Kenji Oi

High Strength Linepipe With Excellent HAZ Toughness

The API 5L-X65 steel plates for low temperature service were produced using the thermo-mechanical control process (TMCP) with the optimum micro-alloying addition. Featuring of the additions are as low amount of titanium, calcium, niobium, and vanadium as possible, for high heat affected zone (HAZ) toughness and strength. Controlling titanium and nitrogen and the Ti/N ratio, a large number of TiN dispersed finely are formed in steel and the austenite grain size near a weld fusion line is refined remarkably owing to strong pinning effect of TiN. Calcium addition promotes ferrite nucleation, so that increase in fine polygonal ferrites makes microstructure of HAZ much finer. Niobium and vanadium content are reduced, because carbide precipitates are formed when the coarse grain HAZ is reheated around 700 degree C and the precipitation hardening deteriorates HAZ toughness. The trial manufacturing of the 19.5mm, 26.9mm and 31.4mm thick X65 grade UOE pipes was finalized with the satisfactory results. The toughness of longitudinal submerged-arc welds was more than 50 J in Charpy V-notch impact test at −30°C.Copyright

Deformation Capacity of Weld Seam of the High Quality HFW Linepipe

Newly-developed high quality high frequency electric resistance welded (HFW) linepipes have recently been used in pipelines in reel-lay applications and low temperature service environments because of their excellent low temperature weld toughness and cost effectiveness. In order to clarify the applicability of these HFW linepipes to the seismic environment, a series of full-scale tests such as bending test with internal pressure and uniaxial compression test were conducted according to the seismic design code in Japan gas association (JGA).Based on the above-mentioned full-scale tests, the safety performance of high quality HFW linepipe to apply to the seismic region is discussed in comparison with the mechanical properties in the small-scale tests such as the tensile and compression property of the base material and weld seam, especially focused on the strain capacity of HFW linepipe from the view points of full-scale performance and geometrical imperfection.Test results of the bending test with internal pressure and the uniaxial compression were complied with the JGA seismic design code for the permanent ground deformation induced by lateral spreading and surface faults.Copyright

Ductile Crack Extension Analysis for X80 Bending Deformation Using Damage-Based Model

This paper presents experimental and analytical results focusing on the strain limit of X80 linepipe. Ductile crack growth behavior from a girth weld notch is simulated by FE analysis based on a proposed damage model and is compared with the experimental results. The simulation model for ductile crack growth accompanied by penetration through the wall thickness consists of two criteria. One is a criterion for ductile crack initiation from the notch-tip, which is described by the plastic strain at the notch tip, because the onset of ductile cracking can be expressed by constant plastic strain independent of the shape and size of the components and the loading mode. The other is a damage-based criterion for simulating ductile crack extension associated with damage evolution influenced by plastic strain in accordance with the stress triaxiality ahead of the extending crack tip.The proposed simulation model is applicable to prediction of ductile crack growth behaviors from a circumferentially-notched girth welded pipe with high internal pressure, which is subjected to tensile loading or bending (post-buckling) deformation.© 2014 ASME

Full Scale Reeling Simulation Test of X65 HFW Linepipe

Reel-lay method is widely recognized cost effective installation process for offshore pipelines. In the process of reel-lay, cyclic plastic strain is expected at pipelines due to reeling, unreeling, aligning and straightening. In order to maintain integrity of pipelines subjected to large bending deformation it is necessary to understand the bending deformability and change in mechanical properties of pipelines.Advanced HFW (High Frequency electric resistance Welded) linepipe “MightySeam®” has superior low temperature toughness at seam weld and base material and has some advantages in application to offshore pipelines from a viewpoint of change in toughness due to pre-straining, coatings and long time exposure.In this paper full scale reeling simulation tests of X65 HFW linepipe were conducted in order to study bending deformability and the change in mechanical properties. The tests were performed by bending pipe alternately to reeling former and straitening former with different radii. Girth welding were applied to test pipes in order to investigate effect of strain concentration around girth weld by strength mismatch or weld reinforcement. Small scale reeling simulation tests with/without aging were also conducted and the results were compared to those of full scale reeling simulation tests.In full scale reeling simulation test, some strain concentration arose around girth weld by strength mismatch and local contact of weld reinforcement to former. Some ovalizations were advanced during tests, however local buckling could not be recognized throughout pipes even if strain concentration occur around girth welds. Tensile properties were changed proportional to last introduced plastic strain as a result of Bauschinger effect and work hardening at strain history. Low temperature toughness of base material and seam weld did not change significantly by pre-straining or aging.Copyright

Effect of Hammer Peening Processing on Fatigue Property of Welded Joints After Overload

One general method for improving the fatigue strength of welded joints is introduction of compressive residual stress by peening. However, there is concern that the fatigue strength of the welded joint may decrease if excessive preloading is applied after peening.It has been found that fatigue strength decreased after applying compressive preloading to a welded joint due to cancellation of the compressive stress at the weld toe.In the present research, the influence of excessive preloading on the fatigue strength of welded joints with compressive residual stress at the weld toe was clarified by experiments using hammer peening with an improved pin.When hammer peening was applied to welded joints, increasing the radius of the weld toe reduced the decrease of compressive residual stress due to excessive preloading. As a result, the decrease of the fatigue strength of the welded joint was also reduced.Copyright