Mechanical Property Assessment of Austempered and Conventionally Hardened AISI 4340 Steel

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2 Mechanical Property Assessment of Austempered and Conventionally Hardened AISI 434 Steel S. S. Sharma1, P. R. Prabhu 2, Gowrishankar M. C 3, A. V. B. Sudhakar 4 1 Professor, Department of Mechanical & Manufacturing Engineering, MIT Manipal 2 Asst. Professor Sr. Scale, Department of Mechanical & Manufacturing Engineering, MIT Manipal 3 Asst. Professor Sr. Scale, Department of Mechanical & Manufacturing Engineering, MIT Manipal 4 Student, Department of Mechanical & Manufacturing Engineering, MIT Manipal 1 ID ss.sharma@manipal.edu 2 ID raghu.prabhu@manipal.edu 3 ID achutha_kini@yahoo.co.in Abstract: The chemical composition and mechanical properties of steel decide its applicability for manufacturing various components in different areas of engineering interests. Heat treatment processes are commonly used to enhance the required properties of steel with or without change in chemical composition. The present work aims to perform conventional hardening and Austempering treatment with experimental investigation of the effect of austempering and conventional hardening (quenching) on AISI 434 steel. Different tests like tensile, torsion, hardness, impact and microstructure analysis are carried out in as bought and heat treated conditions. It was found that Austempering improves tensile, torsional and impact strength whereas a marginal decrease in hardness is found as compared to conventional hardening (direct quenching).lower bainitic and martensitic structures are observed in austempered and conventionally hardened specimens. Keywords: Heat treatment, austempering, martensite, bainite, hardening, tensile. 1. INTRODUCTION In today s world, structural materials require various properties such as high strength, excellent toughness and wear resistant due to the demands for high performance and severe service environments of machine components. In order to meet these demands, many studies have been performed on steels especially alloy steels. However, little attention has been paid to the tensile and torsional behaviour, toughness and hardness of specimens which have been given a heat treatment. Steel over the years has proved to be the most important, multi-functional and most adaptable material in automotive, aircraft and general engineering applications. Nickel, Chromium, Molybdenum, silicon steels are best suited for applications requiring high tensile strength and toughness. In recent years, extensive studies on the improvement of mechanical properties of these materials have been carried out. Austempering as a heat treatment process on engineering materials increases the yield strength, wear resistance, hardness and toughness properties. Engineered systems are often set by intended or unintended stresses due to heavy machining, rapid solidification, bombardment of foreign materials, heat treatment conditions adopted and thermal cycling on components. The conventional hardening process may 134 increase the hardness and ultimate tensile strength but results in the reduction of toughness of the material. Hence, a criterion to enhance the properties of materials such that the maximum load that a component can sustain is paramount importance. Steel is one of the important alloy where a variety of properties are possible by altering heating and cooling cycle i.e., heat treatment. The tailor made properties are possible in steels by selecting suitable heat treatment process according to the application. A wide variety of thermal hardening techniques are available in the heat treatment engineer tool kit like direct quenching, stepped quenching, timed quenching, spray quenching (hardening with self-tempering), martempering, austempering etc. Out of these treatment methods austempering method has the unique advantage of moderate hardness combined with good toughness and tensional strength. At the same time generally there is no retained austenite and residual stresses if the process is designed accordingly. The micro structure consists of needle like ferrite and well dispersed carbides as saturated phases. In appearance it resembles like single phase martensite because of the degree of fineness of the micro constituents [1 4]. Austempered or interrupted quenching steels possess optimum hardness balanced with tensile properties, known as toughness.

