Post-fire behaviour of high-strength quenched and tempered steel under various heating conditions

Journal of Constructional Steel Research 164 (2020) 105785

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Post-fire behaviour of high-strength quenched and tempered steel under various heating conditions Xing-Qiang Wang a, b, Zhong Tao a, *, Md Kamrul Hassan a a b

Centre for Infrastructure Engineering, Western Sydney University, Penrith, NSW, 2751, Australia Department of Civil Engineering, Shandong Polytechnic, Jinan, 250104, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 May 2019 Received in revised form 7 September 2019 Accepted 28 September 2019 Available online xxx

Quenching and tempering (QT) are common procedures in manufacturing high strength steel for building applications to achieve structural, economic and architectural benefits. However, the tempered martensitic microstructure of QT steel can change dramatically when exposing to high temperatures, leading to significant deterioration in mechanical properties. This paper investigates the post-fire mechanical properties of two types of QT steel with nominal yield stresses of 690 MPa and 1070 MPa, respectively. The main test variables include the exposure temperature (100e1200
C), heat soak time (0.5e4 h), and cooling method (cooled in furnace, air or water). It is found that the heat soak time has obvious influence on the residual mechanical properties of QT steel, when the exposure temperature is above 500
C. The cooling method, however, only severely affects the residual properties at a temperature above the phase change temperature (around 724
C). It is also found that QT steel with a higher yield stress experiences more severe strength deterioration after exposure to a temperature of over 300
C. When the exposure temperature is higher than 500
C, however, the residual properties of the two types of steel become similar. Based on the test data in this paper and the literature, empirical equations are proposed to predict the residual elastic modulus, yield stress, ultimate strength, and ultimate strain of QT steel after exposure to elevated temperatures. © 2019 Elsevier Ltd. All rights reserved.

Keywords: High strength steel Quenched and tempered steel Post-fire Residual strength Heat soak time Cooling method

1. Introduction High strength quenched and tempered (QT) steel with a nominal yield stress of 690 MPa or more was developed in the 1960s to meet the needs of high strength material in construction and other industries [1,2]. Compared with normal strength steels, the use of high strength steel can effectively reduce the physical dimensions and the weight of structures, consequently reducing the cost of transportation and on-site construction [3]. Because of its structural, economic and architectural benefits, high strength QT steel is now increasingly used in civil engineering [2,3]. Currently, the most commonly used high strength steels in construction have nominal yield stresses (fy) of 460 MPa or 690 MPa. The use of steel grades with fy in the range of 780e1100 MPa was only reported in a few buildings or bridges [4,5]. Some ongoing research efforts are focused on finding solutions to overcome major hurdles (such as reduced ductility) for using very high strength steel (fy  960 MPa) in construction [4].

* Corresponding author. E-mail address: [email protected] (Z. Tao). https://doi.org/10.1016/j.jcsr.2019.105785 0143-974X/© 2019 Elsevier Ltd. All rights reserved.

The high strength of QT steel results from its tempered martensitic microstructure, which can change dramatically at elevated tempreatures [6]. Compared with mild steel, QT steel is more susceptible to the exposure of elevated tempreatures. Although QT steel in building structures may be protected against fire by insulation materials, the steel can still reach very high temperature after exposure to fire for a certain period. As post-fire mechanical properties of construction materials are important for evaluating a structure after fire exposure, some experimental studies summarised in Table 1 have been conducted in the past to investigate the post-fire residual mechanical properties of QT steels [7e12]. It should be noted that the steel used by Azhari et al. [11,12] was produced by direct quenching to obtain its very high strength. Qiang et al. [7,8] conducted experiments on Grades S690 and S960 QT steels after exposure to temperatures up to 1000
C, where the heat soak time was only 10 min. The test results indicated that the mechanical performance of QT steel was not affected until 600
C, and a significant reduction in strength and elastic modulus was observed after that temperature. For example, the yield stress of S960 QT steel reduces by 63.3% after exposure to 1000
C and the corresponding reduction in elastic modulus reaches 35.1%. Chiew et al. [9] investigated the performance of S690 steel in post-fire

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X.-Q. Wang et al. / Journal of Constructional Steel Research 164 (2020) 105785

Table 1 Summary of previous tests reported in the literature. Source

Steel grade

Delivery condition

Tempering temperature Tp (
C)

Calculated AC1 and AC3 (
C) from Eqs. (1e2)

fy Number of (MPa) data

Soak time ts (min)

Heating temperature T (
C)

Cooling method

Qiang et al. [7] Qiang et al. [8] Chiew et al. [9] Li et al. [10]

S690QL

QTa

640

725/837

789

14

10

20e1000

CIA

S960QL

QT

590

728/858

1045

12

10

20e1000

CIA

RQTS690 Q690 Q690 Grade 1200 Grade 1200

QT

640

720/857

769

6

10

20e1000

CIF

QT QT DQb

590 590 e

714/835 714/835 e

803 803 1170

8 8 9

10 10 20

20e900 20e900 20e800

CIA CIW CIA

DQ

e

737/849

1170

5

20

20e800

CIW

Azhari et al. [11] Azhari et al. [12] a b

QT denotes quenched and tempered steel. DQ denotes direct quenching.

conditions. The specimens were soaked for 10 min and then cooled in furnace with the ventilation system in operation. A slight loss in strength was found between 400 and 600
C, but after exposure to higher temperatures the strength deteriorated dramatically, accompanied by a significant increase in ductility. Li et al. [10] investigated the effect of cooling method (cooled in air or water) on the post-fire behaviour of Q690 steel produced in China. The specimens were heated up to 900
C and soaked for 10 min before cooling down. The experimental results show that the cooling method significantly affects the residual strength and elongation if the exposure temperature is above 700
C, but the elastic modulus is not obviously affected. The post-fire performance of ultra-high strength Grade 1200 steel produced by direct quenching was investigated by Azhari et al. [11,12], in which the soak time was chosen to be 20 min. A strength reduction was observed when the material was heated at 470
C or above and then cooled in air. Scanning electron microscope analysis was conducted on samples. It was found that the initial martensitic structure disappeared after the heat treatment and the steel had a changed microstructure of ferrite grains, which explains the significant strength decrease. However, if the material was heated to 800
C or above and then cooled in water, martensite formed again, leading to a strength increase. The above literature review indicates that most researchers did not report a significant deterioration in elastic modulus of QT steel due to heat exposure except Qiang et al. [7,8]. This needs to be clarified. In particular, the influence of heat soak time and cooling method on residual mechanical properties of QT steel has not been fully understood. As shown in Table 1, the heat soak time in previous studies varied only from 10 to 20 min, which is relatively short. In a real fire scenario, the fire might last for hours or even longer. It has been clear that QT steel experiences a phase change from ferrite/martensite back to austenite after exposure to a certain temperature beyond 700
C, which contributes to the change in post-fire residual strength [6]. Meanwhile, it is reported that the mechanical properties of martensitic steel are very sensitive not only to the treatment temperature but also to the heat soak time and cooling rate during manufacture [6,13]. Furthermore, it can be seen from Table 1 that previous studies mainly focused on QT steel with a nomial yield stress of 690 MPa, only Qiang et al. [8] and Azhari et al. [11,12] studied QT steel with a nomial yield stress of 960 or 1200 MPa. Based on the above research background, two typical types of QT steel are selected, which have nominal yield stresses of 690 and 1070 MPa, respectively. The effects of temperature, heat soak time and cooling method on the post-fire mechanical properties are investigated accordingly to reflect the real fire situation. Based on

