fatigue and tensile tests were cut parallel to the rolling direction into the forms indicated in Fig. 2.

Transcription

1 Stress Cold-Rolled Fatigue Carbon By Masahiko Strength Steel Ogirima* To obta effect cold rollg on fatigue strength, commercial 0.8% carbon eutectoid steel coldrolled to various degrees n fatigued bendg at constant deflection. On or h, cold-rolled strips are always accompanied, fatigue strength is much fluenced on surface cold-rolled sheets. on surface sheet cold-rolled at each rollg measured X-ray method (s2ψ method) which corrected Fourie Analysis. trsic change fatigue strength cold rollg obtaed usg relation between fatigue strength mean bias. As a result, fatigue strength creases monotonously with, creasg fatigue strength with earlier stage cold rollg, if uncorrected, is very large because large compressive. As an example, on surface a cold-rolled sheet changes neir cold rollg at least more than 10% nor one-pass (Received October Usually, mechanical properties such as tensile yield strengths are creased cold rollg. But effect cold rollg on fatigue strength is not exactly known, this problem is reduced to that fatigue strength prestraed material. problem is very complex because cold rollg is always accompanied with, value which varies very widely accordg to crystal structure rollg condition. It is well-known that tensile decreases fatigue strength compressive creases it. To clarify relation between fatigue strength cold rollg, commercial eutectoid steel (SK5) coldrolled to various thicknesses n fatigue tests were carried out. On or h, on ir surfaces measured X-ray method, fatigue strength corrected known relation between fatigue strength mean applied bias. Specimen per pass. Annealg carried out at 750 for 1 hr vacuo, with attention to decarburization. Test pieces for fatigue tensile tests were cut parallel to rollg direction to forms dicated Fig. 2. Sce ir fatigue characteristics is very sensitive to surface defects such as flaws rusts, test pieces were hled with much caution preserved oil. A rectangular measurement No.10 sheets strips. For * Central Research Tokyo, Japan. Trans. JIM (30 50mm) for purpose cut Laboratory, Hitachi 1.9% 4.7%. III. 1. Measurement X-ray from comparg No.1 When dl is specific surface, cled ψ, ( ) where v from vice equal to Ltd., Kokubunji, versa. group pot dψ=d, along dψ, is Young's straight value les le, ) dψ is cross σ modulus. method when v/1+v=s2ψ to formula(1); results a straight if σ 0 (compressive regard space direction d, s2ψ to lattice followg E Moreover, those parallel among called d without space identical surface relation ratio is to measure strip, among which lattice expressed is Poisson's dψ is formula, at only n one s2ψ=v/1+v. In this experiment, measurement carried out usg a diffractonieter. condition is shown 2. Usually, pattern heavily-deformed crystal is very broad. refore, order to determe a correct angle, pattern approximated to a parabola correct angle gaed means least squares method. pattern from heavily-deformed crystal is Procedure re are several methods on surface X-ray method adopted. specimen or material, staless steel to 0.3mm thick Experimental plottg dψ agast s2ψ slope which is positive Preparation Commercial eutectoid steel SK5 (stard chemical composition) used. cold-rollg process is dicated Fig shows thickness,, number passes, mean SK5 with that cold-rolled strip 18Cr-8Ni referred. This cold-rolled about 30%. This II. 1, 1968) overlap (1) very with broad each or, so peaks separation A. L. Christenson E. S. Rowl: 45 (1953), Kα, se Kα2 two Trans. ASM, Vol.10

