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COMPOSITION MODIFICATION ALLOWS INDUCTION OF 450/12 GRADE DUCTILE of differential component in any challenge was to achieve wear resist- vehicle undergoes heavy ance at the machined 50-mm diameter SG iron grade 450/12 is not stresses during operation. In bores which accommodate the sun possible due to its high ferrite tractors, the load is multifold gears. It was determined that the best Awhile negotiating U-turns option available was induction hard- content (as high as 90%). during certain applications such as ening of the bore. The design param- However, modification of the “puddling,” or wet cultivation (a eters specified were a surface hardness chemical composition, common operation in the Pacific Rim of 45 HRC covering the 9.5 mm bore countries) with a half-cage and full- length. The initial pattern length was reduction in cooling time, and cage wheel. The excessive stresses are decided as 12 to 13 mm (full bore other foundry practices can the primary cause of the differential length). case bore that holds sun gears to wear The response of ductile iron (SG achieve higher pearlite out. This requires the bore to have a iron) to induction hardening is content with an acceptable higher wear resistance, which is gen- dependent on the amount of pearlite erally achieved by increased hardness. in matrix of as-cast, normalized, and compromise in mechanical It is necessary simultaneously to have normalized and tempered prior struc- properties, which changes the sufficient ductility in the differential ture[1]. In the as-cast condition, a min- intermediate nonhardening case to withstand the bending loads imum of 50% pearlite is considered on the differential assembly. necessary for satisfactory hardening grade into a hardening grade The differential case design dis- using an induction heating cycle of 3.5 making induction hardening cussed in this article included two dif- s or longer and a hardening tempera- ferential gears. The housing was a one- ture between 955 and 980°C. A mi- possible. piece cast component made of ductile crostructure containing less pearlite (SG) iron compared with a conven- can be hardened by using a higher Udayan Pathak tional two-piece forged component. temperature, but at the risk of having Research Center The ductility and strength requirement retained austenite and formation of Tata Motors Ltd. of housing is best met using 450/12 ledeburite, which damages the surface. Pune, India grade ductile (SG) iron (ASTM A536- The development of maximum hard- 84: 2004, 65-45-12). However, the major ness depends on the carbon content of the matrix, which transforms into austenite upon heating and into martensite during [2]. The short heating time in flame and induc- tion hardening does not normally permit adequate solution of carbon in initially ferritic matrix structures; there- fore, it is important to use fully pearlitic grades of iron for flame or induction hardening. Figure 1 indicates 50% pearlite with minimum expected tensile strength of 650 MPa, while Fig. 2 shows that a 650-MPa ultimate ten- sile strength corresponds to 3% elon- gation[2]. This compromise in elonga- tion was not acceptable considering the bending loads to which the differ- ential design was subjected. The elon- gation requirements were best met with the 450/12 grade. However, be- cause 450/12 is typically a nonhard- ening grade due to high ferrite content up to 90%[1,2], it was decided to modify the 450/12 grade to achieve about 40% Induction hardening inside bore of differential case. Courtesy M/s Induction Equipment (India) Pvt. Ltd., pearlite and still maintain an 11 to 12% Pune India. elongation. PROGRESS • OCTOBER 2008 37 Induct hard.qxp 9/24/2008 8:25 PM Page 2

