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

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1 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 Abstract **) Yapı-Tek Çelik Sanayi A.Ş., Kocaeli, Turkey 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 epected 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 epectation 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 epansion is detected through the change in length. Measuring the thermal epansion 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 B s and M s (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 standard [5]. The

2 influence of cooling rates on phase transformations and microstructures was eamined 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 eit 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 M s (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

3 used to cool the samples. During eperimental 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 ( K/s) measured in the temperature range between ºC. Fig 3. The temperature-time cycle of the dilatometer eperiments. 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.

4 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 ferritepearlite (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 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 K/s. However, at higher cooling rates such 5.46 and 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 maimum cooling rate of 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 CCTdiagram of steels with similar carbon content given in literature [6,7] that martensite formation required higher cooling rates.

5 (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) K/s after austenitization at 900 ºC for 5 min using a dilatometer.

6 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 eperimental method were applied to determine the A e1, M s ve B s critical transformation temperatures. The obtained values from eperimental work, modeling and literature are summarized in Table 2. The M s and A e1 temperatures are calculated according to equation (1) and (2) and calculated M s is used in CCT-diagram. M s = %C-30.4%Mn-17.7%Ni-12.1%Cr-7.5%Mo (1) A e1 = (723) - (10.7Mn) - (16.9Ni) + (29.1Si) + (16.9Cr) + (290As) + (6.38W) (2) A e1 also eperimentally determined as approimately 760 ºC. On the other hand, B s is eperimentally determined from the cooling curve where bainite was first observed. Table 2. Critical temperatures from the eperimental work, modeling and literature. M s B s A e1 Eperimental work Modeling [9] Literature [7] DISCUSSION In this study, the continuous cooling transformation (CCT) diagram of 0.37C-0.87Mn-0.28Si steel is eperimentally 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 eperimental studies were in well agreement with the temperature obtained by modeling and theoretical calculations. Furthermore, the effect of quenching and tempering heat treatment

7 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, [2] H. K. D. H. Bhadeshia and R. W. K. Honeycombe, Steels, Microstructure and Properties, Elsevier, Oford, [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, , [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) [5] DIN 21530, Ausbau für den Bergbau, [6] M. Atkins, Atlas of Continuous Cooling Transformation Diagrams for Engineering Steels, ASM, Sheffield, [7] G. F. V. Voort, Atlas of time-temperature diagrams for irons and steels, ASM Int., OH, [8] K.W. Andrews, J Iron Steel Inst, 203 (1965), 721. [9]