Journal of Mechanical Engineering and Biomechanics

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1 ISSN X Volume 1 Issue 2, Page Journal of Mechanical Engineering and Biomechanics Effects of Thermal Treatment on Mechanical Properties and Grain size of En8 Steel S S Sharma *, Anudeep Mallarapu **, Hiteshwar Upparapalli ***, Chirag Rai **** * Department of Mechanical and Manufacturing Engineering, Manipal Institute of Technology, Manipal University;Manipal;India **Department of Mechanical and Manufacturing Engineering, Manipal Institute of Technology, Manipal University;Manipal;India *** Department of Mechanical and Manufacturing Engineering, Manipal Institute of Technology, Manipal University;Manipal;India **** Department of Mechanical and Manufacturing Engineering, Manipal Institute of Technology, Manipal University;Manipal;India Abstract Steels are generally thermally (heat) treated in order to achieve desirable changes in their microstructure and mechanical properties. The Effects of heat treatment cycles on the microstructure and some mechanical properties of En8 steel have been studied and presented in this paper. The heat treatment cycles given are annealing, normalising and hardening. Sample of steel is purchased and various specimens are prepared for tests of mechanical properties. The changes in properties (tensile yield strength, ultimate tensile strength, fracture strength, percentage elongation, hardness and grain size) are examined and studied. Standard tests are performed to find the properties and the microstructure is observed using metallurgical microscope. Result analysis showed increment in mechanical properties of En8 steel with suitable heat treatment cycles. Research Article Article History Received 18/6/216 Revised 6/7/216 Accepted 7/7/ Published by Rational Publication. Keywords: Heat treatment, En8 steel, mechanical properties, microstructure 1. Introduction Heat treatment is a process of heating and cooling in solid state at a required rate which is applied to change the properties of several engineering materials as per the requirement. Annealing, normalising and hardening are the three most common heat treatment cycles given to steels. Annealing is generally used as a softening treatment [1]-[3]. It is used to relieve internal stresses, restore ductility and enhance machinability. Normalising involves cooling at a slightly faster rate than annealing. Hardening is the process of sudden cooling of the material from a high temperature [4],[5]. Steel is one of the most common materials used in engineering since it can be found in a variety of microstructures and available in different forms. It is an alloy of iron with carbon content less than 2%. There are various types of steel with various compositions of carbon and alloying elements [3]. 2. Material Composition The spectrometric analysis of EN8 steel used in this study is shown in Table1. EN8 is generally used for parts such as general-purpose axles and shafts, gears, bolts and studs. EN8 steel in its heat treated forms possesses good homogenous metallurgical structures, giving consistent machining properties. *Corresponding Author : Anudeep Mallarapu Address: anudeepmallarapu@gmail.com

2 3. Methods used Table1: Spectrometric analysis of EN8 steel Carbon Silicon Manganese Sulphur Phosphorus.39%.13% Specimen Preparation and Heat Treatment The ASTM E18-2 standard specimen is prepared for tensile test and cylindrical specimens are prepared for hardness and microstructure evaluation by turning, shaping and polishing. Three sets out of four sets are subjected to three types of heat treatment namely annealing, normalising and hardening and one set is left without heat treatment. All the three sets to be subjected to heat treatment are first heated to 9o C for two hours. One set to be hardened is quenched in water, second set to be normalised is cooled in air, while the set to be annealing is cooled in the furnace slowly after switching off the furnace. Each set contains three number of identical test specimens for individual tests and average reading of two identical readings is recorded as the property outcome Determination of mechanical properties Heat treated and untreated specimens tensile properties are determined using electronic tensometer. The Brinell hardness numbers are determined using Brinell hardness tester Microstructure examination The heat treated and untreated specimens are finely polished and suitably etched with Nitol to make the microstructure clearly visible and the surface was observed under a microscope. The images obtained by the microscope are observed and analysed. The grain size is obtained using a software called Envision results 4.1. Effect of heat treatment on mechanical properties Table2: Mechanical properties of EN8 steel Treatment BHN Yield Tensile Fracture N/mm 2 Peak - Elongation % Break - Elongation % N/mm 2 N/mm 2 Annealing Normalizing Hardening Un treated Grain Size number The mechanical properties results are shown in Table 2.The changes in the mechanical properties are clearly visible from the Fig.1 to 5. Annealing is the softening process wherein hardness of the material decreases approximately by 2%, almost same in normalising, increases by nearly 17% compared to untreated specimen. Tensile results shows similar trend with respect to heat treatment. The gap between yield, strength and fracture strengths decreases as hardness increases [6],[7]. This is the clear measure of brittleness incurred on the specimen. It is also supported by the reduction in the %elongation. Due to this effect hardened specimens may show lesser impact resistance compared to annealed or normalised. This is due to the microstructural changes happening as per the heat treatment. Generally annealed structure shows pearlitic structure with larger interlamellar distance compared to normalised even though same two phase structure is obtained. Hardened shows single phase martensitic structure which is supersaturated in nature formed by severe lattice distortion [8],[9]. Lattice distortion is the measure of degree of brittleness. 65

