behaviour of structural carbon steel at high temperatures

8/16/2019 Behaviour of Structural Carbon Steel at High Temperatures

1/10

Journal of Materials Science and Engineering A 2 (7) (2012) 501-510

Behaviour of Structural Carbon Steel at High

Temperatures

Juan Antonio Trilleros Villaverde and Irene Huertas González Department of Materials Science and Metallurgical Engineering, Universitas Complutensis of Madrid, Ciudad Universitaria s/n,

Madrid 28040, Spain

Received: May 19, 2011 / Accepted: June 08, 2011 / Published: July 10, 2012.

Abstract: The research develops an experimental study on two carbon steels used for construction of structures on civil works,S355NL and S460NL, to high temperatures with values ranging between 500 °C and 950 °C. In order to obtain experimental evidence

of behavior of steels at elevated temperatures, the chemical and structural characterizations and tested steels were made, deducing thefound microstructures and average grain sizes measured. Tensile tests at elevated temperatures were made and steels mechanical properties were deducted at different temperatures. Also hardness steels tests were made. Creep and plastic deformation phenomenawere observed from micrography and fractography studies.

Key words: Structural steel at high temperature, behavior of steels at elevated temperatures, mechanical properties for steel at hightemperature.

1. Introduction

The weldability of fine grain structural steels

respond to the requirements of the standard EN

10025-3: 2006 [1], defines four levels of mechanical

properties, S275, S355, S420 and S460, where the

numbers represent the minimum yield strength in MPa.

Each quality can be delivered with properties of

resilience are guaranteed to -20 °C (N grades) or -50 °C

for applications at low temperatures (NL grades).

These structural steels are hot rolled and produce a

largest class of commercial steels from 0.05 to 0.20

weight percent of carbon. Their most typical

applications are bridges, electrical towers, otherarchitectural structures and buildings.

The process of hot strip rolling of low carbon steels

can be subdivided into three stages; reheating, rolling

and cooling; and the metallurgical phenomena which

occur in these processing steps are the recrystallization

and grain growth in austenite, the austenite-ferrite

Corresponding author: Juan Antonio Trilleros Villaverde,Ph.D., professor, research field: behaviour of structural carbonsteel at high temperatures. E-mail: [email protected].

phase transformation and the precipitation. The two

last stages essentially determine the mechanical steel

properties, which depend on the character of the

transformation products (ferrite and pearlite), and theferrite grain size [2-7].

When a component that consists of a multiphase

structure is subjected to thermal or mechanical loads

that cause structural changes due to phase

transformation, an assumption is made that the

mechanical and physical properties of the material are a

combination of the properties of the phases. During

phase transformation, three types of parameters are

dominant: temperature fields, phases of metallic

structures and elastic or inelastic stress strainrelationship [8].

The temperature to which the carbon steel is heated

before cooling has begun determines the phases that are

present at the beginning and the end of the cooling

process [9]. For simulate the phase transformations

Serajzadeh [10], proposed a model for predicting

temperature history and microstructural changes

during cooling steel. He took cognisance of the effects

DAVI D PUBLISHING

D


2/10

Behaviour of Structural Carbon Steel at High Temperatures502

of a number of factors including initial austenite grain

size and its role on the kinetics of pearlite and ferrite

transformation, the amount of residual strain within the

cooling material and heat of transformation. Also, it

was observed that non-uniform cooling can cause

inhomogeneous distribution of ferrite grain size in the

final product. Slow cooling of steel to below the

eutectoid temperature results in the formation of ferrite

and cementite; this might appear as proeutectoid ferrite

and a lamellar ferrite-cementite structure called pearlite,

depending on the carbon content of the steel and the

cooling rate. Slower cooling rates produce coarser

microstructures.

Grain size has a market influence on the mechanical properties of steel. Tensile strength, yield strength,

toughness and hardenability can be altered by varying

the grain size of steel [11]. Yield strength is the amount

of stress at which plastic deformation becomes

significant, besides grain size, yield strength is also

influenced by other factors such as chemical

composition, forming process and heat process [12].