3 Commonly austempered steels include AISI 19, 414, 434, 65, EN 31and SAE 521 [5 8]. Austempering is a method of hardening by heating to the austenitizing temperature i.e., 3 C to 5 C above upper critical temperature in the case of hypo eutectoid steel followed by isothermal quench in a medium maintained above temperature, but below the nose of isothermal transformation diagram and holding the steel in this medium until austenite completely transforms into bainite. Lower the temperature range better is the dispersion of two saturated phases, which enhances toughness of steel. The quenching severity must be faster enough so that continuous cooling curve do not cut the transformation beginning curve of isothermal transformation diagram i.e., cooling rate is equal to or greater than critical cooling rate (CCR) and temperature and duration of isothermal holding in later stage is designed in such a way that decomposition of austenite into a well dispersed tiny two phase mixture as ferrite and carbide is fully completed. In the case of conventionally hardenable steels like HSLA, Cr-Mo, Ni-Mo where martensite forms on air cooling, bainite formation also takes place by continuous slow cooling [9 13]. In such cases bainitic formation results with retained austenite and martensite so that bainitic transformation is incomplete. This type of transformation results in marginal residual stresses compared to isothermal transformation. Higher the temperature range of bainite, lower is the hardness and strength with increased ductility [14 19]. In this view different tests like hardness, impact, wear and microstructure analysis, are carried out before and after heat treatment process. It is found that as bought steel has less hardness and more wear prone, while martempered steel is hardest and least vulnerable to wear. Austempered steel has got highest impact strength and it is depend upon isothermal holding duration. Least toughness is observed in conventionally hardened. On the other hand, qualitative and quantitative studies are performed to ascertain the influence of austempering heat treatment process on the properties. 2. EXPERIMENTAL METHOD The chemical composition of the investigated steel is determined by optical emission spectrometer and shown in Table 1. Table1: Composition of steel used between 28 o C and 35 o C (From Isothermal Transformation diagram). One set of as-bought (without heat treatment) specimens are subjected to austempering on heating to 85 o C for 2 hours and quenching in oil bath maintained at 3 o C for about 2-22 minutes isothermally. Second set is conventionally hardened by heating to 85 o C for 2 hours and quenching in oil bath maintained at room temperature (3 o C). The third set is tested without heat treatment to compare the properties between austempering, conventional hardening and without hardening. 2.1 Mechanical testing Tensile test: All the tensile specimens are subjected to tensile test on Electronic Tensometer. The load versus elongation graphs are recorded and analysed. Fig. 1: Tensile test specimen (All dimensions are in mm) Torsion Test: All the torsion testing specimens are subjected to torsion test on torsion testing machine. The torque versus angular deflection graphs are plotted and analysed. Fig. 2: Torsion test specimen (All dimensions are in mm) Component C Si Mn Ni Cr Mo Wt % The specimens are prepared by machining from asbought steel according to ASTM standard in three sets for tensile, torsion and impact. Each set consists of three specimens each for tensile, torsion and impact tests. The lower bainitic temperature range for AISI 434 steel is Fig. 3: Impact test specimen (All dimensions are in mm) Hardness test: The specimens are polished with 2 series of emery papers before the test. The Rockwell hardness tester is employed for the hardness measurement. 135

4 Impact test: The charpy test is conducted for all the samples. The energy absorbed before failure of the specimen is noted in each case. Microstructure examination: samples are prepared by polishing with different grades of emery papers and etched with Nital solution. Micro structure of the nonheat treated, austempered and conventionally hardened AISI 434 steel is recorded using metallurgical microscope. 3. RESULTS AND DISCUSSION 3.1 Tensile test Figures 4 and 5 show the Load versus deformation graphs for as-bought and conventionally hardened steel. The as-bought specimen shows clear cut yield point, the typical ductile behaviour of steel. The conventionally hardened and austempered specimens do not show clear yield points. The ductility of as-bought steel is higher than austempered and conventionally hardened steel. The area under the load versus deformation is larger for austempered one as compared to other two. This is the measure of toughness. The marginal loss in strength is Load Vs Deformation UltimateTensile Load (N) Fig. 5: Load vs. elongation graph for austempered specimen Tensile test results Load in N Fig. 6: Tensile load vs. type of modification Deformation in mm Fig. 4: Load vs. elongation graph for as-bought specimen Peak deformation(mm) observed in austempered specimen over conventionally hardened with the benefit of higher toughness. The increased deformation with higher strength shows the increase in stiffness of the material. This is the typical behaviour of lower bainitic structure. The better dispersion of fine ferrite and carbides is responsible for this behaviour. Tensile results especially ductility is poor in conventionally hardened specimen. A little permanent elongation is recorded in conventionally hardened specimen. The fractured surface shows almost brittle failure without necking. It is the typical behaviour of unaged martensitic structure. Figures 6, 7 and 8 show the tensile behaviour of the specimen in with and without treatment condition. Tensile test results Fig. 7: Peak deformation vs. type of modification Break deformation(mm) Tensile test results Fig. 8: Break deformation vs. type of modification 136

