Evolution of mechanical behaviour in a structural steel ...

Engineering Failure Analysis 9 (2002) 191–200

Evolution of mechanical behaviour in a structural steel subjected to high temperatures J. Setień a,*, J.J. Gonza´lez b, J.A. Alvarez a, J.A. Polanco a a

Divisioń de Ciencia e Ingenierıá de los Materiales, Universidad de Cantabria, Avda. Los Castros s/n, 39005 Santander, Spain b Departamento de Ingenierıá Minera y Metalu´rgica y Ciencia de Materiales, Universidad del Paı´s Vasco, Alameda de Urquijo s/n, 48013 Bilbao, Spain Received 14 November 2000; accepted 26 December 2000

Abstract In this work a study of how exposure to high temperatures can affect the mechanical properties of a structural steel is presented. To this end, several mechanical tests such as tensile, impact and microhardness tests have been carried out in order to compare the properties of the heat-exposed steel with those of an unaffected reference steel. Complementary compositional, metallographic and fractographic analyses have also been performed to try to cast some light on the problem. The results obtained allow us to extract some conclusions on the reutilization of the steel for structural applications. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Structural steel; Mechanical testing; Damage assessment; Fire damage

1. Introduction In this work the studies carried out to characterize the mechanical behaviour evolution of a steel in the structural shapes of a building subjected, in a fortuitous way, to the action of a fire are presented. The aim of such a study is to assess a possible loss of load bearing in the material so as to decide whether or not to keep the affected structural shapes during the rebuilding. For the experimental program pieces of two different structural shapes subjected to overheating during the fire have been used. These two beams were located in two separate regions of the building and consequently, a different exposure to heat and damage was expected. Also, a piece of the structural shape in the original non-affected conditions was used as a reference material. The structural shapes were standard beams (double-tees) with 4.3 mm of thickness in their centers. Obviously, this particular geometry is very restrictive for the shapes and dimensions of the different specimens that can be used in the material’s characterization. Such a characterization, carried out on both the fired and reference steels, included a detailed compositional analysis, an exhaustive microstructural study

* Corresponding author. Tel.: +34-942-201827; fax: +34-942-201818. E-mail address: [email protected] (J. Setień). 1350-6307/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(01)00008-5

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J. Setień et al. / Engineering Failure Analysis 9 (2002) 191–200

and the performance of microhardness, tensile and impact tests along with their corresponding fractographic analysis. The assessment of the obtained results will justify the different behaviours reported and will be a key consideration in taking decisions about the possible reutilization of the affected beams during the rebuilding process.

2. Experimental program The experimental program is based on the comparison of the results obtained from tests performed on the fire-affected steel, henceforth denoted as material F, and the reference steel (material R). The following tests were carried out on each material: 2.1. Chemical composition Conventional chemical analysis was made on coupons of both materials F and R in order to look for compositional differences that could point out some phenomenon due to the action of high temperatures, such as decarburations. Also specific determinations of the contents of elements such as boron, aluminium (total aluminium, Alt, and soluble aluminium, Als) and, specially, nitrogen and oxygen were carefully made. 2.2. Metallographic analysis The main objective of the metallographic examination is to know the original microstructure of the reference material in order to be compared with that of the heat-affected material. Possible microstructural changes such as recrystallizations, oxidations, precipitations or decarburations can be revealed through this analysis. In this way, two coupons of materials F and R, respectively, were subjected to a conventional metallographic preparation using Nital 3 as etchant for observation in light microscope. 2.3. Microhardness tests Vickers microhardness measurements were carried out on three different coupons, two of them from the fired beams (F1 and F2) and the third from the reference material. Measurements were made on crosssections of the centers of the beams. For coupons F1 and F2 the central region of cross-sections as well as the regions near the free surfaces were tested in order to detect any possible local surface phenomenon that could be produced by the action of the fire. Coupon R was analyzed only on its central region of crosssection because no significant differences with any other region were expected. In all cases, a load of 300 g was applied for 20 s and all tests were carried out according to the standards in use [1,2]. The results presented throughout this paper are in each case the mean value of 10 measurements. 2.4. Tensile tests Two tensile flat specimens were machined from each one of the three available beams. Shape and dimensions were selected according to the recommendations of the corresponding standard [3]. The specimens used had the same thickness as the whole center of the beams, being 25 mm wide. In the performance of the tests a displacement velocity of 0.2 mm/s was adequate for all purposes. Besides, 20 mm separated marks in the shaft of the specimens allowed the measurement of elongations at the end of the tests. This parameter can be considered as a good index of the material’s ductility.