the test data in this paper and the literature, empirical equations are proposed to predict the residual properties of QT steel after exposure to elevated temperatures. 2. Experimental investigation 2.1. Preparation of tensile coupons Two types of high strength QT steel (Bisalloy 80 and Bisalloy 400 produced in Australia) were selected to investigate their post-fire mechanical properties. The tensile coupons were fabricated from steel plates in the rolling direction. The delivered steel plates had dimensions of 1500  750  5 mm. Waterjet cutting was used in the coupon fabrication to minimise heat generation and the potential influence on mechanical properties. The coupon dimensions are shown in Fig. 1, where the gauge length is 75 mm and the nominal width is 12.5 mm in the necking region of the coupon. Bisalloy 80 steel has a nominal yield stress of 690 MPa, which is manufactured in accordance with Grade 700 steel in Australian standard AS 3597e2008 [14]. Bisalloy 80 is equivalent to the US steel grade A514, European grade S690 or Chinese grade Q690 steel. Different from Bisalloy 80, Bisalloy 400 is a new steel grade, and has a higher yield stress with a typical value of 1070 MPa. Both types of steel have very similar chemical compositions, as shown in Table 2. The difference in yield stress mainly comes from the variation in tempering temperature. Before quenching, the raw material is heated to 900
C to allow the complete transformation of ferrite to austenite. Then the material is quenched by water within seconds to room temperature, leading to the transformation of austenite to martensite. Due to the phase transformation, the quenched steel has very high strength but low ductility. To improve the ductility, the quenched steel is further tempered at 600
C and 200
C to produce Bisalloy 80 and Bisalloy 400, respectively. As tempering increases ductility of the steel, it also leads to a loss of strength. The higher the tempering temperature, the lower is the final strength. This explains the higher strength of Bisalloy 400 since it has a much lower tempering temperature than Bisalloy 80. 2.2. Heating and cooling of coupons The coupons were heated using an electrical furnace with a maximum temperature capacity of 1200
C, as shown in Fig. 2. The internal diameter and height of the chamber are 250 mm and 350 mm, respectively. The chamber has six vertically distributed heating elements to generate relatively uniform temperature distribution inside the chamber. The coupon was tied to a wire and suspended in the middle of the chamber for heat exposure. A

X.-Q. Wang et al. / Journal of Constructional Steel Research 164 (2020) 105785

3

Fig. 2. Heating and cooling setup.

immediately removed when the soak time was reached, and the suspension wire was then cut to release the specimen from the furnace to cool in air. CIW was achieved by following a similar procedure to release the specimen into a bucket of water. The coupon test was commenced in winter and the water temperature at that time was around 12
C before quenching the steel sample. When cooling other coupons, the initial water temperature was kept consistent at 12
C by either heating the water to increase the temperature or mixing with ice to reduce the temperature. This is to ensure consistent testing conditions. Fig. 3 shows typical measured temperatureetime curves of steel coupons heated to a target temperature of 800
C and then cooled in three different conditions. As expected, the cooling rate in the furnace is relatively slow, whilst the cooling rate in water is the fastest.

Fig. 1. Coupon dimensions (unit: mm).

thermocouple was attached to the middle of the coupon to measure the temperature. After the installation of the coupon, it was heated at a rate of 20
C/min to reach a predetermined target temperature [11]. Eight different target temperatures (T ¼ 100, 200, 300, 500, 700, 800, 1000 and 1200
C) were considered in this study. At present, there is no standard developed to specify heat soak time for post-fire tests of steel [15]. To study the influence of heat soak time (ts) on post-fire mechanical properties of QT steel, ts was varied at three levels (0.5, 1.5, 4 h) to represent the variation in fire scenarios. Based on trial tests, it is found that the residual performance of steel is not sensitive to ts at lower temperatures (100e300
C). Therefore, the heat soak time was chosen to be 4 h at 100
C, whilst 0.5 and 4 h were adopted for 200 and 300
C. For other temperatures (500e1200
C), the coupons were soaked for 0.5, 1.5 and 4 h, respectively. After heating, the coupons were cooled in three different ways (furnace, air, and water cooling) to investigate the effect of cooling rate on the mechanical properties. These three cooling methods are designated herein as cooled in furnace (CIF), cooled in air (CIA) and cooled in water (CIW), respectively. CIF was accomplished by leaving the specimen in the furnace with the chamber closed after the heat treatment. For CIA, the bottom cover of the furnace was

2.3. Phase change temperature test During heating, QT steels undergo changes in microstructure, which significantly affects their post-fire mechanical properties [6]. When reaching a certain temperature, ferrite starts to transform to austenite. The temperature at which austenite begins to form is termed as Ac1. After reaching this temperature, the rate of steel expansion decreases with increasing temperature. Ac3 is the temperature at which the transformation from ferrite to austenite is completed and the expansion resumes to linear increase with increasing temperature. These two critical temperatures can be

Table 2 Chemical composition (wt %). Grade

C

Si

Mn

P

S

Cr

Ni

Cu

Ti

V

B

Nb

Al

W

Bisalloy 80 Bisalloy 400

0.141 0.120

0.274 0.268

1.194 1.080

0.011 0.011

0.001 0.001

0.473 0.190

0.020 0.010

0.020 0.020

0.016 0.022

0.004 0.002

0.001 0.002

0.012 0.013

0.034 0.029

0.002 <0.001

4

X.-Q. Wang et al. / Journal of Constructional Steel Research 164 (2020) 105785

3. Test results and discussion 3.1. Phase change temperature

Fig. 3. Typical temperaturetime curves.

