Stainless Steel Fire Performance & Radiant Heat Transfer

Fire resistance, containment, and prevention are important aspects of building and structure design. The ability of a loaded structural material to retain its strength when exposed to fire can provide additional valuable time for building evacuation or prevent collapse of a structure like a bridge.  Both bare structural sections and stainless steel concrete reinforcement have been studied.

The capacity of a metal to limit the spread of fire is not restricted to structural sections.  Barriers like fire doors, roofing, insulated equipment enclosures and other panel systems, chimney linings, and other metal components can help to reduce the spread of fire.  However, the metals used in architecture exhibit significant performance differences, even with only brief exposure to high temperatures.

Metal Basics

For large projects it is advisable to conduct fire-engineering assessments. Country specific code guidance should be used to determine fire protection requirements for limiting temperature increase during fires.

Copper and aluminum start to experience strength reductions at fairly low temperatures. Aluminum alloys begin to show a reduction in strength at temperatures above 100ºC (212ºF). ⁽¹⁾  At 204ºC (400ºF), the strength of copper has decreased about 25% and that of 6061-T6 aluminum has decreased by about 60%. ⁽²⁾

Steels are able to retain their strength up to higher temperatures.  The traditional method of ensuring sufficient fire resistance is to ensure that the temperature of carbon steel does not rise above 370ºC (700ºF) so that it retains all of its strength. ^{(3, 4)} By the time carbon steel reaches 500ºC (930ºF), it has lost about 30% of its strength. Unprotected weathering steel loses about half of its strength above 538ºC (1000ºF).⁽⁵⁾

Results from research to determine the structural performance of different alloys of stainless steel in fire have been included in both:

• Eurocode 3: Part 1.2 and
• AISC Steel Design Guide 27: Structural Stainless Steel

Austenitic stainless steels generally retain a higher proportion of their room temperature strength than carbon steel above temperatures of about 550ºC (1000ºF). All stainless steels retain a higher proportion of their stiffness than carbon steel over the entire temperature range. There is further information available in these additional resources:

• Online Information Centre of Stainless Steel in Construction
• Design Manual for Structural Stainless Steel
• Stainless Steel in Fire: Summary Final Report, Research Fund for Coal and Steel of the European Community
• Fire Resistance Rating and Testing of Stainless Steels, British Stainless Steel Association

Stainless Steel Fire Testing History

The first comprehensive fire testing on stainless steel was done by Darchem Engineering in the early 1990’s after a North Sea oil platform fire. The program included testing the fire and radiant heat resistance and heat conduction through fittings and walls of:

• galvanized carbon steel
• fiberglass-reinforced plastic (FRP)
• aluminum and
• molybdenum-containing Type 316 stainless steel.

FRP and aluminum failed the tests. Stainless steel surpassed the requirements of all of the tests and had less deflection than carbon steel. A copy of this report, Stainless Steel for Durability, Fire-Resistance, and Safety (10042), can be downloaded from the Nickel Institute website.

View a video clip of the fire resistance and fire radiation testing programs. Credit: Nickel Institute

In the Darchem Engineering 5-minute test for fire resistance, the metals and FRP (also called GRP) were exposed to direct flames at 1000-1050ºC (1832-1922ºF). The results are summarized in Table 1. Both aluminum and FRP failed the test. The galvanized mild steel passed the five-minute exposure requirement and during that period it reached 642ºC (1188ºF) with a deflection of 166.5 mm (6.6 inches). The Type 316 stainless steel cable ladder also passed the 5-minute test. The testing for the Type 316 stainless steel ladder was extended for 45 minutes and it reached 705ºC (1300ºF). At the end of the extended test, the deflection of the Type 316 stainless steel was only 80.5 mm (3.2 inches).

Testing was also done to simulate heating by radiation and the results are summarized in Table 2. The test was continued until the ladder temperature stabilized or structural failure occurred. Both aluminum and FRP failed the test. The galvanized steel ladder reached a stable temperature of 552ºC (1026ºF) in two hours. The stainless steel reached temperature stability at 556ºC (1033ºF) in three hours. The deflection of the stainless steel after three hours was about one-third that of the galvanized steel ladder after two hours.

