Evaluation of Mechanical Properties of Medium Carbon Low Alloy Forged Steels Quenched in Water, Oil and Polymer

Medium carbon low alloy forged steels (EN 18, EN19, EN 24, and EN25) have been investigated with respect to their mechanical properties after heat treatment. For heat treatment solutionizing temperature of 855°C with a soaking period of 60 min was used. Thereafter quenching was carried out in three media, viz., Step water, oil and polymer (polyethylene glycol) separately. The quenched samples were step tempered at 575°C and at 220°C sequentially for 60 min each. Hardness, tensile strength, Charpy impact strength and metallographic were carried out on the untreated and heat treated specimens. The heat treated specimens showed higher hardness (10-30%), higher strength (20-100%) and higher impact energy (20-160%). The specimens quenched in poly ethylene glycol exhibited the best mechanical properties. The heat treated specimens had a structure of fine tempered martensite with small amount of bainite. *Corresponding author: Chandan BR, Department of Mechanical Engineering, MS Ramaiah Institute of Technology, Bangalore, Karnataka, India, Tel: 08023600822; E-mail: chandanbrmech@gmail.com Received November 06, 2016; Accepted January 25, 2017; Published February 04, 2017 Citation: Chandan BR, Ramesha CM (2017) Evaluation of Mechanical Properties of Medium Carbon Low Alloy Forged Steels Quenched in Water, Oil and Polymer. J Material Sci Eng 6: 320. doi: 10.4172/2169-0022.1000320 Copyright: © 2017 Chandan BR, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


Introduction
Medium carbon steels are similar to low-carbon steels except that the carbon ranges from 0.30 to 0.50% and manganese from 0.60 to 1.65%. The uses of medium carbon steels include shafts, axles, gears, crankshafts, couplings and forgings. Steels of 0.40 to 0.60% C ranges are used for rails, railway wheels and rail axles. Therefore medium carbon low Alloy steels (MCLA) are commonly used for engineering applications in automobiles, aircraft and transportation industries. They are readily hot forged usually at temperatures ranging from 1065°C to 1230°C [1]. In industry, quenching in oil or polymers is resorted to especially for heavy sections to avoid quench crack commonly observed during water quenching. Choice of quenching medium for the MCLA forged steels depends on the composition of the steel in addition to the size and shape of the parts. Of the conventional quenching media, (water, oil and polymer), water quenching and in some cases even oil quenching may result in quench cracks in thick sections. Further though mineral oils exhibit good cooling capacity for the majority of alloy steels, they are relatively expensive and toxic. The synthetic polymer quenchant, polyethylene glycol (PEG) [2] has the advantages of lesser risk of cracking and less distortion, resulting in better mechanical properties compared to water, oil and brine solutions.
Microalloyed steels, are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels. They are not considered to be alloy steels in the normal sense because they are designed to meet specific mechanical properties rather than a chemical composition (steels have yield strengths greater than 275 MPa, or 40 ksi). The chemical composition of a specific steel may vary for different product thicknesses to meet mechanical property requirements. The steels in sheet or plate form have low carbon content (0.05 to-0.25% C) in order to produce adequate formability and weldability, and they have manganese content up to 2.0%. Small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium, and zirconium are used in various combinations [3]. The mechanical properties of microalloyed steel result, however, from more than just the mere presence of microalloying elements. Austenite conditioning, which depends on the complex effects of alloy design and rolling techniques, is also an important factor in the grain refinement of hot-rolled steels [4]. Grain refinement by austenite conditioning with controlled rolling methods has resulted in improved toughness and high yield strengths in the range of 345 to 620 MPa (50 to 90 ksi). The present study is aimed at determining a comparative evaluation of mechanical properties achieved in MCLA steels (EN steels) with different quenching media with standard step tempering procedure [5][6][7][8].

Materials and Methods
Four steels, viz., EN 18, EN19, EN 24, and EN25 in the normalized condition were procured from Mumbai market. The compositions were analysed for confirmation and Table 1 gives the results of the composition check.

