The mechanisms of hot cracking of as-cast Fe-Ni-Cr alloys doped with controlled amount of different impurities are discussed. The respective roles of sulfur, phosphorus, boron and nitrogen are particularly studied. The hot deformation behavior of the different alloys is characterized using classical hot tensile tests. The chemistry of the broken interfaces is studied using in-situ hot tensile test in an Auger electron spectroscope (AES), moreover the precipitates present on this interface are studied using transmission electron microscopy (TEM) of replicas of the in-situ broken samples. The combinations of the two techniques allow us to describe the actual chemistry of hot cracked grain boundaries. In the temperature range corresponding to the minimum of ductility, small boron nitrides were found on the broken interfaces, these nitrides were associated with a very high segregation of sulfur and phosphorus. Moreover, the TEM showed that the nitrides were located at the center of small cavities. The mechanisms of crack initiation and propagation are then discussed taking into account the respective roles of the different elements of the alloys.