Yield strength varies proportionately with the square

root of grain size.

Hardness can be roughly defined as a measure of

metal’s resistance to plastic deformation usually by

penetration. Hardness can also be used as an indicator

of a metal’s strength. The correlation of hardness with

tensile strength is generally good. Sleboda [13] report

that polycrystalline materials often show an increase in

hardness and strength with decreasing grain size.

Schiotz [14] showed that hardness of steel increases

sharply for decreasing grain size, reaches a peak at a

critical grain size and decreases inversely to the square

root of grain size.

The current work presents an experimental study of

the behaviour of two structural carbon steels, both at

room temperature as elevated temperature, from 500 °C

to 950 °C at four different temperature levels. In the

study have been available with sufficient material from

hot strip rolling steels by Arcelor Mittal and perform

the chemical and structural characterizations of steels

(grain size and phases). Also have obtained mechanical

properties of steels (tensile and hardness tests), and

fractographic studies of steels tested. All this, whereas

tests at room temperature and at four levels of high

temperature, in order to have accurate information toenable further study the behavior of steels when they

were being subjected under a fire test.

2. Experiments

2.1 Material Tested

The work was carried out using two types of

common structural steels, the steel grades in

EUROCEDE 3, are based on the standard, [1]. The

selection of tested material was S355NL and S460NL.The chemical composition is given on Table 1 and the

mechanical properties on Table 2.

2.2 Testing Program

2.2.1 Samples Preparation for Microstructural

Analysis

Initially the steel samples, S355NL and S460NL, were

200 × 100 × 20 mm. These specimens were cut to do

Table 1 Chemical composition of steels tested.

STEEL C% Si% Mn% P% S% Cr% Ni% Mo% Al% Cu% Co%S460NL 0.127 0.286 1.40 0.019 0.006 0.029 0.510 0.008 0.028 0.013 0.018S355NL 0.169 0.310 1.14 0.018 0.006 0.046 0.244 0.008 0.040 0.216 0.019

Table 2 Mechanical properties of steel at room temperature.

STEEL Effective yieldstrength (MPa)Elastic modulus(GPa)

Proportional limit0.2%(MPa) Area reduction (%) Elongation (%)

Rockwell Bhardness

S355NL 555.8 203 353 75 12 82S460NL 560.9 218 407 77 13 86


3/10

Behaviour of Structural Carbon Steel at High Temperatures 503

slice of 200 × 100 × 20 mm and it was taken different

part of this slices to do the microstructural analysis

along three directions (rolling, cross and longitudinal

section according rolling sections).

All samples were grinded with different sandpapers

(320, 600, 1,200) and polished with diamond paste

(3µm and 1µm). Then, Nital 2% was used (60%-62%

nitric acid and ethyl alcohol 99.5%) to attack the

samples. This solution was chosen because the

material to be investigated was a plain carbon steel

and the effect of Nital is darkens pearlite and give

contrast between pearlite colonies, reveals ferrite

boundaries and differentiates ferrite from martensite.

2.2.2 Microstructural AnalysisThe steels samples of 200 × 100 × 20 mm were

heated at constant temperatures, 620 °C, 820 °C and

920 °C (firstly the furnace was programmed at

required temperature, secondly the furnace reach the

temperature and finally the specimens were introduced

and it were stayed the optimal time was remained to

reach the temperature, 30 min/pulgada) and

subsequent cooling in air. Then, a microstructural

study was done with the steels tested by light opticalmicroscope (LOM). This is one of the most commonly

used techniques for microstructure characterization in

steels. It was carried out using Nikon EPIPHOT 300

light optical microscope equipped with software for

image analysis. The S355NL and S460NL steels were

characterized at room temperature and post-heating at

elevated temperature.

The grain size was calculated using the main linear

intercept method (Heyn procedure) as described in

UNE-EN ºC ISO 643 [15]. This technique is one of the

numerous procedures within the field of stereology that

have been developed to estimate average grain size,

grain shape, grain distribution, grain orientation etc.

