Hot cracks appear when thermal shrinkage together with deformation caused by restraint cannot be accommodated by plastic deformation. This happens during welding to such alloys, which segregate on heating and cooling at near-solidus temperatures, in particular when low-melting and mechanically weak phases form and occur over a wide range of temperatures. To check for susceptibility to the liquation cracking caused by the low-melting, weak phases, hot tensile testing can be used in combination with a thermal cycle resembling that of real welding. This procedure, which can be executed on a Gleeble (TM) thermal-mechanical simulator, comprises tensile testing of a number of cylindrical samples at the temperatures below solidus and determining their hot strength and ductility. For measuring of the brittle temperature range (BTR), the nil strength temperature (NST) is determined and used as the peak point, down from which the ductility recovery temperature (DRT) is searched for. The ductility is measured after the tensile test as a reduction in area at fracture. An alternative to the hot tensile test during the simulated welding cycle is the strain-induced crack opening (SICO) test, in which a rod-like sample mounted in "cold" copper jaws of the Gleeble is heated by electric current and then compressed till formation of a bulge in its uniformly heated central portion and appearance of cracks due to secondary tensile strain developed along the maximum perimeter of this bulge. Next to the studies of liquation cracking and ductility-dip cracking of the reheat-type, the Gleeble procedures can be also used to determine sensitivity to solidification cracking from these gradients, thermal-mechanical strains and strain accommodation phenomena which in turn affect strain hardening and recrystallization, phase transformations and/or precipitation processes. The combinations of different factors, and their importance, may vary substantially, and the correct physical simulation of welding must take into account a number of interacting phenomena appearing in the real application process, such as: 1. The balance between the heat input and electric current flow during heating, and controlled by thermal gradient heat flux, cooling rate and micro-deformation rate during the short thermal cycle of welding, in the case of heat-affected zone during arc welding. 2. The changes of hot ductility and of hot strength of welded material on heating and on cooling and its susceptibility to form liquid phase along grain boundaries at elevated temperatures below solidus, in the case of hot cracking. 3. The accommodation of strains occurring due to multiple welding thermal cycles and due to annealing microstructural restraints in multi-bead austenitic weld metals, in the case of micro-fissuring.