2 Masahiko Fig. 1 Reduction one-pass 1 Ogirima Cold rollg 183 process 2. Fatigue 2 Condition tensile tests fatigue tests were carried out usg a resonancetype testg mache drived electro-magnetic force with about 400 cps. maximum free-free bar is obtaed on surface at center bar. amplitude deflection at center bar measured a microscope. amplitude controlled condenser-type amplitude detector attached to this mache, stability very excellent. tests were carried out air for purpose coolg, air or dry nitrogen gas blown agast center specimens, so that maximum t emperature Fig. 2 Form size test specimens pieces IV. peaks is necessary y were sepad t his case, Kα2 is assumed to t he measurg ratio to get a correct angle. means Fourie analysis(2). In be angle peak 0.5, height Kα1 Kα1 to that difference about 40. In order to avoid stra ageg durg pause fatigue, fatigue tests were carried out contuously till at least 106 cycles. Yield es were obtaed usg Instron-type tensile testg mache. between Kα 2 is obtaed full-annealed material. (2) S. Taira, Y. Yoshioka Y. Sakata: Preprts for he 4th Symposium on X-ray measurements strength materials, (1965), p Experimental Measurements Results obtaed pattern A. Fig. 3 sepad to two elemental patterns B C method mentioned before. cident beam were cosec cled obtaired. θψ is each 20, 25, 45 to state, From calculated, Fig.3. dicated patterns direction like relation Fig.4, Fig.3 s2ψwhere

3 184 θψ is clcd a ψ. angle when calculation pattern Kα Stress Fatigue 1 (curve Gated No.2 No.10 for specimens 3. are always beam carried B pot Fig. 4 is an average value divergence is very small. cident out Each four specimens, Calculated values are es compressive is di- specimens almost same value. If more accu values are required, lattice plane dependence, absorption factor Lorentz polarization factor are necessary to be taken to account, but this experiment se corrections were neglected. Moreover, it must be noted that values dicated 3 is not accu values on surface, but mean values several microns under surface. n correct absolute value on surface is assumed to be a little larger than gaed values. Fig. 3 Separation elementary a pattern patterns (B C) 2. Fatigue Fig. 5 shows S-N curves specimensno. 1, No. 2, No. 5 No. 9 as representative examples. deviation plottg pots is very small. As dicated Fig. 5, cled parts S-N curves are always parallel this case, suggestg that slope cled part S-N curve is dependent not on but on or factors such as chemicalcomposition heat treatment. Fatigue limits obtaed from each S-N curve are plotted agast cold percentage Fig. 6, which shows that fatigue limits crease monotonously with cold percentage. variation fatigue strength at life 105 cycles has same tendency as that fatigue limits. outle fracture surface SK5 much jugged contrast with that 18-8 staless steel. (A) to two Fig. 5 Fig. 4 relation No.10 3 between s cosec cold-rolled strength metallography usg Fig.3). No.1 No.10 Strength Cold-Rolled Carbon Steel S-N curves cold-rolled sheets sample sheets Fig. 6 relation between fatigue limit cold (uncorrected. )

4 Masahiko Photo. 1 Optical Ogirima micrographs An cubation period from itiation macroscopic cracks to fal fracture (so-called3rd period fatigue) long at time fracture, many macroscopiccracks -werevisible on specimen surface. Optical micrographs specimens No. 1 No. 9 are shown Photo. 1(a) (b). Photo. 1(a) is a usual pearlite structure (b) most layers cementite were deformed to pieces. Nonmetallic clusions are very few. 185 SK5( 720 4/5) relations se factors are assumed to be dicated schematically Fig. 9. Pot P is thought to be decided nature material P shifts to right side case, ster* material to left case harder* material. For purpose comparg. SK5 with that ormaterialwhichhas largerdeformability, 18Cr-8Nistalesssteel (30% cold-rolled) measured same methodas mentionedabove. And result is 34.5kg/mm2 V. Discussion 1. Cold rollg As a matter course, a fullannealed material is almost zero. When this material is cold-rolled, arises it. characteristics value is said to be dependent on kd a material method rollg. Fig. 7 Fig. 8 show dependence on mean per one pass, respectively. se figures show that SK5, on specimen 89.7% surface (cold is almost pct) equal % 9.1 (one pass ). Generally, is controlled maly one-pass, Fig. 8 Fig. 9 Fig. 7 relation between cold relation between one-pass schematic relation between cold on surface a cold-rolled sheet * se words mean deformability a material.

