A. The effects of PWHT and thermal exposure on microstructure

Figure 7 shows microstructural evolution of T92 HAZ regions of investigated T92/TP316H weldments depending on their initial PWHT procedure and subsequent thermal exposure at 625 °C for several time durations. The T92 HAZ microstructures in both initial PWHT states [Figs. 7(a) and 7(b)] contain tempered martensite with intragranular MX (M = V, Nb; X = C, N) carbonitrides and intergranular M₂₃C₆ (M = Cr, Fe) carbides which represent typical secondary phase precipitates in 9Cr tempered martensitic creep-resistant steels.^{Reference Hald15} However, the T92 HAZ microstructure processed by “single-step” PWHT [Fig. 7(a)] exhibits additionally the occurrence of non-equilibrium δ-ferrite decorating prior austenitic grain boundaries. Moreover, it is obvious from all subsequently aged T92 HAZ microstructures [Figs. 7(c), 7(e), and 7(g)] that even long-term aging for up to 11,000 h of the weldments initially processed by “single-step” PWHT did not result in δ-ferrite decomposition. On the contrary, the used renormalizing “two-step” PWHT has been identified as a suitable procedure for δ-ferrite suppression in T92 HAZ microstructure [see Figs. 7(b), 7(d), 7(f), and 7(h)]. Regardless the used initial PWHT procedure of studied weldments, the effects of long-term aging on T92 HAZ regions were generally manifested by additional precipitation of carbidic precipitates and newly formed intermetallic Laves phase already after 500 h of thermal exposure [Figs. 7(c), 7(d) and 8], followed by their gradual coarsening [Figs. 7(e)–7(h)]. Qualitative differentiation of individual phases on metallographic cross-section was performed by their local energy dispersive X-ray (EDX) analyses (Fig. 9). It can reasonably be deduced that the kinetics of microstructure evolution was affected by the initial state of T92 HAZ microstructures depending on the initial PWHT procedure used. According to the findings, e.g.,^{Reference Cui, Kim, Kang and Miyahara16} the presence of δ-ferrite grains enriched by strong ferrite-stabilizing elements such as W and Mo in microstructure may promote nucleation of Fe₂(W,Mo)-based Laves phase. Indeed, the local EDX analysis performed in present study confirmed more than twice increased amount of W in δ-ferrite in T92 HAZ, compared to that declared in Table I for the T92 steel overall chemical composition. The amount of Mo in δ-ferrite and matrix was not detected by means of performed EDX analyses. Further details on microstructural features are discussed in correlation with investigated mechanical properties in the following sections.

FIG. 7.

Microstructural evolution of T92 HAZ regions of T92/TP316H weldments depending on initial PWHT procedure and subsequent aging time at 625 °C: (a) “single-step” PWHT + 0 h, (b) “two-step” PWHT + 0 h, (c) “single-step” PWHT + 500 h, (d) “two-step” PWHT + 500 h, (e) “single-step” PWHT + 2500 h, (f) “two-step” PWHT + 2500 h, (g) “single-step” PWHT + 11,000 h, and (h) “two-step” PWHT + 11,000 h.

FIG. 8.

Detailed SEM visualization in back-scattered electrons contrast of T92 HAZ microstructure after aging at 625 °C for 500 h of T92/TP316H weldment initially processed by “single-step” PWHT.

FIG. 9.

Typical EDX spectra corresponding to local EDX analyses focused on individual phases in T92 HAZ microstructure enabling qualitative phase discrimination on metallographic cross-section.

