5. Combination of weld's HAZ simulation with ACT

The ACT procedure appeared useful to study other high strength steels for elevated temperature applications different than thermal power generation and chemical processing.

Table 3 contains results of ACT on nuclear reactor pressure vessel steel grade 15Ch2MFA (Russian), of which creep resistance is not the most important property; however, the strength of its HAZ after welding and PWHT is crucial. Physicallysimulated HAZs on this steel were earlier studied and reported, as regards their microstructures and CVN impact strength [9]. The initial microstructure of 15Ch2MFA steel was high-tempered martensitic (Figure 23), and after the singlecycle 900°C physical simulation of HAZ, it changed into a ferritic-martensitic mixture in the middle of the simulated HAZ (Figure 24), with well-distinguished equiaxed fine bright ferrite grains and darker-etching areas, details of which could not be revealed by light microscopy.

More informative as regards components of this intercritical HAZ are TEM images from thin foil specimens, which reveal small compact martensitic islands with nondissolved carbides in the HAZ region representative to temperatures just above the A1 (Figure 25) and larger fields of homogeneous acicular martensite surrounded by ferrite grains (Figure 26) in the region closer to the 900°C peak

Material ACT

Physical Background and Simulation of Creep in Steels DOI: http://dx.doi.org/10.5772/intechopen.89651

2 15Ch2MF, HAZ 1-cycle simulated, no PWHT

3 15Ch2MF, HAZ 1-cycle simulated + PWHT

4 15Ch2MF, HAZ 2-cycle simulated + PWHT

5 15Ch2MF, HAZ\* 2-cycle simulated + PWHT

Creep life of simulated HAZs in 15Ch2MFA steel.

Initial high-tempered martensitic microstructure of 15Ch2MFA steel.

Simulated intercritically-reheated microstructure of 15Ch2MFA steel HAZ.

Table 3.

Figure 23.

Figure 24.

81

temperature [°C]

1 Base 15Ch2MF steel 625 10.18 203 4183

Time to fracture [ks] Average tensile stress [MPa]

625 8.81 235 2810

625 12.37 187 5547

625 8.12 264 9765

625 10.66 183 5873

Creep life for 100 MPa [h]

Physical Background and Simulation of Creep in Steels DOI: http://dx.doi.org/10.5772/intechopen.89651


### Table 3.

Creep life of simulated HAZs in 15Ch2MFA steel.

Figure 23. Initial high-tempered martensitic microstructure of 15Ch2MFA steel.

Figure 24. Simulated intercritically-reheated microstructure of 15Ch2MFA steel HAZ.

More informative as regards components of this intercritical HAZ are TEM images from thin foil specimens, which reveal small compact martensitic islands with nondissolved carbides in the HAZ region representative to temperatures just above the A1 (Figure 25) and larger fields of homogeneous acicular martensite surrounded by ferrite grains (Figure 26) in the region closer to the 900°C peak

Figures 21 and 22. These figures show the microstructure transformation in P91 weld metal from tempered lath martensite before the ACT to fully recrystallized after the ACT equiaxed ferrite grains with coagulated and spheroidized carbides,

Well-recrystallized ferrite grains with arrays of carbides, in P91 weld metal sample after completed ACT at

Arrays of subgrains with carbides in the initial microstructure of P91 weld metal before the ACT; TEM, thin

The ACT procedure appeared useful to study other high strength steels for elevated temperature applications different than thermal power generation and

Table 3 contains results of ACT on nuclear reactor pressure vessel steel grade 15Ch2MFA (Russian), of which creep resistance is not the most important property; however, the strength of its HAZ after welding and PWHT is crucial. Physicallysimulated HAZs on this steel were earlier studied and reported, as regards their microstructures and CVN impact strength [9]. The initial microstructure of 15Ch2MFA steel was high-tempered martensitic (Figure 23), and after the singlecycle 900°C physical simulation of HAZ, it changed into a ferritic-martensitic mixture in the middle of the simulated HAZ (Figure 24), with well-distinguished equiaxed fine bright ferrite grains and darker-etching areas, details of which could

the last aligned in directions following prior martensite laths.

