3. The simulative accelerated creep test

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 geometry shall be used.

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 as force and calculated engineering stress. The ACT procedure uses small elastoplastic tensile and compressive strains applied to central portion of rod-like samples (Figure 10) subjected to multiple thermal cycles at temperatures

#### Figure 9.

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

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 pre-

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

Creep crack surface with numerous criss-crossing slip lines, in ½ CrMoV steel exploited for 22.5 years at 568°C;

Schematic drawing of voids (a) and cracks nucleation by the voids coalescence (b) caused by dislocations piled

maturely failed P91 steel (Figure 8).

Figure 5.

Figure 6.

Figure 7.

etched.

72

up at grain boundary.

SEM image, no etching.

Creep Characteristics of Engineering Materials

#### Figure 10.

Schematic drawing of cross-weld samples used for ACT, for testing all-weld-metal and for weld's HAZ.

appearing on the surface is often inclined to the longitudinal axis of the sample,

An internal crack formed in the sample perpendicular to loading axis (a) and size of transient zone between the

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

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-

tempered coarse martensitic microstructure (Figure 13), while in its as-welded and

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

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

PACT ¼ ð Þ� 7 þ log t T=100 (1)

test and its temperature can be included in the following parameter:

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

1.5Mo-1.3Co-V-Nb-N-B (FB2) steel, which in as-delivered state has a high-

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 micro-

following the HAZ (Figure 11b).

mounting portion and the gauge portion on the ACT sample (b).

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

4. Results of ACT

Figure 12.

creep.

structure.

75

characteristic of creep. In homogeneous samples, like plain steel bars or all weld metals, often before the crack appearance on the surface, internal cracks are formed perpendicular to the sample's axis. In microstructurally inhomogeneous samples, the cracks follow the "weakest links" like the HAZs of the welds. The ACT speeds up microstructural changes by accumulating elastoplastic tensile and compressive strains in the central portion of the sample during thermal cycling at temperatures characteristic of creep. The compressive strains result in a decrease of yield strength by generation of mobile dislocations, that is, the Bauschinger effect, and this accelerates creep [8]. Then, in the second part of the loading cycle, that is, changing the load to tension, a relaxation time under constant load is applied, during which the sample extends with strain rates equivalent to 10<sup>5</sup> to 10<sup>8</sup> per second, depending on its stiffness (pseudoelasticity modulus E\*) and yield strength.

The shape of the sample's gauge portion after the ACT interrupted before fracturing is given in Figure 11. When the heat flow is symmetric to both cold clamping jaws and the tested material is homogeneous, centrally located slight necking forms on the sample often coinciding with an internally nucleated crack (Figures 11a and 12a). Figure 12b shows on cross-section of this sample the size of transient zone (TZ) between the grip portion and the uniformly-heated central zone (UHZ). In the case of testing cross-weld sample with weld HAZ, the crack

#### Figure 11.

Shape of the cross-weld, all-weld-metal sample after the ACT terminated before entire fracturing (a), and of weld's HAZ with the crack following the HAZ's direction (b).

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

Figure 12.

characteristic of creep. In homogeneous samples, like plain steel bars or all weld metals, often before the crack appearance on the surface, internal cracks are formed perpendicular to the sample's axis. In microstructurally inhomogeneous samples, the cracks follow the "weakest links" like the HAZs of the welds. The ACT speeds up microstructural changes by accumulating elastoplastic tensile and compressive strains in the central portion of the sample during thermal cycling at temperatures characteristic of creep. The compressive strains result in a decrease of yield strength by generation of mobile dislocations, that is, the Bauschinger effect, and this accelerates creep [8]. Then, in the second part of the loading cycle, that is, changing the load to tension, a relaxation time under constant load is applied, during which the sample extends with strain rates equivalent to 10<sup>5</sup> to 10<sup>8</sup> per second, depending

Schematic drawing of cross-weld samples used for ACT, for testing all-weld-metal and for weld's HAZ.

The shape of the sample's gauge portion after the ACT interrupted before fracturing is given in Figure 11. When the heat flow is symmetric to both cold clamping jaws and the tested material is homogeneous, centrally located slight necking forms on the sample often coinciding with an internally nucleated crack (Figures 11a and 12a). Figure 12b shows on cross-section of this sample the size of transient zone (TZ) between the grip portion and the uniformly-heated central zone (UHZ). In the case of testing cross-weld sample with weld HAZ, the crack

Shape of the cross-weld, all-weld-metal sample after the ACT terminated before entire fracturing (a), and of

weld's HAZ with the crack following the HAZ's direction (b).

on its stiffness (pseudoelasticity modulus E\*) and yield strength.

Figure 10.

Creep Characteristics of Engineering Materials

Figure 11.

74

An internal crack formed in the sample perpendicular to loading axis (a) and size of transient zone between the mounting portion and the gauge portion on the ACT sample (b).

appearing on the surface is often inclined to the longitudinal axis of the sample, following the HAZ (Figure 11b).