Materials and Methods Trial 1 – An MF (9 kHz, 150 kW) ma- Various trials were conducted to chine was used for the differential case modify the grade suitably including hardening using a 45-mm OD, single- examination of melt composition, in- turn 12 × 8-mm plain copper coil sec- oculant addition, and casting cooling tion inductor design. A heating time of time on microstructure and physical up to 20 s using up to 70 kW power properties to establish the desired was used. Heating was not considered modification. The chemical composi- adequate based on visual evaluation. tions of the original and modified Surface temperature was estimated to 450/12 grades are given in Table 1. Fig- be 500°C. ures 3 and 4 represent the microstruc- Trial 2 – Flux concentrators made of tures of the original and modified cold-rolled grain oriented (CRGO) (a) grades. Table 2 summarizes trial high stampings were used conditions. to improve coil efficiency for better Induction hardening of the part was heating using the same coil and pa- planned as a final post-machining op- rameters of trial 1. While this improved eration, with target metallurgical spec- heating and surface temperature, the ifications as defined above. Consid- chamfer on the bottom side of the bore ering the criticality of dimensional was overheated and started melting tolerance on performance, the other before reaching sufficient surface tem- major task was to control dimensional perature at other areas. distortion due to excessive heating. After these two trials, it was con- Various induction hardening trials cluded that induction hardening using were conducted using 150 kW MF an MF machine was not suitable to (b) (medium frequency) and 50 and 25 meet requirements. Fig. 1 — Relationship between strength and kW RF (radio frequency) machines. Trial 3 – This trial used a 50 kW RF amount of pearlite. (a) Tensile strength versus The casting design was typical, machine plus a 45-mm OD coil made amount of pearlite in having varying having a very limited space for the of 5-mm square tubing with two turns properties of graphite in a nodular form. (b) 0.2% inductor to approach the area to be offset yield strength versus amount of pearlite in and a 3 mm gap between the two irons having varying properties of graphite in a hardened. A typical arrangement is turns. Operating parameters consisted nodular form. Expected strength is 650 MPa with shown in Fig. 5. of 7 to 7.5 kV and 9 to 9.5 A (37 to 42 50% pearlite. Source: ASM Handbook: Casting, Vol. 15, p 656, 2008. Table 1 — Properties and chemical composition of original and modified 450/12 grade ductile iron (a) Original Modified Chemical composition, wt% C 3.50-3.70 Same Si 2.20-2.40 2.45-2.55(b) Cu 0.15-0.25 0.45-0.55(b) Mn 0.21-0.35 Same P 0.010-0.015 Same S 0.008-0.012 Same Mg 0.032-0.050 Same Sn 0.005-0.012 Same Cr 0.020-0.025 Same Ni 0.027-0.030 Same Fig. 2 — Tensile strength versus elongation; 650 Mo 0.004-0.005 Same MPa corresponds to 3% elongation. Source: ASM Handbook: Casting, Vol. 15, p 654, 2008. Inoculation addition level, wt% 0.25 0.20(b) Casting cooling time in mold, min 70-120 60-80(b) Mechanical properties UTS, MPa 460-480 550-600 YS, MPa 335-350 410-440 Elongation % 15.5-17.0 11.4-12.2 Hardness, BHN (10 mm ball, 3000 kgf load) 150-155 187-210 Microstructure Graphite Graphite nodules in nodules in ferritic matrix ferritic-pearltic with 10-15% matrix with pearlite 40-50% pearlite (a) Results summarized from 13 heats poured by Mahindra Hinoday. (b) Modification to original grade.

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kW) and a heating time 3.5 s. Visual examination of a part cross section estimated a total case depth of about 1.5 mm and a hardening pattern length of 8.5 mm, with a 40-45 HRC surface hardness. However, 10 to 15% (by visual estimation) of the bore area was overheated and partly melted. Bore distortion was 60 to 70 µm at the heated portion and more than 10 µm in the soft portion. There were soft patches and no hardened Fig. 3 — Graphite nodules in ferritic matrix with Fig. 4 — Graphite nodules in ferritic matrix with case after machining (material 10 to 15% pearlite. Etchant: 2% nital. 100× 40 to 50% pearlite. Etchant: 2% nital. 100× Table 2 — Summary of trial conditions for induction hardening 450/12 grade ductile iron Heating Trial # Type of machine Inductor type parameters Results 1 Medium frequency (9 kHz) 150 kW Round single turn 45 mm OD, 70 kW, 20 s Poor heating 12 mm ×8 mm copper tube, no flux concentrators 2 Medium frequency (9 kHz) 150 kW Round single turn 45 mm OD, 70 kW, 20 s Nonuniform heating 5 mm × 5 mm copper tube Melting at corners, with flux concentrators. poor heating in other areas 3 Radio frequency (50 kW) Round two turn 45 mm OD, 37-42 kW, 3.5 s Partial overheating 12 mm × 8 mm copper tube and melting, soft patches, no case depth after machining, 70 µm distortion 4 Radio frequency (50 kW) Round two turn 45 mm OD, 37-42 kW, 3.5 s Partial overheating 12 mm × 8 mm copper tube. and melting, soft Precise flatness and patches, no case depth workmanship. after machining, 70 µm distortion 5 Radio frequency (50 kW) Round three turn 45 mm OD, 37-42 kW, 3.5 s Improved hardening 5 mm square copper tube. coverage up to 12 mm Precise flatness and workmanship. width. 6 Radio frequency (50 kW) Round three turn 45 mm OD, 12-17 kW, 25 s Overheating and 5 mm square copper tube. Precise 1 melting at inner flatness and workmanship. chamfer area. 7 Radio frequency (50 kW) Oval two turn major OD 40 mm, No voltage No heating due to minor OD 37 mm, 5 mm square developed due lack of inductive copper tube. Precise flatness to lack of coupling. Trial and workmanship inductive abandoned. (refer Fig. 6 and 7) coupling 8 Radio frequency (50 kW) Two turn coil, Top loop OD 43 mm, 13-18 kW, 22 s Consistent surface bottom loop OD 43 mm for flange side; hardness 50-55 HRC (refer Fig. 8) 20 s for other except small side portion (refer Fig. 11). 9 Radio frequency (50 kW) Two turn coil, Top loop OD 43 mm, 13-18 kW, 22 s Consistent results bottom loop OD 43 mm. for flange side; with minimal soft Loop construction modified 20 s for other patch with coil as shown in Fig. 11 a-c side (Fig. 11c); 70 µm distortion. 10 Radio frequency (50 kW) Two turn coil, Top loop OD 43 mm, 13-18 kW, 22 s No prominent bottom loop OD 43 mm. for flange side; effect of quench Coil construction modified 20 s for other pressure and as shown in Fig. 11 c. side. elaborate Various quench box arrangement as combinations. per Figs. 9 & 10 on hardness. Hence, simple pipes used for quenching (Fig. 12).