Metallographic measurements are usually made in a

2-D planar section of a volume and in this specific case,

the average grain size calculations were based on

measurements made on LOM micrographs.

After tensile test with the samples tested a

micrographic and fractographic study was done too. It

was used a Nikon EPIPHOT 300 light optical

microscope equipped with software for image analysis

to see the microstructure and a scanning electron

microscopy (SEM) JEOL 6400 JSM at the CNME

which is a 35 kW machine offering resolution of up to

10 nm to see the fracture area.

2.2.3 Mechanical Analysis

It was done the small-scale tensile test of S355NL

and S460NL steels at room temperature and high

temperature. In the tests, the samples studied,

according UNE EN ISO 6892-1: 2009 [16], were

heated up to a specified temperature (500 °C, 650 °C,

800 °C and 950 °C) in electric furnace. After reaching

the preselected temperature, around 21 °C/min,

approximately 10 min was required for the temperature

to stabilize and after the tensile test was carried out.

The loading rate was kept constant of 0.025 mm/sec.

Stress and stain values were first recorded and from the

stress-strain curves the mechanical material properties

could be determined. The testing device is illustrated in

Fig. 1.

Finally, hardness Rockwell B was used for testedsteels to know the resistance to plastic deformation by

penetration. To do the hardness measurements it was

used a hard steel indenter of 1/16 inch, 100 kg load,

a pattern of HRB hardness 76.1 ± 1 and a preload of

10 kg.

3. Test Results

3.1 Structural Analysis and Average Grain Size

The micrographs shows that in the reception state

and in the roll, longitudinal and cross section, both

steels have a one-way striping of fine pearlite (dark

bands), consisting of ferrite and cementite (Fe 3C) and

ferrite (light bands). The average grain size of S460NL

steel was about 8-9 μ m and the S355NL steel had a

slightly higher grain size, about 9-10 microns. Both

steels have an equiaxied shaped grains (Fig. 2).

When steel is maintained during some time and at

High temperatures, the austenite crystals tend to grow


4/10


Fig. 1 High-temperature tensile testing device.

S355NL S460NL

Fig. 2 LOM micrographs at room temperature of S355NL and S460NL steels (longitudinal section), × 100 and × 500.

and increase its size. The growth is proportional to the

temperature achieved and the time of heating. The size

of the steel crystals after cooling, depend on the size

of the austenite crystals reached during the heating

time.

For S460NL steel at 620 °C, according to the

equilibrium diagram Fe-C, hasn´t a significant

difference respect to the initial state. This was due it

did not pass the austenitizing temperature around of

723 °C where the pearlite transforms to austenite.

Therefore the band structure remains and the grain size

was still the order of 8 μ m (Fig. 3a). In the case of steel

S355NL shows the same trend and keep the grain size

in 10 μ m (Fig. 4a).

At 820 °C the ferritic-pearlitic bands start to

disappear. For this temperature the equilibrium

diagram Fe-C shows a biphasic region where there is

ferrite and some of pearlite has transformed in

austenite. After cooling, the pearlite is redistributed.

The ferrite grains and ferrite conglomerates start to

(a) (b) (c)Fig. 3 LOM micrographs at (a) 620 °C, (b) 820 °C and (c) 920 °C of S355NL steel (longitudinal section), × 500.

Thermocoples

Test Piece

Resistor Elements


5/10


6/10


950 °C, were determined from the stress-strain curves

and are presented on Table 4.

For both steels were observed that the strength,

modulus and the limit elastic decrease with thetemperature. At 500 °C and 650 °C, the mechanical

behavior of S460NL steel is greater than S355NL steel.

The elongation and reduction area present a uniform

trend due to thermal creep phenomenon that develops

during the test.

