3rd International Conference on Mechanical, Automotive and Materials Engineering (ICMAME'2013) April 29-30, 2013 Singapore

Effect of Heat Treatment on Mechanical Properties of AISI 4147 Spring

S. S. Sharma, K. Jagannath, C.Bhat, U. Achutha Kini, P. R. Prabhu, Jayashree P. K and Gowrishankar M. C

Abstract - Today steel is the most important resource in this The combination of heating and cooling operations applied to industrialized world. It forms the basic building material of today’s a metal or in the solid state is termed as heat treatment. structure. Moreover with large and vanadium The high temperature phase in steel has the property percentage can be used as spring steels which form the suspension to transform into variety of room temperature phases like system. Prevention of wear and increase in steel life depends coarse , & depending upon the principally on the design and operation on the component, but providing some pre-use treatment on steel can also improve the cooling cycle [1-3]. These phases may be the decomposition quality to a great extent. It has been seen that most of the study products like ferrite & or it may be super saturated focuses on the experimental testing of the steel component and very solid solution. Depending upon the plate thickness and few focuses on the material testing and improving its properties interlammer spacing between ferrite and cementite phases in beforehand. One of the processing routes to alter the properties is pearlite, the property of steel can be altered. The plate heat treatment. Nearly 90% of the springs are used in heat treated thickness and interlammer spacing between ferrite and conditions. The major requirement for the conventional spring steel is cementite is larger, coarser is the pearlite and ferrite, hence toughness, strength & hardness. In this view, it is proposed to study ductility increases. This is possible by slow cooling of the the mechanical and tribological properties of AISI 4147 (EN47) austenitic phase to room temperature, accordingly the spring steel with different heat (thermal) treatments like normalizing, hardening and . All heat treatments are carried out in treatment is known as [4-7]. atmospheric condition. Hardening treatment improves hardness of the If the austenitic phase is cooled at a slightly faster rate, so material, a marginal decrease in hardness value with improved that the decomposition of austenite by the eutectoid reaction is ductility is observed in tempering. Hardening and longer duration possible to form medium or fine pearlite with increased weight tempering show better wear resistance compare to other heat percentage of eutectoid mixture (pearlite), the treatment is treatments. Both mild and severe wear regions are observed. known as normalizing [8-11]. Here the degree of dispersion of Generally mild wear region is observed above 5 hours of continuous ferrite and cementite in pearlite increases to improve the running of the specimen. Microstructural analysis shows the machinability with finer grain size. The normalized steel existence of pearlitic structure in as bought & normalized specimens, contains nearly 50 wt % of pearlite and 50 wt % of lath martensitic structure in hardened specimen. proeutectoid ferrite. If the austenitic phase is cooled in such a way that the cooling rate is greater than or equal to critical Keywords – tempering, hardness, heat treatment, normalizing, toughness cooling rate (CCR), the transformed phase is termed as I. INTRODUCTION martensite. It is supersaturated single phase with body centered TEELS can be subjected to variety of conventional heat tetragonal (BCT) structure. The BCT structure has got a c/a treatments like normalizing, hardening and tempering. ratio higher with more trapped in the lattice. So S hardness and strength increase with considerable amount of thermal stresses because of quenching severity. To decrease S. S. Sharma, Professor, Department of Mechanical & Manufacturing the c/a ratio of BCT martensite, to improve toughness, to Engineering, Manipal Institute of Technology, Manipal - 576104 (Ph: 0820 2925473; e-mail: [email protected]). convert retained austenite into stable phases; to saturate the K. Jagannath, Professor, Department of Mechanical & Manufacturing non-equilibrium BCT martensitic structure and to minimize the Engineering, Manipal Institute of Technology, Manipal - 576104 (Ph: 0820 thermal stresses induced during hardening, tempering 2925473; e-mail: [email protected]). treatment is given. Depending upon the tempering temperature U. Achutha Kini, B Professor, Department of Mechanical & Manufacturing Engineering, Manipal Institute of Technology, Manipal - and duration, the improvement in toughness is possible with 576104 (Ph: 0820 2925473; e-mail: [email protected]). the sacrifice of hardness and strength. Because of high P. R. Prabhu, Asst. Professor - senior scale, Department of Mechanical & brittleness, as quenched spring steels are seldom used for Manufacturing Engineering, Manipal Institute of Technology, Manipal - practical applications [12, 13]. By tempering process, the 576104 (Ph: 0820 2925463; e-mail: [email protected]). Jayashree P. K, Asst. Professor - selection scale, Department of properties of quenched steel could be modified to improve its Mechanical & Manufacturing Engineering, Manipal Institute of Technology, impact resistance. During tempering the resulting Manipal - 576104 (Ph: 0820 2925473; e-mail: [email protected]). microstructure contains bainite or epsilon carbide in a matrix Gowrishankar M. C, Asst. Professor, Department of Mechanical & of ferrite depending on the tempering temperature. Manufacturing Engineering, Manipal Institute of Technology, Manipal - 576104 (Ph: 0820 2925463; e-mail: [email protected]). Spring steels are used in the quenched and tempered C. Bhat, Professor & Head, Department of Mechatronics Engineering, condition which gives optimum strength, toughness and Manipal Institute of Technology, Manipal - 576104 (Ph: 0820 2925441; e- vibration damping. The change in microstructure and strength mail: [email protected]).

