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Surface Finish Effects on Fatigue Behavior of Steel Forgings Ali Fatemi, Professor Sean McKelvey, Graduate Research Assistant The University of Toledo May 2010
Outline Motivation and Objectives Methodology Metallography Specimen Configuration Experimental Program Preliminary Test Results (Work in Progress) Preliminary Conclusions
Motivation Stress or load Forged component Ground and polished specimen Strength reduction factor due to surface finish, k s Fatigue life (log) k s = ( ( σ σ e e ) ) Forged Polished
Motivation Curve derived from data published in 1946 Significant reduction in fatigue strength for the as-forged surface Improvements in forging technology makes this data too conservative Surface Finish Factor, k s 0.8 0.6 0.4 0.2 0.0 Brinell Hardness, (HB) 120 160 200 240 280 320 360 400 440 480 520 1.0 ks = u 272( σ ) 600 800 1000 1200 1400 1600 Tensile Strength, (MPa) 0.995 Allowable Working Stresses, G.C. Noll and C. Lipson, 1946
Motivation Allowable Working Stresses, G.C. Noll and C. Lipson, Proc. Society for Experimental Analysis, vol. 3, No. 2, 1946. Source of data Fundamentals of Mechanical Component Design, K. S. Edwards and R. B. McKee, McGraw-Hill, 1991. This curve appears on p. 224. Machine Design, R. L. Norton, 2nd Edition, 2000. This curve appears on p. 350. Fundamentals of Machine Component Design, R. C. Juvinall and K. M. Marshek, 2nd Edition, Wiley, 1991. This curve appears on p. 268. Mechanical Analysis and Design, A. H. Burr and J. B. Cheatham, 2nd Edition, Prentice Hall, 1995. This curve appears on p. 287. Mechanical Behavior of Materials: Engineering Methods for Deformation, Fracture, and Fatigue, N. E. Dowling, 2nd Edition, Prentice Hall, 1998. This curve appears on p. 443. Fundamentals of Metal Fatigue Analysis, J. A. Bannantine, J. J. Comer, and J. L. Handrock, 1990. This curve appears on p. 13. Metal Fatigue in Engineering, R. I. Stephens, A. Fatemi, R. R. Stephens, and H.O. Fuchs, 2nd Edition, Wiley Interscience, 2000. This curve appears on p. 81.
Objectives Evaluate and quantify forged surface finish effects on fatigue resistance of common forging steel for a broad range of conditions, including: Heating methods (induction heating vs. gas furnace heating) Hardness levels (90 HRB to 45 HRC) Residual stresses (with or without shot cleaning) Fatigue life regimes (short and long lives) Loading conditions (cantilever bending and rotating bending) Effect of machined surface (from forging vs. from bar stock) Trim or flash effect (with or without trim line)
Material Used Element % Weight C 0.380 Mn 0.940 P 0.010 S 0.021 Si 0.240 Ni 0.080 Cr 0.160 Mo 0.030 Cu 0.200 Sn 0.012 Al 0.030 V 0.007 B 0.0017 Ti 0.042 Nb 0.002 N 0.0084 Fe Balance Material: 10B40 Steel Grain Size = ASTM 6.5 As-rolled Hardness = 215 BHN Material and chemical analysis courtesy of Gerdau-MacSteel
Microstructure As-forged, gas furnace heating As-forged, induction heating As-forged and heat treated to 45 HRC, gas furnace heating As-forged and heat treated to 45 HRC, induction heating Metallurgical Evaluation Courtesy of Peter Bauerle of Chrysler
Decarburization As-forged, gas furnace heating As-forged, induction heating As-forged and heat treated to 45 HRC, gas furnace heating As-forged and heat treated to 45 HRC, induction heating Metallurgical Evaluation Courtesy of Peter Bauerle of Chrysler
