T. Revaux et al., HIGH-TEMPERATURE TENSION TESTS AFTER IN-S ITU REMELTING - APPLICATIONTO STEELS AND CAST IRONS, Revue de métallurgie, 93(12), 1996, pp. 1551-1562
The authors have developed a hot tensile test after ''in situ'' remelt
ing apt to characterize metallic alloys immediately after solidificati
on. The originality of this test lies in the utilization of a notched
conical crucible allowing to deport the shrinkage cavity out of the zo
ne of the notch and thus to allow accurate measurements of stresses an
d deformations. Two procedures have been implemented: tensile tests at
an imposed temperature and contraction tests at a controlled cooling
rate from the liquid phase. The two procedures have been used to study
the hot cracking of Nb-V microalloyed steels, austenitic stainless st
eels and phosphorus gray cast irons. Concerning microalloyed steels, s
ignificant ductility gaps between conventional tests and remelting tes
ts have been made obvious. The austenitic grain size is always far mor
e coarse for remelting structures where columnar grains, orientated pe
rpendicularly to the axis of the sample and localized at surface, are
clearly identifiable. intergranular cracks similar to transverse crack
s in continuous casting products have been observed and it has been sh
own that the detrimental effect of niobium is worsened by vanadium add
ition that deepens and displaces the ductility through when risen to h
igh temperatures. The cooling rate from solid state appeared as a fund
amental parameter : its decrease allows the precipitation of coarser t
herefore less harmful Nb(C, N) and V(C, N) particles. For austenitic s
tainless steels, differences between conventional and remelting tests
can be considered negligible. These steels do not present structural t
ransformations under test temperatures typically ranging from 600 to 1
200 degrees C so that the structure and the state of segregation are l
ittle modified during the reheating phase. The sulfur has proven to be
particularly harmful as for the hot ductility. Contraction tests have
appeared to be particularly adapted to the study of the contraction c
racks of gray cast iron. It has been shown that the addition of phosph
orous worsens the so-called ''cooling'' a cracking at temperature lowe
r than 950 degrees C. On the other hand, to temperatures higher than 9
50 degrees C, the liquid film of the phosphorous eutectic facilitates
the stress relaxation and limits the so-called ''solidification'' crac
king.