Controlled hardening of large objects in water-air cooling installations

Abstract

Hardening large massive shafts with complex configuration for welded rotors of turbines in nuclear and thermal power stations is a critical operation in the technological cycle of their heat treatment that creates the required complex of service properties. The devised regime of quenching has to ensure that the required strength, ductile properties and resistance to brittle failure over the entire section of the semiproduct are obtained together with a safe level of the arising quenching stresses. When in the process of hardening of rotor blanks traditional methods of cooling are applied with the use of water and oil tanks, considerable temperature gradient results, and the phase transformations along the object proceed nonuniformly. This leads to a high level of quenching stresses and to crack nucleation, a cause of the expensive products for important purposes having to be scrapped. A promising trend is the use of water--air mixtures ensuring the possibility of controlled cooling of massive objects [1]. The object of the present work is to devise a technology of controlled quenching of large objects on the example of shafts of welded rotors of high-speed turbines with a view to the design features of the cooling installation put into operation at the Kramatorsk plant "Energomashspetsstal ~' [2]. The installation is intended for the controlled quenching of rotor blanks weighing up to 20 tons, up to 1600 mm diameter and up to 4200 mm long, the coolants being water spray, water--air mixture, and fan-blown air. In regard to the technological possibilities the installation is unique among Soviet equipment for the heat treatment of large parts. A diagram of the shaft of the welded rotor is shown in Fig. 1. To choose the conditions of cooling the shafts, we investigated the effect of the rate of cooling from the quenching temperature on the phase transformations of supercooled austenite, on the mechanical properties and the resistance to brittle failure of steel 25Kh2NMFA that is used for making elements of welded rotors, and we also calculated the temperature fields in the process of hardening the shafts. The results of the investigations of the kinetics of the decomposition of supercooled austenite on a dilatometer "Formastor" show that in a wide range of cooling rates (8000-250"C/h) the decomposition of austenite is accompanied by the formation of structures of bainitic type, and only at cooling rates below 250°C/h do we find products of decomposition of austenite in the region of pearlitic transformation. We investigated the effect of the cooling rate on the mechanical properties on a laboratory installation designed at the Central Research Institute of Engineering which made it possible to heat and cool blanks 18 x 18 x 180 mm in size according to a specified program. The range of reproduced heating rates of blanks in the installation is 101000PC/h, and of cooling rates 10--3600"C/h. A detailed description of the design of the installation is presented in [3]. Heat treatment of blanks on the modeling installation included heating to 880°C, holding for 5 h, cooling at the specified rate, and tempering at 640°C for 25 h. Cooling from the quenching temperature was carried out at the rates 900, 700, 500, 300, 200, and 100*C/h. The chosen rates included the possible cooling rates of the central zones of shafts in hardening under industrial conditions. From the heat-treated blanks we made specimens for mechanical tests and for determining the critical brittle point (Tso).