The measured expansionetemperature curve of Bisalloy 80 is shown in Fig. 5. The rate of expansion is constant when the temperature increases from 700 to 724
C, indicating no obvious phase change in this temperature range. After reaching 724
C, the expansion rate decreases due to the formation of austenite. From this figure, Ac1 can be taken as 724
C. At 868
C, the expansion increases linearly again with increasing temperature, due to the completion of the austenite transformation [18]. Thus, the austenite completion temperature (Ac3) of this type of steel is 868
C. After reaching Ac3, the dominant phase in the metal is austenite. However, a phase change will occur again during the cooling process and the actual phase of the material cooled to room temperature depends on the cooling rate, which will be discussed in the following sections. The change in chemical composition of steel has some influence on its phase change temperatures Ac1 and Ac3. Andrews [19] proposed the following empirical formulas to predict Ac1 and Ac3 from

determined from the expansiontemperature curve obtained from thermomechanical analysis (TMA). TMA was conducted using NETZSCH TMA 402F1 instrument in Advanced Materials Characterisation Facility at Western Sydney University. For carbon steels, Ac1 and Ac3 are expected to be higher than 700
C but lower than 900
C according to the ironcarbon diagram [6]. Bisalloy 80 was selected to determine exact values of Ac1 and Ac3 for this type of steel. A sample with a dimension of 12.5  10  5 mm was cut from the delivered plate, and was heated from room temperature to 700
C at a rate of 20
C/min. At this temperature, recrystallisation would occur, leading to the formation of ferrite in the martensite boundaries and inside the grains [16]. Therefore, the sample was kept at 700
C for 30 min to accelerate this transformation. Then the temperature was slowly increased to 900
C at a heating rate of 0.5
C/min. The steel expansion was detected and recorded during the heating process.

2.4. Tensile testing After the heating and cooling of the coupons, tensile tests were conducted using an Instron tensile machine, as shown in Fig. 4. Two uniaxial extensometers were mounted on the specimen to measure the axial strain. The tensile tests were conducted in accordance with Australian standard AS 1391e2007 [17]. A strain rate of 0.015/ min was adopted in the initial stage of the tensile test. When the axial strain reached 10%, the strain rate was increased to 0.03/min until the fracture of the coupon. During the testing, load and extension were recorded to generate full-range stressestrain curves of the steel.

2.5. Microstructure observation To detect the microstructure change of QT steel after heat exposure, Bislloy 80 steel samples were observed using a microscope Axio Imager A2m. The samples had been exposed to two different temperatures, i.e., 700 and 1000
C. The former temperature is lower than Ac1, whereas the latter is higher than Ac1. Meanwhile, samples with different soak times (0.5, 1.5 and 4 h) and cooling rates (CIF and CIW) were also chosen for observation.

Fig. 4. Tensile test setup.

X.-Q. Wang et al. / Journal of Constructional Steel Research 164 (2020) 105785

5

representing a reduction in thickness of 5.8% compared with untreated coupons. Bisalloy 80 was selected to study the variation of coupon thickness due to heat exposure. Fig. 7 shows the remaining coupon thickness as a function of temperature after removal of any rust or oxide layer. In general, the influence of cooling method on the remaining coupon thickness is very minor. When the temperature is 800
C or lower, the change in coupon thickness is negligible. However, a significant reduction in thickness was found at 1000
C and the reduction increases with increasing heat soak time. On average, the reductions at 1000
C are 1.4%, 3.0% and 7.3% when the soak times are 0.5, 1.5 and 4 h, respectively. A more significant reduction in thickness can be found at 1200
C. The corresponding reductions increase to 16.3%, 19.1% and 28.5% for soak times of 0.5, 1.5 and 4 h, respectively. It seems that the loss of cross-sectional area may need to be considered in post-fire structural evaluation if the maximum exposure temperature of steel reaches 1000
C or above.

Fig. 5. Determination of phase change temperatures for Bisalloy 80.

chemical composition, respectively. 3.3. Stressstrain curves

Ac1 ¼ 723  10:7Mn  16:9Ni þ 29:1Si þ 16:9Cr þ 290As þ 6:38W

(1)

pffiffiffi Ac3 ¼ 910  203 C  15:2Ni þ 44:7Si þ 104V þ 31:5Mo þ 13:1W

(2)

Using these formulas, the predicted Ac1 and Ac3 for Bisalloy 80 are 726 and 846
C respectively based on the chemical composition shown in Table 2. As can be seen, the predicted value of Ac1 (726
C) is very close to the measured result (724
C), whereas the predicted value of Ac3 is 22
C lower than the test result. This comparison indicates that Eqs. (1) and (2) are reasonably accurate in predicting phase change temperatures. By using these two equations, the predicted Ac1 and Ac3 for Bisalloy 400 are 722 and 852
C, respectively. It seems Bisalloy 80 and Bisalloy 400 have fairly close phase change temperatures. Values of Ac1 and Ac3 for other steels are also presented in Table 1, which are calculated based on reported chemical composition in the literature. As can be seen, the variation in Ac1 or Ac3 is relatively small for these steels. For example, the range of Ac1 is from 714 to 737
C and the average value is 725
C. Since this average value is very close to the measured value of 724
C for Bisalloy 80, Ac1 may be roughly taken as 724
C for all QT structural steels. 3.2. General observation of heated coupons Fig. 6a demonstrates tested coupons of Bisalloy 80 soaked in heat for 4 h and then cooled in water. As can be seen, the coupon colour remains unchanged when the temperature is 200
C or lower. The colour changes to brown after exposure to 300e500
C. When the temperature increases further to 700
C or above, the coupons become black due to oxidation, and apparent rust can be seen on the coupon surface. It is also found that the cooling method or heat soak time has no obvious influence on the colour change of the coupons. A similar colour change was also observed for Bisalloy 400 and conventional mild steel [20] after exposure to elevated tempeatures. Fig. 6b presents a typical coupon of Bisalloy 80 heated to 1000
C, soaked in heat for 4 h and then cooled in furnace. Before heating, the coupon had a measured thickness of 5.00 mm. But after exposure to heat, its thickness increased to 5.61 mm due to the formation of oxide layer on both sides. After removing the oxide layers, the remaining thickness of the coupon reduced to 4.71 mm,