5. Combination of weld's HAZ simulation with ACT

chemical processing.

80

Figure 21.

Creep Characteristics of Engineering Materials

Figure 22.

600°C; TEM, thin foil.

foil.

not be revealed by light microscopy.

Figure 25.

Ferritic-martensitic microstructure with retained carbides in simulated HAZ region close to A1 (760°C) temperature.

strain-induced precipitation was achieved revealing potential of this steel to strengthen in the HAZ. However, not only the stress intensity but also the locations of post dual-phase microstructure generated by subsequent weld thermal cycles, that is, the second cycle of 920°C peak after the first cycle of 1220°C peak, contributed to the exceptionally high creep life of the HAZ in the case of sample #4 (result in row 4 of Table 3). In classical welding HAZ simulation experiments, the thermal cycles are executed having their peak temperatures in the same locations on the fixed sample (see Figures 27 and 28), and in consequence, the intercritically reheated zone of the second cycle does not overlap with the similar zone from the first cycle, as it appears closer to the peak temperature location than in the first cycle (Figure 29) [10], while in many true multibead welds, these overlap zones are vulnerable to initiation of voids and cracks, in particular during creep. Result #4 from Table 3 shows an exceptionally high value due to the overlap of the intercritically reheated HAZ portion of the second simulation cycle (peak 920°C) with the fine martensitic

microstructure of the previous simulation cycle (peak 1220°C).

The same sample in Gleeble chamber during simulation of HAZ's 2nd cycle.

Physical Background and Simulation of Creep in Steels DOI: http://dx.doi.org/10.5772/intechopen.89651

Figure 28.

Figure 29.

83

Thermal graph of two-cycle HAZ simulation on Gleeble [9].

The intercritically reheated zones after a single-cycle simulation at 1220°C appear at about 1.0–1.5 mm closer to the cold Cu jaws, than at the simulation with the peak temperature at 920°C, with the same span between the Cu jaws. To correct this difference, distance plates (P) can be used in the clamping assembly of the Gleeble, and the first thermal cycle, for example, 1220°C, was being executed with both plates inside, while for the second cycle, for example, 920°C peak, one of the plates is removed, and this type of simulation was marked HAZ\*. It allowed intercritical zones from both cycles to overlap and resulting microstructures of HAZ\* did not differ from those of real welds. To carry out the ACT after the HAZ\* cycle simulation, both distance plates have to be removed and thus the free span

temperature of the simulation cycle. In both cases, the ferrite interacting with martensite contained very high dislocation density. More details of this microstructure in as-simulated state and after conventional tempering were given in previous IIW-Document [9].

Creep life of the weld's simulated HAZ on this steel was tested using the simulated welding thermal cycle(s) followed immediately by ACT, applying setups as shown in Figures 27 and 28. Various simulations of the welding thermal cycles were carried out and then, by manipulation of the stress intensity in ACT, the

Figure 27. The ACT sample in Gleeble chamber during simulation of HAZ's 1st cycle.

Physical Background and Simulation of Creep in Steels DOI: http://dx.doi.org/10.5772/intechopen.89651

Figure 28. The same sample in Gleeble chamber during simulation of HAZ's 2nd cycle.

strain-induced precipitation was achieved revealing potential of this steel to strengthen in the HAZ. However, not only the stress intensity but also the locations of post dual-phase microstructure generated by subsequent weld thermal cycles, that is, the second cycle of 920°C peak after the first cycle of 1220°C peak, contributed to the exceptionally high creep life of the HAZ in the case of sample #4 (result in row 4 of Table 3). In classical welding HAZ simulation experiments, the thermal cycles are executed having their peak temperatures in the same locations on the fixed sample (see Figures 27 and 28), and in consequence, the intercritically reheated zone of the second cycle does not overlap with the similar zone from the first cycle, as it appears closer to the peak temperature location than in the first cycle (Figure 29) [10], while in many true multibead welds, these overlap zones are vulnerable to initiation of voids and cracks, in particular during creep. Result #4 from Table 3 shows an exceptionally high value due to the overlap of the intercritically reheated HAZ portion of the second simulation cycle (peak 920°C) with the fine martensitic microstructure of the previous simulation cycle (peak 1220°C).

