Effect of Heat Treatment on Mechanical Properties of AISI 4147 Spring Steel

Transcription

1 Effect of Heat Treatment on Mechanical Properties of AISI 4147 Spring Steel 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 industrialized world. It forms the basic building material of today s structure. Moreover steels with large chromium and vanadium percentage can be used as spring steels which form the suspension system. Prevention of wear and increase in steel life depends principally on the design and operation on the component, but providing some pre-use treatment on steel can also improve the quality to a great extent. It has been seen that most of the study focuses on the experimental testing of the steel component and very few focuses on the material testing and improving its properties beforehand. One of the processing routes to alter the properties is heat treatment. Nearly 90% of the springs are used in heat treated conditions. The major requirement for the conventional spring steel is toughness, strength & hardness. In this view, it is proposed to study the mechanical and tribological properties of AISI 4147 (EN47) spring steel with different heat (thermal) treatments like normalizing, hardening and tempering. All heat treatments are carried out in atmospheric condition. Hardening treatment improves hardness of the material, a marginal decrease in hardness value with improved ductility is observed in tempering. Hardening and longer duration tempering show better wear resistance compare to other heat treatments. Both mild and severe wear regions are observed. Generally mild wear region is observed above 5 hours of continuous running of the specimen. Microstructural analysis shows the existence of pearlitic structure in as bought & normalized specimens, lath martensitic structure in hardened specimen. Keywords tempering, hardness, heat treatment, normalizing, toughness I. INTRODUCTION TEELS can be subjected to variety of conventional heat treatments like normalizing, hardening and tempering. S S. S. Sharma, Professor, Department of Mechanical & Manufacturing Engineering, Manipal Institute of Technology, Manipal (Ph: ; ss.sharma@manipal.edu). K. Jagannath, Professor, Department of Mechanical & Manufacturing Engineering, Manipal Institute of Technology, Manipal (Ph: ; jagan.korody@manipal.edu). U. Achutha Kini, B Professor, Department of Mechanical & (Ph: ; achutha_kini@yahoo.co.in). P. R. Prabhu, Asst. Professor - senior scale, Department of Mechanical & (Ph: ; raghu.prabhu@manipal.edu). Jayashree P. K, Asst. Professor - selection scale, Department of Mechanical & Manufacturing Engineering, Manipal Institute of Technology, Manipal (Ph: ; jayashreepk@rediffmail.com). Gowrishankar M. C, Asst. Professor, Department of Mechanical & (Ph: ; gowrishankarmc@gmail.com). C. Bhat, Professor & Head, Department of Mechatronics Engineering, Manipal Institute of Technology, Manipal (Ph: ; e- mail: Chandra.bhat@manipal.edu). The combination of heating and cooling operations applied to a metal or alloy in the solid state is termed as heat treatment. The high temperature phase austenite in steel has the property to transform into variety of room temperature phases like coarse pearlite, bainite & martensite depending upon the cooling cycle [1-3]. These phases may be the decomposition products like ferrite & cementite or it may be super saturated solid solution. Depending upon the plate thickness and interlammer spacing between ferrite and cementite phases in pearlite, the property of steel can be altered. The plate thickness and interlammer spacing between ferrite and cementite is larger, coarser is the pearlite and ferrite, hence ductility increases. This is possible by slow cooling of the austenitic phase to room temperature, accordingly the treatment is known as annealing [4-7]. If the austenitic phase is cooled at a slightly faster rate, so that the decomposition of austenite by the eutectoid reaction is possible to form medium or fine pearlite with increased weight percentage of eutectoid mixture (pearlite), the treatment is known as normalizing [8-11]. Here the degree of dispersion of ferrite and cementite in pearlite increases to improve the machinability with finer grain size. The normalized steel contains nearly 50 wt % of pearlite and 50 wt % of proeutectoid ferrite. If the austenitic phase is cooled in such a way that the cooling rate is greater than or equal to critical cooling rate (CCR), the transformed phase is termed as martensite. It is supersaturated single phase with body centered tetragonal (BCT) structure. The BCT structure has got a c/a ratio higher with more trapped carbon in the lattice. So hardness and strength increase with considerable amount of thermal stresses because of quenching severity. To decrease the c/a ratio of BCT martensite, to improve toughness, to convert retained austenite into stable phases; to saturate the non-equilibrium BCT martensitic structure and to minimize the thermal stresses induced during hardening, tempering treatment is given. Depending upon the tempering temperature and duration, the improvement in toughness is possible with the sacrifice of hardness and strength. Because of high brittleness, as quenched spring steels are seldom used for practical applications [12, 13]. By tempering process, the properties of quenched steel could be modified to improve its impact resistance. During tempering the resulting microstructure contains bainite or epsilon carbide in a matrix of ferrite depending on the tempering temperature. Spring steels are used in the quenched and tempered condition which gives optimum strength, toughness and vibration damping. The change in microstructure and strength 102

