C:\Users\raoh\Desktop\Paper_2.xps The Journal of Engineering Research Vol. 8 No. 1 (2011) 12-18 1. Introduction Ni-Cr-Mo alloyed CC steels are widely used for trans- mission gears, as it provides combination of soft ductile core, high fatigue strength and surface hardness, thereby wear resistance of contacting surfaces (Wilks, et al. 1994). Overly fluctuating Ni and Mo cost in the recent times, alarmed automotive industries worldwide, as it affect the final component cost. Vehicle cost plays significant role, when automotive manufacturers compete locally and globally. Hence, automotive industries demand cost effec- tive steels without compromise in mechanical properties, machinability, performance and durability. Melloy et al. (1973) found optimum combination of __________________________________________ *Corresponding author’s e-mail: cengg@yahoo.com hardenability and toughness by adding 15 to 25 ppm of boron. Irvine and Pickering (1957) showed that, 20 ppm of boron results in similar tensile strength, achieved by alloying 1.0% Cr and 0.7% Mn. Stumpf and Banks (2006) found 10 to 30 ppm of boron enhances hardenability of steel through segregation at austenite grain boundaries and hence delays the nucleation of ferrite and pearlite. Boron addition did not affect the hot and cold working properties of as rolled steels. It behaves as plain carbon steel but with higher hardenablity, reports Szuch and Delve (1967). Gear tooth bending strength and loading capacity were ensured by impact strength of steel. Lower impact strength results in gear tooth failure. Impact strength of conventional low carbon boron steel (CBS) Boron Steel: An Alternative for Costlier Nickel and Molybdenum Alloyed Steel for Transmission Gears A. Verma*a, K. Gopinatha and S.B. Sarkarb *aMDS, Mechanical Engineering Department, Indian Institute of Technology, Madras, India (T.N.) 600 036 bMahindra Ugine Steel Company Ltd., Jagdish Nagar, Khopoli (M.H.) India 410216 Received 17 May 2009; accepted 4 October 2009 Abstract: Case Carburized (CC) low carbon steels containing Ni, Cr and Mo alloying elements are widely used for transmission gears in automobile, as it possesses desired mechanical properties. In order to cut cost and save scarce materials like Ni and Mo for strategic applications, steel alloyed with Boron has been developed, which gives properties comparable to Ni-Cr-Mo alloyed steel. In the process of steel development, care was taken to ensure precipitation of boron which results in precipitation hardening. The characterization of the devel- oped boron steel had exhibited properties comparable to Ni-Cr-Mo alloyed steel and superior to conventional boron steel. Keywords: Gear steel, Boron steel, Carburization distortion, Impact strength, Cost 13 The Journal of Engineering Research Vol. 8 No. 1 (2011) 12-18 (20MnCr5B steel) was inferior to Ni-Cr-Mo alloyed steel (EN353 of BS970 standard). In CBS, titanium (Ti) was added to control the nitrogen because of its higher chemi- cal affinity towards nitrogen than boron. Kapadia et al. (1968) reported that, Ti forms TiN and boron goes free in the solid solution, resulting in higher hardenability of steel. However, in absence of Ti, boron forms precipitates of BN. Hence, reduces effective boron, which was respon- sible for increasing hardenability of steel. Boron harden- ability potential was inversely proportional to the carbon content of the steel (Rahrer and Armstrong, 1947; Irvine and Pickering, 1963). Rahrer and Armstrong concluded that, Al deoxidized steel forms AlN with nitrogen, which was thermodynamically more stable than BN. But AlN forms more slowly than BN, in austenite. Considering the above facts, development of low car- bon boron steel (DBS) and its merits over CBS and Ni-Cr- Mo alloyed steel for potential gear applications were reported in this paper. The scope restricts to comparison of impact strength, tensile strength and case carburization distortion. 