Heat treatment effects to the fracture splitting parameters of C70S6 connecting rod/C70S6 svaistikliu terminio apdirbimo itaka skilimo sklidimo parametrams.

Ozdemir, Zafer

1. Introduction

The fracture splitting method is an innovative processing technique in the field of the automobile engine connecting rod (con/rod) manufacturing. Compared with traditional method, the technique has remarkable advantages. It can decrease manufacturing procedures, reduce equipment and tools investment and save energy. Hence the total production cost is greatly reduced. Furthermore, the technique can also improve product quality and bearing capability [1]. It provides a high quality, high accuracy and low cost route for producing connecting rods (con/rods). Sawing and machining processes of the rod and cap, in order to mate two faces can be eliminated, and is believed to reduce the production cost by 25%. Another advantage of this production method is that fracture-splitting connecting rods exhibit 30% higher fatigue strength and 13% less weight than conventional connecting rods, and can be splitted into two pieces (big body and cap) by fracturing with an instant impact load. Compared with powder metal and cast con-rods, it also has lower cost for the whole manufacturing process. Hence, it provides more advantageous production opportunities, and is prefered in manufacturing technology mostly [2].

1.1. Purpose and the content of the study

One of the fracture parameters optimizing methods is to change the microstructure without tampering the chemical structure by heat treatment applications. The fracture capability and effect of microstructure after austempering, optimizing the fracture parameters and comparing the conventional pearlitic C70S6 (produced in compliance with DIN 17100 and inspected according to EN 10204) and bainitic C70S6 have been investigated in this study. Bainite is an important microstructure between martensite and pearlite. It is neither hard as martensite nor soft as pearlite. Because of this, it is noteworthy for academic study to examine especially upper bainite usability as main structure for crackable connecting rods. No reference or research has been met about changing the microstructure of crackable C70S6 con-rod steel by heat tretment methods in literature. The author and his colleagues have studied and made experiments on martensite and tempered martensite [3]. As known, bainite is an important microstructure due to its outstanding mechanical properties. That is because, it has been especially emphasized on bainite. In detail, for crackable connecting rods as far as strength is desired but also toughness is not much allowed for fracture splitting property, theoretically upper bainite could be a perfect choice for crackable connecting rods, also there is no technological use of bainite in crackable connecting rods. The author has proved perfect fracture for upper bainite in his Ph.D. thesis [4]. The parameters for perfect fracture is explained in detail at chapter 2.2. These are the reasons why this study is an original one.

The study consists of austempering, fracture test, metallographic observation and the interpretation of these analysis. The hardness has been measured, the relation of the hardness and microstructure to the impact fracture load has been examined.

1.2. Brief knowledge about bainitic structure

In addition to pearlite, other microconstituents that are products of the austenitic bainite transformation exist; one of these is called bainite.

[FIGURE 1 OMITTED]

The microstructure of bainite consists of ferrite and cementite phases, and thus diffusional processes are involved in its formation. Bainite forms as needles or plates, depending on the temperature of the transformation; the microstructural details of bainite are so fine that their resolution is possible only using electron microscopy. Fig. 1 is an electron microscope that shows a grain of bainite (positioned diagonally from lower left to upper right); it is composed of a ferrite matrix and elongated particles of [Fe.sub.3]C; the various phases in this micrograph have been labeled. In addition, the phase that surrounds the needle is martensite, the topic to which a subsequent section is addressed.

[FIGURE 2 OMITTED]

Furthermore, no proeutectoid phase forms with bainite. The time-temperature dependence of the bainite transformation may also be represented on the isothermal transformation diagram. It occurs at temperatures below those at which pearlite forms; begin, end and half-reaction curves are just extensions of those for the pearlitic transformation, as shown in Fig. 2, the isothermal transformation diagram for an iron-carbon alloy of eutectoid composition that has been extended to lower temperatures. All three curves are C-shaped and have a "nose" at point N, where the rate of transformation is a maximum. As may be noted, whereas pearlite forms above the nose i.e. over the temperature range of about 540 to 727[degrees]C; at temperatures between about 215 and 540[degrees]C, bainite is the transformation product. It should also be noted that pearlitic and bainitic transformations are really competitive with each other, and once some portion of an alloy has transformed to either pearlite or bainite, transformation to the other microconstituent is not possible without reheating to form austenite.

1.3. Findings in literature and previous studies

Although C70S6 is excellent in fracture-splitability thanks to its small deformation during splitting, it has a coarser structure than the ferrite/pearlite structure of the medium-carbon micro-alloyed steels currently used as connecting rod steels. It is therefore low in yield ratio (yield strength/tensile strength) and cannot be applied to high-strength con-rods requiring high yield strength. Moreover, the inferior machinability of C70S6 owing to its pearlite structure has kept the steel from finding extensive utilization.

Because of the problems above, new studies for optimizing the fracture parameters have been carrying out. Steels for fracture-split components have been developed in response to the foregoing needs. The effect of martensite and tempered martensite to the fracture parameters have been studied in detail recently [3]. The research of fracture split of steel was carried out after changing the chemical structure: adding new elements as zirconium, calcium, aluminum [2] and titanium via finite elements method [6] recently.

