212 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII Assessment of pearlite degradation in power industry cast steel after long-term exploitation Justyna Kasińska 1* , Piotr Matusiewicz 2 , Andrzej Czarski 2 , Leopold Barwicki 3 1 Kielce University of Technology, Faculty of Mechatronics and Mechanical Engineering, Department of Applied Computer Science and Armament Engineering, Kielce, Poland, 2 AGH University of Science and Technology, Faculty of Metals Engineering and Industrial Computer Science, Department of Physical and Powder Metallurgy, Kraków, Poland, 3 ENREM–POŁANIEC Sp. z o.o., Połaniec, Poland, * kasinska@tu.kielce.pl The paper presents the results of pearlite degradation analysis. The degradation level was evaluated on three specimens (cuttings) of low alloyed 20HM cast steel, sampled from various locations on the main body of the WP turbine exposed to creep (steam temperature 480°C, pressure 12.7 MPa). The mechani- cal properties (hardness, impact toughness) were performed. Microscopic observations were performed on nital-etched sections in the scanning electron microscope JSM 7100F. Microstructure analysis involved the characterization of microstructure morphology and quantitative metallography for describing pearlite degradation after a long-term exposure to creep. The measurements were performed using MetIlo image analysis program. There were measured the volume fraction of the regions with lamellar morphology, V V L and the pearlite degradation ratio, L ratio . In addition the microstructure class were determined on the basis of a qualitative assessment of changes in the morphology of pearlite regions. Key words: 25CrMo4 cast steel, pearlite degradation, quantitative metallography. Inżynieria Materiałowa 5 (219) (2017) 212÷216 DOI 10.15199/28.2017.5.2 © Copyright SIGMA-NOT MATERIALS ENGINEERING 1. INTRODUCTION Changes of the metals and alloys microstructure under heat activa- tion are caused by the tendency to reduce the free energy. McLean [1] defines this behaviour as internal instability of the system. The result of this instability are processes such as grain growth, sphe- roidization, particle coarsening, coalescence, discontinuous thick- ening etc. Knowledge of these processes from the point of view of their mechanisms and kinetics is very important in cognitive terms, but primarily in utilitarian. This is the basis for analysis of the mor- phological stability of microstructure. As regards the lamellar mi- crostructure such as perlite morphological destabilization is above all the result of the spheroidization and particle coarsening. The process of pearlite spheroidization consists in a change of the shape of the cementite plates into the shape which is approxi- mately spherical, with the preservation of the constant phase vol- ume, while is can be accompanied by diffusion growth [2]. The first stage of the process is fragmentation, i.e. division of the plates into smaller ones. Next, gradual rounding of the plates is observed, until a semi-spherical shape is obtained. In the further stages, coarsen- ing of the spheroidized particles may occur. The complexity of the processes taking place during spheroidization has its source in the substructure of ferrite and cementite, the geometrical characteristics of the phases (deviations from the plate morphology), the properties of the interface boundaries etc. Particle coarsening, in alloys with disperse phase, is the process of particles growth, consisting in a diffusive growth of larger par- ticles at the expense of dissolution of the smaller in the matrix, at a constant volume fraction of the disperse phase (e.g. cementite). Particle coarsening results in increasing of mean particle size, as well as changes in the particles size distribution [3]. Many power plants use ferritic-pearlitic steels formed after nor- malization for pressure vessel elements, such as superheated steam pipelines. After a long-term service of the components, it is neces- sary to evaluate whether they are still serviceable under the boiler or turbine working conditions. Metallographic examinations, includ- ing non-destructive replica techniques, are used to obtain images or copies of the element microstructure for the evaluation of its changes and degradation extent. This allows predictions related to the period of further failure-free use of this element [4]. A long-term exploitation of boilers and turbines under elevat- ed temperature and creep above the limiting temperature leads to changes in the microstructure of an element, resulting from the tran- sitions in the ferritic matrix and carbide precipitation processes. The changes in carbide morphology are induced by spheroidizing heat treatment followed by coagulation and coalescence. These changes together with the privileged precipitation of carbides at the grain boundaries are the major factors in lowering toughness, and creep damage such as voids and microcracks can lead to disrepair and failure. In steels and cast steels with the initial, pre-service, ferritic- pearlitic microstructure, early signs of cracking and separations of cementite lamellae in pearlite are observed after a relatively short service period under creep. Next stages involve precipitate coagu- lation, including cementite in pearlite, the growth in size of some carbide types and the formation of ferritic microstructure with the carbide phase of varied morphology. The sequence of changes in steel microstructure due to long-term service under creep is well known and described in the literature. For industrial purposes, the classification of structural change patterns was developed with im- ages of microstructure of typical steel types after various service periods and the corresponding mechanical properties [4÷6]. Also new technologies are underway to allow the control over the desired structure of ferritic-pearlitic steel, i.e. grain size decrease and the shape and dispersion of cementite precipitates [7÷10]. The methods of quantitative metallography used to describe the microstructure expand the scope of interpretation of the microstruc- ture images and facilitate the evaluation of microstructural degener- ation occurring under long-term service and creep [11]. Additional relationships between quantitative parameters of the microstructure and the basic toughness parameters can be obtained from the tool for more reliable assessment of materials in terms of service safety, utilitarian and especially useful for practical industrial applications. Attempts were made in the studies of X10CrMoVNb9–1 steel sup- plied to the energy sector [12]. The following stereological param- eters were evaluated in the quantitative analysis: relative phase