2. Boron Steel Development The process followed to develop low carbon boron steel was EAF - LF - VD - CCP (EAF-Electric arc furnace, LF- Ladle furnace, VD-Vacuum degassing and CCP- Continuous casting process). The same process was fol- lowed for development of CBS and Ni-Cr-Mo alloyed steel. The required charge was carefully prepared to con- trol tramp elements such as Cu, Sn and P etc. EAF with EBT (Eccentric bottom tilting) facility was used for steel melting, as it had superior slag control. The slag was tapped in a preheated ladle where molten steel was deox- idized with deoxidizers and fresh lime was added for mak- ing fresh slag. The melt was later transferred to the LF sta- tion for addition of ferroalloys in order to achieve the desired chemistry and temperature. A reducing slag was created as it aids in melt refining. The ladle was subse- quently transferred to the VD station where melt degassing was carried out below 10-3 bar. VD with con- tinuous argon purging through porous plug ensured removal of gases like hydrogen, oxygen and nitrogen. Boron in the melt was added after VD. Prior to boron addition, the melt was adequately deoxidized with Al and other deoxidizers. The melt inclusions were controlled by proper deoxi- dation, slag physio-chemical characteristics, vacuum degassing, soft rinsing, argon shrouding and tundish met- allurgy. The molten steel was cast in a 250 X 200 mm cast iron mould by continuous casting process (CCP) setup. The CCP setup had mould EMS (Electro magnetic stirrer), auto mould level control (AMLC) device, well designed tundish and sub entry nozzle (SEN) system. The rectangu- lar billets were rolled to achieve reduction ratio of 1:6 or higher on the steel. The typical chemical composition of the Ni-Cr-Mo alloyed steel, conventional low carbon boron steel (CBS) and developed low carbon boron steel (DBS) were shown in Table 1. The compositions of three steels were measured by Spectromax, CCD spectrometer from Spectro AI Gmbh. 3. Results 3.1 Metallurgical Studies The test specimen for metallurgical studies were pre- pared from the forged and annealed (8800C for an hour) steel bars. Grain sizes of steel were measured by compar- ison chart method as mentioned in the ASTM E112 stan- dard. Mixed grain sizes at 500X were observed for CBS in the Leica make metallurgical microscope. Grain sizes of 5 and finer (9 to 10) were observed for DBS under similar microscope at 500X. The grain size distribution for three steels was shown in Fig. 1. Grains of similar size and uni- form distribution were observed for DBS where as CBS showed grains of mixed size and non-uniform distribu- tion. The etching solution was prepared by mixing 100 ml of distilled water with four grams of picric acid and 0.5% of soap solution. The etching was performed at room tem- perature for over thirty minutes. The cleanliness study of DBS steel was conducted as per ASTM E 45 standard. The microscopic analyses of inclusion ratings were summa- rized in Table 2. Microstructure of boron steel shows lamellar pearlite and ferrite only. Table 1. Chemical compositions of CBS DBS and Ni-Cr-Mo alloyed steel Table 2. Inclusion ratings of the developed boron steel as per ASTM E 45 standard 14 The Journal of Engineering Research Vol. 8 No. 1 (2011) 12-18 3.2 Mechanical Properties Evaluation Impact strength qualification of any steel is desired for consideration as gear steel. Instrumented dynamic impact tests were carried out by Brugger test method (Tikhonov and Palagin, 1994). Brugger test specimens (Fig. 2) were prepared from the forged bar of CBS, DBS and Ni-Cr-Mo alloyed steel. The specimens were treated under identical carburizing, hardening, tempering parameters and cycle durations as used for existing Ni-Cr-Mo alloyed steel gears. Specimens from all three steels were carburized in the same batch, so any differences in mechanical proper- ties due to heat treatment parameters and cycles were eliminated. Impact tests were carried out on Zwick / Roell GmbH make, instrumented dynamic impact testing machine (model RKP 450) at room temperature. Comparison of impact load results of CBS, DBS and Ni- Cr-Mo alloyed steels were shown in Fig. 3. Impact load Figure 1. Microscopic image of grain size distrubution in boron steel at 500 X a) Conventional boron steel (CBS), b) Developed boron steel (DBS), c) Ni-Cr-Mo alloyed steel 15 The Journal of Engineering Research Vol. 8 No. 1 (2011) 12-18 for DBS and Ni-Cr-Mo alloyed steel were comparable (52 kN) where as CBS showed lower impact load (40kN). Mechanical properties of steels (soft condition) were measured on Zwick / Roell GmbH make tensile testing machine (model Z250) and tentative steel costs were com- pared and tabulated in Table 3. Load Vs deflection curve obtained during tensile testing for all the steels were shown in Fig. 4. DBS showed highest tensile strength of 789 MPa where as CBS showed maximum elongation of 11.73 %. Tensile strength and elongation of Ni-Cr-Mo alloyed steel were in between the two boron steels. 