The intercritically reheated zones after a single-cycle simulation at 1220°C appear at about 1.0–1.5 mm closer to the cold Cu jaws, than at the simulation with the peak temperature at 920°C, with the same span between the Cu jaws. To correct this difference, distance plates (P) can be used in the clamping assembly of the Gleeble, and the first thermal cycle, for example, 1220°C, was being executed with both plates inside, while for the second cycle, for example, 920°C peak, one of the plates is removed, and this type of simulation was marked HAZ\*. It allowed intercritical zones from both cycles to overlap and resulting microstructures of HAZ\* did not differ from those of real welds. To carry out the ACT after the HAZ\* cycle simulation, both distance plates have to be removed and thus the free span

Figure 29. Thermal graph of two-cycle HAZ simulation on Gleeble [9].

temperature of the simulation cycle. In both cases, the ferrite interacting with martensite contained very high dislocation density. More details of this microstructure in as-simulated state and after conventional tempering were given in previous

The ACT sample in Gleeble chamber during simulation of HAZ's 1st cycle.

Martensite grain and recrystallized ferrite, in simulated HAZ region close to peak (900°C) temperature.

Ferritic-martensitic microstructure with retained carbides in simulated HAZ region close to A1 (760°C)

Creep life of the weld's simulated HAZ on this steel was tested using the simulated welding thermal cycle(s) followed immediately by ACT, applying setups as shown in Figures 27 and 28. Various simulations of the welding thermal cycles were carried out and then, by manipulation of the stress intensity in ACT, the

IIW-Document [9].

Figure 25.

Creep Characteristics of Engineering Materials

temperature.

Figure 26.

Figure 27.

82

between jaws increased, to secure the uniformly heated hot working zone of about 10 mm length on the ACT sample, as shown in Figure 30. The test depicted on this figure was carried out in a low vacuum of the working chamber, so the "hot" portion of the ACT sample visibly oxidized.

possible (i.e., 0.3 mm) to the fractures of samples (#4) and (#5) revealed substantial differences of ferrite substructures of the 15Ch2MFA steel HAZs, as to the dislocation density and dispersion of ultrafine precipitates (Figures 31 and 32). This strengthening of simulated weld HAZ in the 15Ch2MFA steel as well as increase of its creep life can be related to the strain-induced precipitation forced by multicycle

The up-to-date ACT procedure includes evaluation of true creep life. Out of the collected test's rough data, the stiffness of material, that is, pseudoelasticity modulus E\* and then the creep life, can be calculated for a nominal stress, for example, 100 MPa. Some examples from recent research [11], on forged FB2 steel (9Cr-1.5Mo-1.3Co-V-Nb-N-B) and a weld on it, are given in Table 4 in rows 1–4. In this research, one of the main questions was how much the HAZ on samples taken from real welds has been weaker than the base material, and ACT combined with the

The FB2 grade belongs to the family of new creep resisting steels, developed and tested in EU R&D COST-522 and COST-536 collaborations. In these COSTs, also conventional creep tests were carried out, and for comparison, some of their results are given in Figure 33. The result creep life in ACT at 650°C on FB2 steel, that is, 8656 h, matches quite well with the upper black line of the graph presented.

