Characteristics Evaluation of Normalized and Conventionally Hardened AISI 4340 Steel

Transcription

1 Characteristics Evaluation of Normalized and Conventionally Hardened AISI 4340 Steel Gurumurthy B.M 1, A. Amar Murthy 2, Jamaluddin Hindi 3, S.S Sharma 4, Achutha Kini 5 Abstract The mechanical properties of steel decide its applicability for a particular condition. Heat treatment processes are commonly used to enhance the required properties of steel starting from a simple crankshaft of two wheelers to the robust turbine blades. The type of heat treatment that equipment demands depends on its use and ability of the metal to undergo phase transformation with respect to different non equilibrium cooling rates. The present work aims at experimentally investigating the effects of normalizing, and conventional hardening on the hardness, microstructure and toughness of AISI 4340 steel. The material was machined to ASTM standards and then different tests like microstructure analysis, hardness test, impact test, and were carried out after the heat treatment processes. All the tests were carried as per ASTM standards and compared with as bought steel. It was found that normalized steel has got lower hardness than conventionally hardened steel. The ASTM grain size is used to compare the grain size of different phases obtained with and without heat treatment. An increase in brittleness was observed with the increase in hardness during conventional hardening. Keywords Normalizing, hardening, AISI 4340, microstructure, toughness. T I. INTRODUCTION HIS AISI 4340 steel is a heat treatable, low alloy steel containing chromium, nickel and molybdenum. It has high toughness and strength in the heat treated condition. The addition of molybdenum prevents the steel from being susceptible to temper embrittlement. The addition of nickel improves the hot hardness value with high toughness chromium improves wear resistance of material. These usages tend to fall in to the condition where different combinations of properties are required. In each of these applications, works being carried out to develop higher strength where this type of manufacturing processes, the type of heat treatment and the number,concentration alloying element are taken into consideration. An understanding of the mechanical properties of metals during deformation over a wide range of loading conditions is of considerable importance for a number 1 Assistant Professor Mechanical & Manufacturing Engg. Dept, MIT, Manipal, Karnataka, India. guru.mech402@gmail.com 2 Assistant Professor-Sr.Scale, Mechanical & Manufacturing Engg. Dept, MIT, Manipal, Karnataka, India. amar.murthy@manipal.edu 3 Assistant Professor Mechanical & Manufacturing Engg. Dept, MIT, Manipal, Karnataka, India. jamalhindi@gmail.com 4 Professor, Mechanical & Manufacturing Engg. Dept, MIT, Manipal, Karnataka, India. sharmass_mit@yahoo.co.in 5 Professor, Mechanical & Manufacturing Engg. Dept, MIT, Manipal, Karnataka, India. achutha_kini@yahoo.co.in). of engineering applications. Upon quenching, martensite is formed throughout each austenite crystal in a manner similar to the freezing of solid-solution alloys. A redistribution of carbon takes place and the needle type Martensite plates are formed. Martensite is the non-equilibrium super saturated solid solutions of Iron in body centred tetragonal shaped unit cells, where carbon atoms are lodged in the interstitial space. The height of the cell depends upon the weight percentage of carbon dissolved in austenite phase on heating up to the heat treatment temperature. The hardness of the quenched specimen depends upon the weight percentage of martensite formed i.e. martensite formation is coupled with retained austenite. more the percentage of retained austenite present during martensite formation, lesser in the hardness and strength. Whereas normalising is totally different form conventional hardening where lamellar fine pearlitic two phase structured forms without any retained austenite. The pearlitic structure consists of alternate layers of ferrite and cementite in finer form. Finer the pearlite more is the toughness and hardness with reduced ductility. Normalised structure is stable room temperature phase. It is proposed to give heat treatments in this method involving normalizing and conventional hardening. The structure and properties obtained by these two treatments is compared with as-bought specimen. II. EXPERIMENTAL PROCEDURE The chemical compositions of the investigated samples are shown in table1. In addition to Cr,Si,Mn, traces of phosphorous and sulfur are present at concentrations on the order of a few parts per million. Figure 1 shows a graphical representation of the heat treatment cycle for normalizing (air cooling) and hardening (water quenching). In order to ensure Complete austenitic grain structure upon heating, the material was heated to 900 o C, a temperature slightly above the upper critical temperature. The heating temp range is selected by analyzing the Fe-Carbon equilibrium diagram shown in figure.2 the normalized samples were heated, followed by air cooling at room temperature. The conventionally hardened steels were heated and then quenched in water at room temperature. To evaluate grain size, the microstructures were first observed under an optical microscope under a magnification of 200X. The software used for microstructure analysis was Envision 5.0. To 64