The measured stressestrain (seε) curves are presented in Figs. 8e10 to show the influence of different factors. The strains were obtained from the readings of the two extensometers, and the stress was calculated by dividing the tensile force by the measured undeformed width in the necking region of the coupon and the residual thickness after the removal of any oxide layer. In the figures, furnace, air and water cooling are designated by “CIF”, “CIA” and “CIW”, respectively. Fig. 8 shows the influence of temperature (T) on the seε curve of Bisalloy 80, where the heat soaking time (ts ¼ 4 h) was constant and the coupons were cooled either in furnace or water. When T is 200
C or lower, its influence on the seε curve is very minor for both cooling methods. Under this circumstance, the seε curve has an initial elastic response, followed by a nonlinear response towards fracture without yield plateau. After exposure to a temperature ranging from 500 to 700
C, an obvious yield plateau appears in the seε curve, accompanied by a significant drop in strength and an increase in ductility. After exposure to 1000
C and cooling in furnace, further deterioration in strength is observed. However, after exposure to the same temperature of 1000
C, cooling in water results in a significant increase in strength and decrease in ductility. This is due to the formation of martensite phase upon fast cooling [6], which is also confirmed by the microstructure analysis in Section 3.5. Fig. 9 demonstrates the effect of heat soak time (ts) on the seε curve. The curves of Bisalloy 80 and Bisalloy 400 soaked for 0.5 and 4 h before cooling in furnace or water are depicted in this figure. At 500
C or less, the influence of ts is very minor. But at 700
C or above, a longer soak time leads to a more severe drop in strength. However, it seems that the heat soak time has no obvious influence on the shape of the seε curve. Fig. 10 presents the influence of cooling method on the seε curves of Bisalloy 80 and Bisalloy 400 after soaking for 1.5 h. At 700
C or less, the cooling method does not affect the seε curve significantly. But at 1000
C or above, an obvious increase in strength and decrease in ductility can be found for materials cooled in air or water compared with the material cooled in furnace. The influence of material type on the seε curves can also be seen in Figs. 9 and 10. As the tempering temperature of Bisalloy 400 is lower than that of Bisalloy 80, Bisalloy 400 has much higher strength and lower ductility when T is 300
C or less. But when T reaches 500
C or above, the seε curves generally become similar for the two types of steel.

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X.-Q. Wang et al. / Journal of Constructional Steel Research 164 (2020) 105785

Fig. 6. General observation of coupons after heat exposure.

the following subsections.

Fig. 7. Thickness reduction of Bisalloy 80 after heat exposure.

3.4. Residual mechanical properties Residual mechanical properties, including modulus of elasticity (ET), yield stress (fyT), ultimate strength (fuT) and ultimate strain (εuT), are determined from the measured sε curves of Bisalloy 80 and Bisalloy 400. The corresponding parameters of these two types of steel at ambient temperature are designated by E, fy, fu and εu, respectively. Tables 3e5 present the measured mechanical properties of steel under different cooling conditions, respectively. The influence of temperature, heat soak time, cooling method and material type on the post-fire mechanical properties is discussed in

3.4.1. Modulus of elasticity The modulus of elasticity (ET) is determined according to ISO 6892e1 [21]. Due to nonlinearity of the sε curve, ET is taken as the secant slope measured between 10% and 50% of the yield stress fyT, as shown in Fig. 11. Fig. 12 depicts the influence of different parameters on the residual elastic modulus (ET). To clearly show the influence, the soak times are chosen as 0.5 and 4 h, whilst furnace cooling and water cooling are chosen to present the effect of cooling rate. For Bisalloy 80, the modulus of elasticity is not obviously affected by the heat treatment when the temperature is 500
C or lower, as shown in Fig. 12a. A variation of about 6% is found for the ratio of ET/E in this temperature range, which may have been caused by test error. At 700
C, the ratios of ET/E consistently decrease by about 5.0% for all samples, regardless of the cooling method or heat soak time. When the temperature increases further to 800 or 1000
C, a more significant decrease in ET can be seen from Fig. 12a. In general, ET decreases with increasing heat soak time at 800 or 1000
C. At 1000
C, the average losses of elastic modulus for Bisalloy 80 are 3.5%, 12.4% and 14.8% when the samples were cooled in furnace, air and water, respectively. For Bisalloy 400, there is no significant change in modulus of elasticity before 500
C, as can be seen from Fig. 12b. However, an average loss of 11.3% is found for Bisalloy 400 heated to 1000
C and cooled in water. It was found that a significant drop in ET mainly occurred when QT steel was heated to at least 800
C and cooled fast in air or water. This could be due to the regeneration of a martensitic microstructure under fast cooling conditions, greatly increasing the initial nonlinearity of the stressstrain curves, as shown in Figs. 9 and 10.

Fig. 8. Influence of temperature on stressstrain curves.

X.-Q. Wang et al. / Journal of Constructional Steel Research 164 (2020) 105785

7

Fig. 9. Influence of heat soak time on stressstrain curves.

Fig. 10. Influence of cooling method on stressstrain curves.

Fig. 11 shows a typical nonlinear curve of Bisalloy 400, which was heated to 1000
C, soaked in heat for 4 h and then cooled in water. In this scenario, the determined ET-value is 172,000 MPa, taking as the secant slope measured between 10% and 50% of fyT. However, the initial slope E0 corresponding to 0.1fyT is 219,300 MPa. As can be seen, ET is only 78.4% of E0, representing a significant increase in nonlinearity of the steel after heat treatment. Hutchinson et al. [22] also reported that as-quenched martensitic steel shows remarkable

nonlinearity in its stressestrain curve. Typical test results of ET for S690 [7,9], Q690 [10], and S960 [8] reported by other researchers are also plotted in Fig. 12. It seems that the current test results of ET agree with those reported in Refs. [9,10]. However, for QT steel cooled in air, the significant deterioration in ET at above 600
C reported by Qiang et al. [7,8] was not observed in the current tests and by others [9,10]. As Qiang et al. [7,8] did not explain the reason of the significant deterioration in ET,

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X.-Q. Wang et al. / Journal of Constructional Steel Research 164 (2020) 105785

Table 3 Mechanical properties of steel cooled in furnace. Soak time (h)

e 0.5

1.5

4

T (
C)

20 200 300 500 700 800 1000 1200 500 700 800 1000 1200 100 200 300 500 700 800 1000 1200

Bisalloy 80

Bisalloy 400

ET (MPa)

fyT (MPa)

fuT (MPa)

εuT

ET (MPa)

fyT (MPa)

fuT (MPa)

εuT

217,300

723.5

818.1

0.064

213,500

1137.9

1234.5

0.031

814.3 812.6 784.1 578.4 526.2 435.1 416 790 547.4 498.8 419.8 375.7 806.7 821.1 818.6 766.8 515.7 467.5 372.8 338.8

0.052 0.059 0.071 0.119 0.15 0.174 0.188 0.064 0.106 0.165 0.222 0.15 0.058 0.057 0.058 0.068 0.125 0.176 0.2 0.125

207,500 226,800 199,600 218,900 223,800 200,000 e 231,100 213,700 197,100 191,400 e 228,000 224,400 221,200 219,100 202,200 183,100 237,100 e

1202.2 1127 725.8 496.5 424.4 233.4 e 753.5 445.4 375.8 219.5 e 1155.5 1190.2 1097.1 715.4 383.7 338.3 208.2 e

