Stainless steel and fire resistance

Will stainless steel provide a barrier to flames and if it does, how rapidly will the heat penetrate the barrier sufficiently to cause damage (usually a specific temperature rise) on the far side?

A satisfactory demonstration is supplied by reference BS 647 Part 22 tests carried out for a British Stainless Steel Association (BSSA) member, Stewart Fraser. The company manufactures 316 framed doors which include a cavity filled with non-combustible boards. The results are given at www.bssa.org.uk.

It showed slight discolouration and distortion on the flame impingement side with the sheltered side of the door reaching only 98°C after 60 minutes. The test was continued for another 80 minutes without the failure of flame containment or subsequent opening of the door in its frame. Similar testing was carried out on a 1.5mm thick 2304 duplex sheet fabricated into a simulated ship’s bulkhead with enclosed ceramic wool insulation. With a bright orange glow of an 1100°C metal temperature on the flame side, the “safe” side reached 30°C after 40 minutes and 110°C after 60 minutes. The test was terminated after 120 minutes with containment still satisfying IMO resolution A518 (XIII).

What are the effects (both during and after an event) to the mechanical properties of stainless steel? How do these compare with structural carbon steels?

There are tests as well as a theoretical basis which demonstrate that both austenitic and duplex stainless steels have superior high-temperature properties compared to carbon steel.

Table 1 shows the deflection and failure modes of three-metre long commercial electrical cable trays loaded to simulate actual loadings (Image 2). They were heated with 18 LPG burners to obtain an average temperature of 1000°C to 1050°C for at least five minutes (Nickel Institute publication No. 10042). The publication also considers the life cycle costs (LCC) of the use of aluminium, galvanised steel or stainless steel for stairways, handrails, gratings and firewalls, as well as cladding for corridors and accommodation modules on North Sea platforms. Fire risk controls are obviously a major concern, although corrosion resistance is also critical. On an LCC basis, stainless steel was most economical especially when its reduced requirement for maintenance periods were included.

In addition to the above testing in cable tray applications, substantial research and application work has since been carried out and codified. Installations include 2205 duplex hangers suspending the slab which forms the floor of the emergency ventilation duct in the CLEM7 tunnel in Brisbane [ISSF], see Image 3.

In short term fires such as on balconies or stairways, the temperature rise exposed to an ISO 834 fire temperature profile depends on thickness and emissivity. Polished stainless steels typically have low emissivity of <0.1 and hence a slower temperature rise.

Conservatively, after 30 minutes a 12mm sheet of stainless steel with 0.2 emissivity would reach 620C whereas steel (with no rust) and 0.4 emissivity would reach 750°C.

When considering strength and deflection, the metal temperatures in a conventional fire do not reach levels to anneal the material so any cold work strengthening will raise the temperature for a 50% strength reduction. In addition, as shown in the graph, the reduction in Young’s Modulus, i.e. deflection from a specific load, is less than that of carbon steel for temperatures above ~200°C. By 600°C the modulus retention for stainless steel is 0.75 compared to 0.3 for carbon steel, i.e. less than half the deflection for a given load.

In summary, stainless steel has substantial advantages in structural use when fire risk is considered, and these advantages continue into higher strength and lower deflections at elevated temperatures.