Acta Metall Sin  2019, Vol. 55 Issue (5): 555-565    DOI: 10.11900/0412.1961.2018.00365
 Current Issue | Archive | Adv Search |
Effect of Annealing on Microstructure of Thermally Aged 308L Stainless Steel Weld Metal
Xiaodong LIN1,2,Qunjia PENG1,3(),En-Hou HAN1,Wei KE1
1. Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
2. School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
3. Suzhou Nuclear Power Research Institute, Suzhou 215004, China
Abstract

Austenitic stainless steel weld metal has been widely used as nozzle/safe-end joint and inner surface cladding of reactor pressure vessel, due to its good mechanical property and corrosion resistance. However, long-term thermal ageing at the service temperature (280~330 ℃) could induce hardening and embrittlement of the weld metal. To recover the thermal ageing embrittlement, the annealing treatment has been proposed since the annealing could affect the ageing-induced microstructural changes such as spinodal decomposition and G-phase precipitation in ferrite. However, there is still an incomplete understanding as well as a lack of nanoscale investigation about the annealing effect on the microstructural change of the weld metal. In this work, 308L stainless steel weld metal was thermally aged at 410 ℃ for 7000 h, followed by an annealing treatment at 550 ℃ for 1 h. Since the weld metal has a dual-phase structure of austenite and δ-ferrite, the phase transformation of austenite and δ-ferrite as well as the element segregation at the δ-ferrite/austenite phase boundary were investigated by TEM and atom probe tomography. The results revealed that austenite was unaffected by annealing while the ageing-induced spinodal decomposition of δ-ferrite was completely recovered. In addition, the number density of G phase in δ-ferrite was significantly reduced following annealing. This indicates that austenite has a higher stability compared with δ-ferrite. As for the δ-ferrite/austenite phase boundary, thermal ageing induced the segregation of Ni, Mn and C at the phase boundary, while the contents of Cr, Si and P remained almost unchanged. Following the annealing treatment, the segregation of all elements was eliminated. Further, only a small quantity of Ni and Mn was enriched in austenite near the phase boundary. The results suggested that the microstructure of the annealed specimen was similar to that of the unaged specimen, indicating a good recovery of the microstructure by annealing.

 ZTFLH: TG139.4
Fund: National Natural Science Foundation of China(51571204)
Corresponding Authors:  Qunjia PENG     E-mail:  pengqunjia@cgnpc.com.cn
 Fig.1  Microstructure and bright field image (inset) of 308L stainless steel weld metal, showing vermicular δ-ferrite (δ) in austenite (γ) matrix (a), and HRTEM image at the δ/γ phase boundary region along the $[1ˉ00]$ zone axis of δ-ferrite (inset) showing a disordered structure with a width of about 6 nm (b) (P.B.—phase boundary) Fig.2  Bright field images (a~c) and atom maps of Fe, Cr, Ni, Si and Mn (d~f) of austenite in 308L stainless steel weld metal (The volume analyzed for the atom map shown in Figs.2d~f is 61 nm×61 nm×105 nm, 80 nm×80 nm×95 nm and 70 nm×70 nm×108 nm, respectively)(a, d) unaged (b, e) 7000 h aged (c, f) annealed Fig.3  TEM images of δ-ferrite (a~c) and atom maps and 35%Cr isosurface of δ-ferrite (d~f) of 308L stainless steel weld metal (The volume analyzed in Figs.3d~f is 74 nm×74 nm×51 nm, 40 nm×40 nm×100 nm and 30 nm×30 nm×90 nm, respectively)(a, d) unaged (b, e) 7000 h aged (c, f) annealed Fig.4  Composition profiles (a~c) and concentration frequency distributions (d~f) of Fe and Cr in the spinodally decomposed area in δ-ferriteColor online(a, d) unaged (b, e) 7000 h aged (c, f) annealed Fig.5  Dark field image, electronic diffraction pattern and HRTEM images of G phase(a) dark field image of G phase in δ-ferrite following 7000 h ageing by using the weak diffraction pattern spot circled in the inset along [110] direction(b) electronic diffraction pattern of δ-ferrite along [110] zone axis, indicating no weak electronic diffraction pattern spot of G phase(c, d) HRTEM images of G phase in δ-ferrite following 7000 h ageing (c) and annealing (d), respectively. The corresponding fast Fourier transformation (FFT) patterns inserted show a cube-on-cube relationship between G phase and δ-ferrite along the [311] and [100] zone axes, respectively Fig.6  Atom maps of Ni, Si, Mn, P and Cu in δ-ferrite in the unaged (a), 7000 h aged (b) and annealed (c) specimens (The isosurface of 12%Ni, 5%Si, 3.5%Mn, 0.65%P and 0.6%Cu is superimposed on the atom map in Fig.6b in order to characterize the precipitates following 7000 h ageing. The volume analyzed is 80 nm×80 nm×123 nm, 88 nm×88 nm×160 nm and 71 nm×74 nm×89 nm, respectively) Fig.7  Atom map of Cr and Ni at the vicinity of G phase (The volume analyzed is 15 nm×15 nm×15 nm) (a), and composition profile of Cr and Ni across G phase, suggesting the low Cr content of G phase (b) (The direction of the composition profile is indicated by the arrow in Fig.7a. The range of G phase in Fig.7b is marked according to the variation of Ni content rather than Cr content due to the Cr fluctuation induced by spinodal decomposition)Color online Fig.8  Atom maps and composition profiles of elements in the unaged, 7000 h aged and annealed specimens(a) atom maps of Ni, Mn, C, Cr, Si and P at the δ-ferrite/austenite phase boundary region. The volume analyzed is 97 nm×97 nm×75 nm, 88 nm×88 nm×40 nm and 70 nm×60 nm×30 nm, respectively(b) composition profiles of Ni, Mn and C (The direction of the composition profile is from austenite to δ-ferrite)(c) composition profiles of Cr, Si and P Fig.9  The parameter V of Fe and Cr in the spinodally decomposed area in δ-ferrite in the unaged, 7000 h aged and annealed specimens (V—a parameter for quantitatively characterizing the degree of spinodal decomposition) Fig.10  Atom maps and 1.5% isosurfaces of Ni at the vicinity of δ/γ phase boundary in the 7000 h aged (a) and annealed (b) specimens, both of which showing a Ni-depleted zone (NDZ) in δ-ferrite near the δ/γ phase boundary (The Ni content inside the isosurface is less than 1.5%. The volume analyzed is 88 nm×88 nm×40 nm and 70 nm×60 nm×30 nm, respectively)