Ductility improvement of a low-carbon steel by warm rolling and annealing
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LASER HARDENED 5 STEEL
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Fig. 3--Transverse section of specimen shown in Fig. 2 with Knoop indentations to indicate hardness.
Figure 3 shows a transverse section of the same specimen. The small diamond shaped marking are Knoop hardness indentations. A comparison of the size of the indentations shows that material in the heat affected zone is considerably harder than in the bulk. The microstructure of the heat affect zone shows dissolution of cementite in the pearlite regions; near the surface, dissolution of pro-eutectoid ferrite. A martensitic microstructure is observed within about 10 ~m of the surface at high magnification. To investigate the effect of laser heat treatment on fatigue life, rotating beam fatigue specimens were tested in both the treated and untreated condition. Five specimens were tested at each level of stress as described in Table I. Hardness values for the heat affected zone and base material for several specimens obtained with a load of 200 g are also given in Table I. The hardness values are based on an average of five measurements. Specimens 37 and 38 were not subjected to fatigue testing. The results of the fatigue tests are shown in Fig. 4. The 10, 50 and 90 pct failure lines were determined by a Weibull analysis of the data? Only in the case of one specimen (31) did the non-uniformity and shallowness of the heat affected zone near the shoulder cause premature failure in this region. In the case of the untreated specimens at 52.8 ksi, three of the five specimens tested exhibited fatigue lives exceeding 106 cycles. In the case of the treated specimens, at 68.7 ksi, three of the five specimens tested exhibited fatigue lives exceeding 106 cycles. These results suggest that 52.8 and 140--VOLUME 12A, JANUARY 1981
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105 106 N, CYCLES TO FAILURE---~DFig. 4--Probability S-N curve.
68.7 ksi approximate the fatigue limits of the untreated and treated specimens, respectively. Our results demonstrate that a laser of moderate power (475 W) can be employed to harden 1045 steel to a uniform depth of 100/~m at a surface rate of 12.4 cm 2 min-L Such hardening increases the fatigue life at 74 ksi by a factor of 15 and may improve the fatigue limit by as much as 30 pct. This research was sponsored by the Rockwell International Science Center from the Independent Research Program. The authors wish to thank James Savage for his assistance in the experimental part of this work. I. G. H. Harth, W. C. Leslie, V. G. Gregson, Jr., and B. A. Sanders: J. Met., 1976, vol. 28, p. 4. 2. D. S. Gnanamuthu, C. B. Shaw, Jr., W. E. Lawrence and M. R. Mitchell: Laser-Solid Interactions and Laser Processing--1978, S. D. Ferris, H. J. Leamy, and J. M. Poate, eds., p. 171, American Institute of Physics, 1979. 3. Young-Won Kim, P. R. Strutt and H. Nowotny: Met. Trans
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