3.3 Comparison of Carburizing Distortion Gears from DBS and CBS were forged and machined to similar design and geometrical parameters (weight 5.6 kg, outer diameter (OD) 180 mm and inner diameter (ID) conforms to 76G5 mm of ISO 286-1: 1988) as used for existing Ni-Cr-Mo alloyed steel gears. In order to avoid geometrical / profile variation due to carburizing and tem- pering cycles, gears from three steels, were loaded to same batch in a furnace. Twenty five gears from each of the three steels were checked for gear dimensions (ID and ID width) before and after carburization to evaluate distortion / growth pattern. Mitutoyo, Japan make gauges were used for measurement of gear dimensions. The carburizing cycle of duration eight hours, comprises of heating gears to temperature of 930 oC for carburization and hardened at 830 oC, followed by oil quenching at 110 oC for twenty minutes. The gears were then tempered at 150 oC for more than three hours. All the gears were metallurgically char- acterized and results were reported in Table 4. The grinding / finishing stocks on gear ID post carbur- izing were plotted in Fig. 5. ID distortion signifies the deviation of circularity measured at three locations over gear ID width and average result reported. ID distortion trend of DBS gears, were comparable (average 0.225 mm) to that of existing Ni-Cr-Mo alloyed steel. The average distortion of CBS gears were 0.400 mm, higher compared to DBS and Ni-Cr-Mo alloyed steel by about 0.175 mm. Gears ID width (measured along the gear axis) distortion trend for all three steels were plotted in Fig. 6. DBS gears width growth trend were comparable to Ni-Cr-Mo alloyed steel with average growth of 0.05mm. CBS gears width grows by average of 0.09 mm, higher by 0.04 mm than DBS and existing Ni-Cr-Mo alloyed steel. Gears ID grind- ing stock trend of all three steels were more consistent than gears width distortion trend. All other gear dimen- sions (like lead, profile, crowning and span length) of DBS were comparable to Ni-Cr-Mo alloyed steel and with in the specifications (K templates) when tested on Klingelnberg, GmbH make lead and profile testing machine. 4. Discussions Higher impact strength of Ni-Cr-Mo alloyed steel was attributed to the alloying of Ni and Mo. These elements results in the formation of low carbon martensite and thereby imparts sufficient strength and toughness to the steel (Bepari and Shorowordi, 2004). CBS had lower impact strength compared to DBS. In CBS, titanium fixes nitrogen by forming TiN precipitates and free boron in solution segregated to austenite grain boundaries, reduces the cohesive force and thereby impact toughness (Kapadia, et al. 1968; Azarkevich, et al. 1995). However, in DBS titanium presence was in traces, boron reacts with nitrogen and carbon to form precipitates of boron nitride and boron carbide respectively. These precipitates removed from solution at the grain boundaries and there- by improve the impact properties. This is in line with the findings of Irvine and Pickering (1963). Also, Treppschuh et al. (1967) showed that, toughness of case hardened and cold worked steels can be improved by boron addition, provided boron should combine with nitrogen. Tensile strength of DBS steel was 789 MPa, much higher com- pared to CBS steel (383 MPa) and better than existing Ni- Cr-Mo alloyed steel (679 MPa). Boron nitride and boron carbide precipitates at the grain boundaries resulted in precipitation hardening of DBS steel. These precipitates acts as barrier for movement of dislocations and thereby Figure 2. Brugger impact test specimen Figure 3. Impact test results of Ni-Cr-Mo alloyed steel and boron steels by Brugger method 16 The Journal of Engineering Research Vol. 8 No. 1 (2011) 12-18 imparts higher strength to the DBS. However, in CBS, boron segregation at the grain boundaries resulted in sup- pression