1229.5 1155.4 760.5 566.0 495.6 394 e 781.9 517.3 458.9 387.3 e 1231.8 1220.1 1132.8 749.5 479.5 435.4 346 e

0.036 0.03 0.067 0.104 0.138 0.187 e 0.052 0.122 0.161 0.202 e 0.027 0.024 0.035 0.065 0.133 0.189 0.198 e

228,500 217,300 216,700 210,400 225,000 203,800 196,600 223,700 207,100 214,300 223,200 152,300 224,400 224,300 230,200 226,600 205,300 213,100 202,300 260,600

777.2 781.2 739.2 507.5 429.7 264.8 253.2 748.5 480.2 399.1 248.6 221.6 731.5 787.5 787 717.5 429.5 363.2 227.8 215.7

0

the test results of ET reported in Refs. [7,8] are tentatively ignored in developing an empirical model in Section 4.1. 3.4.2. Yield stress The measured yield stress (fy) for as-received Bisalloy 80 and Bisalloy 400 are 723.5 and 1137.9 MPa, respectively. The lower strength of Bisalloy 80 is due to its higher tempering temperature (600
C) than that of Bisalloy 400 (200
C). An increase in tempering temperature usually results in more pronounced decomposition of the martensitic phase in QT steel, leading to a strength decrease [6]. The residual yield stresses (fyT) are compared in Fig. 13 between Bisalloy 80 and Bisalloy 400 after exposure to elevated temperatures. At 300
C or below, Bisalloy 400 still has a higher fyT than Bisalloy 80. However, at 500
C or above, the difference in fyT between the two types of steel is not very significant. Similar observations are also found for fuT and εuT, which can be

seen from Tables 3e5. Since the tempering temperatures are 600 and 200
C for Bisalloy 80 and Bisalloy 400, respectively, it seems that the deterioration in mechanical properties becomes apparent when the heating temperature is close to or above the original tempering temperature of the steel. Fig. 14 shows the effect of heat exposure on the normalised residual yield stress (fyT/fy) of the two types of steel. A slight increase in fyT can be found for Bisalloy 80 at 200e300
C, and Bisalloy 400 at 100e200
C. This is due to the shape change in the sε curve as shown in Fig. 15. The sε curve of the raw material is rounded in shape without a yield plateau. After exposure to a temperature of 100e300
C, however, the sε curve has a prolonged elastic response accompanied by the appearance of a clear yield plateau. It should be noted that Azhari et al. [11] also reported a slight increase in yield stress (about 1%) for high strength steel (Grade 1200) after exposure to 200e300
C. Hutchinson et al. [22] studied the effect of

Table 4 Mechanical properties of steel cooled in air. Soak time (h)

e 0.5

1.5

4

T (
C)

Bisalloy 80

Bisalloy 400

ET (MPa)

fyT (MPa)

fuT (MPa)

εuT

ET (MPa)

fyT (MPa)

fuT (MPa)

εuT

20

217,300

723.5

818.1

0.064

213,500

1137.9

1234.5

0.031

200 300 500 700 800 1000 1200 500 700 800 1000 1200 100 200 300 500 700 800 1000 1200

224,000 204,800 204,700 207,600 183,300 195,800 172,400 205,300 217,700 186,900 181,300 211,500 217,000 215,800 222,200 212,200 202,600 180,600 194,200 208,600

771.3 780 713.4 523.3 387.2 342 347 714.7 493.9 340.8 312.2 322.6 732.3 783.8 785.5 706.8 449.5 292.6 290.4 298.9

812.1 815.2 760.5 582.6 629 574.1 545 760 564.5 587 507.1 492.3 817.3 819.2 821 755.4 522 603.2 484.4 448

0.056 0.058 0.071 0.109 0.13 0.135 0.102 0.066 0.13 0.134 0.132 0.095 0.056 0.055 0.064 0.066 0.118 0.138 0.146 0.101

223,300 215,400 213,500 186,700 214,400 224,200 e 217,800 203,300 229,800 189,305 e 224,500 213,500 245,000 228,000 189,400 206,400 231,100 e

1187.6 1070.7 723.1 511.2 422.6 317.2 e 712.1 442.8 385 260.3 e 1164.1 1186.9 1086.3 702.4 346.9 359.7 301.9 e

1220.4 1100.3 759.5 572.0 535.9 464.2 e 746.1 512.9 522.2 373.2 e 1240.9 1219.7 1111 739.6 464.7 477.3 427.2 e

0.021 0.022 0.056 0.112 0.142 0.168 e 0.062 0.114 0.163 0.157 e 0.029 0.032 0.025 0.063 0.157 0.176 0.165 e

X.-Q. Wang et al. / Journal of Constructional Steel Research 164 (2020) 105785

9

Table 5 Mechanical properties of steel cooled in water. Soak time (h)

e 0.5

1.5

4

T (
C)

Bisalloy 80

Bisalloy 400

ET (MPa)

fyT (MPa)

fuT (MPa)

εuT

ET (MPa)

fyT (MPa)

fuT (MPa)

εuT

20

217,300

723.5

818.1

0.064

213,500

1137.9

1234.5

0.031

200 300 500 700 800 1000 1200 500 700 800 1000 1200 100 200 300 500 700 800 1000 1200

217,000 213,600 205,200 208,900 160,800 189,500 171,100 213,400 216,600 176,800 201,500 166,300 215,500 210,600 220,200 225,800 202,100 179,100 164,500 199,300

768.1 785.1 723 490.3 478.5 900.1 786.5 726 514.8 520.9 772.1 631.3 722.8 779.1 787.8 701.6 461.9 420.1 670.3 418.3

812.7 816.6 772.7 593.7 869.1 1184.1 974.4 772 576 923.1 1060.8 751.5 808.8 814.3 821.4 751.7 535.1 791 879.2 560.4

0.057 0.063 0.066 0.115 0.085 0.039 0.021 0.07 0.12 0.082 0.037 0.015 0.056 0.056 0.062 0.068 0.131 0.097 0.036 0.032

212,100 215,300 213,300 197,000 227,600 200,600 e 220,100 209,000 202,600 195,500 e 225,200 205,900 209,400 216,800 186,700 198,500 172,000 e

1199.1 1082.3 720.6 530.3 382.4 842.7 e 724 474.5 394.4 764.6 e 1153.5 1186.7 1113.7 704 367.1 357.1 639.7 e

1235.1 1112.2 758.7 593.4 706.9 1114.9 e 753 542.3 742 963.9 e 1232.7 1219.5 1141.8 737.6 481.2 677.8 804.1 e

