Characterization of Multi-layer Weld Metal and Creep-Rupture Behavior of Modified 10Cr-1Mo Welded Joint

doi:10.1007/s40195-020-01012-4

Abstract

Abstract:

The rupture behavior of the modified 10Cr-1Mo steel multi-layer welded joint is determined by the fine-grain zones of the weld metal adjacent to the fusion line during the long-term creep test at 620 °C. The microstructures of multi-layer weld metal before and after the creep tests were characterized in detail, and its role in creep behavior was systematically investigated. Most grain boundaries of subgrains represented the low-angle boundaries in the weld metal adjacent to the fusion line both before and after the creep test. The widths of grains in the fine-grain zones were about 0.5-1 μm. The fracture morphology appeared as “wave” structure due to the cracking initiating from multi-layer grain boundaries in the fine-grain zones. Some W elements that melted into weld metal adjacent to the fusion line altered the thermodynamic and kinetic conditions of the Laves phase formation during long-term creep exposure. Laves phase particles mainly distributed along the grain boundaries due to the faster diffusion and segregation of Mo, W, and Si elements. Moreover, higher-density grain boundaries in the fine-grain zones led to easier nucleation and growth of Laves phase particles. Compared with other areas in the welded joint, the size of Laves phase particles in the fine-grain zones of the weld metal adjacent to the fusion line was the largest ones. The interface between Laves phase particles and the matrix acted as the nucleation site of creep micro-cavities. The creep micro-cavities grew up at the expense of fine-grain boundaries and even grew across the grain boundary deeply into adjacent grains, and then developed to cracks in the fine-grain zones.

Key words: 10Cr-1Mo, Welded joint, Fine grains, Multi-layer, Creep rupture

Chang-Zhen Zhang, Chen-Dong Shao, Hai-Chao Cui, Hua-Li Xu, Feng-Gui Lu. Characterization of Multi-layer Weld Metal and Creep-Rupture Behavior of Modified 10Cr-1Mo Welded Joint[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(6): 808-820.

Figures/Tables 18

Table 1 Main chemical compositions of BM and filler material (wt%)

Elements	C	Si	Mn	Cr	Mo	W	Ni	V	Nb	N	Fe
BM	0.1	0.1	0.45	10.4	1.06	0.81	0.74	0.2	0.08	0.06	Bal.
Filler	0.11	0.25	0.55	8.87	0.97	-	0.46	0.2	0.06	0.05	Bal.

Fig.1 Schematic of the specimens prepared for the creep tests from the welded joint

Fig.2 OM images of the microstructure of the investigated WM before the creep test. a Macrostructure of the WM adjacent to the fusion line, b the microstructure of adjacent two layers of weld bead adjacent to fusion line, c the microstructure of adjacent two layers of weld bead adjacent to the center

Fig. 3 Inverse pole figure (IPF) color images and relative frequency of misorientation angles of the investigated WM before creep. a Macrostructure of the WM, b the microstructure of the FGZ in WM, c the size of grains in FGZ, d the relative frequency of misorientation angles in figure a

Fig. 4 Hardness test result of the specimen before and after the creep test

Fig. 5 Macrostructure of the ruptured specimen at 620 °C under the stress of 150 MPa. a Optical image of ruptured specimen, b the overall macrostructure of the ruptured specimen, c the overall fracture morphology of the ruptured specimen

Fig. 6 SEM images of the creep fracture morphology at 620 °C under the stress of 150 MPa. a The “wave” structure fracture morphology, b the microstructure of lamellar structure, c the microstructure of honeycomb-like structure

Fig. 7 SEM images of precipitate phases of FGZ in WM adjacent to fusion line away from the fracture. a Before the creep test, b after the creep test, c the BSE image of figure b, d the EDS result of Cr-rich phase before the creep test, e the EDS result of Mo-rich phase after the creep test, f the EDS result of Cr-rich phase after the creep test

Fig. 8 Microstructure of the micro-crack in FGZ in WM adjacent to the fusion line in the creep-ruptured specimen. a OM image of micro-crack in FGZ in WM, b the IPF of micro-crack in FGZ in WM, c the SEM image of micro-cavities, d the BSE image of figure c

Fig. 9 Relative frequency of misorientation angles in Fig. 8b

Fig. 10 TEM images of microstructure in FGZ in WM adjacent to the fusion line: a before the creep test, b after the creep test

Table 2 EDS analysis results of precipitates shown in Fig. 10 (wt%)

Elements	Mo	Cr	Fe	C	Si	W
Point A	4.3	43.2	23.6	24.5	-	-
Point B	7.9	50.6	19.6	19.6	-	-
Point C	33.9	8.9	44.6	-	4.5	6.8

Fig. 11 Microstructures of different zones in the welded joint after the creep test: a all zones location in welded joint, b BM, c OTZ, d FGHAZ, e CGHAZ, f WM adjacent to fusion line (WMF), g WM in the center

Fig. 12 Statistic results of Laves phase particles

Fig. 13 Phase diagrams of WM and BM: a WM, b BM

Table 3 Main chemical compositions of the Laves phase in different zones after the creep test (wt%)

Zones	Cr	Fe	Si	Mo	W
WMF	8.5	49.7	3.3	30.37	2.65
CGHAZ	8.09	53.27	2.79	29.15	2.63
FGHAZ	8.4	48.17	1.27	15.34	21.63
OTZ	12.49	48.48	0.8	12.97	20.12
BM	10.96	39.9	0.71	18.32	23.46

Fig. 14 Content of Cr, Mo, and W near fusion line after the creep test

Fig. 15 Schematic of the creep-rupture process of the 10Cr-1Mo multi-layer welded joint at 620 °C under 150 MPa. a, d Micro-cavities nucleating around Laves phases on the grain boundaries, b, e the micro-cavities growing up, c, f the micro-crack forming in FGZ in WM adjacent to fusion line

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