Significant testing has subsequently been conducted to meet the requirements of structural design codes and guides. Stainless steel’s ability to retain stiffness better than carbon steel at elevated temperatures has been repeatedly demonstrated. Furthermore, molybdenum containing Type 316 stainless steel retains its strength better than Type 304 stainless steel.  Figure 1 shows the stiffness retention behavior of stainless and carbon steels at elevated temperatures and Figure 2 compares the strength retention. (See the Eurocode and AISC Design Guide for further information.)⁽⁷⁾

By 800ºC (1472ºF), carbon steel has a stiffness retention level of about 10%, while stainless steel retains approximately 60%. This may make it possible to eliminate stainless steel fire protection if the expected failure mode is governed by stiffness retention.

Figure 1: Comparison of stainless and carbon steel stiffness retention at elevated temperature

Figure 2: Comparison of stainless steel and carbon steel strength retention factors

Darchem Engineering also determined the length of time necessary for heat to be conducted through walls. The experiment was designed to measure how much heat is transferred through insulated cavity walls when structural sections extend through them. The results for stainless steel and aluminum cable ladders are shown in Table 3. The hot side was exposed to flame temperatures of 1,000 – 1,050° C (1832-1922ºF). The aluminum ladder collapsed due to melting after only 1:08 minutes but the test was continued on the remaining end pieces until the cold side stabilized after 37 minutes. The aluminum legs extending through the wall to the cold side were 134° and 152° C (273° and 306° F). There was no deformation of the stainless steel ladder during this test and, after 90 minutes, the cold side legs stabilized at 80° and 58° C (136° and 176° F). This result is not unexpected given the metals’ significant difference in thermal conductivity.

Stainless Steel Concrete Reinforcement Testing
The Steel Construction Institute conducted a tensile test program at temperatures of up to 1000° C (1832° F) on four types of austenitic and duplex stainless steel reinforcing bar produced in accordance with the British Standard for stainless steel concrete reinforcing bar BS 6744. Both plain round and deformed 12 mm (0.472 inch) and 16 mm (0.629 inch) diameter reinforcing bar were studied. (The closest equivalent sizes in ASTM A955/A955M are #4 and #5.) Reduction factors were calculated for the key strength, stiffness and ductility properties and compared to equivalent factors for stainless steel plate and strip, as well as those for carbon steel reinforcement. The test results demonstrate that the strength and stiffness reduction factors for stainless steel plate and strip can also be applied to stainless steel reinforcing bar.⁽⁸⁾

Physical Properties

Although the densities of these metals are similar, there are thermal property differences that need to be considered during design. These properties are provided in Table 2.7 of the Design Manual for Structural Stainless Steel or the AISC Steel Design Guide 27: Structural Stainless Steel Table 2-9.  Both of these documents provide significantly more information on stainless steel properties and fire performance.

Note that the coefficient of thermal expansion for austenitic stainless steels is about 30% higher than that for carbon steel. Where carbon steel and austenitic stainless steel are used together, the effects of differential thermal expansion should be considered in design. The thermal conductivity of austenitic and duplex stainless steels is about 30% of that of carbon steel.  The higher thermal elongation and lower thermal conductivity of stainless steel compared to carbon steel leads to steeper temperature gradients during welding and hence greater weld distortion, especially for austenitic grades.

Continued research and innovations in stainless steel structural design will allow designers and engineers to create ever more compelling structures that capitalize on the fire resistance of bare metal to express details as sculptural design elements.

References:

1. Doran, D.K., Construction Materials Reference Book, Butterworth-Heineman Ltd., Oxford, 1992.
2. Brandes, E. A. and Brook, G. B., Smithells Metals Reference Book, Seventh Edition, Butterworth-Heinemann Ltd, Oxford, 1992
3. Elevated Temperature Properties of Constructional Steels, Metals Handbook Desk Edition, American Society for Metals, Section 4-70
4. Metals Handbook Vol. 1, 10^th Edition, Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM International, 1990
5. Rozen, Harold and Heineman, Tom, Architectural Metals for Construction, McGraw Hill, 1996, New York
6. G. Waller and D.J. Cochrane, Stainless steel for durability, fire-resistance and safety, Nickel Development Institute, technical series 10042, Toronto, Ontario, Canada
7. L. Gardner and K. T. Ng “Temperature development in structural stainless steel sections exposed to fire,” Fire Safety Journal.
8. L. Gardner et al, “Elevated temperature material properties of stainless steel reinforcing bar”, Construction and Building Materials, 114 (2016) 977-997, Elsevier Press