Improved Centerless Grinding Productivity through Reduced Heat Treatment Distortion

Transcription

1 Improved Centerless Grinding Productivity through Reduced Heat Treatment Distortion Dr. Craig Seidelson Chief Engineer Manufacturing Advancement- Timken China & Sr. Visiting Research Fellow Univ. of the West of England Vijayananda Reddy Group Leader- Timken India Huang Lixin Supervisor Process Design Timken China Dong Shumin Supervisor Process Design Timken China Abstract Prior researchers focused on machine set up and operation to improve centerless grinding productivity. A research gap was how changes in upstream operations (i.e. heat treatment) affected centerless grinding productivity. Spherical roller bearings consisted of steel inner and outer rings. The rolling surfaces (termed races) were through hardened. After hardening the races distorted. The researchers sought to answer for thin walled spherical roller bearing outer rings Did stress relieving at optimum quench oil temperature reduce distortion? If so, Did grinding productivity gains justify added heat treatment costs? For stress relieved outer rings which were hardened using low temperature quench oil, out of roundness (OOR) distribution on the races fell 34%. It was possible to increase grinding in-feed rate 120% over the roughing portion of the cycle. The increase in feed rate equated to an 11% increase in overall grind productivity. The researchers concluded it was possible to increase spherical roller bearing grinding productivity by focusing on the heat treatment operation. Findings were applicable in bearing manufacture because the added heat treatment costs were offset multiple times by the grinding productivity gains. Keywords- Centreless grinding; heat treatment distortion; spherical roller bearings I. INTRODUCTION Spherical roller bearings are assembled from steel inner and outer rings. Rings distorted when hardened. Distortion took 3 forms: size changes, taper changes and out of roundness (i.e. OOR). Prior researchers found OOR was one of the main limiting factors in centreless grinding productivity [1]. OOR reduced productivity for 2 reasons: 1. Grind stock was added to remove form error 2. Feed rate was reduced to prevent burn Prior researchers identified causes in ring distortion. Before hardening, rings were lathe turned. Clamping and cutting forces introduced residual stresses [2]. During hardening residual stresses were released and rings deformed. Prior researchers found pre-heating a workpiece (a) removed residual stresses without plastic deformation and (b) minimised plastic deformations during hardening [3]. In addition to preheating, prior researchers found changes in quench oil temperature affected distortion. As quench oil temperature changed viscosity changed; viscosity changes affected how evenly the quenched part cooled. Uneven cooling introduced plastic deformations during hardening [4]. This research sought to build upon prior ring distortion findings by answering 2 questions: 94

2 1. For thin walled spherical roller bearing outer rings, Did stress relieving at optimum quench oil temperature reduce distortion? 2. If distortion was reduced, Did grinding productivity gains justify added heat treatment costs? III. RESULTS Per Fig. 2, all 120 rings OD sizes were larger after hardening than before. The median OD expansion was 0.156mm. Seventy-five percent of parts expanded more than 0.138mm. II. METHODOLOGY The test pieces were spherical roller bearing outer rings (i.e. part number 55ESDC22). A total of 120 rings were lathe turned. Rings were subsequently serial numbered, measured, and divided into 4 groups of 30 pieces each. Each group represented 1 cell of a 2 2 full factorial test space. The test factors were stress relief and quench oil temperature (Table 1). TABLE I. FULL FACTORIAL TEST SPACE 60 C Quench Oil 70 C Quench Oil No Stress Relief 1 3 Stress Relief 2 4 Coding for stress relief was (a) it had been done for 3 hours at 180 C or (b) it had not been done at all. Coding for quench oil temperature was low at 60 C and high at 70 C. After heat treatment, each serial numbered ring s change in size, OOR, and taper were measured on outside diameter (OD) and inside race diameter (ID) per Fig. 1. Figure 2. OD Growth after Hardening The 4 test combinations showed homogeneity of variance in OD size change (Fig. 3). Figure 1. Spherical Roller Bearing Outer Race After quenching it was possible that not all of the austenite converted to martensite [5]. To ensure retained stresses did not confound grind productivity tests, the researchers performed a single vs. double temper test. For the quench oil and stress relief factorial combination found to exhibit minimum distortion, the researchers prepared a batch of 100 rings. Half of the rings were single tempered and half were double tempered. Differences in size, OOR, and taper were evaluated for the 2 groups. Figure 3. Differences in OD Growth Variance across the Heat Treatment Groups Was Rejected 95

