4. Results of investigations. HV, wa 
The X20CrMoV121 steel in an initial state has a tempered martensite structure (fig.1) with a — 240 have bee! 
HV hardness. After 70.000 hours of service with the required operation parameters, an insignificant carbides ( 
degradation of the tempered martensite takes place in the discussed steel (fig.2). This material is 
characterized by hardness at ~ 230 HV level which corresponds to the 1 structure class according 
to VGB-TW 507 (2,3). The extension of the operation time to about 100.000 hours does not evoke a 
any considerable changes in the material structure. The only change to be observed is the 
occurrence of voids, which have both a creep character and are connected with non-metallic 
inclusions, whereas the acicular structure of the tempered martensite is preserved (fig. 3 and 4). The 
hardness decreases to about 210 HV, which allows ascribing to the investigated material, the 2a- 
structure class. 
Fig. 1: Tempered martensite structure of Fig. 2: Tempered martensite after 70.000 
X20CrMoV121 steel in an initial state. The hours of service. The 1 structure class. 
0 structure class.LM 200x LM 400x 
Fie 
Tran 
(he 
The ime 
Fig. 3: Tempered martensite with creep Fig. 4: Acicular structure of tempered A 
voids. The 2a structure class. martensite with creep voids at grain a 
LM 400x boundaries. The 2a structure class. Ba 
SEM 3500x 
Long-term service in conditions insignificantly exceeding the permissible operation parameters for 
this grade of steel (this concerns mainly a too high temperature and pressure as well as a large em 
number of start-ups) have a considerable influence on the structural changes and the material N Kin 
hardness level. A considerable degradation of the material was observed which was manifested by a a 
significant number of creep voids and a decrease in the material hardness to the level of about 200 
238