102 3rd International Conference on Mechanical, Automotive and Materials Engineering (ICMAME'2013) April 29-30, 2013 Singapore after the heat treatment process depends on the cooling rate obtained during quenching. Due to operational safety, springs have to meet increasing performance requirements, which concern mechanical properties as well as fatigue strength. Today oil quenched and tempered springs are widely used for heavy duty spring where high mechanical properties are the main design driver [14-17]. Major requirements of the spring steel are high strength, high proportional limit, and high fatigue strength. These desirable properties of spring can be Fig. 4 Hardness Test Specimen (mm) achieved firstly by a higher carbon content or with suitable alloying elements, and secondly by heat treatment. Steel springs are used in hard, high strength condition. To attain B. Heat Treatment Procedure these properties springs are hardened and tempered. Table I The Electric furnace is used for heating the specimen to the shows the chemical composition of the AISI 4147 steel used in austenitic state. All specimens are prepared from as bought this study. steel and subjected to three types of heat treatments such as TABLE I AISI 4147 STEEL COMPOSITION normalizing, hardening and tempering and compared with as Element % Wt. Element % Wt. bought specimen. Three specimens each for tensile, impact, Carbon 0.496 Sulphur 0.031 wear, microstructure and hardness are used for the analysis in 0.270 Iron 97.30 respective treatments. First set consisting three specimens in Manganese 0.620 Nickel 0.133 respective test is subjected to normalizing and another three Molybdenum 0.015 Aluminium 0.021 sets are hardened. Out of three sets hardened, one set is Vanadium 0.154 Chromium 0.800 tempered at 300oC for one hour and the second set is tempered at the same temperature for five hours followed by slow II. EXPERIMENTAL DETAILS cooling in air. All specimens for normalizing and hardening are heated for two hours at 900oC and normalized specimen is A. Specimen preparation cooled in air. SAE 30 oil is used as quenchant. The average Standard specimens are prepared with the required value of three readings is considered for analysis. dimensions for tensile, impact, hardness, and wear tests. The C. Testing shape and size of standard dimension chosen for the tests are shown in Figures 1, 2, 3 and 4. The heat treated specimens are further subjected to mechanical tests like, tensile (Computer controlled tensometre), impact (Charpy), wear (Pin on disc), hardness (Rockwell) and metallography (Metallurgical microscope). In wear test, diameter of wear track is 88mm, test duration is 5 hours and rpm of the disc is 200 for each specimen. For metallography, specimen is polished with series of emery papers of 100, 200, 300 and 400 micron size and etched with Nital. Fig. 1 Tensile Test Specimen (mm)

III. RESULTS & DISCUSSIONS

A. Micro structural Analysis: The micro structures of as bought, normalized, hardenedand tempered specimens are shown in Figures 5, 6, 7, and 8 respectively. As bought specimen shows well defined pearlitic phase with lamellar packing of ferrite and cementite particles and coarse grains of ferrite and pearlitic colonies. Also there is

no rupturing of grains or no alignment of grains along the Fig. 2 Impact Test Specimen (mm) particular direction. It shows that as bought specimen is standard hot worked or annealed. More weight percentage of proeutectoid ferrite is responsible for the decrease in hardness value. Normalized specimen shows finer grains of ferrite, small colonies of pearlite and an increase in pearlitic weight percentage. This is responsible in the increase of ductility of the specimen with moderate increase in hardness value.

Fig. 3 Wear Test Specimen (mm)

103 3rd International Conference on Mechanical, Automotive and Materials Engineering (ICMAME'2013) April 29-30, 2013 Singapore

(UTS), Yield Strength (YS), Fracture strength (FS) of the specimen with respect to the type of treatment is shown in Figure 10. The hardened specimen shows 100% brittle failure but tempered specimens show partial brittle failure with UTS and FS being closer to each other. Even though a huge difference is existing in respective yield and ultimate tensile strength values in as-bought, normalized and tempered specimens, the fracture behaviour of hardened specimen is totally different [18, 19]. The graph is the evidence for the formation of brittle supersaturated solid solution of martensite. Fig. 5 As-bought specimen at 200X From the % Displacement graph (Figure 11), it is clear that as- bought specimen has got maximum ductility. This also justifies that the as-bought specimen is hot worked. This increase in ductility is due to the increase in weight % of pro-eutectoid ferrite which is coarse in size.

