COMPOSITION MODIFICATION ALLOWS

Transcription

1 COMPOSITION MODIFICATION ALLOWS INDUCTION HARDENING OF 450/12 GRADE DUCTILE IRON Induction hardening of SG iron grade 450/12 is not possible due to its high ferrite content (as high as 90%). However, modification of the chemical composition, reduction in cooling time, and other foundry practices can achieve higher pearlite content with an acceptable compromise in mechanical properties, which changes the intermediate nonhardening grade into a hardening grade making induction hardening possible. Udayan Pathak Engineering Research Center Tata Motors Ltd. Pune, India Adifferential component in any vehicle undergoes heavy stresses during operation. In tractors, the load is multifold while negotiating U-turns during certain applications such as puddling, or wet cultivation (a common operation in the Pacific Rim countries) with a half-cage and fullcage wheel. The excessive stresses are the primary cause of the differential case bore that holds sun gears to wear out. This requires the bore to have a higher wear resistance, which is generally achieved by increased hardness. It is necessary simultaneously to have sufficient ductility in the differential case to withstand the bending loads on the differential assembly. The differential case design discussed in this article included two differential gears. The housing was a onepiece cast component made of ductile (SG) iron compared with a conventional two-piece forged component. The ductility and strength requirement of housing is best met using 450/12 grade ductile (SG) iron (ASTM A536-84: 2004, ). However, the major Induction hardening inside bore of differential case. Courtesy M/s Induction Equipment (India) Pvt. Ltd., Pune India. challenge was to achieve wear resistance at the machined 50-mm diameter bores which accommodate the sun gears. It was determined that the best option available was induction hardening of the bore. The design parameters specified were a surface hardness of 45 HRC covering the 9.5 mm bore length. The initial pattern length was decided as 12 to 13 mm (full bore length). The response of ductile iron (SG iron) to induction hardening is dependent on the amount of pearlite in matrix of as-cast, normalized, and normalized and tempered prior structure [1]. In the as-cast condition, a minimum of 50% pearlite is considered necessary for satisfactory hardening using an induction heating cycle of 3.5 s or longer and a hardening temperature between 955 and 980 C. A microstructure containing less pearlite can be hardened by using a higher temperature, but at the risk of having retained austenite and formation of ledeburite, which damages the surface. The development of maximum hardness 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 induction hardening does not normally permit adequate solution of carbon in initially ferritic matrix structures; therefore, 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 tensile strength corresponds to 3% elongation [2]. This compromise in elongation was not acceptable considering the bending loads to which the differential design was subjected. The elongation requirements were best met with the 450/12 grade. However, because 450/12 is typically a nonhardening grade due to high ferrite content up to 90% [1,2], it was decided to modify the 450/12 grade to achieve about 40% pearlite and still maintain an 11 to 12% elongation. HEAT TREATING PROGRESS OCTOBER

2 (a) (b) Fig. 1 Relationship between strength and amount of pearlite. (a) Tensile strength versus amount of pearlite in irons having varying properties of graphite in a nodular form. (b) 0.2% offset yield strength versus amount of pearlite in irons having varying properties of graphite in a nodular form. Expected strength is 650 MPa with 50% pearlite. Source: ASM Handbook: Casting, Vol. 15, p 656, Fig. 2 Tensile strength versus elongation; 650 MPa corresponds to 3% elongation. Source: ASM Handbook: Casting, Vol. 15, p 654, Materials and Methods Various trials were conducted to modify the grade suitably including examination of melt composition, inoculant addition, and casting cooling time on microstructure and physical properties to establish the desired modification. The chemical compositions of the original and modified 450/12 grades are given in Table 1. Figures 3 and 4 represent the microstructures of the original and modified grades. Table 2 summarizes trial conditions. Induction hardening of the part was planned as a final post-machining operation, with target metallurgical specifications as defined above. Considering the criticality of dimensional tolerance on performance, the other major task was to control dimensional distortion due to excessive heating. Various induction hardening trials were conducted using 150 kw MF (medium frequency) and 50 and 25 kw RF (radio frequency) machines. The casting design was typical, having a very limited space for the inductor to approach the area to be hardened. A typical arrangement is shown in Fig. 5. Trial 1 An MF (9 khz, 150 kw) machine was used for the differential case hardening using a 45-mm OD, singleturn 12 8-mm plain copper coil section inductor design. A heating time of up to 20 s using up to 70 kw power was used. Heating was not considered adequate based on visual evaluation. Surface temperature was estimated to be 500 C. Trial 2 Flux concentrators made of cold-rolled grain oriented (CRGO) high silicon steel stampings were used to improve coil efficiency for better heating using the same coil and parameters of trial 1. While this improved heating and surface temperature, the chamfer on the bottom side of the bore was overheated and started melting before reaching sufficient surface temperature at other areas. After these two trials, it was concluded that induction hardening using an MF machine was not suitable to meet requirements. Trial 3 This trial used a 50 kw RF machine plus a 45-mm OD coil made of 5-mm square tubing with two turns and a 3 mm gap between the two turns. Operating parameters consisted of 7 to 7.5 kv and 9 to 9.5 A (37 to 42 Table 1 Properties and chemical composition of original and modified 450/12 grade ductile iron (a) Chemical composition, wt% Original Modified C Same Si (b) Cu (b) Mn Same P Same S Same Mg Same Sn Same Cr Same Ni Same Mo Same Inoculation addition level, wt% (b) Casting cooling time in mold, min (b) Mechanical properties UTS, MPa YS, MPa Elongation % Hardness, BHN (10 mm ball, 3000 kgf load) 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

