Induct hard.qxp 9/24/2008 8:25 PM Page 1 COMPOSITION MODIFICATION ALLOWS INDUCTION HARDENING OF 450/12 GRADE DUCTILE IRON Induction hardening 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 Engineering 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 quenching[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. HEAT TREATING 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 silicon steel 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 irons 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. 38 HEAT TREATING PROGRESS • OCTOBER 2008 Induct hard.qxp 9/29/2008 1:41 PM Page 3 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.