B. The effects of PWHT and thermal exposure on hardness

Figure 10 shows hardness profiles measured along the Ni WM/T92 HAZ interface of the studied weldments after two different initial PWHT procedures and subsequent long-term thermal expositions. After the “single-step” PWHT, all hardness profiles show a steep hardness gradient within the region of T92 HAZ [see Fig. 10(a)]. Their general course reflects typical microstructural heterogeneity of HAZ regions of tempered martensitic steels and agrees well with observations performed by other authors^{Reference Zhao, Jing, Xu, Han and Xiu17,Reference Zhu, Wang and Xuan18} on similar type of dissimilar weld joints. During the first periods (500–2500 h) of thermal expositions at 625 °C, notable strengthening effects were observed within the T92 HAZ close to the fusion zone with Ni WM. This secondary hardening behavior can be related to the additional precipitation of secondary phases, in particular MX (M = V, Nb; X = C, N) carbonitrides within the grain interiors and M₂₃C₆ (M = Cr, Fe) carbides at the grain boundaries (Fig. 7). Fine MX carbonitrides are known to be the major type of intragranular precipitates in 9Cr creep-resistant steels with dispersion strengthening effect due to their negligible tendency to coarsening.^{Reference Hald15} On the other hand, the effect of intergranular M₂₃C₆ carbides is based rather on the stabilization of grain and subgrain structure.^{Reference Abe, Horiuchi, Taneike and Sawada19,Reference Hu, Huang, Yan, Wang, Sha, Shan and Yang20} According to our observation (Fig. 8) supported also by findings of other studies,^{Reference Kipelova, Belyakov and Kaibyshev21,Reference Abe22} the initial precipitation of intermetallic Laves phase may also play a role in hardening of Grade 92 steels during the first periods of their thermal and/or creep exposure besides the strengthening by MX and M₂₃C₆ precipitates. Kipelova et al.^{Reference Kipelova, Belyakov and Kaibyshev21} observed in a modified P911 tempered martensitic steel that the volume fraction of Fe₂(W,Mo) Laves phase increased to its saturation after about 3000 h of creep test at 650 °C. Abe^{Reference Abe22} and Dudova et al.^{Reference Dudova, Plotnikova, Molodov, Belyakov and Kaibyshev23} reported that the fine Laves phase particles have a significant effect on the stabilization of tempered martensitic lath structure during the initial stage of creep due to their additional pinning force for migrating boundaries. In present study, the Laves phase precipitates have been observed in T92 regions of the weldments already after 500 h of thermal exposure at 625 °C (Fig. 8). The detailed quantification of the Laves phase precipitates in advanced 9–12Cr steels was performed by Dimmler et al.^{Reference Dimmler, Weinert, Kozeschnik and Cerjak24} The increase of aging time above 2500 h led to gradual decrease in hardness values of T92 HAZ [Fig. 10(a)] due to the effect of precipitate coarsening besides thermally activated static recovery processes^{Reference Ghassemi-Armaki, Chen, Maruyama, Yoshizawa and Igarashi25} of highly tempered martensitic microstructure [Fig. 7]. Many other authors e.g., Ref. Reference Yan, Liu, Bao, Weng and Liu26–Reference Jiang, Zhu and Wang28 reported the effect of Laves phase and M₂₃C₆ carbide coarsening to be one of major factors causing strength drop in creep-resistant steels during their long-term thermal or creep exposure. In contrast, gradual hardness increase of Ni WM with increasing aging time [Fig. 10(a)] can be attributed to the fine precipitation of thermally stable NbC carbides which were particularly characterized in Ref. Reference Falat, Výrostková, Svoboda and Milkovič10. The “two-step” PWHT caused the hardness values “equalization” within the whole T92 region of the studied weldment [see Fig. 10(b)] as a result of suppression of T92 HAZ microstructural heterogeneity during the reaustenitization. The influence of additional thermal expositions on the general course of hardness profiles within the T92 regions was rather small. This finding corresponds well with the hardness profile measurements performed on the same type of creep-exposed weldments in our previous study.^{Reference Falat, Čiripová, Kepič, Buršík and Podstranská11} It can be additionally supported by the fact of T92 microstructure homogenization after the “two-step” PWHT, resulting in suppression of both microstructural and chemical gradients.^{Reference Falat, Výrostková, Svoboda and Milkovič10} Thus, the hardness profiles in the T92 regions of the samples after the “two-step” PWHT and subsequent thermal expositions exhibited always stable behavior without any remarkable variations. The hardness behavior of Ni WM after the “two-step” PWHT [Fig. 10(b)] was similar like in the case of “single-step” PWHT [see Fig. 10(a)].

FIG. 10.

Isothermal aging effects on hardness profiles along Ni WM/T92 HAZ interface of T92/TP316H weldments initially processed by: (a) “single-step” PWHT and (b) “two-step” PWHT.