5 186 Stress Fatigue Strength (tensile). In case cold rollg 18-8 staless steel, fluence martensitic transformation course cold rollg must be taken to consideration. 2. Fatigue strength Fig. 6 does not dicate trsic relation between fatigue strength cold rollg, because effect due to cold rollg is cluded fatigue strength obtaed usual testg method. effect on fatigue strength has been studied many researchers, but established ory about relation between fatigue strength has never been proposed. n, is assumed to be regarded as mean bias from experimental can be results related with mean Sires(3), bias fatigue strength σb ( this σω case σb is equal to ) as followg formula: (1) where σy is strength when yield σb=0. σ ω0 is numerical fatigue coefficient 0.5 is almost constant without regard to kd material. yield strength each specimen obtaed from -stra curves tensile tests. Accordg to formula (1), correction Fig. 6 carried out result is dicated Fig. 10. Here, variation durg fatigue test must be taken to consideration. se problems have been studied maly Taira et al.(4) (9), Cold-Rolled Carbon Steel mimum value dicated, course, trsic fatigue strength when does not decrease at all. Accordg to Fig. 10, trsic fatigue strength creases monotonously cold rollg, but change fatigue strength small cold s is larger Fig. 6 because effect. Fatigue strength (both fatigue limit fatigue life) creases cold rollg. fatigue limits 90% cold-rolled specimen reaches about 1.5 times full-annealed one. This crease is thought to be due to high dislocation density, formation fibre structures deformation pearlite structures (Photo. 1 (b)). variation fatigue strength dicated Fig. 10 cludes all se factors. In this experiment, se factors could not be sepad but crease dislocation density is considered to be a predomant factor. Fukui et al. (10) dicated that 0.22% carbon steel, fatigue limits decreased once after small torsional deformation (about several pct) applied creased larger deformation. In order to know effect dividual factors, microscopic vestigations usg a simpler material is expected. accordg to ir results, variation is much, dependent on both itial value on cyclic amplitude. In most cases, has a tendency to decrease monotonously with fatigug, but about 70% itial value remas even after fatigug to 107 cycles, it does not happen that whole vanishes after fatigue tests. Of course microscopic distribution heavily damaged region fatigug is very different from that matrix (not so heavily damaged). But relation dicated formula (1) is that between macroscopic state fatigue strength, this experiment, measured X-ray method a considerably large area, so that correction Fig. 6 usg relation formula (1) is considered to be reasonable. maximum value each plottg pot Fig. 10 dicates trsic fatigue strength when decreases to 70% itial value fatigug, (3) G. Ses: Metal Fatigue (1959), p.145. (4) S. Taira Y. Murakami: J. Japan Soc. Test. Mater., 7 (1958), 591. (5) S. Taira R. Kitano: ibid, 7 (1958), 552. (6) S. Taira Y. Murakami: ibid, 8 (1959), 607. (7) S. Taira Y. Murakami: J. Japan Inst. Metals, 23 (1959), 603. (8) S. Taira Y. Murakami: Treatise Complication Japan Soc. Mach., 25 (1959), 545. (9) S. Taira Y. Murakami: J. Japan Soc. Test. Mater., 9 (1960), 475. (10) S. Fukui S. Sato: Rep. Res. Inst. Sci. Tech. Tokyo Univ., 3 (1949), 311. Fig. 10 relation between fatigue limit cold (corrected ) VI. Summary 0.8% carbon steel (SK5) full-annealed coldrolled to various degrees, fatigue tests were carried out. on surfaces measured X-ray method, trsic relation between fatigue strength obtaed usg relation between fatigue strength mean bias. results are as follows: (1) on surfaceis about -30kg/ mm2 degree mean (compressive) one-pass ( almost ( dependent 10 90%) %).

6 Masahiko Ogirima 187 (2) Fatigue strength cold-rolled strips creases monotonously with cold. (3) Takg account, creasg fatigue strength with degree is Acknowledgments author wishes to express his appreciation to Dr. H. Ohara for his kd support to carry out his work. Thanks are also due to General Manager Central Research Laboratory, Hitachi Ltd., for his permission to publish this paper.