2 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 yield strength, high proportional limit, and high fatigue strength. These desirable properties of spring can be 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 these properties springs are hardened and tempered. Table I shows the chemical composition of the AISI 4147 steel used in this study. TABLE I AISI 4147 STEEL COMPOSITION Element % Wt. Element % Wt. Carbon Sulphur Silicon Iron Manganese Nickel Molybdenum Aluminium Vanadium Chromium II. EXPERIMENTAL DETAILS A. Specimen preparation Standard specimens are prepared with the required dimensions for tensile, impact, hardness, and wear tests. The shape and size of standard dimension chosen for the tests are shown in Figures 1, 2, 3 and 4. Fig. 1 Tensile Test Specimen (mm) Fig. 4 Hardness Test Specimen (mm) B. Heat Treatment Procedure The Electric furnace is used for heating the specimen to the austenitic state. All specimens are prepared from as bought steel and subjected to three types of heat treatments such as normalizing, hardening and tempering and compared with as bought specimen. Three specimens each for tensile, impact, wear, microstructure and hardness are used for the analysis in respective treatments. First set consisting three specimens in respective test is subjected to normalizing and another three sets are hardened. Out of three sets hardened, one set is tempered at 300 o C for one hour and the second set is tempered at the same temperature for five hours followed by slow cooling in air. All specimens for normalizing and hardening are heated for two hours at 900 o C and normalized specimen is cooled in air. SAE 30 oil is used as quenchant. The average value of three readings is considered for analysis. C. Testing 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. III. RESULTS & DISCUSSIONS A. Micro structural Analysis: Fig. 2 Impact Test Specimen (mm) 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 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

3 Fig. 5 As-bought specimen at 200X (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. From the % Displacement graph (Figure 11), it is clear that asbought 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 Fig. 8 Tempered (5hr) specimens at 200X Tempered specimen shows the relaxation of martensitic structures. The supersaturated martensitic structures ages into lesser c/a ratio BCT martensite and cementite. Microstructure indicates white particles of cementite formed during five hours ageing. Microstructure of hardened specimen shows lath or needle type martensite particles formed by quenching austenite phase. B. Tensile test The load versus displacement curves are plotted for all the tensile specimens as shown in Figure 9. Tensile strength and ductility values are found from load versus displacement curves. The distribution pattern of Ultimate Tensile Strength 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 that the as bought specimen is either hot worked or as cast. The hardened specimen shows hardness values almost double that of normalized specimen. It shows the ability of the specimen to harden. The carbon and chromium content is sufficient to get an optimum c/a ratio of BCT martensitic cell. This martensitic transformation is taking place without any quench cracks, so the quenching medium which is employed for hardening is adequate. The decrease in hardness is due to decrease in c/a ratio of martensite during tempering. The marginal decrease in hardness shows the inadequate tempering duration which is 1 hour. When the tempering duration is extended to 5 hours, there is drastic reduction in the hardness value of the hardened specimen. It is shown clearly in the graph. This is due to the relaxation of the BCT martensitic cell i.e. the drastic reduction in c/a ratio of martensite. 104

4 Fig. 10 Tensile test result for UTS, BS, YS Fig. 13 Impact strength Fig. 11 Tensile test result showing Break & Peak displacement % D. Impact test The impact strength of As-Bought specimen is less compared to normalized one. The increase in impact strength of normalized specimen is due to the formation of fine pearlite. The hardened specimen shows brittle failure with no energy absorbed during failure. The broken pieces of hardened specimen clearly show catastrophic failure. The impact strength of the tempered specimen (1 hour) shows considerable improvement over hardened one. As aging duration increases toughness increases. The 5 hours tempered specimen shows excellent impact resistance. This may be due to the reduction in c/a ratio of martensite during ageing. The variation in toughness is shown in Figure 13. Fig. 12 Hardness values Wear Test Fig. 14 Cumulative wt. loss vs Time There is drastic reduction in the wear rate of the tempered specimen during initial period as compared to other specimens [20-23]. Hardened specimen shows better resistance to wear compared to As- Bought specimen. During the initial stages mild wear is observed in tempered specimens whereas severe wear is observed in As- Bought, normalized and hardened specimens. It shows that wear resistance of pearlitic phase is very less compare to martensitic phase but in pearlitic phase, fine pearlite has higher wear resistance compared to coarser one. The wear resistance of tempered specimen is higher compared to hardened specimen because of reduction in c/a ratio of martensitic structure [24]. In hardened specimen there may be 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 bought specimen, which has least hardness. 105

