Table 4.

Thermanit MTS 616. The thickness of the weld was 120 mm. The chemical compositions of the base material FB2 and the filler metal are given in Table 5.

From these welds, smooth cross-weld specimens were fabricated for long-term conventional creep tests (CCT), which were carried out in air at temperatures ranging from 550 to 650°C at stresses from 70 to 220 MPa. Samples from the same weldment were subjected to ACT in order to simulate structure transformations after long-term CCT. ACT was executed at VSB-Technical University Ostrava (Czech Republic) on an HDS-20 physical simulator and partly in the laboratory of the simulator manufacturer (DSI). ACT temperatures ranged from 575 to 650°C (Figure 33).

In its initial, as-delivered state, the steel has a high-tempered martensite microstructure, as given in Figure 13. This microstructure, as seen on TEM images, shows tempered former martensite laths, retaining high density of dislocations and having chains of particles precipitated along the lath boundaries (Figure 34). Most of the precipitates along the former lath boundaries were identified as carbides, while some of the larger-than-average ones appeared to be Laves phase Fe2Mo. In postmartensitic laths and in elongated ferrite grains, numerous recovery-type planar configurations of criss-crossing dislocations can be observed at larger magnifications (Figure 35).

The initial microstructure of the FB2-BM, as seen in light microscope (LM), is highly-tempered martensite of acicular morphology (Figure 36). In the as-welded HAZ, it is changed into a very fine-grained one, hard to resolve by light microscopy (Figure 37).

Details of the HAZ microstructure are revealed on the following TEM pictures taken from thin foil specimens. As the HAZ is expected to be the weakest part of the weld and consequently of the ACT sample, to take the specimens for TEM observations from exact HAZ location just before their fracturing, the tests had to be run


#### Table 5.

Chemical compositions of FB2 and weld metal.

till 85–95% of the expected time to fracture. Therefore, in the ACT program, a force limiting stop was placed in the tensile relaxation portion of the testing cycle. Typical force-time graph of ACT on plain FB2 steel (sample FB22) is given in Figure 38,

Criss-crossing dislocation arrays in high-tempered lath martensite in FB2-AD steel with carbides along former

High-tempered lath martensite microstructure of FB2 steel in as-delivered, i.e., forged and stress-relieved (AD),

In the initial, as-welded microstructure of HAZ, in its under-critical portion at

showing how the initial tensile relaxation force of 2200 kgf decreases to

A1 temperature, the lath martensite recrystallizes, while near to colonies of carbides, first traces of austenite form. Some ferrites get the shape of large laths (Figure 39), while in other ferrite grains next to austenite with carbides, very high

1400 kgf at 90% of the test duration.

FB2-BM microstructure in as-delivered state, before ACT, LM.

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

Figure 35.

Figure 34.

state.

lath border.

Figure 36.

87

#### Figure 33.

Creep rupture plot showing the performance of FB2 parent material and weldments, relative to grade 92 (9Cr-2W) seamless pipe [12].

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

#### Figure 34.

Thermanit MTS 616. The thickness of the weld was 120 mm. The chemical compo-

From these welds, smooth cross-weld specimens were fabricated for long-term

In its initial, as-delivered state, the steel has a high-tempered martensite microstructure, as given in Figure 13. This microstructure, as seen on TEM images, shows tempered former martensite laths, retaining high density of dislocations and having chains of particles precipitated along the lath boundaries (Figure 34). Most of the precipitates along the former lath boundaries were identified as carbides, while some of the larger-than-average ones appeared to be Laves phase Fe2Mo. In postmartensitic laths and in elongated ferrite grains, numerous recovery-type planar configurations of criss-crossing dislocations can be observed at larger magnifi-

The initial microstructure of the FB2-BM, as seen in light microscope (LM), is highly-tempered martensite of acicular morphology (Figure 36). In the as-welded HAZ, it is changed into a very fine-grained one, hard to resolve by light microscopy

Details of the HAZ microstructure are revealed on the following TEM pictures taken from thin foil specimens. As the HAZ is expected to be the weakest part of the weld and consequently of the ACT sample, to take the specimens for TEM observations from exact HAZ location just before their fracturing, the tests had to be run

C Mn Si Cr Ni Mo V Co W Nb N B Al

FB2 steel 0.13 0.34 0.05 9.60 0.17 1.48 0.20 1.32 — 0.059 0.016 0.0079 0.007 MTS 616 0.11 0.32 0.30 8.87 0.57 0.56 0.18 0.15 1.49 0.051 0.018 0.0036 0.009

Creep rupture plot showing the performance of FB2 parent material and weldments, relative to grade 92

Material Element content in wt.pct

sitions of the base material FB2 and the filler metal are given in Table 5.