3 OD size change across the 4 test combinations was normally distributed (Table II). TABLE II. NORMALITY IN OD SIZE GROWTH AFTER HARDENING OD Size Shapiro W p value No Stress Relief- High Oil Temp. 93% 0.06 No Stress Relief- Low Oil Temp. 99% 0.96 Stress Relief- High Oil Temp. 94% 0.08 Stress Relief- Low Oil Temp. 97% 0.6 With normality and homogeneity of variance satisfied, the researchers performed Student s t tests. Rings quenched in low temperature oil and stressed relieved prior to hardening exhibited lower mean OD size growth (Fig. 4). Figure 5. OD Size Growth for One Test Group Was Smaller Across the 4 heat treatment combinations, differences in mean OOR, taper, and size were rejected. However, two form changes of interest were uncovered. One, all 120 rings races contracted. Median contraction was 0.104mm. Seventy-five percent of rings contracted less than 0.08mm (Fig. 6). Figure 4. OD Size Growth after Hardening was Not the Same across Treatments The 0.13mm mean growth in OD size after hardening was 15% less than all other groups (Fig. 5). Figure 6. Hardened Race Contractions Two, the 4 test combinations showed homogeneity of variance in race OOR (Fig. 7). 96

4 By the conservation of volume, as OD expanded ring ID, likewise, expanded and width decreased [8]. A practical application for diametrical expansion of heated rings was the mounting of bearings to a shaft [9]. In this research, 100% of ring OD values increased. But, 100% of the ring ID values decreased. To put mean OD expansion and ID contraction values in perspective, changes as a percentage of nominal ring dimensions were 0.2% and 0.1% respectively. To explain why volume was not conserved, the researchers considered changes in atomic structure during hardening. Figure 7. Variance in Race OOR was Not the same across Heat Treatment Groups When mounting a bearing to a shaft, rings were heated to less than 120 C [10]. However, hardening steel rings required heating above the 840 C austenizing temperature [11]. At this temperature the iron-carbon crystal lattice (Fig. 8) changed from body centered cubic (BCC) to face centred cubic (FCC) [12]. During quenching the rapid temperature drop trapped carbon atoms in the iron lattice. A brittle, body centered tetragonal (BCT) martensitic structure resulted. When quenching in low temperature oil, stress relieved rings showed 35% less standard deviation in race OOR. Through feed centerless grinding OD was multiple times faster than plunge grinding race. Therefore, to understand how reduced heat treatment distortion affected grind productivity, the researchers focused on race grinding. The researchers found when grinding stress relieved rings quenched in low temperature oil this group ground 11% faster than the others. The researchers tested whether retained stresses in the hardened rings were affecting grind productivity results. A 50-piece sample of stress relieved rings was hardened using low temperature quench oil and a single temper. Another 50- piece sample of stress relieved rings was hardened using low temperature quench oil and a double tempered. No statistical differences in size, taper, or OOR existed between the single and double temper groups. The researchers rejected any retained stresses in the rings after heat treatment affected the grind productivity results. IV. DISCUSSIONS When heated, a metal ring OD and ID almost always expanded [6]. For example, given the coefficient of thermal expansion for steel 1 OD Original expanded to OD Final [7]. OD Final ( OD Original 11.9x10 )( C 6 )( T C) 1 Over a 0 to 100 C temperature range for ANSI (1) Figure 8. Steel Lattice Structure Given the atomic radius (r), FCC volume (2) was larger than the BCC (3) [13]. FCC volume = 22.62r 3 (2) BCC volume =12.31r 3 (3) BCT volume was also larger than the original FCC volume [14]. The extent of lattice expansion depended on (a) percent carbon in the martensite and (b) amount of martensite in the ring. Therefore, transformation of the iron-carbon lattice during hardening explained the OD expansion and ID contraction observed. To explain why stress relieved parts quenched at low oil temperature experienced (a) reduced OD size expansion but not (b) reduced race contraction, the researchers focused on residual stresses. Prior researchers noted directionality and magnitude of residual stresses depended on cutting contact 97

