One of the most common defects leading to the rejection of faulty castings made of duplex cast steel is hot cracking. Hot cracking - differently from cold cracking which occur on cooling, when the material exhibits already distinct elastic properties - proceeds in the semi-solid state, particularly when the temperature of solidifying casting is close to the equilibrium solidus point. There exist a range of temperatures in the vicinity of the solidus point within which the solidifying metal shows very small deformation ability and small strength. Therefore even little stresses occurring at the solidification stage and caused by density difference between ferrite and austenite, or restricted shrinkage, or various temperature gradients, can be the reason of hot cracking. Copper addition, as it is used in cast steel grades implemented in Polish power industry applications, decreases the temperature of the end of solidification, thus affecting beneficially the surface reproduction quality, but on the other hand it can promote, along with several other elements contained in duplex cast steel, the peritectic transformation during the last stage of solidification. The presence of peritectic solidification, i.e. strictly speaking the structural stresses accompanying that process, is the well-known and significant reason of the increased steel defectiveness in the continuous steel casting (CSC) technology if the steel with carbon content promoting the peritectic transformation is processed. The authors have presented the examination results of the solidification phenomenon for two duplex cast steel grades - one without copper addition, and the other with addition of about 3.0% Cu.