+ occurrence of relatively small (in the order of a few nanometers) copper precipitates of elongate 
shape in ferrite (fig. 4) as well as in post-bainite (pearlite) regions; qs Tu 
» copper particles show a characteristic contrast (coffee grains), which manifests their coherence NET 
with ferrite (fie. 5) Asa) 
strength ı 
(fig. 56) 
4 coagl 
grain 
4 decre 
bc 
Joss 
i 
Fig. 1: Carbides precipitates at grain boundaries. Fig. 2: Carbides network at ferrite grain 
Visible degradation of bainite regions. SEM 1500x. boundaries. SEM 1500x. 
Fig. 3: Numerous precipitates of copper particles Fig. 4: Dense network of dislocations with 
in ferrite. TEM 17 000x. numerous copper precipitates. TEM 50 000x. 
Mechanical properties of 18CuNMT steel after long term service. 
As a result of the occurrence of structure disintegration processes, changes of its morphology 
and ageing, the drum material is characterized by much lower formability, the measure of which are 
mechanical properties of the material, including mainly impact strength and the ratio Re/Rım. 
The obtained results of mechanical testing at a room temperature have shown that the material 
of the drum tested has a yield point at a temperature of 20°C (547 MPa), at a temperature of 350°C 
(412 MPa) and tensile strength (686 MPa) higher than minimum values. 
Since the a.m. parameters of an assessment of the material condition are in accordance with the 
standard specifications in force, the impact strength level should be defined. A characteristic feature 
of the material tested after its long-time operation is a distinct decrease of ductility expressed by Fig, 
impact strength of much lower values than the required minimum of 35 J/em®. By comparing the ss] ji 
obtained values with structural changes it is possible to explain the considerable decrease of impact Po 
strength by progressing structure degradation (1,3). Rye 
Fracture toughness of 18CuNMT steel after long-term service. At am 
The obtained results of fracture toughness tests have shown that the values of the stress vo 
intensity factor Kj. for the material of the drum shell after 180 000 hours of operation were slightly (045 
higher than those required (K;c=51.9 MPa'm'?) (1,3). ee 
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