4. Results of ACT

A characteristic of creep is the decrease of steel hardness with the progress of time. Therefore, any creep test that results in the increase of hardness of sample must be considered as nonreliable. Table 1 contains examples of initial values for nonexploited materials, for creep exploited materials and for materials subjected to ACT. In the first instance, it shows that during real creep the hardness is most dependent on the time of exposure as well as on the applied stress. Then, the important factor is the initial hardness. Finally, it shows that the drop of hardness after just a few thousand seconds in the ACT follows the tendency of multiyear real creep.

Usually, the tests are run till failure/fracturing, but they can also be stopped anytime and specimens for metallographic, fractographic, and microanalytical investigations can be taken before fracturing. Here, an example is given of the 9Cr-1.5Mo-1.3Co-V-Nb-N-B (FB2) steel, which in as-delivered state has a hightempered coarse martensitic microstructure (Figure 13), while in its as-welded and postweld heat-treated HAZ, such microstructure changes into fine grained, nonacicular with distinguished grains of ferrite. In this pseudo dual-phase microstructure examined by ACT on crosswise samples, microcracks were found when the ACT was interrupted at about 90% of its duration. Such cracks, as shown in Figure 14, were nucleating in intercritically-reheated fine grain portion of heataffected zone, and they often initiated at specific locations of the HAZ, where two subsequent heat cycles of multipass welding overlapped, giving dual-phase microstructure.

As the tests for different materials are run at different temperatures and the response of material results in various stresses equivalent to the yield strength at elevated temperature of the test, to compare the ACT results, the duration of the test and its temperature can be included in the following parameter:

$$P\_{ACT} = (7 + \log t) \times T / 100\,\tag{1}$$

where t = time of test in [ks] and T = temperature in [K].

The ACT result can be inserted onto a graph, having PACT on horizontal axis and average tensile relaxation stress RS on vertical axis. Alternatively, a creep strength


#### Table 1.

Examples of hardness for steels subjected to creep and/or ACTested.

#### Figure 13.

High-tempered martensite of forged 9Cr-1.5Mo-1.3Co-V-Nb-N-B (FB2) steel in as-delivered state, FeCl3 etched.

The results of ACT are usually plotted in the form of strain-time and stress-time graphs, and data from these graphs are used to calculate creep strength and true creep life. The "zero-stress" line on strain-time graph from ACT well resembles the

Typical strain-time and stress-time graphs from ACT on a "hard" alloy, for example, as-deposited (without

Microcracks in IC-HAZ of FB2 steel perpendicular to load axis of ACT sample, developed at 85% of test

Typical strain-time and stress-time graphs from ACT on a "soft" alloy, for example, postweld heat-treated P91

Examples of results obtained at different ACT temperatures for various P91 weld metals in as-welded state (AW) and postweld heat treated (HT) are given in Table 2.

true creep graph from conventional creep tests.

Figure 14.

Figure 15.

weld metal.

Figure 16.

77

PWHT) P91 weld metal.

duration at 625°C.

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

factor in ACT can be given as: FACT = PACT RS/100 and used for comparing materials tested at the same conditions.

During the ACT procedure, the data of stress, strain, strain rate, and temperature, as well as dilatometric information, can be recorded, out of which strain-time and stress-time graphs are produced as shown in Figures 15 and 16.

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

#### Figure 14.

Microcracks in IC-HAZ of FB2 steel perpendicular to load axis of ACT sample, developed at 85% of test duration at 625°C.

#### Figure 15.

Typical strain-time and stress-time graphs from ACT on a "soft" alloy, for example, postweld heat-treated P91 weld metal.

#### Figure 16.

Typical strain-time and stress-time graphs from ACT on a "hard" alloy, for example, as-deposited (without PWHT) P91 weld metal.

The results of ACT are usually plotted in the form of strain-time and stress-time graphs, and data from these graphs are used to calculate creep strength and true creep life. The "zero-stress" line on strain-time graph from ACT well resembles the true creep graph from conventional creep tests.

