Research On Welding And Weld Heat Treatment Technology Of A Low Alloy Steel

CO2corrosion generally exists in the process of oil production and transportation, inrecent years. Low Cr seamless steel pipe with the content of Cr from1%to5%is widely usedin oil field against CO2corrosion. Because welding steel pipe has high precision, uniform wallthickness than seamless steel pip, and has been widely used in oil gas field. In order to meetthe needs of low Cr steel welded pipes in the oil and gas fields, develop the3Cr low alloy lowCr steel welded pipes, steel welding was studied in this thesis. With plasma arc welding andinduction heat treatment test, researches were made on welding parameters influencing onweld appearance and structure performance through the analysis of the welding joint. Study onweld heat treatment parameters influencing on microstructure and property of welded joints,was also made, so as to provide a theoretical basis for the development of3Cr low alloy steelwelded pipe.The results show that the3Cr low alloy steel material organization is given priority towith ferrite and pearlite, base metal aterial yield strength range is466~480MPa, tensilestrength is530~545MPa, elongation is25%~29.4%, HV0.5hardness is211~237. Impact testresults show that the embrittlement transition temperature of parent metal is under minus60℃;Compared with ordinary carbon steel, the high content of Cr element in the parent materialaffects the solderability; JQ.TH650-Ⅱgas shielded welding wire was selected for plasmawelding technology because the wire mechanical properties and chemical composition issimilar to the parent metal; weld microstructure is characteristic of the coarse ferrite andmixed martensite; maximum hardness in welded zone is344HV0.5; the hardness graduallyreduces from the weld heat affected zone to the parent metal. Joint strength is haiher than thatof parent metal and weld toughness is lower than that of parent metal under minus40℃andcrystalline section has reached50%.Using the medium frequency induction heating mode heat treatment test of weld, theweld microstructure is ferrite and martensite. With the improvement of heat treatmenttemperature, the ferrite microstructure becomes finer. At the same time, welded joint, with theimprovement of weld heat treatment temperature, weld hardness and tensile strength decreasesuch that toughness was improved.