸沿晶分布的渗碳体是焊缝金属低温韧性下降的原因,但Mo含量为0.2%时仍能保证大角度晶界比例达67.1%,使焊缝金属的-40 ℃低温韧性达124 J。
To increase transport efficiency and to lower the costs of pipeline construction, longitudinally submerged arc welded (LSAW) pipes with larger diameters and thicker walls have been increasingly used by the pipeline industry. For example, in Russia, the LSAW pipeline in the Bovanenkovo-Ukhta project was recently constructed with K65 steel (the highest grade of the Russian natural gas pipeline), which is similar in specifications and yield strength requirement (550 MPa grade) to API X80 but has a stricter low temperature toughness value of 60 J at -40 ℃ (compared to -20 ℃ for API X80 grade) due to the extreme Arctic environment. Although weld metal with acicular ferrite (AF) has been developed to meet the requirement of low temperature toughness, the main objective of the present work was to clarify the microstructural evolution and the resulting changes in mechanical properties after the bending process. Hot bending pipes are necessary links in the construction of pipeline lying, which make more strin gent standards for the strength and low temperature toughness. That puts forward a challenge especially to the weld bead because of the deterioration of toughness during the hot bending process. In this work, submerged arc welding wire with high strength and toughness was developed for K65 hot bending pipes, and the alloying elements of Mn, Ni, Mo were considered to estimate the microstructure evolution and the effect of low temperature toughness for the weld metal. The results showed the low temperature toughness at -40 ℃ reached 90~185 J and 65~124 J for weld metal of straight seam pipe and hot bending pipe respectively, which reflect the excellent role of alloying elements of Mn, Ni, Mo. Microstructure characterization revealed that the weld metal, which originally consisted mainly of AF in the as-deposited condition, became predominantly composed of bainitic ferrite (BF) after hot bending. In addition, the large size cementite along the grain boundary was also the reason for the deterioration of toughness. It is found that reaustenisation caused a small austenite grain-sized matrix, which brought about a very high volume fraction of bainite. However, the low temperature toughness for hot bending pipe was improved to 124 J for the weld metal with 0.2%Mo, in which about 67.1% of high angle grain boundary were found. It is clear that the process of reaustenitisation during the bending process plays an important role in successful microstructural design for the steel weld metals.
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