C. The effects of PWHT and thermal exposure on tensile strength

Figure 11 shows the values of notch-tensile strength (Rm_V) of T92 HAZ of studied T92/TP316H weldment in dependence of its initial PWHT procedure and subsequent thermal expositions. After the initial “single-step” PWHT, the T92 HAZ region exhibits clearly higher tensile strength in comparison to the state after the “two-step” PWHT. The higher T92 HAZ strength (Fig. 11) and hardness (Fig. 10) after the “single-step” PWHT can be explained in terms of higher level of transformation hardening i.e., lower tempering grade of fresh martensitic microstructure, enhanced by localized solid solution hardening in the fusion zone resulting from mixing of partially melted T92 HAZ substrate with Ni WM. After subsequent thermal expositions, the tensile strength of T92 HAZ initially processed by the “single-step” PWHT gradually decreases down to its minimum value corresponding to the longest duration of thermal exposure (Fig. 11). However, some slight secondary hardening effects could still be indicated for T92 HAZ material aged for 2500 h (Fig. 11). In general, the observed nearly monotonous decrease of tensile strength can be related to successive microstructural degradation related mainly to precipitate coarsening and microstructural recovery processes with increasing aging time (Fig. 7). Similar softening effects have been observed by Yan et al.^{Reference Yan, Liu, Bao, Weng and Liu26} after the tensile testing of thermally exposed 9Cr–3W–3Co martensitic steel at 650 and 700 °C. In contrast, the tensile strength of T92 region of thermally exposed weldments initially processed by the “two-step” PWHT exhibits more complex behavior. After the first 500 h of aging it shows a slight decrease followed by its notable increase reaching the maximum after 2500 h of aging (Fig. 11). This result is likely related to homogenization effects of the “two-step” PWHT followed by additional fine precipitation during the first periods of thermal expositions (Fig. 7). Chen et al.^{Reference Chen, Zhang, Liu, Wang, Yu, Hua, Bai, Zhang, Zhang and Tang29} investigated the microstructures and mechanical properties of T92/Super304H dissimilar welds after their high-temperature aging at 650 °C for time up to 3000 h. They also observed secondary hardening behavior during tensile tests of the weldments after their short-term heat treatments followed by gradual softening effects with further increase of aging time. The observed behavior was explained by dispersion strengthening effects at the early stage of aging followed by the softening effects due to the precipitate coarsening and simultaneous loss of solid solution hardening from carbide-forming alloying elements. The aging temperature of 650 °C was too high to consider the strengthening effect of Laves phase due to its small volume fraction and fast coarsening kinetics in T92 steel at this temperature.^{Reference Chen, Zhang, Liu, Wang, Yu, Hua, Bai, Zhang, Zhang and Tang29} Jang et al.^{Reference Jang, Shim, Komazaki and Honda30} performed nanoindentation experiments and microstructural analysis on high Cr ferritic steel in as-tempered and thermally aged conditions. It has been indicated that thermal softening of tempered martensitic steels is primarily related to matrix softening rather than reduction in grain-boundary strengthening effect. In present study, gradual decreasing of tensile strength of T92 region of the weldment processed by the “single-step” PWHT can also be attributed to its initially lower thermal stability in comparison to the homogenized T92 microstructure immediately after the “two-step” PWHT (Fig. 7). However, it can be clearly seen (Fig. 11) that with increasing duration of thermal expositions, the differences in T92 HAZ tensile strength between the weldments initially processed by different PWHT procedures are getting smaller. This result can be attributed to the similar level of microstructure degradation of the weldments after long-term aging, regardless of their initial PWHT procedure (Fig. 7). It can also be supported by findings of Zhu et al.^{Reference Zhu, Wang and Xuan18} who observed notable homogenization effects of long-term aging on HAZ microstructure of their Ni–Cr–Mo–V steel weldment.

FIG. 11.

Isothermal aging effects on notch-tensile strength of T92 HAZ of the T92/TP316H welds initially processed by different PWHT procedures.