2 evaluate the mechanical properties, hardness number (Vickers) and impact energy in joules (Izod V-notch) were measured at room temperature). 2.1 Specimen Preparation Impact Specimen Fig 1. Heat treatment schedule for hardening and normalising. Fig.3: Impact Specimen (All dimensions are in mm) Fig. 2: Fe-C phase diagram TABLE 1.1 TYPICAL CHEMICAL COMPOSITION Carbon 0.40% Silicon 0.25% Manganese 0.70% Nickel 1.85% Chromium 0.80% Molybdenum 0.25% Iron 95.75% The state is proved and chemical analysis made, rod was cut into 80 mm length pieces using band saw. This specimens are found to reduce the length to 75mm. Drilling was performed at one end in order to hold the work piece with the tail stock. Turning reduces the diameter to mm. Shaping was carried out to obtain 10x10 mm square cross section. V notch was made on the square work piece using shaper. Figure shows the standard specimen for Izod impact test Hardness and Microstructure Specimen Rod was cut into 25 mm pieces using band saw. Facing was done to obtain smooth surface and length was reduced to 20mm. Grinding and polishing were performed to obtain mirror finish on the surface. Fig 4:Microstructure and hardness specimen (All dimensions are in mm) 65

3 2.2 Method Impact Test The specimens are tested for toughness using Izod impact tester. The specimen was placed in the holding vice. Dial gauge was set to maximum (180 joules) and the pendulum is released to strike the specimen with impact load. The energy absorbed before failure is noted. The average values of these trials were recorded Microstructure The microstructure recorded in inverted metallurgical microscope at 200X magnification. The cylindrical specimens are polished with 1/0, 2/0, 3/0, 4/0 emery papers in the order. The final polishing to mirror finish is done on disc polisher with velvet cloth and alumina paste (Al 2 O 3 ). Then it is etched with NITAL Hardness Test Specimens are polished up to mirror like finish. The specimen was placed on the hardness testing fixture and magnification was adjusted to show 400X. Micro-indentation was made on the surface of the specimen by applying 100gf for 15 seconds. Using the eye piece, the length of the diagonals of the indentation was noted down. As per the lengths of the diagonals the machine gives the hardness value. 3 trials were performed and the average value was documented as the hardness value of AISI Fig.5 (b): Microstructure of the conventionally hard ended specimen200x. Figure 6 shows the graph of ASTM grain size number with respect to condition of specimen. Martensitic grains are fine compared to other 2 conditions. Table 2 shows the same result in the tabular form. TABLE 2 GRAIN SIZE ANALYSIS. III. RESULTS AND DISCUSSIONS 3.1 Microstructure analysis. The Microstructure of normalized specimen shows clearly phases as proeututoid ferrite and two phase finer lamellar mixture of ferrite and cementite. The grains are finer, the microstructure of hardened specimen shows band like martensitic structure. Martensitic laths are seen as needles with almost parallel axes. The ASTM grain size number was evaluated using Hayn linear intercept method. The number of lines was varied, and an average value of the ASTM number was reported. Figure 5 shows the microstructure showing well defined pearlite structure and martensitic bands in normalized and hardened conditions respectively. Fig 6: Bar graph representing grain size with respect to condition to specimen Fig.5 (a): Microstructure of the normalized specimen at 200X. 3.2 Hardness Analysis (VHN) The hardness of the conventionally hardened specimen is higher as compared to normalised and as received (as-bought) specimen. The result of hardness test also supports the outcome of microstructure where the conventionally hardened specimen shows martensitic structure with excellent hardness. The as bought specimen hardens also higher than the normalised. This may be due to the fact that the untreated specimen might be cooled at faster rate than that of 66