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 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 Fig. 3 Graphite nodules in ferritic matrix with 10 to 15% pearlite. Etchant: 2% nital. 100 Fig. 4 Graphite nodules in ferritic matrix with 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, 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, kw, 3.5 s Partial overheating 12 mm 8 mm copper tube. and melting, soft Precise flatness and workmanship. patches, no case depth after machining, 70 µm distortion 5 Radio frequency (50 kw) Round three turn 45 mm OD, 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, 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, kw, 22 s Consistent surface bottom loop OD 43 mm for flange side; hardness 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, 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, 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 combinations. arrangement as per Figs. 9 & 10 on hardness. Hence, simple pipes used for quenching (Fig. 12). HEAT TREATING PROGRESS OCTOBER

4 Fig. 5 Schematic arrangement of inductor. Fig. 6 Construction of oval inductor used in trial 7. Fig. 7 Schematic arrangement used in trail 7. 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 increased to 10 mm. Visual examination of a part cross section estimated a total case depth of about 1.5 mm with a 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 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 hardening 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 operating 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 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 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 coupling between the coil and the workpiece was poor resulting in poor surface temperature. Trial 8 A major coil design modification 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 hardening pattern length was 10 mm, and total case depth was 2 mm. Distortion was still 70 to 80 µm. 40 HEAT TREATING PROGRESS OCTOBER 2008

5 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 intermediate 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 requirements were machined after induction 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 process parameters of trial 8 with improved quenching achieved the final process specification requirements. There was an issue of a soft patch beyond the specified 9.5 mm hardening pattern, which was causing heavy rejection 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 observed that quench pressure does not play any role in the hardness pattern or soft patches, only flooding of the bores with continuous water flow is important. So to improve productivity, quench boxes were eliminated, and two ordinary quench pipes were successfully used (Fig12). Conclusion The mean time between failures (MTBF) of differential cases can be drastically improved with little or no compromise of ductility requirements using induction hardened ductile iron grade 450/12 by induction 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 ductility. Trials indicated that a 40% pearlitic matrix makes it possible to Fig. 8 Construction of coil for trial 8. Fig. 9 Quench-box arrangement. Fig. 10 Double quench box. HEAT TREATING PROGRESS OCTOBER

6 (a) (b) induction harden the component, with a surface hardness around 40 HRC, but the hardened microstructure may not be fully martensitic. It is difficult to precisely control the distortion of the part within a few tens of microns during induction hardening of such modified, intermediate grade, and, hence, hardening cannot be considered as a last operation after finish machining. Instead, induction hardening should be considered as an intermediate operation. Process specifications should be developed based on the amount of material removal and end requirements. It is necessary that the design does not impose the requirement of a fully martensitic matrix. 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 overheating 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 induction hardening machine in a job shop. Precise dimensional and positional accuracies (in few tens of microns) cannot be maintained during induction 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 (c) Fig. 11 Effect of coil construction on the induction hardening pattern. Acknowledgement: The authors acknowledge the support of Mahindra Hindoday (formerly DGP Hinoday) automotive castings group (Urse Dist, Pune, India; for developing a modified casting process to obtain the necessary results; and Induction Equipment (I) Pvt. Ltd. (Pune, India; 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, Materials Park, Ohio, USA, p , I.C.H. Hughes, Ductile Iron, ASM Handbook: Casting, Vol. 15, ASM International, Materials Park, Ohio, USA, p , Fig. 12 Quenching arrangement using simple quench pipes. For more information: Udayan Pathak is Assistant General Manager, Materials Technology Group, Engineering Research Center, Tata Motors Ltd., Pune India; udayan.pathak@tatamotors.com; or Udayan. Pathak1@gmail.com. Web site: 42 HEAT TREATING PROGRESS OCTOBER 2008