Today, the most adequate and most used method for studying hot ductility is the hot tensile test, where reduction in area at the final rupture is taken as a measure of the hot ductility.
Experiments in the range of 700 to 1100°C are achieved on a set of C-Mn steels and Nb-V microalloyed steels to establish influence of the temperature path on hot ductility. The results show some significant differences on the ductility curves between tests performed on in situ solidified specimens. It is found that the differences are mainly related to segregation distribution, austenite grain size, ferrite formation kinetics and dynamic precipitation of niobium/vanadium carbonitride.
A new type of high-temp. tensile testing equipment has been tensile testing machine manufacturers developed for testing the hot cracking propensity of steels. The equipment enables the test rod, fitted in a tensile testing machine, to be melted down and to solidify at a predetermined rate to simulate ingot solidification. With the equipment a comparison was accomplished, with respect to hot cracking propensity, between “as-cast” and “in situ solidified” specimens for a carbon steel with 0.5% C.
The tendency for surface-crack propagation was found to increase with Nb and soluble Al addition and to decrease with a rise in the P level. Hot ductility was also found to be affected in a similar manner by these elements. The results have been interpreted in terms of differences in the effectiveness of the NbCN precipitation in pinning the gamma grain boundaries during deformation.
Within the innovative hot forming process for sheet metals, called hot stamping, it is possible to combine forming and quenching in one process step. This affords the opportunity to manufacture components with complex geometric shapes, high strength and a minimum of springback which currently find applications as crash relevant components in the automotive industry.
A new tensile test method which permits the in situ measurement of the mechanical properties of a solidifying shell is described. These properties are of great significance in casting and welding processes. In the apparatus developed, the tensile stress is applied in a direction which is perpendicular to the growth axis of the columnar crystals so that, when alloys are tested, the mechanical behaviour of the mushy zone can be studied, as well as its contribution to the mechanical strength of the shell. This information cannot be obtained from conventional tests made using a Gleeble-type machine because, for example, control of the orientation of the solidification microstructure with respect to the tensile axis is too difficult. The present method was first calibrated by using pure aluminium, which had no associated mushy zone.
The extent to which the hot tensile test can be used to assess the relationship of steel composition to the incidence of transverse cracking in continuous casting has been examined. The influence of S, Ca, Ti, and Nb on the hot ductility of C-Mn-Al steels was investigated for the temperature range 700-1100°C and the results were compared with available commercial data on the effect of these elements on transverse cracking.