Dislocation structure and impact toughness of reactor pressure vessel steel 15Kh2NMFA upon ultrasonic treatment

Abstract

A. V. Mats, V. M. Netesov, UDC 621.789 and V. I. Sokolenko 1The paper addresses the influence of high-temperature ultrasonic treatment on the structure and impact toughness of steel 15Kh2NMFA. The observed increase of impact toughness and decrease of ductile-brittle transition temperature for the steel upon ultrasonic treatment of various durations are due to the formation of a uniform defect structure. Introduction.A study of the influence of ultrasound treatment on metals and alloys, which causes no form changes, but affects their strength characteristics is of considerable interest, insofar as material structure in this case differ from that generated through intensive plastic strains or during static deformation [1]. Such investigations are lucrative for getting deeper insight into physical processes of structural self-organization under external actions. Furthermore, their findings may facilitate the industrial application of ultrasound for treating the steels which are used, for example, in the nuclear reactor industry.Considering that dynamic bending impact tests make it possible to reveal a material susceptibility to brittle fracture, the objective of the present work was to clarify interrelation between the impact toughness behavior and the structural state of the reactor pressure vessel steel 15Kh2NMFA subjected to ultrasonic treatment (UST).Experimental. The investigation was carried out on a reactor pressure vessel steel 15Kh2NMFA which is of ferrite-pearlite type. Billets for test specimens were cut from a segment of a mill forged piece upon the standard mechanical and thermal treatment as involved in the manufacture of a reactor pressure vessel. For impact tests to be performed using a pendulum-type testing machine in the temperature range from -100 to 100°C we have prepared Charpy V-notched bars of rectangular cross section (10 10 mm) and 55 mm long. The UST procedure and methods for determination of the range of ultrasonic stresses s us in the test specimens are detailed in [2]. In this particular case, it was the specimen central portion which was subjected to UST (Fig. 1).With this end in view, the side surface of the specimen was tightly pressed, by a special captive nut, to the tip of the ultrasonic waveform concentrator. The UST frequency was 20 kHz. During the treatment the specimens were cooled with running water. Microhardness was measured using a Mod. PMT-3 tester. Structure investigations were performed by means of a Mod. ÉMV-100BR electron microscope. Foils for electron microscopic observations were cut from the Charpy specimens at a fixed distance from the V-notch, in the UST zone produced under various UST conditions. The density of dislocations and the angle of disorientation of neighbor microvolumes were determined by the procedures detailed in [3].Results and Discussion. Some researchers [4,5] have demonstrated that structural alterations in a crystal lattice under the action of ultrasonic vibrations at a fixed temperature depend on the nature of the initial structure and the range...

Impact toughness of a gradient hardened layer of Cr5Mo1V steel treated by laser shock peening

“…By reducing the mean grain size of a Mg-3Al-Zn alloy to less than 3 µm, Liao et al [7] reported that the impact toughness is greatly improved due to enhancement of the dynamic strength and dynamic plastic deformation ability. It was reported by Mats et al [9] that the increase of the impact toughness for the steel 15Kh2NMFA upon high-temperature ultrasonic treatment of various durations is due to the formation of a uniform defect structure. For the LSP processing technology, the mechanical properties of the hardened layer are not uniform [1,2,[10][11][12].…”

“…It was found by Yasnii et al [4] that the impact toughness of 15Kh13MF steel increases up to twofold after LSP with a power density ranging from 5 × 10 8 to 2 × 10 9 W · cm −2 . The mechanism of the increase of impact toughness is mainly from two aspects: residual stresses and dislocations in the hardened layer [1,[5][6][7][8][9]. Prefabricated residual compressive stress can improve the stress threshold of crack growth.…”