5 Wear resistance of as bought and normalized specimens is very less compared to tempered specimen. Increasing tempering duration from 1 to 5 hours significantly improves wear resistance and toughness. Tempering decreases hardness value of the hardened specimen. As- Bought &Normalized specimens show higher ductility. Hardened and tempered specimen show the lath or needle type martensitic structure whereas As- Bought &Normalized show pearlitic phases. REFERENCES [1] Wilson D V, Russel B., 1960, The contribution of precipitation to strain ageing in low carbon steels, Acta Metallurgica, Vol 8, pp [2] Brintly B J., 1970, The effect of dynamic strain-ageing on the ductile fracture process in mild steel, Acta Metallurgica, Vol 18, pp [3] Farrell R.M., & T. S. Tyre, 1970, The relationship between load and sliding distance in the initiation of mild wear of steel, Journal of Wear, Vol 15, pp [4] Davies RG., 1979, Early stages of yielding and strain ageing of a vanadium containing dual phase steel, Metal Transformation, Vol 10A, pp [5] Chang P., 1984, Temper-aging of continuously annealed low carbon dual phase steel, Metal Transformation, Vol15A, pp [6] Lou S, Northwood DO., 1995, Effect of temperature on the lower yield strength and static strain ageing in low-carbon steels, Journal of Materials Science, Vol 30, pp [7] Sarwar M, Priestner R., 1996, Influence of ferrite martensite microstructural morphology on tensile properties of dual-phase steel, Journal of Materials Science, Vol 31, pp [8] Ahmed E, Priestner R., 1998, Effect of rolling in the intercritical region on the tensile properties of dual phase steel, Journal of Materials Engineering Performance, Vol 6, pp [9] Abdalla A J, Hein LRO, Pereira MS, Hashimoto TM., 1999, Mechanical behaviour of strain aged dual phase steels, Materials Science Technology, Vol 15, pp [10] Sun S, Pugh M., 2002, Properties of thermo mechanically processed dual-phase steels containing fibrous martensite, Mater Science Engineering A, Vol 335, pp [11] Erdogan M, Priestner R., 2002, Effect of martensite content, its dispersion, and epitaxial ferrite content on Bauschinger behaviour of dual phase steel, Journal of Materials Science Technology, Vol 18, pp [12] Hozumi Goto & Yoshufumi Amamoto, 2003, Effect of varying load on wear resistance of carbon steel under unlubricated conditions, Journal of Wear, Vol 254, pp [13] Ng a* D.H.L., Cho a K.S., Wong a M.L., Chan b S.L.I., Mac X.-Y., Lo d C.C.H., 2003, Study of microstructure, mechanical properties and magnetization process in low carbon steel bars by Barkhausen emission, Elsevier Journal, Materials Science and Engineering, A358, pp [14] Prof (Dr) Mitra P K, Dr. Paul S, Chatterjee S, 2004, Treatment, Structure, Corrosion, Correlation of AISI 8640 Steel, IE (I) Journal MM, Vol 85, pp [15] Yan-qiu Zia, Wei-min Liu & Qun-JiXue, 2005, Comparative study of the tribological properties of various modified mild steels under boundary lubrication condition, Journal of Wear, Vol 38, pp [16] Anijdan S H M, Vahdani H., 2005, Room-temperature mechanical properties of dualphase steels deformed at high temperatures, Materials Letter, Vol 59, pp [17] Larour P, Verleysen P. Bleck, W., 2006, Influence of uniaxial, biaxial and plane strain pre-straining on the dynamic tensile properties of high strength sheet steels, Eighth international conference on mechanical and physical behaviour of materials under dynamic loading, Journal of Physics, Vol 134, pp [18] Pradeep L Menezes, Kishore, & Satish V Kailas, 2006, Influence of surface texture on coefficient of friction and transfer layer formation during sliding of pure Mg pin on EN 8 steel plate, Journal of Wear, Vol 261, pp [19] Anand Prakash Modi, 2007, Effects of microstructure and experimental parameters on high stress abrasive wear behaviour of a 0.19 wt % Carbon dual phase steel, Tribology International Journal, Vol 40, pp [20] Sarwar M, Ahmad E, Qureshi K A, Manzoor, 2007, Influence of epitaxial ferrite on tensile properties of dual phase steel, Materials Design, Vol 28, pp [21] Suleyman gunduz & Atilla Torun, 2008, Influence of straining and aging on the RT mechanical properties of dual phase steel, Journal of materials &DSN, Vol 29, pp [22] Gunduz S, Demir B, Kacar R., 2008, Effect of aging temperature and martensite by volume on strain aging behaviour of dual phase steel, Iron making and Steel making, Vol 35, pp [23] Erdal Karadeniz, 2008, Influence of different initial microstructure on the process of spheroidization in cold forging, Elsevier Journal, Materials and Design, Vol 29, pp [24] Qamar S.Z.,, 2009, Effect of heat treatment on mechanical properties of H11 tool Steel, Journal of Achievements in Materials and Manufacturing Engineering, Vol 35, Issue 2, pp