Prediction of the temperature dependence of fracture toughness of 12Cr–2Ni–Mo refractory steel

Abstract

We study the influence of temperature and the size of the specimens on the characteristics of static crack resistance of 12Cr-2Ni-Mo refractory steel. It is shown that, in the temperature range 20-450°C, the increase in the thickness of specimens leads to an insignificant increase in fracture toughness obtained along a 5% secant line according to the standards of evaluation of the characteristics of crack resistance. The evaluation of the characteristics of crack resistance of 12Cr-2Ni-Mo steel with regard for the scale effect according to an earlier developed numericalexperimental model reveals the existence of satisfactory agreement with the experimental data in the entire investigated temperature range.Keywords: fracture toughness, refractory steel, stress intensity factor, scale effect.Statement of the Problem. Disks are among the most heavily loaded structural elements of gas-turbine plants (GTP). Their fracture is not localized in the body of the product and may lead to catastrophic failures. The cross-sectional sizes of the disk change in passing from the hub to the rim. Moreover, the rim part contains a large number of stress concentrators in the form of herringbone grooves, holes for air cooling, places for mounting balance weights, various sites of milling, etc., which play the role of the sources of crack initiation in the process of operation. To prevent sudden failures of the disks, it is proposed to use the concept of operation of GTP according to their current operating state. To make the proper choice of the amount and frequency of scheduled maintenance, it is necessary to perform the experimental investigation of the characteristics of crack resistance of the material of the disks for different loading modes with regard for the operating (temperature) conditions and structural features of the product (sizes and shape).The analysis of the available literature sources shows that the shape and sizes of the body may significantly affect the characteristics of crack resistance. This is why the prediction of the influence of the scale effect on the values of the static and cyclic fracture toughness and the justification of the possibility of application of the critical values of the characteristics of crack resistance obtained for laboratory specimens to the evaluation of the fracture toughness of actual structural elements (disks) are important and urgent problems of fracture mechanics.The aim of the present work is to perform the experimental investigation of the temperature dependence of the static fracture toughness of 12Cr-2Ni-Mo refractory disk steel in the operating temperature range 20-450°C for compact specimens 25 (CT-1) and 10 mm (CT-0.5) in thickness. In addition, on the basis of the results of this investigation, we check an earlier developed procedure [1] for the prediction of fracture toughness of large-scale specimens according to the results of testing of specimens with small cracks.Experimental Results and Their Analysis. The fracture toughness of compact specimens 25 and 10 mm in thi...