3 The microstructure after different heat treatment cycles are observed in metallurgical microscope and the grain size number is found out using software installed in the set up. As the cooling rate increases grains are becoming finer [1].This is due to the increase in the number of nuclei during the increase in cooling rate or degree of super cooling. The rate of nucleation is higher and rate of crystal growth is faster as the cooling rate or degree of super cooling increases. Accordingly hardened specimen shows finer grains compared to normalised and annealed is coarser. The grain size number distribution of microstructure in different heat treatment conditions are shown in Fig Yield Yield Fig. 1 Yield with respect to heat treatment conditions Tensile Tensile Fig 1 Tensile with respect to heat treatment conditions 66

4 Fracture Fracture Fig. 2 Fracture with respect to heat treatment conditions Peak - Elongation % Break - Elongation % Fig. 3 Percentage Elongation with respect to heat treatment conditions Annealing Normalizing Hardening Ascast Fig. 4 Brinell Number with respect to heat treatment conditions 67

5 Grain Size Annealing Normalizing Hardening Un treated Fig. 6 Grain size number with respect to heat treatment conditions 5 Conclusions The following conclusions are arrived based on the experimental results: The reference steel is successfully heat treated with considerable alterations in the properties considered. Hardening has resulted in higher tensile strength and hardness as compared to annealing and normalising. There is considerable reduction in ductility in hardening process due to the formation of distorted structure martensite. Normalising has resulted in increasing the strength and hardness slightly whereas the ductility reduced. Annealing enhanced the ductility but reduced the values of hardness and strength. Hardened specimen shows finer grains compared to normalised and annealed is coarser. References [1] Yadav D, Gaikwad A. Comparison and testing of tensile strength for low & medium carbon steel. International Journal of Mechanical. 215 Aug-Sep; 4(5):1-8 [2] Valeria L, Lorusso HN, Svoboda HG. Effect of Carbon Content on Microstructure and Mechanical Properties of Dual Phase Steels. Procedia Materials Science. 215 Dec 31;8: [3] Manoj MK, Pancholi V, Nath SK. Mechanical properties and fracture behavior of medium carbon dual phase steels. Int. J. Res. Advent Technol. 214 Apr;2(4): [4] Clark DS, Varney WR. Physical metallurgy for engineers. Van Nostrand Reinhold Company; [5] Tukur SA, Usman MM, Muhammad I, Sulaiman NA. Effect of Tempering Temperature on Mechanical Properties of Medium Carbon Steel. [6] Odusote JK, Ajiboye TK, Rabiu AB. Evaluation of mechanical properties of medium carbon steel quenched in water and oil. Journal of Minerals and Materials Characterization and Engineering. 212 Sep 25;11(9):859. [7] Handbook AS. Volume 4: Heat Treating. ASM International. 1991:72-4. [8] Lakhtin Y. Engineering Physical Metallurgy, 3 rd edition. Moscow. Mirpublishers. (1968) [9] Gulyaev A. Physical Metallurgy 2, 5 th edition. Moscow. Mir publishers. (198). [1] Sidney H. Avner. Introduction to Physical Metallurgy. 2 nd edition. New Delhi. Tata McGraw Hill (1997) 68