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removal of 0.2 mm plus 70 µm distortion). Trial 4 – Using the same inductor as in trial 3, coil turns were made flat and parallel, and various coil positions with respect to the bore top face were tried. The hardening pattern length in- creased to 10 mm. Visual examination of a part cross section estimated a total case depth of about 1.5 mm with a 40- 45 HRC surface hardness. However, 10 to 15% (by visual estimation) of the bore area was overheated and partly melted. Bore distortion was 60 to 70 µm at the heated portion and more than 10 µm in the soft portion. There were soft patches and no hardened case after machining (material removal Fig. 5 — Schematic arrangement of inductor. of 0.2 mm plus 70 µm distortion). Trial 5 – To increase the hardening pattern length, a three-turn coil made of 5-mm square copper tube was used with a gap of 1 mm between turns. The machine parameters were the same as trial 4. The induction hard- ening coverage was 12 mm, but the top and bottom chamfer of the bore was partly melted. Bore distortion was up to 70 µm. Trial 6 – The same coil of trial 5 was used with 25 kW machine with oper- ating parameters of 4.5 to 5.5 kV and 4.5 to 5.0 A (12 to 17 kW). The lower chamfer had overheating and melting. Surface hardness was 50-55 HRC, and distortion still was 70 µm. Trial 7 – To avoid nonuniform heating, the coil design was changed to a minor diameter 37 mm and major Fig. 6 — Construction of oval inductor used in trial 7. diameter 40 mm with entry from top of the bore. This inductor construction facilitated rotation of the differential case. Figures 6 and 7 show the schematic layout. However, the cou- pling between the coil and the work- piece was poor resulting in poor sur- face temperature. Trial 8 – A major coil design modifi- cation was made using two loops of different diameters instead of two loops of similar diameter. The upper loop was 43 mm in diameter and the lower loop 47 mm in diameter with a 2.5 mm gap between the loops (Fig. 8). A 25-kW RF machine was used with process parameters of 5.2 to 6.5 kV, 4.2 to 4.5 A (13 to 18 kW), and heating times of 22 s for the flange side (the side with more back-up material) and 20 s for the opposite side (with less back-up material). Surface hardness was 50 to 55 HRC, the induction hard- ening pattern length was 10 mm, and total case depth was 2 mm. Distortion Fig. 7 — Schematic arrangement used in trail 7. was still 70 to 80 µm.

