DILATOMETRIC INVESTIGATIONS OF THE EFFECT OF...

23. - 25. 5. 2012, Brno, Czech Republic, EU

DILATOMETRIC INVESTIGATIONS OF THE EFFECT OF HEAT TREATMENT ON THE MICROSTRUCTURE OF HOT ROLLED MINING SUPPORT PROFILE

Serap Gümüş*, Ersoy Erişir*, Muzaffer Zeren*, Günay Köse**, Erkut Fındık**

*) Kocaeli University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Kocaeli, Turkey

**) Yapı-Tek Çelik Sanayi A.Ş., Kocaeli, Turkey

Abstract

It is known that the mechanical properties of steels are dependent on the final microstructure resulting from

phase transformation during heat treatments. Hot rolled mine support profiles are produced generally from

unalloyed quality steels. After the hot rolling, heat treatments of quenching and tempering result in a

tempered martensite microstructure with high strength and toughness. The retained austenite phase may

also be expected in the microstructure due to the lack of alloying elements. In this study a microstructural

characterization of a mine support profile is carried out before and after the quenching and tempering to

investigate the effect of heat treatments on the microstructure. A dilatometric analysis is performed to detect

the phase transformation temperatures. Dilatometric analysis is carried out at different cooling rates from

austenite phase region. Depending on the different cooling rates, ferrite, pearlite and bainite phases are

determined using dilatation curves, light microscope, and Vickers hardness tests. The results indicate that

critical temperatures can be determined by dilatometric analysis.

Keywords: Dilatometric analysis, Microstructure, Phase transformations, Mining support profiles, Unalloyed

steels

1. INTRODUCTION

Hot-rolled, nonalloyed quality steels are widely used in modern construction applications. The manufacturing

methods of this type of steel should be optimized continuously due to the expectation of reducing cost and

improving properties. Therefore, final heat treatment of long products come into importance as well as the

thermomechanical processes. Parameters like temperature, time and cooling rate should be controlled in an

appropriate manner to achieve the desired microstructure after the heat treatment. Ferrite, bainite,

martensite or cementite phases can form depending on the cooling rate. Through the principles of physical

metallurgy, the size, amount and morphology of these phases can be changed during the heat treatment

proces [1,2].

Measuring the physical properties of materials is a method of determining the temperature and time of phase

transformations.The change in physical properties indicates the temperature and time of lattice structure

changes of material. The dilatometer method is more precise for solid-solid phase transformations. In this

method, the change in specific volume accompanied by a phase transformation or thermal expansion is

detected through the change in length.

Measuring the thermal expansion with high accuracy is also a reason for commonly using of dilatometer as

thermal analysis method [3]. In a dilatometric analysis of Jung et al. [4] have precisely determined the Bs and

Ms (bainite and martensite start) temperature of a medium-carbon steel in their dilatometer work.

Phase transformations are very critical to determine the heat treatment parameters. The heat treatment of

TH profiles used for mining applications is carried out in 3 stages consisting of austenitization, quenching

and tempering. After the heat treatment process, TH profiles are bended to produce bow-shaped steel

supports that can be attached to each other by sliding joints. In this study, microstructural investigations of

TH profiles before and after heat treatment are presented for 0.37C-0.87Mn-0.28Si steel. The TH profiles

produced in a continuous heat treatment line in compliance with the DIN EN 21530 standard [5]. The


influence of cooling rates on phase transformations and microstructures was examined using dilatometer and

light optical microscopy.

2. EXPERIMENTAL

A schematic drawing of the continuous heat treatment line is shown in Fig 1. Heating system is supplied via

a pusher type furnace. The temperature of TH profile is about 900 ºC at the exit of furnace. The surface of

profile is cooled with water jets resulting in different cooling rates from the surface to the center as shown in

Fig 2. After the martensitic phase transformation, the temperature rise again to the temperature above the

Ms (martensitic start). Therefore, the structure will be in a sense of self-tempered condition. Finally, the

hardened structure formed after quenching is tempered at 570 ºC in an annealing furnace. The chemical

composition of 0.37C-0.87Mn-0.28Si steel is given in Table 1.

