An Assessment of Mechanical Properties of Medium Carbon Steel under Different Quenching Media

Transcription

1 An Assessment of Mechanical Properties of Medium Carbon Steel under Different Quenching Media M. B. Ndaliman Department of Mechanical Engineering, Federal University of Technology Minna, Nigeria Abstract The mechanical properties of medium carbon steel (0.36C) were investigated under two different quenching media (water and palm oil). The properties are: the strengths, impact and hardness of the material. The investigations centered on unheat - treated, normalized; water and palm oil quenched, and tempered conditions. The tempering temperature is 200 C. An AISI steel of grade C1035 was used for comparison of the properties. Result indicated that water quenched steel produced its best properties in strength and hardness, while palm oil quenched steel has its best property in impact strength. Keywords: Quenching, tempering, hardness, yield strength, tensile strength, austenites, normalizing, ferrite, pearlite, martensite Introduction Steel is essentially an alloy of iron and carbon or of iron, carbon and other alloying elements (Lacktin 1977). The carbon content of steel is between 0.05% and about 1.2%. The other elements may be controlled by impurities or alloying elements that are introduced to alter the response to heat treatment or to produce some special properties. Plain carbon steel is the one in which the only alloying element is carbon. Carbon being a powerful alloying agent can give a variety of strength and hardness by varying its composition in the steel. It is in this regard that carbon steel can be classified as low, medium and high carbon steel. It has been shown (Lindberg 1977) that carbon steel with carbon content between 0.3 and 0.6% is termed medium carbon steel. While those with lower and higher are respectively classified as mild and high carbon steel. The aim of this paper is to examine the mechanical properties of some steel products from the Delta steel company (DSC) in Nigeria and also the effect of the media used in heat treating them. Some authors have earlier investigated certain aspects of the products from DSC. Some notable investigations include: the use of DSC slag in soil conditioning (Ayakwo and Ozara 1995), and utilization of steel slag from DSC as industrial raw materials (Abubakre 2001). In this paper, some mechanical properties of medium carbon steel products from the plant were investigated under different quenching media: water and palm oil. Water has been used as quenching medium for quite a long time now, but palm oil being a local viscous fluid is not being used for this purpose. Therefore, one aim of this research is to explore the palm oil s performance as a quenching medium. Water is being used here to serve as a standard. Such properties include: tensile strength, impact strength, hardness and ductility of the material. The response of this material to heat treatment, using these quenching media is reported in the following paragraphs. Methods Methodology and Test The steel material used for this investigation is a medium carbon steel. The 100

2 mechanical properties investigated were, the tensile strength, impact strength and hardness. Since standard test procedures were used, standard test specimens were prepared for all the tests. A set of specimens was prepared for conducting pre-heat treatment investigations. A total of nine specimens were prepared for each of the three tests. Another set was prepared for the investigations conducted after heattreatment. Each test was conducted three times and the average taken represents the results recorded in the Tables. Properties Investigated The properties investigated and the methods used have been summarized in Table 1. Table 1. Methods used in determining the properties The tensile strength is given according to Pearce (1977) as UTS = P max /A o Where P max = maximum load applied A o = Original Cross sectional area. The percentage elongation after fracture is given as ε % =l u - l o x 100 /l o Where l o = the original gauge length l u = the final gauge length. The percentage reduction in area is also given as RA = A o - A u x 100/ A o Where A o = the original cross-section area A u = the minimum cross-sectional after fracture. Properties Tensile strength Impact strength Hardness Tensile Test Standard specimen Type BS standard round test piece (Φ20mm x 450mm length) British standard beam-type test piece 10 x 10 x 60mm length Φ17mm x 16mm length Mode of Evaluation Avery testing machine of 600KN and gauge Charpy impact testing machine Rockwell hardness testing machine The tensile test is a standard test which was conducted using the Avery testing machine. The test specimens were as specified in Table 1. From the tests, the yield and tensile strengths, the percentage elongation and the percentage reduction in area were determined. Impact Test The impact strength is the measure of the energy absorbed by the specimen when it failed as a result of the strike on it by the pendulum of the measuring device. The specimens dimensions were as indicated in Table 1. They weighed separately and recorded. Each of them was placed in the vice of the supported beam. A heavy pendulum mounted on the ball bearing was allowed to strike the specimen after swinging from a height. The material failed at the strike of the pendulum and the energy absorbed by the specimen was recorded. Hardness Test The hardness of the specimen is indicated by the depth of penetration of the indenter on the steel specimen. This was read directly from the calibrated dial gauge of the machine. Heat Treatment All the specimens which were already prepared for the various tests were loaded in to an electric furnace, and allowed to heat up to a temperature of 850 C. They were allowed to 101