0.026 0.023 0.056 0.119 0.155 0.038 e 0.053 0.137 0.124 0.022 e 0.023 0.022 0.038 0.059 0.151 0.127 0.036 e

tempering on the yielding behaviour of martensitic steel and found that tempering at a low temperature of 200
C led to a shape change in the sε curve and a slight increase in the yield stress. The change in curve shape could be explained by the reduction of internal residual stresses and rearrangement of dislocation structure [22]. For Bisalloy 80, its yield stress starts to deteriorate at 500
C or above, as shown in Fig. 14a. However, the corresponding critical temperature for Bisalloy 400 reduces to 300
C. The strength deterioration can be explained by the ease of distortion energy inside the martensite and formation of ferrite [6]. The earlier strength deterioration of Bisalloy 400 can be explained by the fact that Bisalloy 400 is tempered at a lower temperature than Bisalloy 80. Since Bisalloy 400 has a higher fy than Bisalloy 80, the reduction in fyT/fy is faster for the former. As T increases further, fyT decreases quickly. At 700
C, values of fyT reduce by 37% and 61% on average for Bisalloy 80 and Bisalloy 400, respectively. At 700
C or below, the influence of cooling method on fyT is negligible. However, heat soak time has some influence when T is in the range of 500e700
C.

Fig. 11. Method to determine elastic modulus.

For example, when the furnace-cooled Bisalloy 80 is soaked at 700
C for 0.5 h, the decrease in fyT is 30%. But when the heat soak time increases to 4 h, the corresponding reduction in fyT increases to 41%. Since the exposed temperature of 700
C is still lower than Ac1 (724
C), the heat exposure has a similar effect as tempering on the steel properties, i.e., yield stress decreases with increasing tempering temperature and time [13]. In general, the decrease in fyT for Bisalloy 400 is more significant than that of Bisalloy 80 in the temperature range of 300e700
C. If the heating temperature is beyond Ac1 (724
C), austenite forms in the steel [6]. During cooling, the cooling rate will affect the final phase of the steel. Ferrite and perlite are likely to form in steel cooled in a furnace at a relatively slow cooling rate, whereas air cooling with a faster cooling rate may result in the formation of bainite in the steel [23]. In contrast, cooling the steel in water is similar to quenching adopted in the manufacture of QT steel. The fast cooling rate leads to the regeneration of martensite phase in the steel, which is hard and brittle [6,23]. The microstructure change of QT steel under various cooling conditions will be further discussed in Section 3.5. Because of the difference in microstructure, QT steel cooled from a temperature above Ac1 could have different values of fyT under different cooling conditions. Steel cooled in furnace has the lowest fyT, followed by steel cooled in air; whereas steel cooled in water has the highest fyT. The highest heating temperature in previous studies was 1000
C for QT steels [7e12]. But in real fire scenarios, the maximum temperature in a compartment could exceed 1000
C. Therefore, Bisalloy 80 was chosen to investigate the influence of heat exposure to 1200
C. As shown in Fig. 14a, the residual yield stress at 1200
C is very close to that at 1000
C if the steel is cooled in furnace or air. However, compared to fyT at 1000
C, a lower fyT at 1200
C is obtained for water-cooled material, which is due to the significant increase in prior austenite grain size and packet size of lath martensite at a very high temperature of 1200
C [24]. Test results of fyT for S690 [7,9], Q690 [10], S960 [8] and Grade 1200 [11,12] reported by other researchers are also presented in Fig. 14. As can be seen, the measured fyT/fy ratios from the current study are comparable to or lower than the corresponding values reported by others. This is due to the relatively long heat soak time adopted in this paper.

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X.-Q. Wang et al. / Journal of Constructional Steel Research 164 (2020) 105785

Fig. 12. Variation of ET/E with temperature.

3.4.3. Ultimate strength The effect of heat exposure on the normalised residual ultimate strength (fuT/fu) is presented in Fig. 16. The heat exposure does not affect the ultimate strength when the temperature is lower than 500
C for Bisalloy 80 or 200
C for Bisalloy 400. Beyond this temperature, a decrease in fuT can be found. From the comparison

Fig. 13. Comparison of residual yield stresses between Bisalloy 80 and Bisalloy 400.

between Figs. 14 and 16, it can be concluded that the effects of different factors (i.e., temperature, soak time, cooling method, and material type) on fuT are very similar to those on fyT, which have been thoroughly discussed in the last subsection. The test results of fuT for S690 [7,9], Q690 [10], S960 [8] and Grade 1200 [11,12] reported in the literature are also plotted in Fig. 16 for comparison. It can be concluded that the measured fuT/fu ratios from the current study are comparable to or lower than the corresponding values reported by others, which is due to the relatively long heat soak time adopted in this study. 3.4.4. Ultimate strain The measured residual ultimate strain (εuT) corresponding to fuT is normalised with respect to εu. The ratio of εuT/εu is demonstrated in Fig. 17 as a function of temperature (T). When the temperature range is 100e500
C for Bisalloy 80 or 100e300
C for Bisalloy 400, the heat exposure has no significant influence on εuT. But a decrease in εuT can often be observed in the temperature range of 100e300
C, especially for Bisalloy 400. The decrease in εuT could be due to the decomposition of retained austenite, which has a higher amount in Bisalloy 400 tempered at a lower temperature. When the temperature range is 500e700
C for Bisalloy 80 or 300e700
C for Bisalloy 400, εuT increases with increasing temperature, which is due to the transformation of martensite to ferrite. For example, the average increase in ultimate strain is 91%

Fig. 14. Variation of fyT/fy with temperature.

X.-Q. Wang et al. / Journal of Constructional Steel Research 164 (2020) 105785

11

Fig. 15. Change in the shape of sε curve after exposure to a low temperature.

Fig. 16. Variation of fuT/fu with temperature.

for Bisalloy 400 at 500
C. In general, the percentage increase in εuT for Bisalloy 400 is higher than that of the Bisalloy 80. When T increases further to 800
C or above, the cooling method has a significant influence on εuT. When cooled in furnace or air, εuT tends to further increase. But εuT decreases at 1200
C for Bisalloy 80 except for the sample soaked for 0.5 h and cooled in furnace. When cooled in water, εuT starts to decrease after 700
C for Bisalloy 80 or 800
C for Bisalloy 400 due to the regeneration of a martensitic microstructure under fast cooling conditions. Prior to 500
C, the influence of heat soak time on εuT is not

obvious. However, in the temperature range of 700e1000
C, εuT generally increases with increasing heat soak time. For instance, Bisalloy 80 soaked at 700
C for 0.5 h reached an average ultimate strain of 1.79εu. The corresponding value increases to 1.95εu when soaked at the same temperature for 4 h. Test results of εuT for S690 [7,9], Q690 [10], S960 [8] and Grade 1200 [11,12] reported by other researchers are also presented in Fig. 17. As can be seen, the measured εuT/εu ratios from the current study are comparable to or higher than the corresponding values reported by others except a few data reported for the Chinese steel

Fig. 17. Variation of εuT/εu with temperature.