Examples of results obtained at different ACT temperatures for various P91 weld metals in as-welded state (AW) and postweld heat treated (HT) are given in Table 2.

factor in ACT can be given as: FACT = PACT RS/100 and used for comparing

High-tempered martensite of forged 9Cr-1.5Mo-1.3Co-V-Nb-N-B (FB2) steel in as-delivered state, FeCl3

No. Material—sample/exposure or test Hardness HV0.1/(HV30)

1 1/2CrMoV pipe‑exp. 22.5 years at 568°C and 16.6 MPa (180) (131) 2 1/2CrMoV pipe‑exp. 20.5 years at 568°C and 3.8 MPa (180) <sup>158</sup> 3 P91 antler‑exp. 9 years at 600°C and 16.5 MPa (250) <sup>229</sup> 4 P91 weld‑exp. 9 years at 600°C and 16.5 MPa (265) <sup>208</sup> <sup>5</sup> P91 rolled pipe‑exp. 3 years at 568°C (250) <sup>231</sup> 6 P91 weld with PWHT‑exp. 3 years at 568°C (265) <sup>258</sup> 7 P91 forged bottle‑exp. 3 years at 568°C (260) <sup>243</sup>

10 P22 rolled pipe + PWHT/ACT 600°C, 9.5 ks 234 188 11 P22 weld metal + PWHT/ACT 600°C, 11.3 ks 238 202 12 P22 rolled pipe HAZ + PWHT/ACT 600°C, 6.4 ks 227 196 13 P91‑std. weld with PWHT/ACT 600°C, 28.3 ks <sup>287</sup> <sup>230</sup> 14 P91‑std. weld with PWHT/ACT 600°C, 82.6 ks <sup>285</sup> <sup>223</sup> 15 P91‑std. weld with PWHT/ACT 625°C, 37.6 ks <sup>285</sup> <sup>217</sup>

17 P92‑std. weld with PWHT/ACT 625°C, 21.3 ks <sup>294</sup> <sup>240</sup> 18 P92‑rolled pipe HAZ/ACT 625°C, 22.1 ks <sup>268</sup> <sup>228</sup>

8 12Cr1MoV‑exp. 16 years, 550°C, 15 MPa + ACT 600° C, 19.5 ks

Creep Characteristics of Engineering Materials

9 15Cr1Mo1V‑exp. 18 years, 550°C, 15 MPa + ACT 600° C, 30.4 ks

16 P91‑temper-bead weld, no PWHT/ACT 620°C, 51.3 ks

Examples of hardness for steels subjected to creep and/or ACTested.

Table 1.

Figure 13.

etched.

76

Initial or (standard) After exposure or testing

188 164

176 157

298 193

and stress-time graphs are produced as shown in Figures 15 and 16.

During the ACT procedure, the data of stress, strain, strain rate, and temperature, as well as dilatometric information, can be recorded, out of which strain-time

materials tested at the same conditions.

The result of P91-HT2 is complimentary to sample #16 from Table 1, to which after temper bead welding the PWHT was applied for 2 h at 720°C in furnace, and then the ACT was run. Longer durations of some tests (P91-HT2 and P91-HT5) resulted from smaller loading displacements applied in such lower intensity ACT program. The tests are usually run at prevacuum of about 10<sup>3</sup> Tr maintained in the testing chamber of physical simulator; however, when clean fracture surfaces are expected after the ACT, high vacuum of better than 10<sup>5</sup> Tr may be applied. In these experiments on P91 steel, no difference in test duration due to vacuum level was observed. Visible difference in test duration was noted when water mist became introduced into the testing chamber; the result of P91-HT5m in Table 2 shows shorter test duration, lower tensile relaxation stress, and in consequence lower creep strength factor. On cylindrical surface of this sample's gauge portion, numerous fine cracks appeared (Figure 17), which for the sample tested in high vacuum could not be seen up to the macrocrack appearance on the surface (Figure 18).

To verify the results of ACT by proving that the test adequately simulates situations appearing in real creep, metallographic investigations were carried out. Figure 19 shows dislocation substructure of P91 weld metal at half-life of the ACT. In recovered postmartensitic subgrains criss-crossing, a/2 <111> screw dislocations appear in vast amount and form planar arrays.


