Study On Microstructure And Properties Of Welding Joints Of Hot-Rolled H-Beam For Accumulation Of Oil And Gas In Alpine Region

As the exploitation of marine resources from shallow sea to deep sea, the offshore platform steel's grade is higher and higher. But the increase in strength will cause deterioration of weldability. Requirements of toughness under lower temperature is more stringent, especially welding joints of offshore platform in alpine region. In this paper, crack sensitivity and the effect on microstructure and properties of joints under different welding processes of hot-rolled H-beam for accumulation of oil and gas in alpine region are studied. Flux cored arc welding(FCAW) and submerged arc welding(SAW) are used for welding. Mechanical properties of tensile, impact, hardness test, etc are carried out on welding joints. Optical microscope(OM),scanning electron microscope(SEM) and other analysis methods are used to analyze microstructure of seam, heat affected zone and fracture morphology to make reasonable welding process. What we have done is to afford technical support for welding production practices of hot-rolled H-beam for oil and gas in alpine region.The results showed that microstructure of hot-rolled H-beam for accumulation of oil and gas in alpine region was banded structure which was composed of ferrite structure and pearlite banded structure approximately parallel to the rolling direction. The impact energy of hot-rolled H-beam is stable and the energy is no less than 137 J. By calculating, the carbon equivalents(Ceq) and cold crack sensitivity indexes(Pcm) of the H-beam are 0.369% and 0.199%,respectively. The Y-groove cracking tests showed the average section crack rate is 10.93%without preheating. And the maximum micro hardness in HAZ is 306.5 HV10.Microstructure of FCAW and SAW joints are basically identical. Weld seam is composed of massive acicular ferrite and pearlite. Microstructure in both CGHAZ mostly is composed of coarse ferrite and pearlite. Microstructure in both FGHAZ mostly is massive fine ferrite and pearlite. Microstructure in ICHAZ mostly is fine ferrite and pearlite, and also contains some coarse ferrite. But microstructure of FCAW joint is more homogeneous and refined.The tests indicated that Tensile specimens of FCAW and SAW joints were broken at base metal. The strength can meet the requirements of EN10225-2009. The impact energy of welding joints under the two different methods both meets the requirements of EN10225-2009.Impact energy in bottom of FL+5 on FCAW joint is the lowest and the value is 59 J, account for34.71% of that of the H-beam. But the value in remaining positions is better. The value ofimpact test of SAW joint which is lower than that of base metal are located in the top of FL-2, FL and in the top, middle, bottom of FL+5, account for 33.53%, 38.24 and 44.12%,53.53%, 68.23%, respectively.The trend in hardness of joints under two welding methods is almost same. Hardness of different position on joints is higher than that of H-beam. The maximum hardness is located in CGHAZ. And maximum hardness of FCAW and SAW joints are 210.7HV and 193.8HV,respectively. The hardness of SAW joints is about 10% less than FCAW joints.