Creep Characteristics of Engineering Materials

(Figure 33).

cations (Figure 35).

(Figure 37).

Table 5.

Figure 33.

86

(9Cr-2W) seamless pipe [12].

Chemical compositions of FB2 and weld metal.

conventional creep tests (CCT), which were carried out in air at temperatures ranging from 550 to 650°C at stresses from 70 to 220 MPa. Samples from the same weldment were subjected to ACT in order to simulate structure transformations after long-term CCT. ACT was executed at VSB-Technical University Ostrava (Czech Republic) on an HDS-20 physical simulator and partly in the laboratory of the simulator manufacturer (DSI). ACT temperatures ranged from 575 to 650°C

> High-tempered lath martensite microstructure of FB2 steel in as-delivered, i.e., forged and stress-relieved (AD), state.

#### Figure 35.

Criss-crossing dislocation arrays in high-tempered lath martensite in FB2-AD steel with carbides along former lath border.

Figure 36. FB2-BM microstructure in as-delivered state, before ACT, LM.

till 85–95% of the expected time to fracture. Therefore, in the ACT program, a force limiting stop was placed in the tensile relaxation portion of the testing cycle. Typical force-time graph of ACT on plain FB2 steel (sample FB22) is given in Figure 38, showing how the initial tensile relaxation force of 2200 kgf decreases to 1400 kgf at 90% of the test duration.

In the initial, as-welded microstructure of HAZ, in its under-critical portion at A1 temperature, the lath martensite recrystallizes, while near to colonies of carbides, first traces of austenite form. Some ferrites get the shape of large laths (Figure 39), while in other ferrite grains next to austenite with carbides, very high

Figure 37. FB2 weld's HAZ microstructure in as-delivered state, before ACT, LM.

Figure 38. Typical force-time graph from ACT on FB2 steel.

coarse-grained, which transforms to acicular martensite on cooling. To take the TEM thin foil specimens from the HAZs on ACT samples, from the first tested cross-weld sample, the time to fracture was learned and the force stop equivalent to 90% of this was placed in the program. Applying ACT to the as-welded HAZ microstructures forces their high tempering with very intensive precipitation of carbides. The ferrite in the intercritical portion of HAZ, which contains very few carbides, recrystallizes generating equiaxed grains, while along this ferrite, dense bands with agglomerates of carbides are formed (Figure 41). Some largest precipitates in these bands look like undissolved carbides from the initial HAZ

High dislocation density between nondissolved carbides in as-welded FB2 HAZ microstructure of FB2 in

IC-HAZ in FB2 with recrystallized ferrite and precipitates, after ACT at 625°C.

Agglomerates of precipitates in FB2 weld IC-HAZ after ACT at 625°C.

Figure 40.

Figure 41.

Figure 42.

89

as-delivered state, before ACT,TEM 15,000.

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

dislocation density is generated (Figure 40). In intercritical portion of HAZ, the austenite forms more compact islands, while some of the carbides still remain undissolved. Finally, above the A3 temperature, austenite becomes more finegrained homogeneous and close to the fusion surface with the weld metal

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

#### Figure 40.

High dislocation density between nondissolved carbides in as-welded FB2 HAZ microstructure of FB2 in as-delivered state, before ACT,TEM 15,000.

Figure 41. IC-HAZ in FB2 with recrystallized ferrite and precipitates, after ACT at 625°C.

Figure 42. Agglomerates of precipitates in FB2 weld IC-HAZ after ACT at 625°C.

coarse-grained, which transforms to acicular martensite on cooling. To take the TEM thin foil specimens from the HAZs on ACT samples, from the first tested cross-weld sample, the time to fracture was learned and the force stop equivalent to 90% of this was placed in the program. Applying ACT to the as-welded HAZ microstructures forces their high tempering with very intensive precipitation of carbides. The ferrite in the intercritical portion of HAZ, which contains very few carbides, recrystallizes generating equiaxed grains, while along this ferrite, dense bands with agglomerates of carbides are formed (Figure 41). Some largest precipitates in these bands look like undissolved carbides from the initial HAZ

dislocation density is generated (Figure 40). In intercritical portion of HAZ, the austenite forms more compact islands, while some of the carbides still remain undissolved. Finally, above the A3 temperature, austenite becomes more finegrained homogeneous and close to the fusion surface with the weld metal

Large ferrite lath in as-welded FB2 HAZ at location near to A1 temperature.

