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www.outokumpu.com Duplex Stainless Steel in Fire acom 1 – 2012 A corrosion management and applications engineering magazine from Outokumpu e duplex stainless steel grades LDX 2101®, 2304, LDX 2404® and 2205 have been tested to generate data for fire design. LDX 2101® , 2304 and LDX 2404® have not been tested before and are so far not included in Eurocode 3 [1]. 2205 is already included in [1]. e testing shows that the present values in [1] for 2205 are too high, even though all duplex grades showed good fire properties. e retention factors of the duplex grades are lower at the higher temperatures compared to standard austenitic grades, but the higher room temperature strength still gives a higher strength in fire up to 700 – 800 °C. 2304 has lower fire resistance properties compared to LDX 2101® , LDX 2404® and 2205. e Young’s modulus of stainless steel decreases less with temperature compared to carbon steel, and this may in many cases be as important as the proof strength decrease for the fire design. Abstract

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Page 1: Aço Inox Duplex

www.outokumpu.com

Duplex Stainless Steel in Fire

acom1 – 2012A corrosion management and applications engineering magazine from Outokumpu

The duplex stainless steel grades LDX 2101®, 2304, LDX 2404® and 2205 have been tested to generate data for fire design. LDX 2101®, 2304 and LDX 2404® have not been tested before and are so far not included in Eurocode 3 [1]. 2205 is already included in [1]. The testing shows that the present values in [1] for 2205 are too high, even though all duplex grades showed good fire properties. The retention factors of the duplex grades are lower at the higher temperatures compared to standard austenitic grades, but the higher room temperature strength still gives a higher strength in fire up to 700 – 800 °C. 2304 has lower fire resistance properties compared to LDX 2101®, LDX 2404® and 2205. The Young’s modulus of stainless steel decreases less with temperature compared to carbon steel, and this may in many cases be as important as the proof strength decrease for the fire design.

Abstract

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Duplex Stainless Steel in Fire

E. Schedin, B. Ivarsson, M. Andersson and R. Lindström, Outokumpu Stainless AB,

Avesta Research Centre, Avesta / Sweden

IntroductionFour duplex grades, LDX 2101®, 2304, LDX 2404® and 2205 were tested. The work was initiated as a response to the increased use of duplex material in load-carrying applications where there is a risk of fire e.g. buildings, tunnels and bridges. Mechanical properties at elevated temperature are required for determination of the load-bearing capacity of structures under fire conditions. The common understanding is that duplex stainless steel has very low strength at high temperature and that brittle phases are formed between 300 – 1000°C resulting in inferior fire properties compared to austenitic grades. During most fires however, it will take some time before the temperature of the steel will reach the temperatures where the duplex grades have this low strength. Further, the difference in strength between steel grades at the highest fire temperatures, where all steels are “soft” can be neglected in most cases. The time of exposure is most often also too short to form brittle phases in a critical quantity.

Keywords: Duplex stainless steel, Fire design, Mechanical Strength, Anisothermal, Elevated temperature, Young’s modulus.

1. MaterialsThe materials tested were LDX 2101®, 2304, LDX 2404® and 2205 produced as hot rolled plate. Chemical composition of the heats can be found in Table 1. The mechanical properties at room temperature and thickness of the tested heats can be found in Table 2.

Chemical composition of the tested heats in weight per cent. Table 1

EN Designation C Mn Cr Ni Mo N

1.4162 LDX 2101® 0.03 5.0 21 1.5 0.3 0.22

1.4362 2304 0.02 1.5 23 4.8 0.3 0.09

1.4662 LDX 2404® 0.02 3.0 24 3.6 1.6 0.27

1.4462 2205 0.02 1.5 22 5.7 3.1 0.17

Mechanical properties and plate thickness of the tested heats. Table 2

Thickness Rp0.2 Rp1.0 Rt2.0 Rm EN Designation (mm) (MPa) (MPa) (MPa) (MPa)