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2.5. Impact tests In order to evaluate possible impact toughness variations in the material as a consequence of the high temperatures reached, six Charpy-type specimens were machined from each one of the three available beams. Only sub-sized Charpy specimens can be achieved by machining of the center of the beams. Following the recommendations of the standard in use [4], type-A specimens 10 mm wide, 2.5 mm thick and 2 mm deep of the notch were selected. In order to determine the complete transition curve, specimens were tested at different temperatures ranging from room temperature down to 196 K. 2.6. Fractographic analysis A detailed fractographic study has been carried out on both tensile and impact specimens after tests in order to know the fracture mechanisms involved in each case.

3. Results The results obtained from the proposed experimental program can be summarized in the following basic remarks. 3.1. Chemical composition Table 1 shows the results of the compositional analysis carried out on coupons R and F, respectively. All results are expressed in weight percentages or ppm if applicable. It seems clear from these results that we are dealing with two mild steels from different heats as the C, Mn and S contents make evident. It is important to underline the lack of deoxidizing elements such as Al and Si. It is also worth noting the high oxygen content, the nitrogen content being normal and, as a consequence, the original material can be considered as a rimmed steel. The presence of such elements will be decisive for explaining the posterior behaviour of the material. 3.2. Metallographic analysis The metallographic study carried out on the reference material shows, as predicted, a typical ferritic microstructure with a content of pearlite strongly conditioned by the low carbon content of the steel. Equiaxed grains of uniform size (ASTM No. 8 [5]) are very usual for this kind of steel. Figs. 1 (80) and 2 (400) show micrographic images of the general appearance and a detail, respectively, of the reference material’s microstructure. In the same way, Fig. 3 (80) shows the microstructure of burnt material F2 in a region near its surface. As can be clearly seen the iron oxidation has produced big black spots and a light decarburation around

Table 1 Chemical composition of the considered materials (wt.%) Steel C R F

Mn Si

P

S

Cr

Ni

Mo

V

Cu

Alt

Als

Sn

Ti

B

Nb

N

O

0.18 0.81 0.03 0.01 0.023 0.01 0.03 < 0.01 0.003 < 0.01 0.004 0.003 0.001 0.001 1 ppm < 0.02 47 ppm 145 ppm 0.14 0.87 0.03 0.01 0.015 0.01 0.03 < 0.01 0.003 < 0.01 0.003 0.002 0.002 0.001 1 ppm < 0.02 24 ppm 134 ppm

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Fig. 1. Microstructure of reference material, nital 3% (80).

them can also be reported. Nevertheless, the inward penetration in the steel of such oxidation is nonexistent and consequently it can be considered as a surface phenomenon of the fired beams. In fact, no modifications by fire action are detected in the microstructure of inner regions in coupons F1 and F2, as can be seen in Fig. 4 (400). As a consequence, one might think that the temperatures during fire in the beams were kept below the characteristic recrystallization temperature (or any other characteristic transformation temperature) of the material (typically about 720 C). 3.3. Microhardness tests Microhardness tests carried out on F1 material gave values of 155 HVN near the free surface of the coupon and 136 HVN in its central region. The difference in these values is not very significant but it seems that a light hardening of the material in the regions near the surfaces affected by fire has occured, although this effect can also be produced during the rolling process. Something similar has been reported for the F2 coupon, with values of 150 HVN near the surface and 141 HVN in the central region. The mean value of the results obtained for coupon R is 138 HVN, which can be considered as a normal value for this kind of steels and microstructures. 3.4. Tensile tests Fig. 5 shows the load-displacement curves obtained in tensile tests from the F1, F2 and R specimens. As can be clearly seen, ductility in the reference material is markedly higher than in materials F1 and F2 and a significant reduction in the ductility of the original material by heat action can be, therefore, inferred. The


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Fig. 2. Detail of ferritic–pearlitic microstructure of reference material, nital 3% (400).