The plastic deformation areas decrease very

strongly with the temperatures tested. For temperature

800 °C or above there is not plastic deformation and

necking. For the temperature interval of 500 °C and

650 °C, a plastic deformation occurs with formation of

neck and reduction of maximum load resistance. For

temperatures of 800 °C and 950 °C a plastic

deformation develops along the total length of the

sample and therefore also the necking. The

deformation occurs at constant load and it matches

with the maximum strength, (it will be seen later, in

micrographic and fractographic studies in Fig. 9).

The comparison between steels indicates that the

S460NL steel tensile properties are greater thanS355NL ones. Therefore S460NL will be a more

resistant steel with the temperature but its elongation is

the same order that S355NL steel.

In both steels, the grain size decreased slightly with

temperature tested but its variation was not significant.

The measures hardness just presented variation with

temperatures tested. The hardness decreases slightly,

the variation does not exceed 5%, this means that steels

recover the property during the cooling process, bynatural convection and almost in equilibrium.

In the fracture zone from tensile test, specific values

have been measured for hardness RB, taking account

that the measurement surface was very small due to

the size and shape of the samples broken. For S355NL

hardness values were 70, 63, 46, 31 and 25 RB for

(20 °C, 500 °C, 650 °C, 800 °C and 950 °C) and for

S460NL were 61, 63, 68, 66 and 45 RB at same

temperatures. For the first steel, thoses hardness

decrease significantly. While on the second steel,

variation of hardness with the temperature remained

almost constant in the tested interval except at 950 °C

which declined. However, measures hardness

decreases were lower with relationship to the first

tested steel.

Finally on the first graph of Fig. 6 is shown the

experimental reduction factor of yield strength for

both steel tested and the standard values of EN 1993-1.

It can see that for temperatures of 450 °C and 650 °C,experimental reduction factors were lower than those

who predicted the standard. This fact is of interest and

must be taken into account above all in the initial

moments of fire propagation where the resistance of

steels would be lower which predicts the standard. On

Tables 4 Mechanical properties for S355NL and S460NL steels at different temperatures obtained from steady-state test.

Steel/Temperature (ºC)

Effective yieldstrength(MPa)/Reduction factor

Elastic modulus(GPa)/Reductionfactor

Proportional limit0.2% (MPa)

Reductionarea(%)

Elongation(%)

Averagegrain size(µm)

HardnessRockwell B

S-355NL (22) 535/1 908/1 372 70 14 10 82S-355NL (500) 283/0.53 668/0.74 265 85 18 10 82S-355NL (650) 119/0.22 420/0.46 118 86 18 10 81S-355NL (800) 71/0.13 257/0.28 64 46 16 11 80S-355NL (950) 38/0.07 154/0.17 35 41 18 11 80S-460NL (22) 580/1 1003/1 416 60 11 8 87S-460NL (500) 339/0.58 794/0.79 310 85 16 8 86S-460NL (650) 203/0.35 575/0.57 198 85 19 8 86S-460NL (800) 69/0.12 251/0.24 66 67 42 9 81S-460NL (950) 40/0.07 291/0.29 35 45 17 9 81


7/10


Fig. 6 Comparison of reduction factor for yield strength and elastic range experimental values with the standard values of

EN 1993-1.

the second graph is presented the experimetal and

standard reduction factor of elastic range. The

experimental values were greater and tested steels

would suffer a greater deformation in the development

of a fire, which will mean a fact to be taken into

account if the case that were happen. By extrapolacion

of experimental values, it can be deduced that the

deformation of steels would take place from the 390 °C.

3.3 Micrography Study

Three areas were selected to observe the

deformation for the mechanical test samples. No

deformed area, and in the useful area, the part farthest

and nearest of the fracture. The micrographs presented

were made at 22 °C and after high temperature tensile

test (500 °C, 650 °C, 800 °C and 950 °C) for S355NL

and S460NL steels.

In the micrographs of S355NL and S460NL steels, it

was shown that fracture areas with minimumdeformation and without deformation were the same

for all temperatures. For 22 °C, 500 °C and 650 °C, on

the nearest zone of the fracture, there was maximum

deformation and grain elongation. For 800 °C and

950 °C, on fracture zone it was not observed a

lengthening of grain, due to the effect of thermal creep

(explained in the next section). And for 950 °C, the

grain size increases slightly.

