Degradation of microstructure and properties of X20CrMoV121 steel after long- 
term service 
A. Hernas *, J. Dobrzanski **, A. Kielbus *, 
*Silesian University of Technology, Katowice, Poland 
** Institute for Ferrous Mettalurgy, Gliwice, Poland 
I. Abstract. 
The paper presents some results of X20CrMoV121 steel investigation of microstructure and 
properties after long-term service of power plant superheaters components at creep conditions. 
Majority of superheaters in 360 and 500 MW boilers is made from X20CrMoV121 steel. They have 
operated at about 550°C temperature with 10+18 MPa pressure. After long-term service the 
martensite degradation processes, M»3Cs carbides precipitates at grain boundaries, creating a 
characteristic carbide network in material of high degradation generating of cavities and decrease of 
mechanical properties are observed. 
il attr long-tem 
After long-term 2. Introduction. 
puns degradation A long-term service of power plant components in the conditions of an elevated temperature 
cubes at gan and stress brings about changes in the structure of the material and a decrease of utility properties. 
ess eee. The changes of properties which limit durability are mainly caused by repeatable start-ups of a 
ale fhe ested boiler after its scheduled and emergency shut down . If the start-up proceeds incorrectly, the 
nse permissible conditions are exceeded, which accelerates the process of the material wearing or leads 
wn This tay to a breakdown. 
oeriments have In spite of the fact that the pressure systems operating in power industry and heat engineering are 
+ he cases when designed for an exactly specified operation time, in practice, there are many systems, which safely 
higher than the operate even when their calculated durability has been considerably exceeded. The calculated 
durability of 100.000 hours assumed in designing is regarded as a safe operating time. However, 
a 130 thousand there are some examples of a damage of elements brought about by a loss of the utility properties. 
ofthe materi This led to the withdrawal of these elements from service before the end of the operating operation 
yu by the time of time (1). 
In the 70’s in Germany martensite X20CrMoV121 steel was developed which was used for 
of inna nd superheaters in Belchatow and Kozienice power stations. Currently, the operation time of these 
power system elements (including superheater’s coils manufactured from the X20CrMoV121 steel, 
reaches or has exceeded 100.000 hours. Therefore it becomes important to answer the following 
question: what is the technical condition of the investigated components and how much longer can 
ervice” Dolor they be safely used without one’s own experience in this field? 
gee service 
il 3. Material for investigations. 
sol gis The material for the investigations was sections of superheaters coils after long-term service. 
vd y i The sections were made from X20CrMoV 121 (DIN 17175) steel. The superheaters operated at T = 
6 m a 540+570°C temperature and with P = 16+19 MPa pressure. The total operation time was in the 
N SM. Rabka, range of 70+120.000 hours. 
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