Test specimen preparation
A set of specimens was prepared for Tensile, Impact, Hardness and Microstructural analyses. The standards used for samples to carry out the various tests are listed out in Table 2.

Heat treatment/quenching and step tempering
An electric furnace with maximum temperature of 1200°C was used for both solutionizing and step tempering. The heat treat temperatures were kept same for all four steels studied, as shown in Table 3. After solutionizing, the samples were directly quenched in water, oil [9] or polymer solution. The quenched samples were thereafter tested for their mechanical properties.

Mechanical tests
A standard Brinell Hardness Tester was used for measurement of indentation hardness. The tests were conducted using a 10 mm diameter steel ball and 3000-kg load. The tensile tests were carried out using an electrically powered Hounsfield tensometer with a capacity of 20 KN. Impact energy to failure was found using a Charpy impact tester. Table 4 shows the mechanical properties of the as-received and heat treated steel samples with the three quenchants. Figures 1-4 are plots of variations of mechanical properties when different quenchants are used. Figure 1 shows the variation of hardness for the four EN steels when the quenchant is changed from water to oil and then polymer. It is observed that the maximum hardness value of 362 BHN is obtained in EN 25 with water quenching in general, water quenching has the maximum impact on hardness followed by oil quenching and polymer quenching as the least defect [10][11][12]. However, in case of EN 19 polymer quenching resulted in highest hardness of 286 BHN. Figure  2 depicts the defect of varying the quenchant on the UTS of EN steels. The variation of the UTS generally follows the same behaviour as the variation of hardness. It is observed that once again the highest UTS value is obtained in water quenched EN 25 steel (1280 MPa) which is more than twice the value (580 MPa) for the as received steel. Figure 3 depicts the change of impact energy of the four EN steels considered. Once again it is evident that step tempering after quenching for hardening has improved impact energy of steels except in case of water quenching of EN18 and EN19. This may appear as in congruent with Figure 4 where the percentage of elongation of the forged steels is highest in the as received condition in the lower in the heat treated conditions (except for oil quenching).

Results and Discussions
The explanation for higher impact energy (and therefore higher toughness) of the heat treated steels lies in 20-50% increase strength values while the elongation is reduced by maximum 20% only.

Effect of quenching in different media on the microstructure of EN steels
The microstructural investigation was performed using a Carl Zeiss optical microscope. In sequence, the steps include sectioning, mounting, coarse grinding, fine grinding, polishing, etching and      microscopic examination, and the general procedure followed by earlier investigators was employed [6]. The samples were polished using a series of emery papers of grit size varying from 1000 µm to 1500 µm. The samples were etched with nital solution, 100 mL ethanol and 1-10 mL nitric acid for about 10-20 seconds before observation in the optical microscope [13][14][15][16][17].
The mechanical properties of microalloyed steels result, however, from more than just the mere presence of microalloying elements. Austenite conditioning, which depends on the complex effects of alloy design and rolling techniques, is also an important factor in the grain refinement of hot-rolled steels. Grain refinement by austenite conditioning with controlled rolling methods has resulted in improved toughness and high yield strengths in the range of 345 to 620 MPa (50 to 90 ksi).

Observations
The microstructure of water-quenched specimens appears to be a mixture of martensite and lower bainite. For oil-quenched specimens, a mixture of upper bainite, lower bainite and some martensite is obtained. In case of polymer quenched steels, the microstructure consists of fine tempered martensite with a small amount of ferrite.             Thus polymer quenching would improve ductility, toughness and impact strength values.

Conclusions
1. It has been established that polymer can also be used as a quenching medium for MCLA forged steels.
2. The study has shown that using of water, oil and polymer as quenchants improves the mechanical properties when compared to the untreated steels.
3. Polymer quenching improves the ductility of the steel because of its lower and uniform cooling rate compared with water and oil; also there is lesser risk of cracking and distortion in the parts. The uniform low cooling rates also result in better mechanical properties for the polymer quenched steels.
4. Microstructural analysis corroborates the changes in mechanical properties observed.