Study On Microstructures And Properties Of Welding Joint Of X80 And 30CrMo

Deep water drilling riser is important Equipment unit in deep sea drilling and exploiting oil and gas. Welding of X80 and 30CrMo is used in manufacturing of deep water drilling riser. Welding procedure and welding joint properties is important to the localization of riser manufacturing technology and riser product.Microstructures and properties of welded joint are studied in this paper, the laws of effecting of welding procedure and microstructure on hardness and impact toughness of welding joint are intensively studied. Microstructures of different zones in welding joint are observed and analysed, The results show that the microstructures of welding joint are discontinuous, So the mechanical properties of welding joint is also discontinuous. Macrohardness of different zones in welding joint are tested by Vickers, The results show that local hardening and local softening exist in HAZ(heat affected zone) of 30CrMo. The results also show that local softening exists in HAZ of X80 and the hardness of weld metal is moderate. The reason of local hardening of 30CrMoHAZ is forming of martensite and upper bainite in this zone. Plastic deformation resistance of these microstructures is high, so the local hardening is tseted in this zone. Local softening of 30CrMoHAZ is caused by forming of coarse granular ferrite, coarse carbides and low carbon austenite decomposition products, Leading to hardness of this zone is lower than hardness of 30CrMo base matel. Softening of X80HAZ is originated from acicular ferrite, coarse polygonal ferrite and small quantities of carbide distributing among ferrite.Through 500℃×1.5h postweld heat treatment, the hardness of welding joint is changed. The hardness of local hardening zone and local softening zone in 30CrMoHAZ is reduced, which is caused by microstructure transformation and Welding residual stress releaseing. Through postweld heat treatment, microstructure of 30CrMo local hardening zone transform into tempered sorbite, ferrite band and granular ferrite. During postweld heat treatment, carbide precipitation, aggregation and growth occurred in ferrite of 30CrMo local softening zone. Strength rule of carbide is decreased, so the hardness of this zone is reducing after post weld heat treatment.Impact test show that embrittlement occurrs at X80 fusion-line+1mm because of forming of granular bainite, impact toughness of this zone is not improved through postweld heat treatment. Impact test show simultaneously that embrittlement occurred at 30CrMo fusion-line+1mm and 30CrMo fusion-line+2mm. After heat treatment, the toughness of these two zones are improved, impact fracture is dimple. Embrittlement is caused by brittle microstructure, there are upper bainite and hard and brittle martensite in the zone of 30CrMo fusion-line+1mm, and granular bainite martensite in the zone of 30CrMo fusion-line+2mm. Through heat treatment, the microstructures of the zone of 30CrMo fusion-line+1mm transform into granular ferrite, tempered sorbite and dispersed carbide. The microstructures of the zone of 30CrMo fusion-line+2mm transform into granular ferrite, granular bainite and granular tempered sorbite. These microstructure have good toughness, so the toughness of the two zones are improved.