The fracture parameters and microstructures have been examined by author and his colleagues in detail recently [7, 8]. Liming, Z. et al. investigated the lazer effect to the starting notch and fracture parameters [9]. Deen, Z. et al. investigated the lazer effect to the starting notch depth and Radius [10]. Roman C. et al. examined the fractured surfaces of connecting rods [11]. Kou S.Q. et al. composed the starting notch with laser and investigated the effect of this to the fracture parameters [12]. Iwazaki S. et al. designed and created a machine to manufacture crackable connecting rods [13]. Guirgos S. tried a different kind of method. In this method, crackable connecting rod's stress in the fracture area increases in a controlled atmosphere by a stress-increasing device and as the stress increases, the sudden fracture occurs [14].

1.4. Heat treatments effect to the microstructure evaluation

In this article it is aimed to observe the mechanical behaviour of the fracture and the properties of the microstructure after austempering. It is aimed to understand the effect of austempering to the sudden (instant impact force) fracture. The metallography of the fracture surfaces have shown us some typical microstructures of bainite.

2. Examination

The examination consists of heat treatment application (austempering), fracture experiments, metallographic observation and the interpretation of these analysis. The hardness has been measured, the relation of the hardness and microstructure has been examined. Fractured specimens' optical photos were carefully examined at the Nikon MA 100 Metal Microscope.

2.1. Austempering

Austempering is applied to two C70S6 crackable con-rod steels as shown in Fig. 3. A technical drawing of C70S6 steel is shown in Fig. 4. Two C70S6 crackable connecting rods have been applied austenitizing 800[degrees]C for 1 hour in the controlled heat treatment furnaces separately and soonly after it was taken to salt bath in 450[degrees]C for 3 hours, then quenched in still air as shown in Fig. 5.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

2.2. Fracture tests

The testing apparatus for evaluating fracture-splitability consisted of a split die and a 100 ton hydrolic press. The fracture has been started from starting notches (Fig. 6). The split die had the shape of a cylinder formed on a rectangular steel member. A wedge hole was machined in the mating faces of the two semicylinders. In the fracture-split test, the test piece was clamped in the split die, a wedge was inserted, and the assembly was placed on the hydrolic pressure. In these examples, fracture-splitting was conducted by 100 ton hydrolic press 150 mm. with an impact load (Figs. 7-9).

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

The fracture tests have been executed con-rods separately and a perfect brittle fracture has been obtained. Perfect fracture has to ensure some parameters; these are:

1. No material loss during fracture in two pieces (cap and rod).

2. No elastic deformation in fracture surfaces.

3. Exact match of the two surfaces after fracture so as to ensure the rough cleavage surfaces ensure the perfect match of cap and rod and have larger joint surface area than conventional machined smooth surfaces, so the processing accuracy, product quality and bearing capability are dramatically improved.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

As seen from Fig. 11, in 450[degrees]C the austenite transforms to upper bainite. It was chosen upper bainite transformation temperature (450[degrees]C), because the cementite particles here are thin (not very thin) enough to obtain brittle fracture without elastic or plastic deformation and enough toughness. This formation has been obtained due to the austempering by heat treatment process. The structure is neither too hard as martensite nor too soft as pearlite. It is considered that upper bainitic structure is to be an ideal form for crackable con-rods. These remarks are going to be explained in optical-SEM microstructures.

Lower bainite could be another research area. Lower bainite is not considered for crackable con-rod use due to its very thin cementite structure, because it is too elastic and tough, so brittle fracture could not be obtained.

The first line con-rod hardness value is 278 HB approximately and the other one is 306 HB. These values are desired results for fracture parameters.

[FIGURE 12 OMITTED]

The austempering has been applied to the sample before fracture splitting tests. The sample was prepared with 2% picric acid. Untransformed ferrite and bainite could be seen appearently (Figs. 12 and 13).

[FIGURE 13 OMITTED]

2.3. SEM analysis of bainitic microstructure

SEM analysis has been conducted with SEM LEO Gemini Electron Microscope FEI/QUANTA 400 FEG (25 kV) device.

[FIGURE 14 OMITTED]

A perfect brittle fracture could be observed in Fig. 14. It could also be seen granular and cleavage fracture in the fracture surface.

Microstructure transforms from austenite to upper bainite. Feathery appearence of bainite could also be seen clearly (Fig. 15).

[FIGURE 15 OMITTED]

[FIGURE 16 OMITTED]

The surface is a typical brittle and cleavage fracture surface (Figs. 15 and 16). Feathery bainitic appearance and [Fe.sub.3]C particles could be seen appearently. Microstructure is a typical upper bainite. Upper bainite: there are carbide particles in present inside lower bainitic ferrite, since it forms in lower temperatures, no layer form occurs. It has same chemical composition as pearlite but harder than it. It is a cleavage and brittle fracture in splitting surfaces just as desired in fracture splitting process. Feathery appearance of upper bainite is obvious.