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X.-Q. Wang et al. / Journal of Constructional Steel Research 164 (2020) 105785

[10] shown in Fig. 17a. It seems that the relatively long heat soak time adopted in this paper has a potential to increase steel ductility. 3.5. Microstructural analysis Bisalloy 80 is selected to demonstrate the microstructural change of QT steel under different heating conditions. The presence of lath-shaped martensitic microstructure, ferrite and carbides can be clearly seen in the as-received material, as shown in Fig. 18a. When soaked at 700
C (
ET ¼ E



1 1  1:38  105 ðT  300Þ  7:76  107 ðT  300Þ2

4. Prediction of residual mechanical properties Residual mechanical properties such as modulus of elasticity, yield stress, ultimate strength and ultimate strain are important parameters in the assessment of fire-damaged structures [25]. Empirical models are often developed to predict these parameters. In 2013, Tao et al. [20] proposed empirical equations to predict mechanical properties of cold-worked and heat-treated steels after exposure to elevated temperatures. In developing the equations at that time, however, only 13 test data were available for QT

1 1 1  2:5  104 ðT  500Þ

4.1. Residual elastic modulus Tao et al. [20] proposed a model to predict ET for QT steel after exposure to elevated temperatures, as shown in Eq. (3).

The combined test data of ET in this study and the literature are compared in Fig. 19 with the predictions from Eq. (3). It can be observed that Eq. (3) generally underestimates ET of QT steel after

T  300
C T > 300
C

strong and brittle. For QT steel soaked at 1000
C and subsequently cooled in furnace, a prolonged soak time can also cause microstructure change. From the comparison between Fig. 18d and f, it can be seen that a longer heating time facilitates the growth of grains. The increase in grain size is also partly responsible for the strength decrease with increasing soak time, as reported in Section 3.4.

8 ET < ¼ E :

structural steel S690QL (fy ¼ 789 MPa), as reported by Qiang et al. [7]. Due to the lack of test data, 44 test data of QT reinforcing bars (fy ¼ 496e616 MPa) were also used in proposing the equations. In recent years, there has been growing interest in the post-fire mechanical properties of QT structural steels. More test data are now available in the open literature [7e12], as summarised in Table 1. In total, there are 179 test data available, including 62 data reported by others and 117 data from the current study. In the empirical equations proposed in Ref. [20], temperature is the only factor that determines the residual mechanical properties of QT steel after heat exposure. However, the current study shows that the tempering temperature of the as-received steel, heat soak time and cooling method also affect the residual mechanical properties. Therefore, the empirical equations proposed in Ref. [20] need to be revised to consider the possible influence of multiple factors, including heating temperature (T), tempering temperature of the as-received steel (Tp), heat soak time (ts) and cooling method (CIF, CIA or CIW). It should be noted that the unit for T and Tp in this paper are
C and that for ts is h. Due to the limited test data at 1200
C, the equations proposed in the following subsections are only valid for temperatures up to 1000
C.

(3)

exposure to a temperature above 300
C. As shown in Fig. 19a, the heat exposure has no significant influence on ET for furnace-cooled steel if considering the variation in test data. For air-cooled or water-cooled steel shown in Fig. 19b, ET reduces to some extent only at above 500
C due to the increased nonlinearity in stressstrain curves, as discussed in Section 3.4. Using the combined test data, Eq. (4) is proposed to predict ET based on regression analysis. Considering the significant variation in test data, the heat soak time is not included as a variable in Eq. (4). The predictions from both the previous model [Eq. (3)] and the revised model [Eq. (4)] are compared in Fig. 19 with test data. For the previous model, the mean value (m) and standard deviation (SD) of the ratio of (ET/E)test/(ET/E)predicted are 1.11 and 0.18, respectively. In contrast, the corresponding values are 0.99 and 0.08 respectively for the revised model, indicating an improvement in prediction accuracy.

T  500
C 500 < T  1000
C for CIF 500 < T  1000
C for CIA or CIW

(4)

X.-Q. Wang et al. / Journal of Constructional Steel Research 164 (2020) 105785

13

Fig. 18. Microstructure images of Bisalloy 80 (  1000).

4.2. Residual yield stress The original model proposed by Tao et al. [20] to predict fyT for cold worked or heat treated steels after heat exposure is expressed by Eq. (5).

fyT ¼ fy



1 1  1:33  106 ðT  300Þ2

T  300
C T > 300
C

(5)

As discussed in Section 3, four factors, including temperature (T), tempering temperature Tp, heat soak time (ts) and cooling method, affect fyT to some extent. However, T is the only variable

considered in Eq. (5). To improve the prediction accuracy, the influence of Tp, ts and cooling method are also considered. Accordingly, Eq. (6) is proposed to predict fyT based on regression analysis using the current test data and those reported by others as summarised in Table 1. It should be noted that Tp in Eq. (6) and the following equations should not be smaller than 200
C. Accordingly, Tp is taken as 200
C in the calculations for Grade 1200 specimens reported in Refs. [11,12]. The predictions from the previous and the new models are compared with the combined test data in Fig. 20. As can be seen, the new model gives more accurate predictions for fyT. For the previous model expressed by Eq. (5), the values of m and

14

X.-Q. Wang et al. / Journal of Constructional Steel Research 164 (2020) 105785

Fig. 19. Comparison between predicted and measured elastic moduli.

Fig. 20. Comparison between predicted and measured yield stresses.

SD for the ratio of (fyT/fy)test/(fyT/fy)predicted are 0.95 and 0.39, respectively; whereas the corresponding values are 1.02 and 0.12 respectively for the revised model. Clearly, SD for the revised model is much smaller than that for the previous model.

4.3. Residual ultimate strength

in which a0 is determined by Eq. (7).

 fuT 1 T 300
C ¼ 2 4 7 11:4110 ðT 300Þ5:6210 ðT 300Þ T >300
C fu

  a0 ¼ fy  1:05 724  0:85Tp ðts þ 1Þ0:2

The prediction model for the residual ultimate strength proposed by Tao et al. [20] is expressed by Eq. (8).