3 get soaked by maintaining them at this temperature for a period of 1:30 hrs. The soaked specimens were grouped into three parts. One part was cool in air. The remaining two parts were quenching in water and palm oil respectively. These quenched specimens were tempered for 1hour at a temperature of 200 C and cooled in air. Results Results and Discussions The results comprise of chemical composition of the steel sample analyzed, the value of the properties obtained from the plain specimens which are not heat-treated, and those obtained from the specimen that were either normalized or quenched in water or oil and tempered. Table 2 presents the result of analysis of the composition of the steel sample used for this investigation. Table 2. Composition of the steel sample investigated Elements Percentage C 0.36 Si 0.19 Mn 0.90 P S Cu 0.05 The Mechanical properties of the specimens which were not heat-treated are presented in Table 3 along side the properties of standard ones. This allows for a good comparison of the sample to be made with those obtained elsewhere. The properties of the heat-treated specimens are also presented in Table 4. Table 3. Mechanical properties of the medium carbon steel compared with the standards Properties Test Sample *C1035 Yield Strength Tensile Strength Percentage Elongation (%) Percentage Reduction in Area (%) Impact Strength (J) RHN B94 * From AISI (Spotts 1988) Discussion Chemical Composition The purpose of analyzing the chemical composition of the steel sample is to enable its classification to be made. Based on Table II, the steel can be classified as medium carbon steel, since both AISI and SAE classified steel whose carbon content ranges between %, manganese content ranges between % and maximum sulphur content of 0.05% to be medium carbon steel. The steel composition also satisfies the minimum carbon point requirement for it to be materially affected by heat treatment, since it has 36 points of carbon which is higher than 25 (Lindberg 1977). Pretreatment Properties The properties of the medium carbon steel which is not heat treated are presented in Table 3 along side the AISI C1035 steel. C1035 is the steel material whose composition is about the same range with the one 102

4 investigated. For the purpose of comparison, the values of yield strength, tensile strength, elongation and reduction in area are in close comparison. The impact strength has no basis for comparison with C1035. The values indicated is the energy absorbed when a striking energy of 300J was initiated at the notch created on the specimen. It is the notch that actually promotes the crack that led to failure. This material can find applications in axles, gears and drop forgings. Properties of Heat-treated Specimens The steel material with approximate 0.36 carbon when heated to 850 C and soaked for 1:30 hrs, would have the carbon present dispersed to form austenite structures. This structures that were slowly cooled in air precipitates ferrites. The cooled material is there fore made of ferrites and pearlites in laminated form. Table 4. Mechanical properties of the heattreated medium carbon steel Condition Yield Strength Tensile Strength %age Elongation %age Reduction in Area Impact Strength (J) RHN On the other hand, the quenched specimens would have their austenites transformed to martensites. These are fine, needle-like structures which are very strong and hard, but very brittle. The re heating of martensites during tempering would enable it to be transformed into sorbite or troostite. These are fine dispersions of carbide in a ferrite matrix. Strengths From Table 4, the yield and tensile strengths of the normalized structures compared favourably with that of C1035 steel. The strength values are higher than those obtained for the materials that are not heat treated. The increase is a result of the rearrangement of the laminated ferrite and pearlite structures in the normalized material. For the tempered martensites the yield strength was eliminated in both the water and palm oil quenched materials. There is however general increase in values of the tensile strengths. This is as expected because the tempered structure has fine carbide dispersion in the ferrite matrix. It is important to note that water quenched steel possesses the highest strength values of N/mm². The ductility has also improved going by the values of the percentage elongations shown in Table IV. Impact Strengths Normalized Water Quenched Palm oil Quenched C B The impact strength of the unheat- treated specimen was higher than those of the heattreated materials. This strength is an indication of the level of toughness of the material. Results revealed that the normalized material possesses impact strength (43.00J) higher than the quenched materials. Even the palm oil quenched steel is tougher than the water quenched one. The results showed that the materials behaved in opposite manner as compared to their tensile strength behavior. Hardness The hardness values of the heat- treated specimens are generally higher than that of unheat-treated steel. In this case, the ability of 103

5 the material to resist plastic deformation under indentation was used to evaluate hardness. The highest value of hardness (41.7) is obtained on water quenched steel. This is in agreement with Kempster (1976) who stated that water should be used if plain carbon steel is to have a high value of hardness. Conclusion and Recommendations The properties of the heat-treated medium carbon steel from DSC compared favourably well with standard steel products. They have excellent values in terms of tensile strengths and elongation when quenched and tempered in both water and oil. The normalized steel was found to possess good properties in yield strength ( N/mm²), tensile strength (706 N/mm²) and impact strength of 43.0 J. The quenched steel materials have their yield points eliminated. The palm oil quenched steel is found to be exhibiting higher level of toughness. It is recommended that these mechanical properties be examined under different tempering temperatures to see their variations. References Abubakre, O.K Utilization of steel slag, from DSC as industrial raw materials. Nigeria J. Ed. and Tech. 2(1&2): Ayakwo, C.N.; and Ozara, N.A The use of DSC slag in soil conditioning, Proc. Ann. Conf. Nigerian Metall. Soc., Owerri, pp Kempster, M.H.A Material for Engineers. Hodder and Stoughton, London, England, pp Lakhtin, Y.M Engineering Physical Metallurgy and Heat Treatment. Mir Publ. Lindberg. R.A Processes and Materials of Manufacture, 2 nd ed. Allyn and Bacon, Boston, MD, USA. Pearce, R Metrology: Tensile Testing, Metallurgia and Metal Forming, Spotts, M.F Design of Machine Elements, 6 th ed., Prentice-Hall of India, New Delhi, India, pp Notations and Symbols AISI American Iron and Steel Institute A o Original Cross-sectional Area A u Minimum Cross-sectional after Fracture DSC Delta Steel Company l o Original Gauge Length l u Final Gauge Length P max Maximum Load Applied RA Percentage Reduction in Area RHN Rockwell Hardness Number SAE Society of Automotive Engineers UTS Ultimate Tensile Strength Ε % Percentage Elongation after Fracture 104