Fig 1. Schematic view of continuous heat treatment line of TH profile.

Fig 2. Cooling regime during quenching of TH profiles.

Table 1. Chemical composition of investigated steel.

Chemical composition (w-%)

C Si Mn P S Cr Mo Ni Al

0,37 0,28 0,87 0,020 0,006 0,04 0,02 0,08 0,047

The microstructure of samples taken from TH profiles is investigated. The samples were metallographically

prepared by grinding with 120, 600 and 1000 mesh SiC paper and followed by polishing with diamond paste.

Finally, the sample surface was etched by 3% nital solution.

In principle, TTT (Time-Temperature-Transformation) diagrams can be drawn by using values obtained from

transformation temparature and time determined from dilatometric analysis and measuring the phase amount

in the final microstructure. In this study, Netzsch 402C type dilatometer is used to obtain the TTT diagrams.

The samples of diameter 5mm and length 10 mm are heated inductively in a vacuum chamber. Helium gas is


used to cool the samples. During experimental procedure, the samples were firstly heated to 900 ºC,

austenite region at a constant heating rate of 50K/min and austenitized for 5 min at constant temperature

(Fig 3). After austenitizing the samples were cooled at different cooling rates (18.75 - 0.02 K/s) measured in

the temperature range between 800-500 ºC.

Fig 3. The temperature-time cycle of the dilatometer experiments.

3. RESULTS

3.1 Microstructural investigations

It is seen that the microstructure of TH profiles is in ferritic-pearlitic microtructure before heat treatment (Fig

4a). Widmanstaetten ferrite formation is observed in the microstructure due to the small amount of rapid

cooling effect. The Vickers Hardness measurements indicate that the TH profile hardness is about 182 HV.

After quenching and tempering heat treatment, the microstructure is transformed to tempered martensite and

some amount of retained austenite (Fig 4b). The average hardness is increased to 245 HV by the formation

of martensite.

a. b.

Fig 4. Light microscope microstructures of the specimens TH profile, a) before and b) after the quenching

and tempering heat treatments.


3.2 Phase transformations

Fig. 5 shows the length change (dilatation) curves of 0.37C-0.87Mn-0.28Si samples measured during

continuous cooling from 900 ºC. The length change values are normalized to curve of 0.02 K/s for easier

comparison. It can be seen from curves that the temperature of transformation from austenite (A) to ferrite-

pearlite (F+P) is shifted to lower temperature by increasing cooling rate. In this manner the phase

transformation start and finish temperature can be determined for each cooling rate from curves of change in

length.

Fig 5. Dilatation curves measured during continuous cooling at rates of 0.08-0.47 K/s after austenitization at

900 ºC for 5 min. The cooling rates are the average cooling rates measured between 500 and 800 ºC.

Inflection points of the curves indicate the start and finish temperatures of phase transformation.

Continuous cooling transformation (CCT) diagram is obtained in accordance with the measured

transformation temperatures. Microstructural investigations as well as the results of dilatometer analysis

have also been used to obtain the CCT-diagram. As seen in Fig. 6, allotriomorphic ferrite and pearlite are

observed in microstructural investigations after cooling from 900 ºC at cooling rates of 0.02-1.46 K/s.

However, at higher cooling rates such 5.46 and 18.75 K/s, some amount of bainite packets are detected

besides Widmanstaetten ferrite and pearlite phases.

Ferrite (F), pearlite (P), bainite (B) and martensite (M) phase regions are indicated in the CCT-diagram given

in Fig 7. The transformation curve lies in a temperature range from ~760 °C to ~280 °C. Phase

transformation temperatures and final hardness values depending on the cooling rate are also shown on the

CCT-diagram. In this study, the maximum cooling rate of 18.75 K/s could be reached due to the cooling

capacity of the dilatometer. Therefore, martensite phase can not be obtained. It is seen also from CCT-

diagram of steels with similar carbon content given in literature [6,7] that martensite formation required higher

cooling rates.