Decarburization 50 45 Hardness vs Depth from Surface 40 Hardness (HRC) 35 30 25 20 Legend 15 10 0.000 0.005 0.010 0.015 0.020 0.025 0.030 Depth From Surface (in) Gas Furnace Heating 45 HRC Induction Heating 45 HRC Gas Furnace Heating 35 HRC Gas Furnace Heating 25 HRC
Grain Flow Through the Trim Line At 90 º to the Trim Line Metallurgical Evaluation Courtesy of Peter Bauerle of Chrysler
Specimen Configuration 178 Trim Line 64 51 Ø8 R83 Ø16 Gas Furnace Heating Induction Heating
Roughness Measurements R a = 9 µm for 45 HRC forged specimens R a = 10 µm for 25 HRC forged specimens R a = 0.14 µm for 45 HRC polished specimens R a = 0.12 µm for 25 HRC polished specimens
Cantilever Bending Fatigue Tests Load-controlled constant amplitude cantilever bending fatigue tests (R = -1) Fixture designed to eliminate axial load Maximum Stress
Load Fixture Verification Load, (lbf) 60 40 20 0-20 -40 Gage 1 y = 0.0341x - 1.022 R 2 = 0.9996 y = -0.0328x - 1.3429 R 2 = 0.9998 Gage 2 y = 0.0326x - 1.4499 R 2 = 0.9997 y = -0.0336x - 1.9009 R 2 = 0.9998 Gage 1 (top) Gage 1 (bottom) Gage 2 (top) Gage 2 (bottom) -60-2000 -1500-1000 -500 0 500 1000 1500 2000 Strain, (µε)
Rotating Bending Fatigue Tests Four-point rotating bending Compare as-forged surface to polished surface Most fatigue data are obtained from rotating bending tests Maximum Stress
Stress Calculations 1400 1200 Stress Amplitude vs Position Amplitude Stress Amplitude, (MPa) 1000 800 600 400 200 Legend IF45 GF45 IF35 IF25 σ = Mc I 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Position Amplitude, (mm) Stress calculations based on elastic stress equation
Cracking vs. Displacement Amplitude 2.0 Forged Legend Position Amplitude, (mm) 1.0 0.0 4 2 Machined and Polished 1E5 2E5 Legend IF45 GF45 IF35 IF25 GF25 IM45 GM45 IM35 IM25 0 5E4 Applied Cycles 1E5
Preliminary Experimental Results Gas furnace heating vs. induction heating Hardness effect Machined from forged vs. from bar effect Flash (trim) effect Cantilever bending vs. rotating bending Comparison to old data predictions
Induction vs. Gas Furnace Heating 2000 Gas Forged vs Induction Forged 45 HRC Stress Amplitude, σ a (MPa) 1000 900 800 700 600 500 400 300 200 Legend GF45 IF45 100 10 3 10 4 10 5 10 6 10 7 Cycles to Failure, N f
Induction vs. Gas Furnace Heating 2000 Gas Forged vs Induction Forged 25 HRC Stress Amplitude, σ a (MPa) 1000 900 800 700 600 500 400 300 200 Legend GF25 IF25 (4) (4) 100 10 3 10 4 10 5 Cycles to Failure, N f 10 6 10 7
2000 Composite Plot of Induction Forged Hardness Effects Stress Amplitude, σ a (MPa) 1000 900 800 700 600 500 400 300 200 Legend IF45 IF35 IF25 100 10 3 10 4 10 5 10 6 10 7 Cycles to Failure, N f
Hardness Effects 2000 Composite Plot of Induction Machined Stress Amplitude, σ a (MPa) 1000 900 800 700 600 500 400 300 Legend IM45 IM35 IM25 200 10 3 10 4 10 5 10 6 10 7 Cycles to Failure, N f
Machined-Forged vs. Machined-Bar 1000 900 Stress Amplitude, σ a (MPa) 800 700 600 500 Legend IM25 R25 400 10 3 10 4 10 5 10 6 10 7 Cycles to Failure, N f
Flash Effect 2000 Flash Effect Gas Forged 45 HRC Stress Amplitude, σ a (MPa) 1000 900 800 700 600 500 400 300 200 Legend GF45 GF45 (Flash) 100 10 3 10 4 10 5 10 6 10 7 Cycles to Failure, N f
Flash Effect 2000 Flash Effect Gas Forged 25 HRC Stress Amplitude, σ a (MPa) 1000 900 800 700 600 500 400 300 200 Legend GF25 (4) GF25 (Flash) 100 10 3 10 4 10 5 10 6 10 7 Cycles to Failure, N f