D. The effects of PWHT and thermal exposure on impact toughness

Figure 12 shows the values of Charpy V-notch impact toughness (CVN) of T92 HAZ of T92/TP316H weldment in dependence of its initial PWHT procedure and subsequent thermal exposure. With respect to the effects of initial PWHT procedures on resulting T92 HAZ mechanical properties of studied weldments, it can be stated that the CVN dependencies (Fig. 12) show in principle complementary behavior to the Rm_V dependencies (Fig. 11). Specifically, after the initial “single-step” PWHT, the T92 HAZ exhibits lower impact toughness (Fig. 12) and higher tensile strength (Fig. 11) in comparison to the T92 region after the “two-step” PWHT. However, after long-term thermal expositions above 5000 h, clear reversions of mechanical properties dependencies were observed in favor of the “single-step” PWHT (see Figs. 11 and 12). The higher impact toughness after the initial “two-step” PWHT can be attributed to the microstructure homogenization in terms of more uniform prior austenite grain size and alloying elements redistribution leading to considerable suppression of both structural and chemical gradients.^{Reference Falat, Výrostková, Svoboda and Milkovič10} Gao et al.^{Reference Gao, Liu, Di, Yu and Yan31} studied martensite transformation in the modified high Cr heat-resistant steel during continuous cooling and indicated that with increasing cooling rate the aspect ratio of martensite laths decreases and their density increases. This finding can also support our observation of increased impact toughness of re-austenitized, water quenched, and tempered T92 HAZ corresponding to the performed “two-step” PWHT. On the other hand, Moitra et al.^{Reference Moitra, Parameswaran, Sreenivasan and Mannan32} investigated the HAZ toughness of 9Cr–1Mo steel weldments after conventional “single-step” PWHT and observed its minimum within coarse-grained region adjacent to the weld fusion zone which also qualitatively agrees with findings of present study. Chen et al.^{Reference Chen, Zhang, Liu, Wang, Yu, Hua, Bai, Zhang, Zhang and Tang29} reported the local occurrence of non-equilibrium δ-ferrite in T92 HAZ of their T92/Super304H weldments to be one of significant factors influencing the resulting mechanical properties. As already discussed in Sec. III. A of our present investigation, the occurrence of δ-ferrite was really observed along prior austenite grain boundaries within the T92 HAZ of the weldments initially processed by the “single-step” PWHT [Figs. 7(a), 7(c), 7(e), and 7(g)], whereas the weldments after the “two-step” PWHT were found to be free of this phase [Figs. 7(b), 7(d), 7(f), and 7(h)]. Detrimental effects of δ-ferrite on toughness and creep strength of high Cr tempered martensitic steels have been widely reported in the literature, e.g., Ref. Reference Lomozik, Zeman and Jachym33 and Reference Meng, Lu, Cui, Ding, Tang and Huo34. The results of present study show that the CVN values of the samples initially processed by the “two-step” PWHT decrease gradually with increasing thermal exposure duration, whereas the CVN dependency of the samples initially processed by the “single-step” PWHT does not show simple monotonous decreasing (Fig. 12). The observed differences in mechanical behavior can likely be associated with the differences in kinetics of microstructure degradation due to the different initial microstructures after the individual PWHT procedures. In contrast to the Rm_V dependencies (Fig. 11), it is obvious that with increasing duration of thermal expositions the differences in CVN values between the weldments initially processed by different PWHT procedures are increasing (Fig. 12). Consequently, after the longest thermal exposure at 625 °C for 11,000 h, the T92 HAZ region of the weldment initially processed by the “two-step” PWHT exhibits the lowest impact toughness of all tested specimens which indicates its most severe thermal embrittlement. Thus the obtained results indicate that after long-term thermal expositions, a more suitable combination of T92 HAZ mechanical properties (i.e., the higher impact toughness and acceptable tensile strength) is achieved for the weldments initially processed by the “single-step” PWHT (Figs. 11 and 12).

FIG. 12.

Isothermal aging effects on impact toughness of T92 HAZ of the T92/TP316H weldments initially processed by different PWHT procedures.