3. Conclusion

1. No material loss due to the perfect fracture was observed during the fracture tests.

2. SEM and optical microscopy analysis clearly disclose upper bainite.

3. Perfect brittle fracture surfaces shows a typical cleavage fracture necessary for crackable con-rods so as to ensure perfect match of cap and rod and have larger joint surface area than conventional machined smooth surfaces

4. Hardness values (278 and 306 HB) is nearly same as conventional pearlitic structure used in crackable con-rods.

4. Discussion

Upper bainitic microstructure could be an important alternative to the pearlitic microstructure because of perfect brittle fracture, no material loss and much more tough without sacrificing the brittleness. Bainite is important because it can be thought as a mixture of martensite (too hard) and pearlite (tough). The sole disadvantage may be economic production. This could be overcome with the growing and developing technology in austempering. Bainite could also absorbe impact forces, this is another advantage. Untransformed ferrite decreases the hardness a little, this lessens the brittleness a little and increases the toughness. No additional tempering is necessary for bainite, this is important for cost.

References

[1.] Gu, Z.; Yang, S.; Ku, S.; Zhao, Y.; Dai, X. 2005. Fracture splitting technology of automobile engine connecting rod, Int J Adv Manuf. Springer-Verlag London Limited 25: 883-887. http://dx.doi.org/10.1007/s00170-003-2022-2.

[2.] Kubota Manabu; Teramoto Shinya. 2010. Hot-Forging Micro-Alloyed Steel And Hot-Rolled Steel Excellent In Fracture-Splitability And Machinability, And Component Made Of Hot-Forged Microalloyed Steel, United States Patent Application, Tokyo (JP), US 2010/0143180.

[3.] Ozdemir, Z.; Ozdemir, T.; Aksoy, Z.; Eruslu, O. 2013. An examination of different heat treatment effects to the fracture parameters of connecting rod made from C70S6 steel, Marmara University Science Institute Journal, Turkey, 25(2), 75-90.

[4.] Ozdemir, Z. 2013. Optimization of the Fracture Parameters In Manufacturing Crackable Connecting Rods, Ph.D. Thesis, Balikesir University, Mechanical Engineering Department.

[5.] Callister, W.; Rethwisch, D. 2008. Fundamentels of Material Science and Engineering, The University of Iowa.

[6.] Qiu, J.W.; Liu, Y.; Liu, Y.B.; Liu, B.; Wang, B.; Earle, R.; Tang, P. 2011. Microstructures and mechanical properties of titanium alloy connecting rod made by powder forging process, Materials and Design Elsevier, 213-219. http://dx.doi.org/10.1016/j.matdes.2011.07.034.

[7.] Ozdemir, Z.; Aksoy, Z.; Ozdemir, T. 2012. A metallographic examination of fracture splitting C70S6 steel used in connecting rods, Marmara University Science Institute Journal, Turkey, 24(2), 45-58.

[8.] Ozdemir, Z.; Aksoy, Z.; Ozdemir, T. 2012. An examination of fracture splitting parameters of crackable connecting rods, Sakarya University Science Institute Journal, Turkey, 16(2), 113-122.

[9.] Liming, Z.; Shuqing, K.; Shenhua, Y.; Lili, Li.; Fei, Li. 2009. A study of process parameters during pulsed Nd:YAG laser notching of C70S6 fracture splitting connecting rods, Optics and Laser Technology 42: 985-993. http://dx.doi.org/10.1016/j.optlastec.2010.01.019.

[10.] Deen, Z.; Harris, S.J.; McCartney, D.G.; Pashby, I.R.; Towell, J.; Shipway, P.H.; et al. 2008. The effect of laser transformation notching on the controlled fracture of a high carbon (C70S6) steel, Material Science and Engineering 489, 273-284. http://dx.doi.org/10.1016/j.msea.2007.12.040.

[11.] Roman, C.; Boris, A.; Dimitrij, K. 2007. A metallographic examination of a fractured connecting rod, Institute of Metals and Technology 42, 93-95.

[12.] Kou, S.Q.; Wang, J.W.; Gao, Y. 2010. Microstructure and fracture splitting properties of a fracture splitting notch produced in a connecting rod (C70S6) using pulsed laser grooving, Lasers In Engineering 20, 381-395.

[13.] Iwazaki, S.; Isobe, T.; Kubato, T. 2007. Split connecting rod and engine, United States Patent, 159, 1-31.

[14.] Guirgos, S. 2004. Process to fracture connecting rods and the like with resonance fatigue, United States Patent Application Publication, 225, 1-13.

[15.] George, H.T. 2006. Steel Heat Treatment Metallurgy and Technologies, Second Edition, Portland State University, Portland, 148-150.

Received October 11, 2013

Accepted May 30, 2014

Zafer Ozdemir

Balikesir University, Department of Mechanical Engineering at Balikesir, 10100, Balikesir, Turkey, E-mail: krebnatlazafer@gmail.com

cross ref http://dx.doi.org/10.5755/j01.mech.20.3.5384

Table 1

Chemical composition of C70S6 steel, %

C       Si      Mn      P      Al      Cu     Cr

0.692   0.182   0.507   0.02   0.005   0.15   0.13

Ni      Mo      W       S      Sn      V      Fe

0.06    0.015   0.001   0.06   0.005   0.05   Bal.