3.4 Fractography Study

The macrographs made of the fracture area of the

S355NL and S460NL steels are shown below. The

figures presented are for the same steel (S460NL and

S355NL) and it was present a comparison between the

temperatures.

In all figures, the macrographs and micrographs show

that the behavior of both steels was quite similar. For

room temperature, 500 °C and 650 °C it was observed

the size of the base necking is almost the same. During

the rupture was formed a neck by nucleation and

coalescence of holes. The internal distribution holes can

be seen at 400 magnifications and it matches at

distribution holes for 500 °C and 650 °C. The break was

ductile, the surface is fibrous and during deformation a

neck was developed (Fig. 10).

At 800 °C, the reduction area decrease (Fig. 9), the

base of the tear was greater than for lower temperaturesand internal distribution appears to be less porous and

homogeneous (Fig. 10). At the end of the test, the

device detected the failure of the material, but if the

specimen is observed, it needed a few seconds to see

the separation of the parts. All of this was due the large

plastic deformation. A lot of cracks appeared on the

sharp edges of the samples and perpendicular to them

(Fig. 8). To 950 °C the changes in the morphology of


8/10


No deformed Minimum Maximum No deformed

(a) (b) (c) (d) Fig. 7 Micrographs of S-355-NL and S-460-NL steels after tensile test, × 500. (a) S355NL no deformed at 22 °C, 500 °C, 650 °C,800 °C and 950 °C, (b) S355NL minimum deformation at 22 °C, 500 °C, 650 °C, 800 °C and 950 °C, (c) S355NL maximumdeformation at 22 °C, 500 °C, 650 °C, 800 °C and 950 °C, (d) S460NL no deformed at 22 °C, 500 °C, 650 °C, 800 °C and 950 °C.

Fig. 8 Macrographs of S-355-NL(up) and S-460-NL(down) steels at 22 °C × 20, 500 °C × 20, 650 °C × 20, 800 °C × 20 and950 °C × 15.


9/10


Fig. 9 Macrographs of S-355-NL (up), S-460-NL (down) steels at 22 °C × 40, 500 °C × 40, 650 °C × 40, 800 °C × 27 and 950 °C × 27.

Fig. 10 Macrographs of S-355-NL(up) and S-460-NL(down) steels at 22 °C, 500 °C, 650 °C, 800 °C and 950 °C, all pictures × 400.

the materials tested were even greater.

The process of creep will be dominant when the

material temperature when is between 30% and 60%

of the corresponding melting temperature. This

circumstance occurs in steels tested at 800 °C and

950 °C and was checked on stress-strain curves where

there was a large plastic deformation on the material

tested. This is due to the dislocations movement

influenced by the high value of the temperatures,

which leads to the material to a plastic collapse with

the emergence of numerous fractures in the material

without contraction of the section on the material

tested.

4. Conclusions

The micrographs of steels for temperatures between

500 °C and 900 °C were obtained, when steels were

subjected to load and without it. It checked how the

ferritic-pearlitic band structure begins to disappear

from the 820 °C. No record of the structure in band was

found for 950 °C. It was found as appeared certain

areas in acicular Widmanstätten structure. The average

grain size tends to grow with the temperature in both

steels on one micrometer, being most clearly this trend

from the 920 °C.

The stress-strain curves and mechanical properties of

steels at temperatures of 20 °C, 500 °C, 650 °C, 800 °C

and 950 °C were studies and it saw how changed the

plastic deformation of steels with the temperature.

Reduction factors for effective yield strength resistance

and the elastic modulus were deducted along with the

values of the Euronorm for the interval of temperature


10/10


between 400 °C and 650 °C and it found that there was

one greater decrease for steels tested, so it should be

seen in the designs under fire. Also the values of the

hardness in two steels have been obtained and values

ranged around 5% for temperatures tested.