E. The effects of PWHT and thermal exposure on fracture characteristics

Figure 13 shows some typical SEM fractographs of the T92 HAZ notch-tensile samples of studied T92/TP316H weldments in dependence of their initial PWHT procedure and subsequent thermal exposure. The sample initially processed by the “single-step” PWHT and subsequently thermally exposed at 625 °C for 500 h (i.e., the sample with higher tensile strength, see Fig. 11) shows a mixed fracture consisting of coarse transgranular cleavage facets and relatively fine ductile dimples [Fig. 13(a)]. On the other hand, the sample initially processed by the “two-step” PWHT and the same thermal exposition (i.e., the sample with lower tensile strength, see Fig. 11) shows mostly ductile dimples with different sizes and morphologies and occasionally some signs of quasi-cleavage fracture [Fig. 13(b)]. However, after the longest thermal exposure for 11,000 hours, the samples initially processed by different PWHT procedures show very similar fracture behavior characterized by typical ductile dimple tearing [Figs. 13(c) and 13(d)]. This behavior correlates well with decreasing differences in tensile properties with increasing aging time of the samples initially processed by different PWHT procedures. At the bottom of larger dimples, brittle cracking of coarsened secondary phase precipitates is clearly visible on both fracture surfaces [Figs. 13(c) and 13(d)].

FIG. 13.

SEM fractographs of T92 HAZ regions after notch-tensile tests of T92/TP316H weldments initially processed by different PWHT procedures and subsequently aged at 625 °C for several durations: (a) “single-step” PWHT + 500 h, (b) “two-step” PWHT + 500 h, (c) “single-step” PWHT + 11,000 h, and (d) “two-step” PWHT + 11,000 h.

Figure 14 shows some typical SEM fractographs of the T92 HAZ impact toughness samples of studied T92/TP316H weldments in dependence of their initial PWHT procedure and subsequent thermal exposure. The fracture surface of T92 HAZ of the weldment after “single-step” PWHT (i.e., the sample with lower impact toughness) exhibits majority of transgranular cleavage fracture besides some small portion of ductile dimple fracture [Fig. 14(a)]. On the contrary, the fracture surface of T92 region of the weldment after “two-step” PWHT (i.e., the sample with higher impact toughness) shows by rule ductile dimple fracture with small portion of fine transgranular cleavage facets [Fig. 14(b)]. After long-term thermal exposure for 5000 h, the T92 HAZ fracture surface of CVN sample of the weldment initially processed by “single-step” PWHT is completely formed of transgranular cleavage [Fig. 14(c)]. On the other hand, the long-term isothermally aged T92 region of the weldment initially processed by the “two-step” PWHT shows even more pronounced thermal embrittlement after the impact test. It shows a mixture of both transgranular cleavage and intergranular decohesion [Fig. 14(d)]. This observation fairly correlates with intensive embrittlement of the identically processed weldments exhibiting decohesion creep failures in long-term creep conditions.^{Reference Falat, Výrostková, Svoboda and Milkovič10,Reference Falat, Čiripová, Kepič, Buršík and Podstranská11} The observed fracture behavior corresponds well with the course of impact toughness values depending on the initial PWHT processing and duration of subsequent thermal exposure (Fig. 12). During long-term thermal exposition the initial cleavage fracture mechanism is usually changed from dislocation triggered to carbide triggered cleavage nucleation.^{Reference Pluvinage, Azari, Kadi, Dlouhý and Kozak35} The occurrence of intergranular decohesion is usually taken as the evidence of impurity effect.^{Reference Hadraba, Nemec and Dlouhý36} In the case of present investigation the impurity segregation effect might be enhanced by synergic effects of increased grain size after the “two-step” PWHT and subsequent coarsening of intergranular precipitates during long-term aging.^{Reference Janovec37} From that point of view, although ductile and cleavage fracture micromechanism was more beneficial in initial microstructures, it changed to more detrimental for “two-step” PWHT comparing to “single-step” PWHT due to cumulative effects of embrittling factors.

FIG. 14.

SEM fractographs of T92 HAZ regions after impact toughness tests of T92/TP316H weldments initially processed by different PWHT procedures and subsequently aged at 625 °C for several durations: (a) “single-step” PWHT + 0 h, (b) “two-step” PWHT + 0 h, (c) “single-step” PWHT + 5000 h, and (d) “two-step” PWHT + 5000 h.