1.4162 LDX 2101® 25 451 498 515 678

1.4362 2304 24 403 502 521 659

1.4662 LDX 2404® 15 552 617 630 762

1.4462 2205 24 452 546 565 734

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2. Anisothermal Test TechniqueNormally, mechanical properties at elevated temperatures are determined by using standardized isothermal tensile test methods. However, for the determination of fire design data, a more complex, anisothermal method has traditionally been used. This method is often referred to as “transient state test”, and resembles more closely the conditions during a real fire exposure. Although it is commonly used in a number of evaluations [2], this method is not standardized. Therefore, Outokumpu has developed an internal testing and analysis procedure. The main points are as follows:

The load was increased to a fixed stress level, see Figure 1 below. Then the temperature was increased with a constant rate of 10 °C /min until failure occurred or the specimen temperature exceeded 1000 °C, see Figure 2 below. During both stages, the longitudinal strain of the specimen was recorded.

The recorded (isostress, or rather “iso-load”) strain-temperature curves cannot be used directly in the design calculations. Instead, “pseudo-isothermal” curves are derived, by subtracting parasite strains (due to thermal expansion of the specimen and the testing apparatus), making stress-strain-temperature cross-plots, and curve fitting/smoothing.

From these derived curves, the relevant proof strength (Rp0.2 and Rt2.0) values are evaluated. Tensile strength (Rm) values are determined by plotting the stress levels for the individual tests against the temperature at which the specimen ruptured.

This methodology may give strength values that differ considerably from the values obtained in “truly” isothermal tests, but it varies from steel to steel which method will give the highest values.

3. ResultsAll fire dimensioning data are given as relative, so called retention factors, i.e. the strength values determined for elevated temperatures are divided with the corresponding room temperature value.

The retention factors of the Rp0.2 proof strengthas a function of temperature are shown in Figure 3. The Rp0.2 retention curves for LDX 2101®, LDX 2404® and 2205 are similar while 2304 has higher values up to 500 °C and lower for higher temperatures.

Figure 4 shows the retention factors for the ultimate tensile strength Rm. LDX 2101® has higher values than the other duplex steels at low temperatures and 2304 lower values at intermediate temperatures. In general, all duplex steels have similar reduction of Rm with increasing temperature.

Fig. 1 Load-time-curve. Fig. 2 Temperature-time-curve.

Load

�0

t0 Time

Temperature

�u

t0 Time

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3.1 LDX 2101® and 2304 compared to 1.4301 and 1.4401 in Eurocode 3

Retention factors for 1.4301 and 1.4401 can be found in [1] and [3]. These data are for cold rolled 3 – 4 mm thick material, while in this work, hot rolled plate material with gauge around 25 mm has been used. In this section a comparison is made between these [1] data for 1.4301 (designated 4301EC3) and 1.4401 (designated 4401EC3) and the corresponding duplex grades LDX 2101® and 2304. This comparison may be affected to some extent by the fact that cold rolled and hot rolled plate materials are compared, but typically in the conservative direction when it comes to the data from this work, i.e. 4301EC3 and 4401EC3 are expected to be somewhat lower in 25 mm gauge than indicated in [1].

The retention factors of LDX 2101® are lower compared to the values in [1] for the austenitic grades 4301EC3 and 4401EC3 especially at temperatures above 500 °C, as can be seen in Figure 5. 2304 has only half of the relative strength left compared to 4401EC3 above 500 °C.

However, the absolute proof stress values Rp0.2 are higher for LDX 2101® than for 4301EC3 and 4401EC3 up to some 700 – 800 °C and is then equal to 4301EC3, see Figure 6. 2304 has higher proof stress values compared to 4401EC3 up to 550 °C and lower above. The absolute proof strength in Figure 6 is not based on the actual measured proof strength in Table 2. Instead it is based on the retention factor multiplied by the standardised EN minimum values for the grade since this is the way the retention factors are used by the designer. The same procedure was used for 4301EC3 and 4401EC3.

