2. Physical simulation of creep

Considering creep as a plastic deformation appearing at elevated temperature, with small strains and very small strain rates [2], next to the short duration demanded from the ACT are the following conditions:


Then the simulative test must be in agreement with the following observations: The creep voids nucleate preferentially at grain boundaries of recrystallized grains or recovered subgrains, in which oriented slip is observed (Figure 1), while inside several creep voids, escape of slip lines can be seen (Figure 2). Such voids very seldom nucleate at any larger precipitates.

In the substructure of the crept steels, a/2<111> screw dislocations dominate, and these dislocations when not pinned to precipitates are highly mobile. In thin foil of thickness about 260 nm, relatively medium density of such dislocations could be seen (Figure 3), while in thicker foil of about 380 nm, their density is much higher, especially when interacting with precipitates and on intersections of two families of these screw dislocations, the a<001> edge dislocations could be identified (Figure 4). These last dislocations are mobile with Burgers vectors in {110} planes [3].

When the sample taken from a crept material containing internal "clean" creep voids not exposed to atmosphere is fractured below its ductile-to-brittle transition

temperature, the voids open up, and on their walls, a characteristic pattern of crisscrossing perpendicular lines can be seen (Figure 5) [4]. This evidence confirms the micromechanism of creep microcracks nucleation by piling up mobile dislocations in slip planes at grain boundaries [5] and, after identifying the dislocations and their glide planes, tells that these slip planes are two perpendicular of {110} type. The schematic two-dimensional drawing in Figure 6 is a model illustrating this

½ CrMoV steel crept for 20.5 years at 568°C; a/2<111> dislocations interacting with precipitates; TEM, thin

½ CrMoV steel crept for 22.5 years/16.6 MPa at 568°C; FeCl3 etched, SEM SE-image; slip lines visible inside

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

½ CrMoV steel crept for 20.5 years at 568°C; precipitates and a/2<111> dislocations in ferrite; TEM, thin foil

situation.

71

Figure 4.

foil 380 nm thick.

Figure 2.

creep void.

Figure 3.

260 nm thick.

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

#### Figure 2.

2. Physical simulation of creep

Creep Characteristics of Engineering Materials

real creep‑just a few pct in total.

cracks must not be different.

very seldom nucleate at any larger precipitates.

planes [3].

Figure 1.

70

Considering creep as a plastic deformation appearing at elevated temperature,

2.The final deformation at fracture, in particular crosswise strain, must be like at

3.The depletion of steel matrix in alloying elements must be achieved similar to that of crept steels, and the carbide phases or other precipitates at onset of

which thousands of grains or hundred of thousands of subgrains take part, the portions of samples for ACT that undergo transformation must have adequate

Then the simulative test must be in agreement with the following observations: The creep voids nucleate preferentially at grain boundaries of recrystallized grains or recovered subgrains, in which oriented slip is observed (Figure 1), while inside several creep voids, escape of slip lines can be seen (Figure 2). Such voids

In the substructure of the crept steels, a/2<111> screw dislocations dominate, and these dislocations when not pinned to precipitates are highly mobile. In thin foil of thickness about 260 nm, relatively medium density of such dislocations could be seen (Figure 3), while in thicker foil of about 380 nm, their density is much higher, especially when interacting with precipitates and on intersections of two families of these screw dislocations, the a<001> edge dislocations could be identified (Figure 4). These last dislocations are mobile with Burgers vectors in {110}

When the sample taken from a crept material containing internal "clean" creep voids not exposed to atmosphere is fractured below its ductile-to-brittle transition

½ CrMoV steel crept for 22.5 years/16.6 MPa at 568°C; FeCl3 etched for slip lines, SEM SE-image.

4.As creep in the matrix of ferritic-martensitic steels is the phenomenon in

"bulk" size, so the first creep voids and cracks in the test nucleate

predominantly in most vulnerable sites of microstructure.

with small strains and very small strain rates [2], next to the short duration

of nonequilibrium phases, as well as strain-induced precipitation.

