Deformation and fracture of high-temperature steels at different temperatures and loading rates

Abstract

S. V. Lenzion, UDC 539.4 and V. V. KharchenkoCharpy impact tests were conducted at different temperatures and loading rates. Temperature dependences of the crack initiation and propagation energies were determined for carbon steel 45, steels St. 3 and 15Kh2NMFA from the load-time curves obtained with due account of the impact test results. The effect of the loading rate on the temperature dependence of the impact toughness was analyzed within the range from 1 to 4.4 m/s. Keywords: high-temperature steels, Charpy specimen, instrumented impact testing machine, impact toughness, crack initiation and propagation energy, brittle-to-ductile transition temperature, crack propagation rate.Introduction. Brittle-to-ductile transition temperature (BDTT) is one of the major material characteristics when it comes to assessing strength and reliability of structures and constructions. It is determined using various test methods. Charpy impact testing is one of the simplest and most widespread approaches. In the course of such tests, the BDTT is evaluated by processing the dependences of the impact toughness on the test temperature [1-3]. Here, the fact that the impact toughness is an integral energy characteristic, which includes the specimen strain energy and the crack initiation and propagation energies, should be taken into account.In order to get more information about the material behavior, instrumented impact testing machines are used nowadays, which ensure recording of the force vs time curves during testing [1,[4][5][6][7]. The application of modern high-speed data acquisition systems extends appreciably the possibilities of processing the information obtained from the impact tests. This information allows analyzing different approaches to the determination of the BDTT and the energy spent at different stages of deformation and fracture of Charpy specimens in the course of impact tests [2].The objective of this work is to study the behavior of high-temperature steel 15Kh2NMFA versus St. 3 and 45 steels at different temperatures and loading rates using an instrumented vertical impact testing machine.Experimental Technique and Processing of Results. Impact tests were carried out using an instrumented vertical impact testing machine equipped with a multichannel system for high-speed recording of forces and strains and a system for specimen cooling and heating in the temperature range from -150 to 400°C [8]. Standard Charpy specimens of size 55 10 10´mm of St. 3, 45, and 15Kh2NMFA steels were the objects of the study [9,10]. The impact velocity V 0 varied from 1.0 to 5 m/s and the temperature from -135 to 315°C.From the force vs time curves, P t ( ), obtained with a sufficiently high resolution in both coordinates (Fig. 1), it is possible to determine the energy spent on the specimen fracture and to divide it into components, namely the energy prior to the instant of crack formation (initiation) and the energy of the ductile and/or brittle crack propagation [5]. Methods for determining the values of the above energy compone...