HIGH-TEMPERATURE TENSION TESTS AFTER IN-S ITU REMELTING - APPLICATIONTO STEELS AND CAST IRONS

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.