Changes in the microstructure of selected structural alloy steel grades identified after their simulated exposure to fire temperature

Results of research on changes in microstructure of selected steel grades used in construction, observed in cooled state after prior exposure to fire temperature are presented in this paper. The following steel grades have been analyzed in detail: S355J2+N, X20Cr13, X6CrNiTi18-10 and X2CrNiMoN22-5-3. These grades are considered to be representative for classic low alloy structural steel of ferritic-pearlitic structure, and several special grades of stainless steel varying in microstructure: martensitic, austenitic and mixed, austenitic-ferritic of duplex type. Analysis of post-fire susceptibility to initiation and unconstrained propagation of brittle failure was the basic purpose of this research. This property seems to be the key one in the context of potential post-fire continued service of these steels when subjected to load. The presented results seem to correlate well with the results of toughness test conducted on the same steels (presented in [6]). The scenarios of sample heating and long term keeping up in the high temperature, and in particular the temperature levels assumed by the authors have been intentionally selected to enable or exclude the chance of expected structural changes occurring in the material subjected to experiment. The varied effective cooling modes have been designed to simulate the slow cooling in the open air or alternatively, rapid cooling during firefighting action conducted by fire brigade.