For both steels has found that fluency with large

plastic deformation phenomena were very important to

800 °C and 950 °C temperatures, and specific hardness

in areas of steels broken during tensile tests declined

clearly. This situation should be taken into

consideration for the design structural steels under

fully developed fire.

Acknowledgments

The authors gratefully acknowledge the financial

support of the Ministry of Science and Innovation of

Spain, the project BIA2008-06705-C02-02 and Dr.

Víctor López Serrano and Dra. Mª Jesús Bartolomé

García of Physical Metallurgy Department of CENIM,

for the help provided in the metallurgical study.

References

[1] Hot rolled products of structural steels: Part 3. Technicaldelivery conditions for normalized rolled weldable finegrain structural steels, EN Patent, 10025-10030 (2006).

[2] M. Hever, F. Schröter, Modern Steel: High performance material for high performance bridges, Int.Symposium on Steel Bridges, Barcelona, Spain, 2003, pp.80-81.

[3] M. Militzer, Modelling of microstructure evolution and properties of low carbon steel, Acta Metallurgica Sinica,13 (2) (2000) 574-580.

[4] M. Militzer, E.B. Hawbolt, T.R. Meadowcroft,Microstructural model for hot strip rolling of high-strengthlow alloy steel, Metall. Mater. Trans. A 31 (2002)1247-1259.

[5] B. Eghbali, A. Abdollah, The influence ofthermomechanical parameters in ferrite grain refinement

in a low carbon nb microalloyed steel, Scripta Materialia53 (2005) 41-45.

[6] Y. Zhang, D. Li, Y. Li, Modeling of austenite deposition in plain carbon steels, J. Materials Processing Technology171 (2006) 175-179.

[7] Y. Zhang, H. Zhang, G. Wang, S. Hu, Application ofmathematical model for microstructure and mechanical

property of hot rolled wire rods, Applied MathematicalModelling 33 (2009) 1259-1269.

[8] M. Coret, A. Combescure, A mesomodel for the numericalsimulation of the multiphasic behaviour of materials underanisothermal loading application to two low carbon steels,Int. J. Mech. Sci. 44 (2002) 1947-1963.

[9] T. Miokovic, V. Schulze, O. Vohringer, D. Lohe,Prediction of phase transformation during laser surfacehardening of AISI 4140 including the effects oninhomogeneous austenite formation, Mater. Sci. Eng.435/436 (2006) 547-555.

[10] W. Shen, L.H. Peng, C.Y. Tang, A anisotropic damage based plastic yield criterion and its application to analysisof metal forming process, Int. J. Mech. Sci. 47 (2005)1897-1922.

[11] K. Muszka, J. Majta, L. Bienias, Effects of grainrefinement on mechanical properties of microalloyed steel,Met. Foundary Eng. 32 (2006) 87-97.

[12] I. Sen, S. Tamirisakandala, B.D. Miracle, U. Ramamurty,Microstructural effects on the mechanical behaviour of bmodified Ti-6Al-4V Alloys, Acta Meteralia 55 (2007)4983-4993.

[13] T. Sleboda, J. Kane, R.N. Wrigth, N.S. Stoloff, D.J.Duquette, The effect of thermomechanical processing onthe properties of Fe-40 at % Al alloy, Mat. Sci. Eng. A 386(2004) 332-336.

[14] J. Schiotz, Simulation of nanocrystalline metals at theatimic scale, What can we do? What can we trust? In: Proc.22nd Riso Int., Symposium on Metarials Science Roskilde,Denmark, 2001, 127-139.

[15] UNE-EN ISO 643:2003, Steel, Test methods forDetermining average grain size. (in Spanish)

[16] UNE EN ISO 6892-1: 2009, Metallic materials, TensileTest: Part 1. Test method at room temperature. (inSpanish)

[17] L. Habraken, De ferri metallographia I, PressesAcademiques Europeennes S.C. Bruxelles, 1996.

behaviour of structural carbon steel at high temperatures

Documents