final elongations including necking, measured after tests over broken shafts, gave values of 22.5% for F1, 25.6% for F2 and 42.5% for the reference material. A small reduction in both yield strength and tensile strength has also been reported in materials F1 and F2 with regard to the reference material, but no changes in Young’s modulus values have been recorded. Finally, a light Portevin-Le Chatelier effect in the plastic region of specimens F1 and R can be detected although no influence in the overall mechanical behaviour of the materials is expected. 3.5. Impact tests The absorbed energy vs. temperature curves for the considered materials after impact fracture of specimens 2.5 mm thick are plotted in Fig. 6. In this graph experimental data have been fitted to an hyperbolic tangent-type function. The transition temperature is in this case the variable reflecting the damage in the material. This is because of the strong tendency to develop plane stress fracture conditions in specimens of such a small thickness instead of plane strain conditions and consequently the variations in the amounts of absorbed energy in the ductile region are not reliable. In this way, the reference material has a transition temperature of 75 C, being 65 C and 50 C for F2 and F1 respectively in this type of test and with this particularly thin specimen. 3.6. Fractographic analysis Featureless results have been obtained from the fractographic analysis carried out on both tensile and impact specimens. Characteristic cleavage fractures were reported in specimens broken at low temperature

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Fig. 3. Microstructure of material F2 near surface, nital 3% (80).

tests as can be seen in Fig. 7. A detailed inspection of fracture surfaces can also reveal some incipient intergranular behaviour tendency as shown in Fig. 8. A possible justification of such behaviour can be attributed to the fact that the steel has been produced without a deoxidation process and consequently there is high oxygen content at interstitial positions and grain boundaries with oxygen segregations [6].

4. Analysis of results According to the fire-fighters brigade report, the duration of the fire was around 10 h and the estimated temperature over the structural shapes was kept below 700 C. Under these conditions it can be considered that the steel has undergone a long subcritical annealing as a heat treatment. In a rimmed steel, such treatment is not advisable according to the bibliographical sources [7] and the steel manufacturers experience because it produces the effects that have been reported throughout this paper: a light softening of the material with a small reduction of strength associated with a marked loss of ductility and a considerable drop in toughness. In this case, the performance of fracture toughness tests (CTOD or J-contour integral) is not really significant with the available material thickness, which is below that recommended by the standards for validating the tests; therefore, this type of test has not been considered. These harmful mechanical effects are generally associated with the thermally-activated diffusion of interstitial elements such as carbon, nitrogen and above all in this case oxygen, to the dislocations [8]. The slip of such dislocations is then hindered and also their number is reduced in a kind of recovery of ferrite as a consequence of high temperatures. Therefore, the drop in strength associated with a simultaneous reduction in ductility can be justified by the difficulty of dislocations motion in ferrite BCC.


Fig. 4. Inner microstructure of material F2, nital 3% (400).

Fig. 5. Load–displacement curves for all materials tested.

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198


Fig. 6. Ductile–brittle transition curves for the three materials studied.

Fig. 7. Cleavage fracture in specimens tested at low temperatures (500).


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Fig. 8. Incipient intergranular behaviour tendency in overall cleavage fracture of impact specimens.

5. Conclusions In this work the assessment of damage in the steel of different beams subjected to high temperatures during a fire is presented. The following concluding remarks can be derived from the analysis of the results obtained: . after fire, a slight reduction of strength and a simultaneous marked loss of ductility are noticeable; . in the explanation of these phenomena, the high content of oxygen in the steel plays a fundamental role due to its condition as an interstitial element, acting after diffusion processes as a dislocations stopper; . the damage to the steel is significant though not dramatic and consequently can be used again in a non critical metallic structure. This is because its mechanical properties after fire meet the minimal requirements of the corresponding standards [9,10].

References [1] Standard test method for Vickers hardness of metallic materials. E92-82. Annual book of ASTM standards, vol. 03.01, 1997. [2] Standard test method for microhardness of materials. E384-89. Annual book of ASTM standards, vol. 03.01, 1997. [3] Ensayos de traccioń a temperatura ambiente de productos de acero. Norma UNE 36-401-81. Aceros para construccioń meta´lica, AENOR, 1987. [4] Standard test methods for notched bar impact testing of metallic materials. E23-96. Annual book of ASTM standards, vol. 03.01, 1997. [5] Standard methods for determining average grain size. E112-84. Annual book of ASTM standards, vol. 03.03, 1984. [6] Honeycombe RWK, Badeshia K. In: Arnold E, editor. Steels: microstructure and properties. 2nd ed. 1995.

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