Fig. 3 Retention curves for Rp0.2

Ret

entio

n fa

ctor

for R

p0.2

0.6

0.8

1.0

0.4

0.2

0.00

Temperature (°C)400 600 800 1000200

22052304LDX 2101®

LDX 2404®

Fig. 4 Retention curves for Rm

Ret

entio

n fa

ctor

for R

m

0.6

0.8

1.0

0.4

0.2

0.00

Temperature (°C)400 600 1000 1200200 800

22052304LDX 2101®

LDX 2404®

Fig. 5 Rp0.2 retention for corresponding duplex and austenitic grades.

Ret

entio

n fa

ctor

for R

p0.2

0.6

0.8

1.0

0.4

0.2

0.00

Temperature (°C)400 600 800 1000200

LDX 2101®

4301 EC323044401 EC3

Fig. 6 Decrease of EN minimum Rp0.2 for corresponding duplex and austenitic grades.

Proo

f str

ess

Rp0

.2 (M

Pa)

300

400

500

200

100

00

Temperature (°C)400 600 800 1000200

LDX 2101®

4301 EC323044401 EC3

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Figure 7 shows the ultimate tensile strength Rm retention factors of LDX 2101® and 2304 compared to those for 4301EC3 and 4401EC3. LDX 2101® has a higher Rm retention factor than 4301EC3 up to 550 °C and has somewhat lower retention factor above this. Compared to 4401EC3, the Rm retention factors for LDX 2101® is higher up to 400 °C while the corresponding crossover temperature for 2304 is 250 °C.

Figure 8 shows the reduction of the absolute value of the ultimate tensile strength Rm for LDX 2101® and 2304 compared to 4301EC3 and 4401EC3. The level of Rm is as for Rp0.2 based on the retention factor multiplied by the EN minimum values. LDX 2101® has higher tensile strength compared to 4301EC3 up to 700 °C and has the same tensile strength in the temperature range above this. Compared to 4401EC3, the tensile strength of LDX 2101® is higher up to 600 °C and is somewhat lower above. 2304 has higher tensile strength up to 400 °C compared to 4401EC3. Above this temperature, the tensile strength is lower.

3.2 Duplex stainless steel in Eurocode 3 and a new proposal

Figure 9 shows Rp0.2 retention factors for 2205 and the present values in [1] for 2205EC3. The values in the present [1] are based on two test series performed by RWTH and CTICM and the values are the average of data from these test series. The values are high compared to those in this work, and the explanation is that the values from the CTICM tests are considerably higher compared to any other duplex test series performed, while the RWTH data are in the same level as in this work. The reason for the higher values has not been explained.

Fig. 7 Rm retention for corresponding duplex and austenitic grades.

Ret

entio

n fa

ctor

for R

m

0.6

0.8

1.0

0.4

0.2

0.00

Temperature (°C)400 600 800 1000200

LDX 2101®

4301 EC323044401 EC3

Fig. 8 Decrease of EN minimum Rm for corresponding duplex and austenitic grades.

Tens

ile s

tren

gth

Rm

(MPa

)

400

500

600

700

300

200

100

00

Temperature (°C)400 600 800 1000200

LDX 2101®

4301 EC323044401 EC3

Ret

entio

n fa

ctor

for R

p0.2

0.6

0.8

1.0

0.4

0.2

0.00

Temperature (°C)400 600 800 1000200

22052205 EC3

Fig. 9 Rp0.2 retention for 2205 from this work and 2205 in [1].

Fig. 10 Proposed new Rp0.2 retention classes for [1].

Ret

entio

n fa

ctor

for R

p0.2

0.6

0.8

1.0

0.4

0.2

0.00

Temperature (°C)400 600 800 1000200

Duplex IDuplex II

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A new proposal for possible inclusion in [1] has been generated based on the data in this work [4]. In this proposal, the duplex stainless steels are divided into two groups, Duplex I and Duplex II. Duplex I includes both 2205 and LDX 2101® and is the average of the data in this work and the present Eurocode 3 data. Duplex II only includes 2304 today, and is based on the data from this work. The idea with the new proposal is to simplify the stainless steel fire design by rationalizing the steel grades into generic groups with similar retention factors.