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It was concluded from these trials that induction hardening could not be the last operation after machining due to stringent requirements of diameter sizes, ovality, and positional accuracies on this product. Hence, induction hardening was planned as an interme- diate operation after roughing of the bores. Material removal was 0.3 mm diametrically on bores, and all critical diameters (both soft and hard) and faces having positional accuracy re- quirements were machined after in- duction hardening. Accordingly, final process specifications for induction hardening were to achieve a surface hardness of 55 to 62 HRC, an effective case depth of 30 HRC at 2 to 2.5 mm, and a 9.5-mm minimum induction hardening pattern. Trial 9 – Using the inductor and Fig. 8 — Construction of coil for trial 8. process parameters of trial 8 with im- proved quenching achieved the final process specification requirements. There was an issue of a soft patch be- yond the specified 9.5 mm hardening pattern, which was causing heavy re- jection during the boring operation. Various quench options were used to avoid this rejection. Figure 9 shows quench box details for internal quench by scanning, and Fig.10 shows a schematic layout of the double quench box design. Refinement was done in the construction of the overlapping area to reduce the occurrence of soft patches. Fig 11 shows the effect of the coil construction on the soft patch. During production runs, several trials were conducted regarding quenching pressure, and it was ob- served that quench pressure does not play any role in the hardness pattern or soft patches, only flooding of the Fig. 9 — Quench-box arrangement. bores with continuous water flow is important. So to improve productivity, quench boxes were eliminated, and two ordinary quench pipes were suc- cessfully used (Fig12).

Conclusion The mean time between failures (MTBF) of differential cases can be drastically improved with little or no compromise of ductility require- ments using induction hardened ductile iron grade 450/12 by induc- tion hardening wear-prone surfaces. The chemical composition of ductile iron grade 450/12, cooling time, and other foundry practices must be modified to get about 40% pearlitic matrix without compromising duc- tility. Trials indicated that a 40% pearlitic matrix makes it possible to Fig. 10 — Double quench box.

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induction harden the component, with a surface hardness around 40 HRC, but the hardened mi- crostructure may not be fully martensitic. It is difficult to precisely control the distortion of the part within a few tens of microns during in- duction hardening of such modified, intermediate grade, and, hence, hardening cannot be consid- ered as a last operation after finish machining. In- stead, induction hardening should be considered as an intermediate operation. Process specifica- tions should be developed based on the amount of material removal and end requirements. It is necessary that the design does not impose the re- quirement of a fully martensitic matrix. (a) During hardening of bores with higher width using lower rating machine, it is seldom necessary to use multiturn coils. The overlap of the coil induces the soft patch or appreciable variation in pattern length or width as shown in Fig. 11. The diameters of the coil should be adjusted in such way that heating during induction hardening should start from the massive section and progress toward the edges or corners. Utmost care should be taken to avoid over- heating and melting in such areas. Quench pressure does not play major role in surface hardness, but sufficient flow is important for proper quenching. Induction hardening of difficult-to-access bores can be successfully done using proper coil design and process parameters with a readily available (b) induction hardening machine in a job shop. Pre- cise dimensional and positional accuracies (in few tens of microns) cannot be maintained during in- duction hardening as a last post-machining operation. In the case study discussed here, the field failure rate due to bore wear out was reduced from 20,000 ppm to zero ppm, and mean time between failure (MTBF) was improved from 200 h to 1,100 h, against an expected 1,000 h. Within two years of consistent production quality using induction hardening, the part was successfully developed with a 4% rejection. HTP

Acknowledgement: The authors acknowledge the sup- (c) port of Mahindra Hindoday (formerly DGP Hinoday) – automotive castings group (Urse Dist, Pune, India; Fig. 11 — Effect of coil construction on the induction hardening pattern. www.hinoday.com) for developing a modified casting process to obtain the necessary results; and Induction Equipment (I) Pvt. Ltd. (Pune, India; www.induc- tionindia.com) for conducting various trials in induction hardening.

References 1. K.B. Rundman, Heat Treating of Ductile Iron, ASM Handbook: Heat Treatment, Vol. 4, ASM International, Ma- terials Park, Ohio, USA, p 682-692, 1997. 2. I.C.H. Hughes, Ductile Iron, ASM Handbook: Casting, Vol. 15, ASM International, Materials Park, Ohio, USA, p 647-666, 1998.

For more information: Udayan Pathak is Assistant Gen- eral Manager, Materials Technology Group, Engineering Research Center, Tata Motors Ltd., Pune 411 018 India; e-mail: [email protected]; or Udayan. Fig. 12 — Quenching arrangement using simple quench pipes. [email protected]. Web site: www.tatamotors.com.

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