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1 Great Designs in Steel is Sponsored by: AK Steel Corporation, ArcelorMittal Dofasco, ArcelorMittal USA, Nucor Corporation, Severstal North America and United States Steel Corporation

2 Surface Finish Effects on Fatigue Behavior of Steel Forgings Ali Fatemi, Professor Sean McKelvey, Graduate Research Assistant The University of Toledo May 2010

3 Outline Motivation and Objectives Methodology Metallography Specimen Configuration Experimental Program Preliminary Test Results (Work in Progress) Preliminary Conclusions

4 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

5 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 Brinell Hardness, (HB) ks = u 272( σ ) Tensile Strength, (MPa) Allowable Working Stresses, G.C. Noll and C. Lipson, 1946

6 Motivation Allowable Working Stresses, G.C. Noll and C. Lipson, Proc. Society for Experimental Analysis, vol. 3, No. 2, Source of data Fundamentals of Mechanical Component Design, K. S. Edwards and R. B. McKee, McGraw-Hill, This curve appears on p Machine Design, R. L. Norton, 2nd Edition, This curve appears on p Fundamentals of Machine Component Design, R. C. Juvinall and K. M. Marshek, 2nd Edition, Wiley, This curve appears on p Mechanical Analysis and Design, A. H. Burr and J. B. Cheatham, 2nd Edition, Prentice Hall, This curve appears on p Mechanical Behavior of Materials: Engineering Methods for Deformation, Fracture, and Fatigue, N. E. Dowling, 2nd Edition, Prentice Hall, This curve appears on p Fundamentals of Metal Fatigue Analysis, J. A. Bannantine, J. J. Comer, and J. L. Handrock, 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, This curve appears on p. 81.

7 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)

8 Material Used Element % Weight C Mn P S Si Ni Cr Mo Cu Sn Al V B Ti Nb N Fe Balance Material: 10B40 Steel Grain Size = ASTM 6.5 As-rolled Hardness = 215 BHN Material and chemical analysis courtesy of Gerdau-MacSteel

9 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

10 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

11 Decarburization Hardness vs Depth from Surface 40 Hardness (HRC) Legend Depth From Surface (in) Gas Furnace Heating 45 HRC Induction Heating 45 HRC Gas Furnace Heating 35 HRC Gas Furnace Heating 25 HRC

12 Grain Flow Through the Trim Line At 90 º to the Trim Line Metallurgical Evaluation Courtesy of Peter Bauerle of Chrysler

13 Specimen Configuration 178 Trim Line Ø8 R83 Ø16 Gas Furnace Heating Induction Heating

14 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

15 Cantilever Bending Fatigue Tests Load-controlled constant amplitude cantilever bending fatigue tests (R = -1) Fixture designed to eliminate axial load Maximum Stress

16 Load Fixture Verification Load, (lbf) Gage 1 y = x R 2 = y = x R 2 = Gage 2 y = x R 2 = y = x R 2 = Gage 1 (top) Gage 1 (bottom) Gage 2 (top) Gage 2 (bottom) Strain, (µε)

17 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

18 Stress Calculations Stress Amplitude vs Position Amplitude Stress Amplitude, (MPa) Legend IF45 GF45 IF35 IF25 σ = Mc I Position Amplitude, (mm) Stress calculations based on elastic stress equation

19 Cracking vs. Displacement Amplitude 2.0 Forged Legend Position Amplitude, (mm) Machined and Polished 1E5 2E5 Legend IF45 GF45 IF35 IF25 GF25 IM45 GM45 IM35 IM25 0 5E4 Applied Cycles 1E5

20 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

21 Induction vs. Gas Furnace Heating 2000 Gas Forged vs Induction Forged 45 HRC Stress Amplitude, σ a (MPa) Legend GF45 IF Cycles to Failure, N f

22 Induction vs. Gas Furnace Heating 2000 Gas Forged vs Induction Forged 25 HRC Stress Amplitude, σ a (MPa) Legend GF25 IF25 (4) (4) Cycles to Failure, N f

23 2000 Composite Plot of Induction Forged Hardness Effects Stress Amplitude, σ a (MPa) Legend IF45 IF35 IF Cycles to Failure, N f

24 Hardness Effects 2000 Composite Plot of Induction Machined Stress Amplitude, σ a (MPa) Legend IM45 IM35 IM Cycles to Failure, N f

25 Machined-Forged vs. Machined-Bar Stress Amplitude, σ a (MPa) Legend IM25 R Cycles to Failure, N f

26 Flash Effect 2000 Flash Effect Gas Forged 45 HRC Stress Amplitude, σ a (MPa) Legend GF45 GF45 (Flash) Cycles to Failure, N f

27 Flash Effect 2000 Flash Effect Gas Forged 25 HRC Stress Amplitude, σ a (MPa) Legend GF25 (4) GF25 (Flash) Cycles to Failure, N f

28 Rotating Bending Fatigue 2000 Rotating Bending vs. Reverse Bending Gas Forged 45 HRC Cantilever Bending Stress Amplitude, σ a (MPa) Legend GF45 Reverse Bending GF45 Rotating Bending Rotating Bending Cycles to Failure, N f

29 Rotating Bending Fatigue 2000 Rotating vs Reverse Bending Gas Forged 25 HRC Cantilever Bending Stress Amplitude, σ a (MPa) Legend GF25 Reverse Bending GF25 Rotating Bending Rotating Bending Cycles to Failure, N f

30 2000 Gas Forged vs Gas Machined 45 HRC Comparison to Old Data Stress Amplitude, σ a (MPa) GF45 GM45 Legend Prediction based on old data Cycles to Failure, N f

31 Comparison to Old Data 2000 Induction Forged vs Induction Machined 35 HRC Stress Amplitude, σ a (MPa) Legend IF35 IM35 Prediction based on old data Cycles to Failure, N f

32 Comparison to Old Data 2000 Induction Forged vs Induction Machined 25 HRC Stress Amplitude, σ a (MPa) IF25 IM25 Legend Prediction based on old data Cycles to Failure, N f

33 Typical Fracture Surfaces Gas Furnace Heating Induction Heating Rotating Bending Flash Testing 25 HRC 35 HRC 45 HRC

34 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.

35 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.

36 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

37 1000 Stress Amplitude, S a (MPa) 100 Legend 1045 QT 4340 QT 8630 Cast Cast Mg(AZ91E-T6) Ti-6Al-4V 2024-T T6 A356 Cast Cycles to Failure

38 300 Legend 1045 QT 4340 QT 8630 Cast Cast Mg(AZ91E-T6) Ti-6Al-4V 2024-T T6 A356 Cast S a * = S a /ρ Cycles to Failure

39 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)

40 Great Designs in Steel is Sponsored by: AK Steel Corporation, ArcelorMittal Dofasco, ArcelorMittal USA, Nucor Corporation, Severstal North America and United States Steel Corporation