Microstructure of 10% Cr martensitic heat-resistant steel welded joints and type IV cracking behavior during creep rupture at 650°C

Effect of Postweld Heat Treatments on Type IV Creep Failure in the Intercritical Heat-Affected Zone of 10% Cr Martensitic Steel Welded with Haynes 282 Filler

“…Meanwhile, W and Mo-rich Laves phase indicated by the yellow arrows were precipitated along the newly formed fine PAGBs as well as the former PAGBs. It is well known that Laves phases are formed preferentially on the grain boundaries adjacent to Cr-rich M 23 C 6 carbides, and then they gradually swallow the M 23 C 6 carbides in close vicinity due to the rearrangement of the alloy elements (Cr, Mo, and W) [11,12,22,[39][40][41][42], which is in agreement with the results of this study (Figure 7b). However, a lot of the Wand Mo-rich Laves phases were also observed at the newly formed fine PAGBs without M 23 C 6 carbides after the creep (Figure 7a,c).…”

“…The precipitation of the Laves phase is considered to be the main cause of Type IV cracking in 9-12% Cr steel because the nucleation and growth of the Laves phase promote diffusion of W or Mo from the matrix to the Laves phase, which results in the loss of solid solution strengthening and deteriorates the creep properties. When the Laves phase precipitates on the grain boundaries, the sliding of grain boundary during creep could lead to stress-strain concentration at the Laves phase/matrix interface, which results in the nucleation and growth of the creep void [10][11][12]19,32,37].…”

“…Power plants operating under ultra-super-critical (USC) or advanced USC (A-USC) conditions require heat-resistant materials with enhanced creep resistance [1][2][3][4][5][6]. Normally, Ni-based superalloys are employed for high-temperature applications above 670 • C, and advanced high-chromium (9-12% Cr) martensitic steels are used at the temperature lower than 670 • C [6][7][8][9][10][11][12]. Consequently, dissimilar metal welds (DMWs) between Ni-based superalloys and 9-12% Cr martensitic steels are inevitable in USC and A-USC power plants.…”

A Review of Austenite Memory Effect in HAZ of B Containing 9% Cr Martensitic Heat Resistant Steel

“…Because of rapid heating and cooling during welding, non-equilibrium microstructures were formed in heat affected zone (HAZ) [8,9], leading to the degradation of mechanical properties of HAZ and thus weakening the whole welded joint [10]. For the martensitic steel welded joint used in thermal power unit, the service life mainly depended on its creep performance and type IV crack in HAZ was a common creep fracture mode which led to premature failure of the welded joint [11][12][13][14][15][16][17][18][19][20][21]. As they moved away from fusion line, the sub-zones of HAZ of conventional martensitic heat resistant steel, i.e., P91, were classified into coarse-grained HAZ (CGHAZ, also known as overheated sub-zone of HAZ), fine grained HAZ (FGHAZ, also known as normalized sub-zone of HAZ), inter-critical HAZ (ICHAZ) and over-tempered base metal (OTBM), which were distinguished with the different peak temperature and prior austenite grain size [22], as shown in Figure 1 [23].…”

“…If the temperature rises far above A C3 , the fine equiaxial austenite grains continue to grow and eventually coarse-grained austenite forms, and after cooling, CGHAZ will form. Type IV crack always occurred in FGHAZ of martensitic steel during service [16][17][18][19][20][21]. Coarsening of precipitates, lack of adequate precipitates pinning grain boundaries, matrix softening, and grain refinement in FGHAZ contribute to the formation of type IV crack [13,14,18,[24][25][26][27][28][29][30][31][32].…”