4 normalising or steel might be cold worked after casting. Table 3 shows the hardness result of the specimen in different condition and same represented with the help of graph in figure 7. TABLE 3 HARDNESS TEST RESULTS IN VHN. Fig 8: A bar graph showing Impact Energy with respect to condition of specimen. Fig.7: bar graph showing of hardness in VHN with respect to specimen treatment. 3.3 Impact energy analysis. The toughness (energy observed before failure ) of the asbought specimen is higher than the other two. This supports the argument that the as- bought specimen is not clod worked. It would have been cold worked the toughness would have been reduced. Toughness of the Martensitic structure is least as compared to other two. This is fundamental quality of Martensitic structure, which is very week in tensile and impact but strong in hardness.the required toughness would have been incorporated if martensitic structure would have been tempered. IV. CONCLUSIONS The steel under consideration is effectively and efficiently heat-treated by conventional hardening and normalising techniques. Mechanical properties are determined by standard tests. The tests are in good argument with literature outcome. However following conclusion is arrived. There was 37.26% increase in hardness value in conventional hardening compared to the as-bought specimen. The impact resistance value of specimen in conventional hardening is too low compared to normalized specimen. The microstructure shows two phase pearlitic structure in normalized and needle like martensitic single phase structure in conventional hardening. The grain size of martensitic structure is finer than normalized. V. RECOMMENDATIONS By analyzing the properties in as bought, normalized and hardened conditions in the order, the trend line for hardness and impact resistance be fitted as shown in figure 9(a) and (b). TABLE 4 IZOD TEST RESULTS Fig 9(a): VHN trend line. 67

5 Fig 9(b): Impact Energy Trend line Dr. S. S. Sharma holds Bachelor s degree in Industrial & Production Engineering (Mysore University, India 1987), Master s degree in Materials Engineering (Mangalore University, India 1996) and PhD degree in Materials Engineering (Manipal University, India 2007). He has 24 years of teaching experience. His area of interest includes Heat Treatment, Deformation of Metals, Material Characterization, Composite Materials. He has published 28 research papers in journals and presented 47 research papers in conferences. Dr. Achutha Kini holds Bachelor s degree in Mechanical Engineering (Mysore University, India 1985), Master s degree in Engineering Management (Mangalore University, India 1991) and PhD degree in Corrosion Engineering (Manipal University, India 2012). He has 24 years of teaching experience. His area of interest includes Corrosion Science and Engineering, Composite Materials. He has published 19 research papers in journals and presented 34 research papers in conferences. REFERENCES [1] TATA Steels High Tensile Steel AISI 4340 properties. [2] Laser surface heat treatment of AISI 4340 steel: A microstructural study by Palaniappa A. Molian. [3] Mechanical properties and microstructural features of AISI 4340 highstrength alloy steel under quenched and tempered conditions. [4] Vijaya K.Sharma,Scrtipta Materialaia 37; ,1997. [5] M.Gupta and M.K Sarappa,Materials research bulletin,30 ;8: ,1995 [6] M.Gupta,S quin,l.w.chin,j Mater,65 (1997) [7] DJ Loyod,Int.Mater.Rev 39(1) (1994) 13 [8] L.Sun,S Q Chen,JF MAO,DJ Yang,Meter,Chem Phys.36 (1994) 217 [9] M Gavgali,Ytotik,R sadeler,materials Letters 57 (2003) [10] Advances in the Metallurgy and Applications of ADI,Adel Nofel [11] AMS Hamouda,S.SulaimanT R Vijayram,M.Sayuti;Journal of Achievements in Materials and Manufacturing;Vol 25,Isseu 2,Dec 2007 [12] The effect of austenitizing temperature on the microstructure and mechanical properties of as-quenched 4340 steel [13] Baozhu, G., Krauss, G. Journal of Heat Treating, 4, 365, (1986) [14] Tomita, Y., Okabayashi, K., Metallurgical Transactions, 14, 485, (1983) [15] Mirak, A.R. and Nili-Ahmadabadi, M., Effect of modified heat treatments on the microstructure and mechanical properties of a low alloy high strength steel, Materials Science and Technology 20 (7): , (2004) [16] Sanjib Kumar Jaypuria, A Project report on Heat treatment of low carbon steels. 23, (2008) [17] Mechanical properties and microstructural features of AISI 4340 highstrength alloy steel under quenched and tempered conditions, , (1999) Mr.Gurumurthy.B.M is the corresponding authormechanical received his Beachelor s Degree in Engineering (VTU, Belgaum, India 2008) and Master s Degree in Mechatronics Engg (NIT K Surathkal, India 2010). He has four years in teaching and research. His area of interest includes Mechatronics, Manufacturing, Heat treatment and Metallurgy. He is a faculty member at Manipal Institute of Technology, Manipal and. He has published 3 research papers in journalsconferences. Mr. A. Amar Murthy is working as Assistant Professor-Sr scale in mechanical dept, He has 6years in teaching and research. design Manufacturing, Heat treatment and Metallurgy. He is a faculty member at Manipal Institute of Technology, Manipal Mr. Jamaluddin Hindi is working as Assistant Professor- in mechanical dept, He has 3 years in teaching and research.design Manufacturing, Heat treatment and Metallurgy. He is a faculty member at Manipal Institute of Technology, Manipal 68