Al) wi a HV, which classifies this steel to 2b-structure class. (fig. 5) In case when the operation parameters 
(is aif have been significantly exceeded, a complete decomposition of tempered martensite into ferrite and 
This nar di carbides of M23Ce6 type with hardness at the 190 HV level is observed. This allows classifying this 
8 . . «qe zz . 
: dass egy tng structure to 3a class. (fig. 6 and 7) Even longer service of this material in extreme conditions brings 
S does not le about additional occurrence of micro-cracks in the material (3b structure class) (fig. 8), decrease in 
observed is the hardness to about 170 HV which consequently leads to failure of the superheater coil. REN 
it —_— . i | Lk 7 Ze 
ie Sad) Te CE a ! 
Material, the 22. Cy a ' i ' " Cw 
wae 
ERASE da 
FESS RAN A eo 
SRR Ce wl REX 
Fig. 5: Tempered martensite with numerous Fig. 6: Ferritic structure with carbides and 
creep voids. The 2b structure class. creep voids. The 3a structure class. 
LM 200x LM 250% 
ite ater Ul 
ure Class. 
4x 
Fig. 7: Ferritic structure from releasing Fig. 8: Ferritic structure with carbides. 
carbides at the grain boundaries. Visible micro—cracks. The 3b structure 
The 3a structure class. SEM. 1000x class. SEM 700x 
The investigations on the mechanical properties carried out at temperature of 550°C revealed that 
regardless of the X20CrMoV121steel structure class, the yield point is higher than the minimum 
ge af tempered value required by the DIN 17175 250 MPa standard. In case of a material of the highest degradation 
ids at grain degree (3b structure class), the yield point at temperature of 550°C equaled 250 MPa. 
Joss. 
3500x 
5. Summary 
(on caramels OF The investigations carried out allowed to assess the structural changes, which took place in 
5 well 252 large the structure, and properties of superheaters pipes made from X20CrMoV121 steel after long-term 
and ie material service. 
ns manifested by 2 
al of about 200 
230