(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig 6. Light microscope microstructures of the specimens cooled at (a) 0.02 K/s, (b) 0.08 K/s, (c) 0.17 K/s,

(d) 0.47 K/s, (e) 0.88 K/s, (f) 1.46 K/s, (g) 5.46 K/s, and (g) 18.75 K/s after austenitization at 900 ºC for 5 min

using a dilatometer.


Fig 7. CCT diagram of 0.37C-0.87Mn-0.28Si steel (discontinuous lines are obtained from empirical equations

[8] and modeling [9]).

On the other hand, Andrews equations [8] and modelling [9] as well as the experimental method were

applied to determine the Ae1, Ms ve Bs critical transformation temperatures. The obtained values from

experimental work, modeling and literature are summarized in Table 2. The Ms and Ae1 temperatures are

calculated according to equation (1) and (2) and calculated Ms is used in CCT-diagram.

Ms = 539-423%C-30.4%Mn-17.7%Ni-12.1%Cr-7.5%Mo (1)

Ae1 = (723) - (10.7Mn) - (16.9Ni) + (29.1Si) + (16.9Cr) + (290As) + (6.38W) (2)

Ae1 also experimentally determined as approximately 760 ºC. On the other hand, Bs is experimentally

determined from the cooling curve where bainite was first observed.

Table 2. Critical temperatures from the experimental work, modeling and literature.

Ms Bs Ae1

Experimental work - 530 760

Modeling[9]

360 500 700

Literature[7]

355 - 720

4. DISCUSSION

In this study, the continuous cooling transformation (CCT) diagram of 0.37C-0.87Mn-0.28Si steel is

experimentally determined by dilatometer analysis, metallography and hardness measurement. Phase

transformation investigations done by dilatometer shows that ferrite, pearlite and bainite phases can form at

cooling rate up to 18,75 K/s. The experimental studies were in well agreement with the temperature obtained

by modeling and theoretical calculations. Furthermore, the effect of quenching and tempering heat treatment

x

x

x

x x

x x

x

x

x

x

x

x

x

x

x


on microstructure of TH support profiles developed for mine application is investigated. It can be concluded

after the heat treatment that the ferritic-pearlitic structure of the steel is transform to tempered martensite and

a small amount of retained austenite due to the high cooling rate effect.

ACKNOWLEDGEMENT

This study is realized under the project “Development of Processing line of TH Profiles “ of Tübitak-

Teydep 1501 Support Program for Industry R&D.

REFERENCES

[1] G. Krauss, “Steels; Processing, Structure and Performance”, ASM, Materials Park, OH, 2005.

[2] H. K. D. H. Bhadeshia and R. W. K. Honeycombe, “Steels, Microstructure and Properties”, Elsevier, Oxford, 2006.

[3] C. Garcia de Andres, F.G. Caballero, C. Capdevila, L.F. Alvarez, “Application of dilatometric analysis to the study

of solid–solid phase transformations in steels”, Materials Characterization 48, 101– 111, 2002.

[4] M. Jung, M. Kang, Y. –K. Lee, “Finite-element simulation of quenching incorporating improved transformation

kinetics in a plain medium-carbon steel”, Acta Materialia 60 (2012) 525–536.

[5] DIN 21530, “Ausbau für den Bergbau”, 2003.

[6] M. Atkins, “Atlas of Continuous Cooling Transformation Diagrams for Engineering Steels”, ASM, Sheffield, 1980.

[7] G. F. V. Voort, “Atlas of time-temperature diagrams for irons and steels”, ASM Int., OH, 1991.

[8] K.W. Andrews, J Iron Steel Inst, 203 (1965), 721.

[9] http://calculations.ewi.org/vjp/secure/TTTCCTPlots.asp

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