Effect of High Temperature Ageing on Microstructure and Stress-Relief Cracking Susceptibility of Coarse Grain Heat Affected Zone in T23 steel
WANG Xue1,2(), LI Yong2,3, WANG Jiaqing3, HU Lei1
1.Key Laboratory of Green Fabrication and Surface Technology of Advanced Metal Materials, Ministry of Education, Anhui University of Technology, Ma'anshan 243032, China 2.School of Power and Mechanics, Wuhan University, Wuhan 430072, China 3.Da Tang Boiler Pressure Vessel Inspection Center Co. , Ltd. , Hefei 230088, China
Cite this article:
WANG Xue, LI Yong, WANG Jiaqing, HU Lei. Effect of High Temperature Ageing on Microstructure and Stress-Relief Cracking Susceptibility of Coarse Grain Heat Affected Zone in T23 steel. Acta Metall Sin, 2021, 57(6): 736-748.
Owing to its high creep rupture strength, good weldability, and low costs, T23 steel is an ideal material for manufacturing the heating components of water walls, superheaters and reheaters in ultra-super critical plants. However, its coarse grain heat affected zone (CGHAZ) is prone to stress-relief cracking (SRC) during post-weld heat treatment or high-temperature service. The mechanism of SRC is controversial and an effective method for forecasting and preventing SRC in T23 components is currently lacking. Clarifying the mechanism of SRC in the CGHAZ of T23 steel, and developing a practical engineering technique for predicting and preventing SRC generation, are therefore essential. In this work, CGHAZ specimens of T23 steel were simulated in a thermo-mechanical simulator, and aged at 650oC for 0-48 h. After simulating the microstructure evolution of the as-welded CGHAZ during service, the SRC susceptibility of the CGHAZ was evaluated. The microstructural changes and carbide precipitation were observed by OM, SEM, TEM, and EDS. The as-welded CGHAZ of T23 steel was composed of mixed martensite and bainite with high hardness. After ageing at 650oC, the structure recovered and recrystallized with a lower dislocation density and larger sub-grains than the as-welded CGHAZ. Carbides such as M23C6, M7C3, and MX gradually precipitated inside the grains and grain boundaries, decreasing the hardness. The SRC susceptibility was high in the as-welded CGHAZ, but decreased with increasing ageing time. When the ageing time exceeded 24 h, the sample was SRC-resistant. The main cause of SRC in the CGHAZ was precipitation and growth of M23C6 on the grain boundaries, which induced the formation of softened zones in the matrix near the grain boundary, and promoted the formation of micro-voids. During ageing, the unstable microstructure in the as-welded CGHAZ transformed as carbides precipitated and the matrix recrystallized, thereby reducing the intragranular strength. Meanwhile, the depletion of alloy elements near the grain boundary was eliminated. The microstructural evolution decreased the difference between the intragranular and intergranular strengths in the CGHAZ. Finally, the CGHAZ showed significantly improved ductility and low SRC susceptibility. The hardness of the aged CGHAZ was positively related to the SRC susceptibility. At hardnesses above 250 HB, the CGHAZ was SRC-susceptible, but at hardnesses below 250 HB, the CGHAZ was SRC-resistant.
Fig.1 Schematic illustration of the sample (unit: mm) for stress-relief cracking susceptibility evaluation (a) and thermal cycle curves (b) (tp/8—cooling time from peak temperature to 800oC, t8/5—cooling time from 800oC to 500oC, t5/3—cooling time from 500oC to 300oC)
Fig.2 OM images of coarse grain heat affected zone (CGHAZ) in T23 steel samples in as-welded condition (a), aged at 650oC for 6 h (b), 24 h (c), and 48 h (d)
Fig.3 SEM images of CGHAZ in T23 steel samples in as-welded condition (a), aged at 650oC for 6 h (b), 24 h (c), and 48 h (d) (M-A—martensite-austenite)
Fig.4 TEM images of CGHAZ in T23 steel samples as aged at 650oC for 6 h (a, b), 24 h (c, d), and 48 h (e, f) (Insets show the corresponding EDS results of the carbides in the circles)
Fig.5 Hardnesses of CGHAZ in T23 steel samples in as-welded condition and aged at 650oC for different time
Fig.6 Reduction of area vs test temperature of CGHAZ in T23 steel samples
Fig.7 Fracture surfaces at 750oC of CGHAZ in T23 steel samples in as-welded condition (a), aged at 650oC for 6 h (b), 24 h (c), and 48 h (d) (Insets in Figs.7a and d are corresponding high magnified images of fracture surface)
Fig.8 Longitudinal section views of CGHAZ in T23 steel samples fractured at 750oC (Insets in Figs.8b and d show the high magnified images of grain boundaries with void)
Fig.9 TEM images of CGHAZ in T23 steel samples fractured at 750oC
Fig.10 TEM image (a) and typical EDS line scans (b) done across prior austenite grain boundaries in fracture samples of as-welded CGHAZ of T23 steel sample tested at 750oC
Fig.11 TEM image (a) and typical EDS line scans (b) done across prior austenite grain boundaries in fracture samples of CGHAZ of T23 steel aged at 650oC for 48 h tested at 750oC
Fig.12 Illustration of fracture mechanism of as-welded (a) and aged (b) CGHAZ (PAGB—prior austenite grain boundary)
T / oC
6 h
24 h
48 h
550
611
2444
4889
500
9659
38635
77269
Table 1 Equivalent heat exposure conditions of CGHAZs of T23 steels aged at 650oC for different time
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