5 3.2 Torsion Test Figures 9, 1, 11 and 12 show the torsional behaviour of the specimen in the given condition. Higher torque is observed in austempered one as compared to as bought specimen. Conventionally hardened specimen also shows lesser torque with lesser yield angular displacement. Austempered shows higher yield angular deflection and is at par with as bought specimen. It also indicates the increase in shear strength of the material during Austempering. This behaviour is due to the uniform dispersion of fine ferrite and carbide phases. Fig. 12: Angular deflection vs. type of modification 3.3 Hardness test Figure 13 shows the bulk hardness of the specimen with respect to the treatment given. Excellent hardness value is observed in conventionally hardened specimen compare to as bought. A marginal decrease in hardness is due to the behaviour of super saturated solid solution martensite structure. Fig. 9: Torque vs. Angular deflection graphs for as bought specimen Fig. 13: Rockwell hardness number vs. type of modification 3.4 Impact test Fig. 1: Torque vs. Angular deflection graphs for austempered specimen Torque (Kg-cm) Figure 14 shows the ability of the specimen to resist impact load. The energy absorbed before failure under impact load is extremely higher in austempered specimen compare to the other two conditions. It also suggests that further tempering may not be required after the treatment. Torsion test results Fig. 11: Torque vs. type of modification Fig. 14: Energy absorbed vs. type of modification 137

6 3.5 Microstructure examination Proceedings of the 2 nd International Conference on Current Trends in Engineering and Management Figure 15 shows the microstructure of different specimens in all the three conditions at 5X magnification. Clear distinguished carbide and ferritic phases are seen in as bought specimen. Conventionally hardened specimen Shows typical band like single martensitic phase. Austempered one shows needle type well dispersed fine phases. This is the typical pattern of bainitic structure. (a) 4. CONCLUSIONS The UTS of conventionally hardened and austempered specimens are comparable but peak and break elongation of conventionally hardened is far less than that of austempered specimen. This indicates the increase in elastic limit of the material during austempering compare to conventionally hardened one. However, the following conclusions are made during metallography, tensile, torsion, impact and hardness tests. Tensile graph shows clear and sharp yield strength in as bought specimen. Ductility of as bought specimen is higher than austempered and least in conventionally hardened. Yield torque of austempered one in torsion test is higher but angular deflection is comparable with as bought specimen. Torsional strength of conventionally hardened is far less compare to heat treated one. Hardness of austempered and conventionally hardened are almost similar but far higher than that of as bought specimen. The toughness (energy absorbed before failure) of the austempered specimen is far ahead compare to as bought and conventionally hardened. It indicates the ability of the specimen to undergo self-tempering during austempering. It also reduces the processing cost of the specimen to induce toughness compared to conventionally hardened one. Microstructure reveals the clear martensitic structure in conventionally hardened, needle type bainitic structure in austempered and ferritic and carbide structure in as bought specimen. There is overall improvement in mechanical properties of austempered one compared to conventionally hardened one. (b) 4. REFERENCES (c) Fig. 15: Microstructure of (a) as bought (b) Conventionally hardened (c) Austempered specimen at 5X 1. F. Abbasi, A. J. Fletcher, and A. B. Soomro, A critical assessment of the hardening of steel by martempering, International Journal of Production Research, vol. 25, No. 7, pp , Chang P., Temper-aging of continuously annealed low carbon dual phase steel, Metal Transformation, Vol15A, pp , Sarwar M and Priestner R., Influence of ferrite martensite microstructural morphology on tensile properties of dual-phase steel, Journal of Materials Science, Vol 31, pp , Abdalla A J, Hein LRO, Pereira MS and Hashimoto TM., Mechanical behaviour of strain aged dual phase steels, Materials Science Technology, Vol 15, pp , Erdogan M and Priestner R., Effect of martensite content, its dispersion, and epitaxial ferrite content 138

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