Evolution of microstructure and acoustic damping during creep of a Cr–Mo–V ferritic steel

Abstract

The microstructural evolution of the Cr–Mo–V ferritic steel ASTM A193-B16, subjected to a tensile creep test at 923 K, was studied by monitoring the shear wave attenuation and velocity using electromagnetic acoustic resonance (EMAR). This study revealed an attenuation peak independent of the applied stress at around 30% of the creep life and a minimum value at 50%. This novel phenomenon is interpreted as resulting from microstructural changes, including strain hardening and dislocation recovery. This interpretation is supported by transmission electron microscopy observations of the dislocation structure. The relationship between attenuation change and microstructure evolution can be explained with the string model for dislocation vibration. EMAR is shown to have the potential to assess the progress of creep damage and predict the remaining creep life of various metals.

Introduction

Around the world there are many 30- to 40-year-old thermal power plants that have exceeded their design lives of 100,000 h. Structural metals used in plant components are subject to aging from a combination of fatigue, creep, and corrosion [1], [2], [3]. Exposure to elevated temperatures promotes creep. Aged metals lose toughness, or the ability to absorb energy at stress above the yield point, and cannot endure an occasional high load without fracturing. Creep is one of the most critical factors for determining the structural integrity of components. To save energy and to meet recent regulatory requirements for CO₂ emissions, as well as to improve thermal efficiency, steam pressures and operating temperatures in the components have been increased, resulting in accelerated material degradation. Furthermore, economic and environmental circumstances prohibiting the construction of new plants increase the severity of this problem. Non-destructive evaluation (NDE), for characterizing microstructures and damage initiation and growth, may become more widely used because of growing awareness of the benefit of using NDE techniques to assess the performance of components. In particular, NDE is becoming more significant for assessment of material degradation. An NDE technique that enables evaluation of the present state of materials and predicts their remaining life has long been sought [4], [5], [6], [7].

There are several candidates for creep-damage evaluation: magnetic techniques, surface replication (in situ optical metallography), small-angle neutron scattering (SANS), and ultrasonic techniques [3]. The magnetic techniques measure the change in the magnetic properties of a metal as they evolve due to creep progression [3], [4], [6], [7], [8]. This method effectively senses the damage, but it is limited to ferromagnetic materials. The replication method is capable of detecting the microstructural state in highly localized surface regions [3], [9], [10] and provides accurate information about microstructural changes. However, this method requires taking numerous microstructure samples from different locations and at different depths in order to obtain more reliable statistical information on the damage. In addition, it requires empirical judgment and careful analysis and is time-consuming and labor-intensive. As regards SANS, it has been well established that a beam of neutrons entering a material undergoes scattering by microstructural features. SANS is very sensitive to changes in second-phase precipitation and void formation [3], [11], [12], [13], but it does not evaluate grain coarsening and dislocation structure evolution and requires large-scale instruments. What is required, then, is a technique that provides simple and quick measurements of a large number of objects and gives accurate information about microstructural changes.

Ultrasonic methods have a unique potential for detecting internal damage relatively easily. Ultrasonic techniques have been extensively studied for estimating creep damage in a wide range of materials [4], [14], [15], [16], [17], [18], [19], [20]. In these applications, ultrasonic velocity, attenuation, and backscattering echoes of acoustic waves traveling through a material have been measured using contacting piezoelectric transducers. In a few cases, the velocity changes more than the attenuation. A sharp decrease in the velocity indicates that microcracks have formed in the material. In the early stages, however, a change in the velocity does not reflect damage progression, and the dislocations control the creep mechanism. The dislocation and microstructural change during creep causes attenuation changes. However, conventional contacting measurements cannot precisely reflect the creep damage because background energy loss occurs at the transducer–sample interfaces and obscures sensitivity to microstructural evolution [21]. This problem is especially serious for attenuation measurements because mechanical contacts allow the probing ultrasonic waves to propagate in the sample, as well as in the transducers, couplant, and buffer, where acoustic energy is absorbed during wave propagation. Therefore, non-contact measurements with electromagnetic acoustic transducers (EMATs) [22] are preferable, but low transduction efficiency prevents one from accurately measuring the ultrasonic characteristics. To overcome this dilemma, we combined an EMAT with the resonant technique so as to superimpose many signals coherently and compose large signal amplitudes. This is the electromagnetic acoustic resonance (EMAR) [22], [23], [24] method developed for contactless measurement of phase velocity and attenuation. There are only a few studies that have attempted to assess creep damage using this method [25], [26], [27].

The objective of this study was to quantify the ability of the EMAR method to identify and assess ultrasonic attenuation and velocity during creep and to establish the relationship between the microstructure and ultrasonic response. The Cr–Mo–V ferritic steel ASTM A193-B16 was used for the samples. It has the optimum fine-grained bainitic structure for service at elevated temperatures. Addition of 0.25% vanadium results in a stable dispersion of vanadium carbide (V₄C₃) [28]. This steel is commonly used for high-temperature bolts that are critical for maintaining the efficiency, integrity, and safety of industrial plants operating at high temperatures and pressures, particularly in steam turbine power generation equipment. In the present work, a bulk-wave EMAT [22], [23], [24] was used to measure the ultrasonic attenuation and velocity in the thickness direction of specimens as creep progressed.