Fig. 6 Normalized specimen at 200X

Fig. 7 Hardened specimen at 200X

Fig. 9 A typical Load - Displacement Diagram

C. Hardness test The Fig. 12 explains that the hardness of As-Bought specimen is nearly 30% less than that of normalized specimen. It clearly indicates Fig. 8 Tempered (5hr) specimens at 200X that the as bought specimen is either hot worked or as cast. The hardened specimen shows hardness values almost double that of Tempered specimen shows the relaxation of martensitic normalized specimen. It shows the ability of the specimen to harden. structures. The supersaturated martensitic structures ages into The carbon and chromium content is sufficient to get an optimum c/a ratio of BCT martensitic cell. This martensitic transformation is lesser c/a ratio BCT martensite and cementite. Microstructure taking place without any quench cracks, so the quenching medium indicates white particles of cementite formed during five hours which is employed for hardening is adequate. The decrease in ageing. Microstructure of hardened specimen shows lath or hardness is due to decrease in c/a ratio of martensite during needle type martensite particles formed by quenching austenite tempering. The marginal decrease in hardness shows the inadequate phase. tempering duration which is 1 hour. When the tempering duration is extended to 5 hours, there is drastic reduction in the hardness value B. Tensile test of the hardened specimen. It is shown clearly in the graph. This is The load versus displacement curves are plotted for all the due to the relaxation of the BCT martensitic cell i.e. the drastic tensile specimens as shown in Figure 9. Tensile strength and reduction in c/a ratio of martensite. ductility values are found from load versus displacement curves. The distribution pattern of Ultimate Tensile Strength

104 3rd International Conference on Mechanical, Automotive and Materials Engineering (ICMAME'2013) April 29-30, 2013 Singapore

Fig. 13 Impact strength

Fig. 10 Tensile test result for UTS, BS, YS

Fig. 14 Cumulative wt. loss vs Time Fig. 11 Tensile test result showing Break & Peak displacement % Wear Test D. Impact test There is drastic reduction in the wear rate of the tempered The impact strength of As-Bought specimen is less compared to specimen during initial period as compared to other specimens [20- normalized one. The increase in impact strength of normalized 23]. Hardened specimen shows better resistance to wear compared to specimen is due to the formation of fine pearlite. The hardened As- Bought specimen. During the initial stages mild wear is observed specimen shows brittle failure with no energy absorbed during in tempered specimens whereas severe wear is observed in As- failure. The broken pieces of hardened specimen clearly show Bought, normalized and hardened specimens. It shows that wear catastrophic failure. The impact strength of the tempered specimen (1 resistance of pearlitic phase is very less compare to martensitic phase hour) shows considerable improvement over hardened one. As aging but in pearlitic phase, fine pearlite has higher wear resistance duration increases toughness increases. The 5 hours tempered compared to coarser one. The wear resistance of tempered specimen specimen shows excellent impact resistance. This may be due to the is higher compared to hardened specimen because of reduction in c/a reduction in c/a ratio of martensite during ageing. The variation in ratio of martensitic structure [24]. In hardened specimen there may be toughness is shown in Figure 13. brittle fracturing of contacting surface during sliding which increases wear. The wear behavior is explained in Fig. 14.

IV. OBSERVATION & CONCLUSIONS • Tempered specimen has lesser strength but better ductility and toughness compared to hardened specimen. • As Bought specimen shows least ultimate tensile strength whereas hardened specimen shows maximum strength. • Normalized and tempered specimens have almost same value of tensile strength. • As- bought specimen shows higher ductility due to coarser pearlitic phase. • Longer the tempering duration higher is the toughness value. • Hardened specimens possess excellent Rockwell hardness followed by tempered, normalized and as Fig. 12 Hardness values bought specimen, which has least hardness.

105 3rd International Conference on Mechanical, Automotive and Materials Engineering (ICMAME'2013) April 29-30, 2013 Singapore

• Wear resistance of as bought and normalized specimens conference on mechanical and physical behaviour of materials is very less compared to tempered specimen. under dynamic loading, Journal of Physics, Vol 134, pp 1085– 1090. • Increasing tempering duration from 1 to 5 hours [18] Pradeep L Menezes, Kishore, & Satish V Kailas, 2006, “Influence significantly improves wear resistance and toughness. of surface texture on coefficient of friction and transfer layer • Tempering decreases hardness value of the hardened formation during sliding of pure Mg pin on EN 8 steel plate”, Journal of Wear, Vol 261, pp 578-591. specimen. [19] Anand Prakash Modi, 2007, “Effects of microstructure and • As- Bought &Normalized specimens show higher experimental parameters on high stress abrasive wear behaviour ductility. of a 0.19 wt % Carbon dual phase steel”, Tribology International • Hardened and tempered specimen show the lath or Journal, Vol 40, pp 490-497. [20] Sarwar M, Ahmad E, Qureshi K A, Manzoor, 2007, “Influence of needle type martensitic structure whereas As- Bought epitaxial ferrite on tensile properties of dual phase steel”, &Normalized show pearlitic phases. Materials Design, Vol 28, pp 335–340. [21] Suleyman gunduz & Atilla Torun, 2008, “Influence of straining REFERENCES and aging on the RT mechanical properties of dual phase steel”, Journal of materials &DSN, Vol 29, pp 1914-1918.

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