Rotating Bending Fatigue 2000 Rotating Bending vs. Reverse Bending Gas Forged 45 HRC Cantilever Bending Stress Amplitude, σ a (MPa) 1000 900 800 700 600 500 400 300 200 Legend GF45 Reverse Bending GF45 Rotating Bending Rotating Bending 100 10 3 10 4 10 5 10 6 10 7 Cycles to Failure, N f
Rotating Bending Fatigue 2000 Rotating vs Reverse Bending Gas Forged 25 HRC Cantilever Bending Stress Amplitude, σ a (MPa) 1000 900 800 700 600 500 400 300 200 Legend GF25 Reverse Bending GF25 Rotating Bending Rotating Bending 100 10 3 10 4 10 5 10 6 10 7 Cycles to Failure, N f
2000 Gas Forged vs Gas Machined 45 HRC Comparison to Old Data Stress Amplitude, σ a (MPa) 1000 900 800 700 600 500 400 300 200 GF45 GM45 Legend Prediction based on old data 100 10 3 10 4 10 5 Cycles to Failure, N f 10 6 10 7
Comparison to Old Data 2000 Induction Forged vs Induction Machined 35 HRC Stress Amplitude, σ a (MPa) 1000 900 800 700 600 500 400 300 200 Legend IF35 IM35 Prediction based on old data 100 10 3 10 4 10 5 10 6 10 7 Cycles to Failure, N f
Comparison to Old Data 2000 Induction Forged vs Induction Machined 25 HRC Stress Amplitude, σ a (MPa) 1000 900 800 700 600 500 400 300 200 IF25 IM25 Legend Prediction based on old data 100 10 3 10 4 10 5 10 6 10 7 Cycles to Failure, N f
Typical Fracture Surfaces Gas Furnace Heating Induction Heating Rotating Bending Flash Testing 25 HRC 35 HRC 45 HRC
Preliminary Conclusions Although gas furnace heating for forging results in deeper decarburization layer and surface discontinuities than induction heating, differences in fatigue behaviors appear to be small (Tests at HRC 25 and HRC 45). The fatigue limit increases as the hardness increases for the machined and polished specimens, as expected. However, the 25 HRC and 35 HRC as-forged specimens experienced a higher fatigue limit than the as-forged 45 HRC specimens.
Preliminary Conclusions The effect of flash (or trim line) on fatigue behavior of forgings appears to be negligible, based on tested 25 HRC and 45 HRC specimens. A lower fatigue limit is obtained under rotating bending loading, compared to non-rotating cantilever bending loading. This is mainly due to the size effect and a larger stressed volume in rotating bending. Experimental data generated so far indicates much less reduction in fatigue strength due to forged surface finish, compared to what the surface finish factors based on the data from the literature predict.
Work to be Completed Experimental Test 90 HRB specimens Test for shot-cleaning effects Perform tensile tests Complete testing of other conditions Analytical Modeling of surface finish effect with hardness for different fatigue life regimes Modeling the effect of residual stresses Publications
1000 Stress Amplitude, S a (MPa) 100 Legend 1045 QT 4340 QT 8630 Cast Cast Mg(AZ91E-T6) Ti-6Al-4V 2024-T3 7075-T6 A356 Cast 10 3 10 4 10 5 10 6 10 7 10 8 Cycles to Failure
300 Legend 1045 QT 4340 QT 8630 Cast Cast Mg(AZ91E-T6) Ti-6Al-4V 2024-T3 7075-T6 A356 Cast S a * = S a /ρ 100 10 3 10 4 10 5 10 6 10 7 10 8 Cycles to Failure
Acknowledgments Funding FIERF (Board of Trustees, Karen Lewis) AISI (Bar Applications Group, David Anderson) Materials Gerdau-MacSteel (Bob Cryderman & Paul Dimitry) Forging (Induction Heating) Keystone Forging Company (Joe Cipriani) Heat treatment and metallography Chrysler LLC (Peter Bauerle)
Great Designs in Steel is Sponsored by: AK Steel Corporation, ArcelorMittal Dofasco, ArcelorMittal USA, Nucor Corporation, Severstal North America and United States Steel Corporation