Table 2. Examples of ACT results.

In Figure 20, the fracture surface of ACT sample is shown, after the removal of surface oxides by electropolishing and then etching. On this surface, the traces of slip lines can be recognized, thus confirming the fracture nucleation by slip and

Fracture surface of ACT sample of P91 steel showing high density of carbides, electro-polished and FeCl3 etched,

Subgrains with recovered dislocation configurations at half-life in ACT, in P91 weld metal sample tested at

The final verification has to be done on TEM images of the tested steel or weld metal before and after the ACT, like on the examples presented here in

Macrocrack on ACT sample tested at 600°C with high vacuum in simulator's working chamber.

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

piling up dislocations in planar arrays.

Figure 18.

Figure 19.

Figure 20.

SEM image.

79

600°C; TEM, thin foil.

### Figure 17.

Cylindrical surface of gauge portion on P91 ACT sample after test at 600°C with the presence of water mist in working chamber.

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

#### Figure 18.

The result of P91-HT2 is complimentary to sample #16 from Table 1, to which after temper bead welding the PWHT was applied for 2 h at 720°C in furnace, and then the ACT was run. Longer durations of some tests (P91-HT2 and P91-HT5) resulted from smaller loading displacements applied in such lower intensity ACT program. The tests are usually run at prevacuum of about 10<sup>3</sup> Tr maintained in the testing chamber of physical simulator; however, when clean fracture surfaces are expected after the ACT, high vacuum of better than 10<sup>5</sup> Tr may be applied. In these experiments on P91 steel, no difference in test duration due to vacuum level was observed. Visible difference in test duration was noted when water mist became introduced into the testing chamber; the result of P91-HT5m in Table 2 shows shorter test duration, lower tensile relaxation stress, and in consequence lower creep strength factor. On cylindrical surface of this sample's gauge portion, numerous fine cracks appeared (Figure 17), which for the sample tested in high vacuum could not be seen up to the macrocrack appearance on the surface

To verify the results of ACT by proving that the test adequately simulates situations appearing in real creep, metallographic investigations were carried out. Figure 19 shows dislocation substructure of P91 weld metal at half-life of the ACT. In recovered postmartensitic subgrains criss-crossing, a/2 <111> screw dislocations

> ACT time to fracture [ks]

P91-AW1 625 22.0 305 228 P91-HT1 625 37.6 273 210 P91-AW2 620 38.0 395 303 P91-HT2 620 51.3 177 138 P91-HT3 600 28.3 325 240 P91-HT4 600 26.3 318 233 P91-HT5 600 82.6 336 262 P91-HT5m 600 57.2 266 203

Cylindrical surface of gauge portion on P91 ACT sample after test at 600°C with the presence of water mist in

Tensile relaxation stress RS [MPa]

Creep strength factor FACT [MPa]

appear in vast amount and form planar arrays.

Creep Characteristics of Engineering Materials

ACT temp. [°C]

(Figure 18).

Material and state

Table 2.

Figure 17.

78

working chamber.

Examples of ACT results.

Macrocrack on ACT sample tested at 600°C with high vacuum in simulator's working chamber.

#### Figure 19.

Subgrains with recovered dislocation configurations at half-life in ACT, in P91 weld metal sample tested at 600°C; TEM, thin foil.

#### Figure 20.

Fracture surface of ACT sample of P91 steel showing high density of carbides, electro-polished and FeCl3 etched, SEM image.

In Figure 20, the fracture surface of ACT sample is shown, after the removal of surface oxides by electropolishing and then etching. On this surface, the traces of slip lines can be recognized, thus confirming the fracture nucleation by slip and piling up dislocations in planar arrays.

The final verification has to be done on TEM images of the tested steel or weld metal before and after the ACT, like on the examples presented here in

#### Figure 21.

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

#### Figure 22.

Well-recrystallized ferrite grains with arrays of carbides, in P91 weld metal sample after completed ACT at 600°C; TEM, thin foil.

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, the last aligned in directions following prior martensite laths.
