Austenitic stainless steel grades have excellent strength properties in temperatures between 500°C to 1000 °C and they retain a higher proportion of their room temperature strength than carbon steel above temperatures of about 550°C. For the carbon steels, there is a large drop in strength in the temperature range 500-700°C and this is the obvious reason that constructions from carbon steel require both passive and active extra fire protection measures. In temperatures above 800 °C austenitic stainless steels have the highest strength.
In a fire design case study made by H. L. Groth, B. Ivarsson, and E. Schedin the strength properties of three stainless steel grades (Duplex EN 1.4462, Austenitic EN 1.4301 and Ferritic 1.4003) and carbon steel grade (S235JRG) were tested and compared as compressed columns in high temperatures to generate data for further fire design choices. The results were presented as a comparison of a critical time in standard fire.
Duplex stainless steel is a superior choice
A very interesting result from the study was that the duplex stainless steel 1.4462 was much stronger than both the carbon steels and other tested stainless steel grades with five times longer fire exposure time than carbon steel S235JRG. In this respect, duplex stainless steel can be a superior choice for structures that need to resist fire in time spans up to 30 minutes. The second-best grade was austenitic 1.4301 and the third-best was ferritic grade 1.4003. All tested 1.4301 and 1.4003 columns resisted fire for at least 15 minutes. The carbon steel columns lasted only 10 minutes.
To conclude, according to the study the results in three main aspects that can be applied to structures that are made without additional fire protection are:
- The strength of duplex steels can be utilized in a fire situation.
- Duplex and austenitic stainless steels are superior to and ferritic grade 1.4003 is much better than carbon steels.
- Stainless steels can be promoted for fire-resistant, load-bearing structures that will be used without coating.
AUSTENITIC AND DUPLEX STAINLESS STEELS IN FIRE, H. L. Groth, B. Ivarsson and E. Schedin, Outokumpu Stainless AB, Avesta Research Centre, Avesta, Sweden (2012)
DESIGN MANUAL FOR STRUCTURAL STAINLESS STEEL 4TH EDITION, SCI Publication (2017)