(7)

(8) As shown in Fig. 21a, the prediction accuracy of Eq. (8) is not

8 > > > fy > >   > > > fy  1:05 T  0:85Tp ðts þ 1Þ0:2 > > > > > > a  0:8ðT  724Þ > > < 0 " #  a fyT ¼ ðT  724Þ2 0 >  1 ðT  724Þ  a  > > > 0 200 550 > > > > # " > > > > ðT  724Þ2 > > ðts þ 1Þ0:3 a0  0:7 ðT  724Þ  > > 80 :

T  0:85Tp 0:85 Tp < T  724
C 724 < T  1000
C for CIF 724 < T  1000
C for CIA 724 < T  1000
C

for CIW

(6)

X.-Q. Wang et al. / Journal of Constructional Steel Research 164 (2020) 105785

15

Fig. 21. Comparison between predicted and measured ultimate strengths.

satisfactory when compared with the combined test data. To improve the prediction accuracy, the influence of Tp, ts and cooling method on fuT is further considered. Based on regression analysis, Eq. (9) is proposed to predict the ultimate strength after heat exposure. Fig. 21b demonstrates the comparison between predicted values of (fuT/fu)predicted and measured values of (fuT/fu)test. As can be seen, a significant improvement in prediction accuracy has been achieved compared with the predictions from the previous model proposed in Ref. [20]. This improvement is also confirmed by the comparison of statistical parameters (m ¼ 0.91, SD ¼ 0.27 for the previous model and m ¼ 0.99, SD ¼ 0.09 for the new model).

8 fu > > > >   > > > fu  1:05 T  0:85Tp ðts þ 1Þ0:2 > > > > > > b  0:6ðT  724Þ > > < 0 # "  fuT ¼ b0 ðT  724Þ2 >  0:4 ðT  724Þ  b  > > > 0 1000 1300 > > > > " # > > > > ðT  724Þ2 > > ðts þ 1Þ0:3 > : b0 þ 5 ðT  724Þ  550

T  0:85Tp 0:85Tp < T  724
C 724 < T  1000
C for CIF 724 < T  1000
C for CIA 724 < T  1000
C

in which b0 is determined by Eq. (10).

  b0 ¼ fu  1:05 724  0:85Tp ðts þ 1Þ0:2

(10)

4.4. Residual ultimate strain Eq. (11) was originally proposed by Tao et al. [20] to predict the ultimate strain after heat exposure.

εuT ¼

8 > > > <

100

fyT ET

fy 300 MPa

h  i f > > > : 1000:15 fy 300 yT ET

the influence of all four factors, including T, Tp, ts and cooling method, are considered in proposing a revised model for εuT. It should also be noted that Eq. (11) is not applicable for steel with a yield stress above 800 MPa. Based on regression analysis, Eq. (12) is proposed herein to predict the ultimate strain of QT steel after exposure to elevated temperatures. Although εuT is not directly expressed as a function of T, Tp and ts, their influence has been considered when calculating ET and fyT. The predictions from Eq. (12) are compared with the combined test data of εuT, as shown in Fig. 22b. For the previous model, the mean value m and standard deviation SD of (εuT/εu)test/ (εuT/εu)predicted are 0.68 and 0.33, respectively. For the revised

(11) 300
Predictions from Eq. (11) are compared with the combined test data in Fig. 22a, indicating a low prediction accuracy. Accordingly,

(9)

for CIW

model, the corresponding values are 0.93 and 0.20, respectively. As can be seen, the variation in prediction accuracy for the new model is much less significant. It should be noted that the measurement of ultimate strain is affected by many factors, such as the chemical composition and treatment of steel, tensile loading rate and accuracy of strain measuring device. Significant variation in ultimate strain is also found for normal hot-rolled steels at room temperature [26]. By taking this into account, the prediction accuracy of Eq. (12) can be considered acceptable.

This study indicates that the original tempering temperature Tp of QT steel is an important factor that affects the residual mechanical properties. In some cases, however, the information of Tp may not be readily available. As can be seen from Table 1, Tp normally ranges from 590 to 640
C for QT steels with a nominal fy of 690 MPa. As this type of steel is currently the most widely used QT

16

X.-Q. Wang et al. / Journal of Constructional Steel Research 164 (2020) 105785

Fig. 22. Comparison between predicted and measured ultimate strains.

steel, its Tp may be taken as 600
C if no information is available.

5. Conclusions

(4) Quenched and tempered steel Bisalloy 400 with a higher original strength has a higher strength loss after exposure to elevated temperatures of over 300
C. When the exposure temperature is higher than 500
C, however, the residual properties of Bisalloy 80 and Bisalloy 400 become similar.

This paper presents an experimental study on the behaviour of

8 > > > > > > > > > > > > > > > > <

T  0:85Tp  2 ! fyT fyT 0:29  90  85 0:85 Tp < T  724
C  εu ET ET  2 ! εuT ¼ fyT fyT > 0:29  90  85 724 < T  1000
C for CIF or CIA > > > E ET T > > > > > >  2 ! > > fyT fyT > > 0:29  100  100 724 < T  1000
C for CIW > > : ET ET εu

quenched and tempered high strength steels (Bisalloy 80 and Bisalloy 400) after exposure to elevated temperatures up to 1200
C. The phase change temperature of Bisalloy 80 was identified by thermomechanical analysis. The influence of heating temperature, heating time and cooling method on the post-fire mechanical properties were investigated. The conclusions from this experimental study include: (1) Heating temperature has a significant influence on residual mechanical properties. Its influence becomes apparent when the heating temperature is close to or beyond the tempering temperature. At 700
C, the yield stress on average reduces by 37% for Bisalloy 80 and 61% for Bisalloy 400. (2) Heat soak time affects the residual mechanical properties: a longer soak time generally leads to a lower strength. This effect is more significant for steel with a higher strength. (3) Cooling method has minor influence when the temperature is lower than the austenitising temperature Ac1 (724
C on average). Beyond Ac1, its influence becomes significant. Furnace cooling results in the lowest residual strength and the highest ductility, whereas the water-cooled material has the highest strength and lowest ductility due to the regeneration of martensite phase in the steel.

(12)

(5) Based on regression analysis, empirical equations are proposed to predict ET, fyT, fuT and εuT considering the influence of multiple factors, including the heating temperature (100e1000
C), the tempering temperature (200e640
C), heat soak time (0.5e4 h) and cooling method (cooled in furnace, air or water). Within the investigated parameter ranges, the prediction accuracy of the proposed equations is much better than that of previous models in the literature. The proposed equations are suitable for QT steels with nominal yield stresses from 690 MPa to 1200 MPa.

Acknowledgments The authors would like to express sincere appreciation to Dr. Richard Wuhrer and Dr. Laurel George in Advanced Materials Characterisation Facility, Western Sydney University, for their assistance in conducting the TMA test. The authors would also like to extend special thanks to Dr. Xin Yu for his assistance in testing chemical composition and conducting microstructure characterisation. The authors would also like to express special thanks to Bisalloy Steels and SSAB for providing technical information of the quenched and tempered steel.

X.-Q. Wang et al. / Journal of Constructional Steel Research 164 (2020) 105785

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