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Acta Metall Sin  2018, Vol. 54 Issue (12): 1767-1776    DOI: 10.11900/0412.1961.2018.00051
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Influence of Inter-Pass Temperature on Residual Stress in Multi-Layer and Multi-Pass Butt-Welded 9%Cr Heat-Resistant Steel Pipes
Lei HU1, Xue WANG1,2(), Xiaohui YIN1, Hong LIU3, Qunshuang MA1
1 School of Materials Science and Engineering, Anhui University of Technology, Ma'anshan, Anhui 243032, China
2 School of Power and Mechanics, Wuhan University, Wuhan 430072, China
3 Dong Fang Boiler Group Co., Ltd., Dong Fang Electric Corporation, Zigong 643001, China
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9%Cr heat-resistant steels have been abundantly used in boilers of modern thermal plants. The 9%Cr steel components in thermal plant boilers are usually assembled by fusion welding. Many of the degradation mechanisms of welded joints can be aggravated by welding residual stress. Tensile residual stress in particular can exacerbate cold cracking tendency, fatigue crack development and the onset of creep damage in heat-resistant steels. It has been recognized that welding residual stress can be mitigated by low temperature martensitic transformation in 9%Cr heat-resistant steel. Nevertheless, the stress mitigation effect seems to be confined around the final weld pass in multi-layer and multi-pass 9%Cr steel welded pipes. The purpose of this work is to investigate the method to break through this confine. Influence of martensitic transformation on welding stress evolution in multi-layer and multi-pass butt-welded 9%Cr heat-resistant steel pipes for different inter-pass temperatures (IPT) was investigated through finite element method, and the influential mechanism of IPT on welding residual stress was revealed. The results showed that tensile residual stress in weld metal (WM) and heat affected zone (HAZ), especially the noteworthy tensile stress in WM at pipe central, was effectively mitigated with the increasing of IPT. The reasons lie in two aspects, firstly, there is more residual austenite in the case of higher IPT, as a result, lower tensile stress is accumulated during cooling due to the lower yield strength of austenite; secondly, the higher IPT suppresses the martensitic transformation during cooling of each weld pass, thus the tensile stress mitigation due to martensitic transformation was avoided to be eliminated by welding thermal cycles of subsequent weld passes and reaccumulating tensile residual stress. The influence of IPT on welding residual stress relies on the combined contribution of thermal contraction and martensitic transformation. When the IPT is lower than martensite transformation finishing temperature (Mf), thermal contraction plays the dominant role in the formation of welding residual stress, and tensile stress was formed in the majority of weld zone except the final weld pass. While, compressive stress was formed in almost whole weld zone due to martensitic transformation when the IPT is higher than martensite transformation starting temperature (Ms).

Key words:  9%Cr heat-resistant steel      multi-layer and multi-pass welding      inter-pass temperature      residual stress      numerical simulation     
Received:  02 February 2018     
ZTFLH:  TG404  
Fund: Supported by National Natural Science Foundation of China (Nos.51374153 and 51574181) and Science and Technology Program of Sichuan Province (No.2018JY0668)

Cite this article: 

Lei HU, Xue WANG, Xiaohui YIN, Hong LIU, Qunshuang MA. Influence of Inter-Pass Temperature on Residual Stress in Multi-Layer and Multi-Pass Butt-Welded 9%Cr Heat-Resistant Steel Pipes. Acta Metall Sin, 2018, 54(12): 1767-1776.

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Fig.1  Simulation model (unit: mm) (a) and finite element meshes near the welding zone (WCL—center line of weld, HAZL—center line of heat affected zone) (b)
Fig.2  Welding thermal cycles of the first two weld passes at point A in Fig.1 (Ti—inter-pass temperature)
Fig.3  Simulation results of axial (a, c, e, g) and hoop (b, d, f, h) residual stresses on the cross-section in Weld A (a, b), Weld B (c, d), Weld C (e, f) and Weld D (g, h)
Fig.4  Effect of Ti on axial (a, c) and hoop (b, d) residual stresses along WCL (a, b) and HAZL (c, d) in Fig.1b
Fig.5  Schematic of simulation model and restraint conditions of Satoh tests (unit: mm)
Case Tp1 / ℃ Ti / ℃ Tp2 / ℃
I 1350 - -
II 800 - -
III 1350 102 1350
IV 1350 375 1350
V 1350 102 800
VI 1350 375 800
Table 1  Simulation conditions of Satoh tests
Fig.6  Simulation results of Satoh tests for Case I (a), Case II (b), Case III (c), Case IV (d), Case V (e) and Case VI (f)
Fig.7  Contour maps for axial (a, c, e, g) and hoop (b, d, f, h) stresses after the 3rd weld pass in Weld A (a, b), Weld B (c, d), Weld C (e, f) and Weld D (g, h)
Fig.8  Contour maps for axial (a, c, e, g) and hoop (b, d, f, h) stresses after 7th weld pass in weld A (a, b), weld B (c, d), Weld C (e, f) and Weld D (g, h)
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