1.The basic temperature and applied strains in the ACT must prevent odd transformations like secondary dissolution of carbides or intensive formation

demanded from the ACT are the following conditions:

½ CrMoV steel crept for 22.5 years/16.6 MPa at 568°C; FeCl3 etched, SEM SE-image; slip lines visible inside creep void.

#### Figure 3.

½ CrMoV steel crept for 20.5 years at 568°C; precipitates and a/2<111> dislocations in ferrite; TEM, thin foil 260 nm thick.

#### Figure 4.

½ CrMoV steel crept for 20.5 years at 568°C; a/2<111> dislocations interacting with precipitates; TEM, thin foil 380 nm thick.

temperature, the voids open up, and on their walls, a characteristic pattern of crisscrossing perpendicular lines can be seen (Figure 5) [4]. This evidence confirms the micromechanism of creep microcracks nucleation by piling up mobile dislocations in slip planes at grain boundaries [5] and, after identifying the dislocations and their glide planes, tells that these slip planes are two perpendicular of {110} type. The schematic two-dimensional drawing in Figure 6 is a model illustrating this situation.

#### Figure 5.

Creep crack surface with numerous criss-crossing slip lines, in ½ CrMoV steel exploited for 22.5 years at 568°C; SEM image, no etching.

3. The simulative accelerated creep test

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

etry shall be used.

Figure 9.

73

gauge length.

Figure 8.

TEM, thin foil.

Taking into account the mentioned phenomena associated with creep and implementing the factors speeding up microstructure transformation, the simulative accelerated creep test was designed. It relies on the response of tested material, and for intensifying transformation of microstructure, various programmed testing procedures are used. The ACT was implemented on Gleeble physical simulator using samples with central gauge portion of 12 mm length 10 mm diameter, mounted like in Figure 9. When the span between the "cold" mounting copper jaws is 32–35 mm, the uniformly-heated zone of about 10 mm length is created in the middle of the sample, due to direct resistance heating of the Gleeble balanced by controlled heat flow toward the "cold" Cu jaws. Sizes of the ACT samples are given in Figure 10. For testing base steel without weld, plain samples of the same geom-

Substructure of ferrite containing a<001> edge dislocations in P91 steel after service for 3 years at 568°C;

The ACT samples mounted in the Gleeble's "pocket jaws" assembly, like in Figure 9, are subjected to programmed cycles of low-cycle thermal-mechanical fatigue, run till failure or till predetermined stress or strain. Loading of the samples is executed by displacement (stroke) control and response of the sample is recorded

elastoplastic tensile and compressive strains applied to central portion of rod-like

Mounting of the ACT sample in "cold" Cu-jaws set of Gleeble, with TC percussion welded in the middle of

as force and calculated engineering stress. The ACT procedure uses small

samples (Figure 10) subjected to multiple thermal cycles at temperatures

#### Figure 6.

Schematic drawing of voids (a) and cracks nucleation by the voids coalescence (b) caused by dislocations piled up at grain boundary.

#### Figure 7.

Microcrack developed near HAZ in P91 steel after service for 3 years at 568°C; light microscope image, FeCl3 etched.

The a<001> edge dislocations in steel ferrite matrix are known from earlier work as effectively transporting interstitial elements, thus enhancing precipitation rate. During "in-situ" heating experiments in TEM on HSLA steel specimens containing preformed configurations of such dislocations, ultrafine precipitates in fractions of seconds appeared in sites of these dislocations escape and annihilation [6]. In prematurely failed P91 steel higher than average density of fine precipitates appears near to cracks (Figure 7). And the generation of substructures containing large amount of such a<001> edge dislocations was observed during deformation of dual-phase steels [7], and also they appeared in this prematurely failed P91 steel (Figure 8).

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

Figure 8.

Substructure of ferrite containing a<001> edge dislocations in P91 steel after service for 3 years at 568°C; TEM, thin foil.