Figure 37.

Figure 38.

Figure 39.

88

Typical force-time graph from ACT on FB2 steel.

FB2 weld's HAZ microstructure in as-delivered state, before ACT, LM.

Creep Characteristics of Engineering Materials

exposure to real creep. The carbides and Laves phase precipitates, after ACT, did not reach sizes of these after real creep, but their amount and density appeared substantially larger. The more intensive nucleation of precipitates has been characteristic of electro-thermal treatment [14] and this way the direct resistance heating in Gleeble [1] plays a role in acceleration of the process. The larger density of precipitates, despite their smaller sizes, results in a similar or even more substantial depletion of matrix in alloying elements like in the long-term real creep. The strains applied in each cycle of the ACT procedure are too small to homogenously deform the tested material in any of the individual cycles to cause dynamic recovery or recrystallization. The dislocation configurations generated in each deformation cycle accumulate from cycle to cycle mainly in the preferentially oriented components of microstructure and after reaching critical density annihilate, gradually contributing to generation of voids and cracks, simulating the real creep in this way. Finally, on the crack surfaces after ACT, very large amounts of precipitates are present and also characteristic lines of dislocation escape appear. As to the chemical compositions of precipitates after ACT, in an earlier study on P91 steel [15], the microanalysis results from precipitates after ACT matched those of the crept steels

Up to now, the ACT procedure was applied in a study on development of P24 grade welding consumables [16] and new steels for supercritical components of power generation systems [17], as well as for determining remnant life of crept steels and repair welds [18]. Finally, in combination with weld thermal cycle simulation, it offered an opportunity of fast optimization for welding procedures to

The ACT procedure does not force the material to fail by overloading. By cyclic compressing and stretching of the tested material, ACT records response of the material while choosing the most prone sites in microstructure for nucleation of voids and cracks, thus well simulating the real creep. Programmed various amounts of compressive and tensile strains in combination with thermal cycles affect the intensity and duration of test as well as promote precipitation, making in intended manner the ACT plausible for study of creep-fatigue situations, importance of the last has been growing recently due to possible instability of grid by incorporating various energy sources. The conventional standard long-term creep tests cannot

1.Low-cycle thermal-mechanical fatigue of martensitic-ferritic creep resisting steels, applying direct electric resistance heating, accelerates transformation of microstructure and intensifies precipitation of secondary phases, causing depletion of matrix in alloying elements similar to that occurring during the

2.A substantial role in the accelerated microstructure transformation is played by meta-stable a<001> edge dislocations, which effectively transport

3.From the accelerated creep tests as well as from conventional case studies appears that the applied stresses and accumulated strains speed-up weakening

of the matrix and promote local failures in the most vulnerable sites of

interstitials, thus accelerating precipitation.

and complied with ThermoCalc equilibrium predictions.

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

address problems of weakening heat-affected zones of welds.

address this last issue.

8. Conclusions

real creep.

microstructure.

91

Figure 43. Carbides in FB2 HAZ at location above A3 temperature, ACT at 625°C.

Figure 44. Carbides in FB2 HAZ at location near to fusion line, ACT at 625°C.

microstructure, while numerous finer ones should be considered as of tempering type (Figure 42).

In the HAZ portion representing above A3 temperature, that is, of mainly austenitic as-welded microstructure, high density of partly spheroidized carbides appear and the largest precipitates seldom appear (Figure 43), while in the HAZ portion near to the fusion surface, the carbides in high-density colonies are partly elongated and aligned in the directions of former martensite laths (Figure 44); here, some larger precipitates do appear again.

ACT executed on the weld HAZ and interrupted before fracturing of sample allows taking proper specimens for TEM observations, which in turn show clearly how the separation of phases, ferrite and austenite, in the intercritical region produces during creep the microstructure vulnerable to premature failure.
