Testing of Post-Fire Susceptibility to Brittle Cracking for X6CrNiTi18-10 Stainless Steel with Austenitic Structure

The results of experimental research on the post-fire brittleness exhibited by the X6CrNiTi18-10 stainless, acid-resistant, chromium–nickel steel of austenitic structure are presented and discussed in detail. Formal reasoning is based on the results obtained in instrumented Charpy impact test. The samples have been cooled down after initial heat treatment simulating a'priori fire action. Conditions of the test conformed to the steady-state heating regime. For comparison two fire scenarios have been considered, namely a fire of short duration with one-hour long heating period and a fire of long duration with heating period extended to ten hours. The samples have been heated in a muffle furnace with constant speed of 100⁰C/min until reaching one of two intentionally selected levels of temperature, i.e. 600⁰C or 800⁰C. First of these two levels, by assumption, was too low while the second was sufficiently high in order to induce in the samples examined a’posteriori permanent changes in the microstructure, preserved in the tested material after cooling. Two qualitatively different cooling modes of the samples have been applied. In the first mode the samples have been cooled slowly in the muffle furnace to simulate self-extinguishing of the fire, while in the second mode the samples have been cooled rapidly, in water mist, to simulate the action of a fire brigade. The impact tests of cooled samples have been conducted at +20⁰C to model the expected behaviour of the tested steel during summer, and, for comparison, at –20⁰C to check the properties of the same steel grade when subjected to winter conditions.