Creep Behavior and Life Assessment of a Novel Heat-Resistant Austenite Steel and Its Weldment

doi:10.1007/s40195-018-0822-5

Abstract

Abstract:

In the present study, creep activation energy for rupture was obtained as 221-348 kJ/mol for 22Cr15Ni3.5CuNbN due to the precipitation-hardening mechanism. The extrapolation strength of creep rupture time of 10⁵ h at 923 K for 22Cr15Ni3.5CuNbN is more valid (83.71 MPa) predicted by the Manson-Haferd method, which is superior to other commercial heat-resistant steels. The tensile creep tests ranging from 180 to 240 MPa at 923 K were conducted to investigate creep deformation behavior of welded joint between a novel heat-resistant austenite steel 22Cr15Ni3.5CuNbN and ERNiCrCoMo-1 weld metal. Apparent stress exponent value of 6.54 was obtained, which indicated that the rate-controlled creep occurred in weldment during creep. A damage tolerance factor of 6.4 in the weldment illustrates that the microstructural degradation is the dominant creep damaging mechanism in the alloy. Meanwhile, the welded joints perform two types of deformation behavior with the variation in applied stress, which resulted from the different parts that govern the creep processing. Also, the morphology evolution of the fracture surfaces confirms the effects of stress level and stress state.

Key words: Heat-resistant steel weldment, Creep deformation, Life assessment, TTP (time-temperature parametric) method

Yu Zhang, Hong-Yang Jing, Lian-Yong Xu, Yong-Dian Han, Lei Zhao, Xi-Shan Xie, Qiu-Hua Zhu. Creep Behavior and Life Assessment of a Novel Heat-Resistant Austenite Steel and Its Weldment[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(5): 638-650.

Figures/Tables 22

Table 1 Chemical compositions 9 wt% of 22Cr15Ni3.5CuNbN (BM) and weld metal (WM)

Material	C	Si	Mn	Cr	Ni	Mo	Cu	Co	W	Nb	Ti	N	Fe
BM	0.07	0.38	0.64	22.6	15.7	0.12	3.5	0.06	0.008	0.53	0.006	0.34	56.03
WM	0.065	0.11	0.12	21.92	47.89	7.1	0.94	8.38	0.27	0.11	0.31	0.044	13.04

Table 2 Specifications of the tubes and the welding parameters

Material	Outside diameter (mm)	Wall thickness (mm)	Welding current (A)	Welding voltage (V)	Interpass temperature (°C)	Groove angle (°)
22Cr15Ni3.5CuNbN	51.0	10.3	90-120	10-15	100	70

Fig. 1 22Cr15Ni3.5CuNbN weldment and the schematic representation of the sampling location for creep strain tests

Fig. 2 Variations between creep lifetime and applied stress at different temperatures

Fig. 3 Variations between creep lifetime and applied stress at 923 K

Fig. 4 Variations between creep lifetime and applied stress at 973 K

Fig. 5 Creep ductility variations of the BM and weldment at 923 K

Fig. 6 Partial view of strain-stress curves for the BM and weldment at 923 K

Fig. 7 Arrhenius plots of the BM at different applied stresses

Fig. 8 Selection of Larson-Miller constant (C)

Fig. 9 Isostatic curves for M-H method plotted with the optimal (Ta, ta)

Fig. 10 Creep lifetime data and the master curve of the M-H method

Fig. 11 Creep lifetime data and the isothermals of the OSD method

Fig. 12 Creep lifetime data and the isothermals of Wilshire’s equation

Table 3 Creep life prediction by different methods

Methods	$\sigma_{{10^{5} }}^{923}$ (MPa)	RSS	RMS
Power law	132.49	0.0436	0.0222
Laron-Miller	137.12	0.0383	0.0049
Manson-Herford	83.71	0.0090	0.0138
Orr-Sherby-Dorn	119.9	0.0226	0.0364
Wilshire equation	69.18	0.00137	0.02911

Fig. 13 Accuracy comparison for the different methods

Fig. 14 Comparison of creep data at 923 K with other commercial heat-resistant steel

Fig. 15 Curves of a creep strain-time, b creep rate-normalized time under different applied stress levels

Table 4 Creep data list of welded joint in the present study

Applied stress, σ_a (MPa)	Minimum creep rate, $\dot{\varepsilon }_{\hbox{min} }$ (h^-1)	Creep life, t_r (h)	Creep rupture strain, ε_f (%)
180	6.71E-07	8947	6.47
200	1.53E-06	4949	5.75
220	2.33E-06	3966	4.63
240	4.72E-06	3144	3.35

Table 5 Creep-strength reduction factors for welded 22Cr15Ni3.5CuNbN tube at 923 and 973 K

Creep lifetime (h)	1000	3000	5000	8000
Reduction factors at 923 K	1.227	1.063	1.010	0.904
Reduction factors at 973 K	1.053	0.793	0.757	0.656

Fig. 16 Variations of a time to the specific moments versus ruptured time; b minimum creep rate versus applied stress; c damage tolerance factor versus creep ruptured time

Fig. 17 Creep fractographs of specimens after creep at 923 K under 180-240 MPa

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