Dislocation damping and microstructural evolutions during creep of 2.25Cr-1Mo steels

Abstract

We studied the microstructural evolution of 2.25Cr-1Mo steels subjected to tensile creep at 923 K through monitoring of shear-wave attenuation and velocity, using electromagnetic acoustic resonance (EMAR). Contactless transduction based on the magnetostrictive mechanism is the key to establishing a monitor for microstructural change in the bulk of the metals with a high sensitivity. In the short interval, 50 to 60 pct of the creep life, attenuation experiences a peak, being independent of the applied stress. This novel phenomenon is interpreted in terms of the drastic change in dislocation mobility and rearrangement, which is supported by transmission electron microscopy (TEM) observations for dislocation structure. At this particular period, the dense dislocation structure starts to transform to subgrain boundaries, which temporally accompanies long, free dislocation, absorbing much ultrasonic energy to produce the attenuation peak. The EMAR has the potential to assess the damage advance and to predict the remaining creep life of metals.way of evaluating the internal damage state with comparatively simple and easy instrumentation, while many others, including X-ray, replication, hardness, and magnetic properties, merely inspect the surface or near-surface region of the materials.Ultrasonics have been studied for estimating the creep damage. [5][6][7][8][9][10][11] In these applications, the ultrasonic attenuation and velocity have been measured with a contacting piezoelectric transducer. In some cases, the velocity decreased with damage accumulation; in others, this tendency was not observed. The velocity was often more sensitive to the damage than the attenuation. The creep mechanism is dominated by dislocations, which also dissipate acoustic energy causing ultrasonic attenuation (or internal friction). However, the attenuation changes measured using the conventional transducer cannot exactly reflect the creep damage because the as-measured attenuation includes not only the attenuation of the sample, but also the damping through the transducer, the couplant and the buffer, the reflection and transmission losses at the interface, and the energy leakage into the transducer. Because the attenuation within the sample has to be considered in evaluating the creep damage, these extra losses are much greater than the attenuation within the sample [12]

Creep-Induced Microstructural Evolution and Acoustic Characterization in Tempered Martensitic Stainless Steel

“…[23][24][25][26][27][28] The results confirmed that an attenuation coefficient showed a peak at a specific time during the creep life and that the time does not depend on the stress and specimen shape.…”

“…As for 2.25 Cr-1Mo steel, Cr-Mo-V steel, Ni-based superalloy, and stainless steels, [23][24][25][26][27][28] the peak and minimum values of the attenuation and local minimum of the velocity were observed in the creep progression of quite different materials under the different test conditions. Shown in Table III is t/t r , where the attenuation shows the peak and the minimum in these materials.…”

“…[21,46] It has been reported that they cause only negligible change in the attenuation coefficient. [23][24][25][26][27][28] Therefore, only the dislocation damping can explain the observed acoustic response.…”

“…It was clear that the attenuation coefficient with EMAR clearly enabled the detection of the dislocation structure evolution in the fatigue progress of pure copper and low-carbon steel [49][50][51][52] and in the creep progress of a Cr-Mo-V, 2.25 pct Cr-1 pct Mo, Ni-based superalloy and austenite stainless steels. [23][24][25][26][27][28] One expects that the attenuation in a type-403 tempered martensitic steel changes because the dislocation structure changes during the creep. Figure 16 shows the results obtained by the digital image analysis of the TEM micrographs.…”

Laser-ultrasonic Monitoring of Austenite Recrystallization in C–Mn Steel

“…In a paramagnetic phase such as austenite, the ultrasonic velocity mainly depends on texture, 5,6) dislocation structure [16][17][18] and on scattering effects in general. Scattering is caused by acoustic inhomogeneities, like grain boundaries, porosity, cracks, etc.…”