1 School of Power and Mechanics, Wuhan University, Wuhan 430072, China 2 State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China 3 Henan No.1 Thermal Power Construction Company, Zhengzhou 467001, China
Cite this article:
Xue WANG,Lei HU,Dongxu CHEN,Songtao SUN,Liquan LI. Effect of Martensitic Transformation on Stress Evolution in Multi-Pass Butt-Welded 9%Cr Heat-Resistant Steel Pipes. Acta Metall Sin, 2017, 53(7): 888-896.
It has been recognized that low temperature martensitic transformation can reduce harmful tensile stress and generate beneficial compressive stress in weld zone of single pass welded joints. The influence of martensitic transformation is even greater in 9%Cr steel because of its high hardenability and low transformation temperature (about 100~400 ℃). However, compressive stress was confined in certain parts of weld zone in multi-pass butt-welded 9%Cr steel pipes. In this work, stress evolution in a multi-pass butt-welded 9%Cr steel pipe was predicted using Abaqus software, and the effect of martensitic transformation was further investigated. The simulated results show that the overall pattern for the axial and hoop stresses appears to be similar, despite the lower magnitudes for axial stress. The maximum compressive stress was found in the final weld pass, and the maximum tensile stress was formed in the weld pass adjacent to the final weld pass. Stress in weld passes adjacent to weld root is relatively low. Tensile stress due to thermal contraction in the final weld pass was relieved by martensitic transformation and clear compressive stress was formed. However, little effect of martensitic transformation was found on the significant tensile residual stress in weld passes adjacent to the final weld pass. The final weld pass has the primary effect on the formation of residual stress. Compressive stress was indeed generated by martensitic transformation in former weld pass, however it was relieved by weld thermal cycle of latter weld pass. As a result, the effect of martensitic transformation appears to be confined to the final weld pass. The influence of martensitic transformation is greater around outer surface than that around inner surface.
Fund: Supported by National Natural Science Foundation of China (Nos.51374153 and 51574181) and State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology (No.AWJ-Z15-02)
Fig.1 Dimensions of the pipes and geometry of the groove used in the simulation (unit: mm)
Pass number
Welding method
I / A
U / V
v / (cmmin-1)
Tp (or Ti) / ℃
1
GTAW
120
11
5.2
300
2
GTAW
170
11
6.5
200~250
3~11
GMAW
250
30
25~40
200~250
Table 1 Welding conditions for each pass [15]
Fig.2 Finite element meshes near the weld zone and sequence of the weld pass
Fig.3 Martensite distribution after welding
Fig.4 Evolutions of temperature, martensite fraction fm and austenite fraction fγ with time at point A in Fig.3 in the weld thermal cycle of the 1st weld pass
Fig.5 Simulation results of hoop residual stress (a, b) and axial residual stress (c, d) with (a, c) and without (b, d) martensitic transformation effect
Fig.6 Hoop (a, b) and axial (c, d) components of residual stress distributions along longitudinal direction on inner surface (a, c) and outer surface (b, d) (Dotted lines highlight the WM/HAZ region, HAZ—heat affected zone, WM—weld metal, SSRT—solid-state phase transformation)
Fig.7 Hoop stress evolutions in the multi-pass welding after 1st weld pass (a, b), 3rd weld pass (c, d), 5th weld pass (e, f) and final weld pass (g, h) with (a, c, e, g) and without (b, d, f, h) martensitic transformation effect
Fig.8 Axial stress evolutions in the multi-pass welding after 1st weld pass (a, b), 3rd weld pass (c, d), 5th weld pass (e, f) and final weld pass (g, h) with (a, c, e, g) and without (b, d, f, h) martensitic transformation effect
Fig.9 Stress evolutions in the 3rd pass at point B in Fig.8c (Ms, Mf —martensite transformation start and finish temperatures, respectively)
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