4. DiscussionThe results demonstrate the potential of utilising duplex stainless steel in constructions where fire resistance must be considered. By utilising the data, it is often possible to reduce the gauge and thereby the cost of the construction compared to using austenitic grades.

Stainless steel can often replace carbon steel in constructions due to a lower life cycle cost, but sometimes because of a lower overall installation cost. When it comes to fire design, the fire behaviour of Young’s modulus becomes as important as the as the proof strength, for example when stiffness is the design criterion. In Figure 11, the decrease of Rp0.2 for two carbon steels are compared to that of LDX 2101® utilising the retention factors from [3] and this work together with the EN minimum proof strength level for all steels. As can be seen, LDX 2101® has a higher strength than the carbon steel II (min Rp0.2 = 235 MPa) in the whole upper temperature range, while it is similar to the carbon steel I (min Rp0.2 = 355 MPa) in the higher temperature range and has lower strength in the lower temperature range.

However, the Young’s modulus retention with temperature is much better for stainless steel compared to carbon steel, see Figure 12, and this is perhaps more interesting for many applications when stiffness is the main design criterion. Since the Young’s modulus of stainless steel and carbon steel is more or less the same at room temperature, these curves also reflect the actual Young’s modulus decrease with temperature. This may be more important for many design cases than the strength retention determined in this work.

Fig. 11 Rp0.2 retention for LDX 2101® from this work and two carbon steels in [1].

Proo

f str

ess

Rp0

.2 (M

Pa)

300

400

500

200

100

00

Temperature (°C)400 600 800 1000200

LDX 2101®

Carbon steel ICarbon steel II

Fig. 12 Young’s modulus retention for stainless steel and carbon steel from [1].

Ret

entio

n fa

ctor

for E

-mod

ulus

0.6

0.8

1.0

0.4

0.2

0.00

Temperature (°C)400 600 800 1000200

Stainless steelCarbon steel

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5. Conclusion • The duplex grades have somewhat lower strength retention than corresponding

austenitic grades in the higher temperature range of a fire. However, the absolute strength level at room temperature is still maintained up to the higher temperature range where the strength level is similar to the austenitic grades.

• The existing [1] retention values for 2205 are considered to be too high. As a result of the data in this work a new proposal for Eurocode 3 has been presented, resulting in two duplex groups Duplex I and Duplex II.

References[1] EN 1993-1-2 Eurocode 3, Design of steel structures, Part 1–2, General rules,

Structural fire design. [2] Mäkeleinen P., Outinen J and Kesti J., (1998). Fire design model for

structural steel S420M based upon transient-state tensile test results, Journal of Constructional Steel Research 48, 47– 57

[3] Design Manual for Structural Stainless Steel (Third Edition), Euro Inox & SCI (2006)

[4] Gardner, L., Insausti, A., Ng, K. T. and Ashraf, M. (2010). Elevated temperature material properties of stainless steel alloys. Journal of Constructional Steel Research. 66(5), 634– 647.

Presented at the 7th European Stainless Steel Conference – Como, 21 – 23 settembre 2011, organised by AIM

Page 8: Aço Inox Duplex

www.outokumpu.com

1506EN

-GB

Art 58. February 2012

Outokumpu Stainless AB, Avesta Research Centre

Box 74, SE-774 22 Avesta, Sweden

Tel. +46 (0) 226 - 810 00, Fax +46 (0) 226 - 810 77

Comments on acom and its articles or suggestions on future articles are appreciated and should be sent to the editor Andreas Persson at [email protected]

This document is for information only and seeks to provide professionals with the best possible information to enable them to make appropriate choices. Although every effort has been made to ensure the accuracy of the information provided in this document, Outokumpu can not accept any responsibility for any loss, damage or other consequence resulting from the use of this publication. The information provided herein may be subject to alterations without notice.

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What makes Outokumpu special is total customer focus – all the way, from R&D to delivery. You have the idea.We offer world-class stainless steel, technical know-how and support. We activate your ideas