5 length and force [15]. When turning 1mm off ID and OD, the tool s 3mm contact length on OD was 300% longer than on ID. The 0.3mm/rev feed rate when cutting on OD was 20% faster than on ID. OD residual stresses were significant enough stress relieving reduced OD expansion. To explain why stress relieved parts quenched at low oil temperature experienced reduced standard deviation in race OOR, the researchers again focused on residual stresses. Prior researchers noted clamping forces established residual circumferential stresses [16]. In this research, turned races were clamped on OD. Stress relieving reduced the variation in circumferential residual stresses released when quenching at low oil temperature. What remained was to explain why the interaction effect on distortion between stress relieving and quench oil temperature. Increased quench oil temperature reduced viscosity; reduced viscosity, in turn, increased wetting [4]. With increased wetting (i.e. more uniform collapse of the vapour blanket during quench) parts cooled more evenly [17]. More even cooling reduced distortion and stress [3]. However, the researchers found less distortion for stress relieved rings quenched at low oil temperature. The rings 4mm wall thickness explained the break with conventional wisdom. Prior researchers found thin walled parts showed less distortion when quenching at higher viscosity (i.e. lower oil temperature) [4]. The researchers were able to grind stress relieved rings quenched at low oil temperature at 11% higher productivity. The gains in productivity traced back to OOR and the roughing portions of the grind cycle. For this heat treatment group OOR standard deviation was reduced 35%. When roughing these parts at 120% higher feed rate grinding burn was not a problem. The higher variations in race OOR among the other 3 heat treatment groups caused grinding burn problems at the higher feed rate. V. CONCLUSIONS In the manufacture of spherical roller bearings, outer rings were lathe turned, through hardened, and then centerless ground. During hardening rings distorted. Distortion introduced taper, size, and roundness errors. Distortion reduced centerless grinding productivity for 2 reasons: 1. Grind stock was added to remove form errors 2. Feed rate was reduced to prevent grinding burn The researchers found for thin walled spherical roller bearing outer rings: 1. Stress relieving at optimum quench oil temperature reduced distortion. 2. Grinding productivity gains justified added heat treatment costs? When stress relieving rings at 180 C for 3 hours and quenching in 60 C oil, mean OD expansion reduced 15% and standard deviation in race OOR reduced 35%. Since race grinding was the bottleneck, the researchers focused on this operation. For the stress relieved low quench oil temperature rings grind productivity increased 11%. Productivity gains traced back to increased feed rates during the roughing portions of the grind cycle. At reduced OOR standard deviation, when these rings with ground at 120% higher feed rate grinding burn was not a problem. The researchers concluded it was possible to increase centerless grinding productivity by adding a stress relieving step in combination with optimum quench oil temperature. The increase in heat treatment costs was $0.02 per ring. Findings had applicability in bearing manufacture because the added heat treatment costs were offset multiple times by the gains in grinding productivity. VI. FURTHER RESEARCH Across a wide variety of spherical roller bearing outer ring geometries, it may be possible to uncover regressions between part geometry and distortion. By being able to predict ring distortion, designers would be able to lathe turn rings to minimize grind stock. At reduced grind stock, productivity gains would be possible. REFERENCES [1] Zhou, S. A Dynamic Study of the Plunge Centerless Grinding Process and its Effect on Out-of-Roundness. Dissertations Collections for the Univ. of Connecticut. Paper AAI (1994) [2] Brinksmeier, E., Solter, J., Grote, C. Distortion Engineering Identification of Causes for Dimensional and Form Deviations in Bearing Rings. CIRP Annals- Manufacturing Technology. vol. 56, issue 1, (2007) p [3] Toften, G., Howes, M., Inoue, T. Handbook of Residual Stress and Deformation of Steel. ASM International, Materials Park: OH. (2002) [4] Information on Magazine/News-Info-From-Industrial-Heating-Magazine/Secrets-of- Effective-Hot-Oil-Quenching.html [5] Information on hart2010/technicalarticlespring_2010.pdf [6] Information on < [7] Information on =aisi_52100&prop=cte&page_title=alloy%20steel%20aisi% [8] Information on [9] Information on ing_and_inspection [10] Information on 0Lubrication/Bearing%20Mounting%20Basics.pdf 98

6 [11] Stickles, C. & Peters,C (1977) Compressive Strain Induced Austenite Transformation of Steel. Metal and Materials Transactions. Vol 8, #7, (1977), p [12] Information on [13] Information on /2009/12/iron-changing-bcc-fcc.htm [14] Information on 52&id=16862 [15] Ulutan, D., Alaca, B., Lazoglu, I. Analytical Modeling of Residual Stresses in Machining. Journal of Materials Processing Technology, vol 183, (2007), p [16] Kessler, O., Sackmann, T., Nowag, L., Surm, H., Frerich, F., Lubben, T., Zoch, W., Experimenatl Study of Distortion Phenomena in Manufacturing Chains. Materialwissenschaft und Werkstofftechnik. vol. 37, (2005), p [17] Information on Dr. Craig Seidelson Dr. Seidelson holds a PhD in grinding from the University of the West of England. In addition to working as the Timken Company s Chief Engineer for Manufacturing Advancement in China, he is a Senior Visiting Research Fellow in the Faculty of Environment and Technology at the University of the West of England. 99