An additional question was if by physical simulation of welding thermal cycles, similar creep properties of HAZ can be obtained and the answer was not straightforward. Physical simulation of two HAZ thermal cycles "one on another," with the first peak temperature of 1220°C followed by the second one of 920°C peak and PWHT "in situ" at 720°C for 15 min, gave in ACT an optimistic result of almost 25,000 h at nominal stress of 100 MPa and temperature of 625°C. The application of the modified HAZ\* simulation procedure, mentioned before for the 15Ch2MFA steel, gave after ACT a result similar to creep life of HAZs in real FB2 welds, and the result of this modified HAZ\* simulation followed by ACT is included in row 6 of

The ACT samples for this research were taken from the FB2 material used for the preparation of dissimilar weld joint from forgings of two rings (external diameter 600 mm and thickness 200 mm) made of steel types COST FB 2 and COST F [13]. The heat treatment after the forging of FB2 was 1070°C/6.5 h + 570°C/ 12.5 h + 710°C/24 h. The weld was manufactured by automated TIG hot wire method in a narrow gap with internal protection by argon, using filler material PSM

> Average tensile stress [MPa]

Creep life for 100 MPa [h]

Time to fracture [ks]

1 FB2 base 675 18.04 134 4156 2 FB2 base 650 19.99 149 8656 3 FB2 base 625 9.89 144 27,169 4 FB2 HAZ 625 8.11 196 16,670 5 FB2 HAZ 625 10.59 182 18,636 6 FB2 sim HAZ\* 625 9.64 192 17,423

intensive loading during ACT.

6. Evaluation of creep life from ACT data

Physical Background and Simulation of Creep in Steels DOI: http://dx.doi.org/10.5772/intechopen.89651

HAZ simulation allowed to answer this.

Material ACT

Creep life calculated from ACT data.

temperature [°C]

Table 4.

Table 4.

85

The last row (#5) in Table 3 shows the creep life of two-cycle HAZ\*-simulated sample after ACT carried out at lower stress intensity, and the fracture on this sample appeared eccentrically close to the location in which both intercritically reheated zones overlapped. Thin foils for TEM observations taken as close as

Figure 30. The HAZ-simulated sample in Gleeble chamber during the ACT, before cracking.

Figure 31.

High density of ultrafine precipitates in ferrite in the two-cycle 1220°C/920°C HAZ-simulated sample (4) with PWHT, after ACT of high stress intensity; foil thickness 360 nm.

#### Figure 32.

Dislocations with fine precipitates in ferrite in a single-cycle 1220°C HAZ-simulated sample (3) with PWHT, after ACT of normal stress intensity; foil thickness 450 nm.

between jaws increased, to secure the uniformly heated hot working zone of about 10 mm length on the ACT sample, as shown in Figure 30. The test depicted on this figure was carried out in a low vacuum of the working chamber, so the "hot"

The last row (#5) in Table 3 shows the creep life of two-cycle HAZ\*-simulated sample after ACT carried out at lower stress intensity, and the fracture on this sample appeared eccentrically close to the location in which both intercritically reheated zones overlapped. Thin foils for TEM observations taken as close as

portion of the ACT sample visibly oxidized.

Creep Characteristics of Engineering Materials

The HAZ-simulated sample in Gleeble chamber during the ACT, before cracking.

PWHT, after ACT of high stress intensity; foil thickness 360 nm.

after ACT of normal stress intensity; foil thickness 450 nm.

High density of ultrafine precipitates in ferrite in the two-cycle 1220°C/920°C HAZ-simulated sample (4) with

Dislocations with fine precipitates in ferrite in a single-cycle 1220°C HAZ-simulated sample (3) with PWHT,

Figure 30.

Figure 31.

Figure 32.

84

possible (i.e., 0.3 mm) to the fractures of samples (#4) and (#5) revealed substantial differences of ferrite substructures of the 15Ch2MFA steel HAZs, as to the dislocation density and dispersion of ultrafine precipitates (Figures 31 and 32). This strengthening of simulated weld HAZ in the 15Ch2MFA steel as well as increase of its creep life can be related to the strain-induced precipitation forced by multicycle intensive loading during ACT.