of ferrite and pearlite formation (Brownrigg, 1973). This results in lower strength of CBS steel. Ferrite strengthening effects of Ni and Mo results in higher strength of the Ni-Cr-Mo alloyed steel (Davis, 2000). Thermal stresses introduced during case carburization were responsible for dimensional changes or component distortion (Thelning, 1974). These dimensional changes needs to be incorporated in machining tolerance and removed by grinding and finishing. The amount of retained austenite exhibits significant effects on dimen- Table 3. Mechanical properties and cost comparison of CBS, DBS and Ni-Cr-Mo alloyed steel Figure 4. Load deflection curve for Ni-Cr-Mo alloyed steel and boron steels Number of Gears Figure 5. Grinding stock of gears made of Ni-Cr-Mo alloyed and boron steels after CC 17 The Journal of Engineering Research Vol. 8 No. 1 (2011) 12-18 sional stability. After quenching, steels contain some retained austenite along with martensite which increases with amount of alloying elements dissolved during austenitization. During carburizing, the amount of carbon in the case increases with applied carbon potential, but its increase leads to higher austenite retention (Bensely, 2008). Higher the alloying elements results in higher retained austenite and thereby lower distortion (Stumpf and Banks, 2006; Mohanty, 1995). In addition, presence of Ni suppresses Ms temperature which further enhances retained austenite content. Hence, Ni-Cr-Mo alloyed steel had lower ID and width distortion. Higher retained austen- ite reduces strength and residual stresses but subsequent tempering reduces retained austenite by transforming to martensite (Parrish, 1999). In CBS, gear ID and width dis- tortion values were higher compared to Ni-Cr-Mo alloyed steel. This was obviously due to lower retained austenite and high carbon martensite in CBS (boron has no effect on Ms temperature). Lower distortion in DBS may be due to controlled and uniform grain size distribution, in contrast to the mixed grain size distribution observed in CBS. 5. Conclusions 1. DBS was comparable to the existing Ni-Cr-Mo alloyed gear steel (EN353 of BS 970 standard) and superior to CBS (20MnCr5B). DBS specimens failed at an average impact load of 53.1 kN, comparable to 54 kN of Ni-Cr-Mo alloyed steel and higher than 38.68 kN of CBS. 2. Tensile strength of DBS was highest (789 MPa) where as elongation of CBS was highest (11.73%). Tensile strength (679 MPa) and elongation (9.18%) of Ni-Cr-Mo alloyed steels was in between the two Table 4. Metallurgical characterization of gears after CC and tempering Number of Gears Figure 6. Width distortion trend of gears made of Ni-Cr-Mo alloyed and boron steels after CC 18 The Journal of Engineering Research Vol. 8 No. 1 (2011) 12-18 boron steels. 3. Distortion of gears ID and width due to carburization of DBS and Ni-Cr-Mo alloyed steel gears were com- parable to average of 0.225 mm and 0.05 mm respec- tively. However, distortion of ID and width of CBS gears were higher to 0.400 mm and 0.09 mm respec- tively. 4. Lower distortions of Ni-Cr-Mo alloyed steel were associated with higher retained austenite compared to CBS. However, lower distortion of DBS gears could be due to controlled and uniform grain size distribu- tion. 5. Lower distortion in the DBS against CBS will save finished component cost in terms of tool repair or replacement period, thereby increasing the productiv- ity. 6. With this huge potential of enhanced mechanical prop- erties, lower raw material cost, reduced machining requirements and higher cost savings, DBS can be considered as alternative steel for transmission gears against costly Ni-Cr-Mo alloyed steels and inferior CBS. Acknowledgment The authors are grateful to Mr. K. Parthasarathy, Retd. Dy. General Manager, Product Development, Materials Engineering Department, Ashok Leyland Limited Chennai, India, for their support and technical help. The authors express sincere gratitude to the management of MUSCO steel plant, India for their guidance and co-ordi- nation. References Azarkevich, A.A., Kovalenko, L.V. and Krasnopolskii, V.M., 1995, "The Optimum Content of Boron in Steel," J. of Metal Science and Heat Treatment, Vol. 37(1-2), pp. 22-24. 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