42801 elope 
we In the case when the material ductility is unsatisfactory (low impact strength, values of the 
Bein pomp stress intensity factor Kj. close to a minimum limit) and the other functional properties are 
© contained within required limits, restorative heat treatment can be applied. 
As a result of the application of regenerative heat treatment carried out for the purpose of impact 
strength improvement, some processes occurred which generally can be called material over ageing 
(fig. 5, 6) (1): 
$ coagulation of M3C and M-;Cz3 carbides occurring in ferrite and postbainite grain as well as at 
grain boundaries; 
% decrease of the density pile-up of dislocations on copper particles; 
occurrence of recovery processes; 
loss of coherence of copper particles with matrix; 
grain boundaries migration. 
rite gram 
Fig.5: Coagulated carbides at grain Fig.6: Rearrangement of dislocation 
boundaries. SEM 1000x. configurations. Recovery effects. 
TEM 100 000x. 
„3000$ Wit 5 | _ 1 KCV ” After restorative heat treatment a 
TEM 0 00x 0s J Ke w considerable increase of impact strength 
i, of the drum shell material took place 
J > 6 (fig.7). An increase of ductile fracture 
hich ae £5 participation up to 40% was found. 
RR 5 a9 — Ki min Restorative treatment also considerably 
) Role il 2 10 RCV min improves crack resistance by increasing 
hae i E45 the stress intensity factor up to 97 
erature 9f 22 ol MPa-m'? (fig. 7)(1,5). 
ihe 0.1 _ On the basis of impact strength tests 
eran ® 180 000 hours Regenerative of the material after operation and after 
erste ELE . of service heat reatmen’ regenerative heat treatment conducted at 
ii expressed by Fig. 7: Influence of heat treatment on 18CuNMT different temperat i NDT 
' comparing te steel impact strength and crack resistance after 180 ) ture FA Tr a ee ; € q 
crease of impact thousand hours of service and after regenerative heat *CMPCTALUFG so was determined. 
treatment. 
At ambient temperature, the material after operation was characterised by brittle fracture and 
de SES transition temperature of ~75°C. After restorative heat treatment a shift of the temperature FATTso 
© aly to ~45°C was found. At ambient temperature, the material revealed mixed transcrystalline fracture 
won ef SEE with numerous ductile regions (fig. 8) (1). 
735