| Summary: | 博士 === 國立臺灣大學 === 材料科學(工程)研究所 === 82 === Laser surface hardening and tempering using a Carbon dioxide
laser were performed on several Cr-Mo and stainless steels. Two
mathematical models were proposed to predict the influence of
processing variables and the initial microstructure of
specimens on the case depth (Zc) of laser-hardened zones. The
fatigue crack growth rate (da/dN) of laser-treated steels was
also evaluated. In the laser hardening process, a parameter M
which is simply a function of hardness was introduced to index
the ease of forming a hardened layer in tempered Cr-Mo steels.
By calculating the superheating above Ac3 (△T) or the
characteristic carbon diffusion distance (dc), and taking the
Zc of as-quenched specimens as calibrated standards, M vs Zc
curves could be con- structured for each steel. These curves
are useful for predicting the case depth of tempered steels in
the laser hardening process. In fatigue crack growth tests, the
results indicate that laser treating of 304 stainless steel
could retard the da/dN of these specimens. This was attributed
to the existence of residual stresses which enhanced the
closure effect in front of a crack tip. For laser-treated 4130
steel, the da/dN was affected not only by residual stresses but
also by the change in microstruc- tures within laser-impinged
areas. As far as the microstructure is concerned, the presence
of brittle martensite in the structure increased the da/dN of
laser-hardened specimens. On the other hand, laser-tempered
specimens which were originally in the as- quenched condition
could retard the da/dN. The complex nature of residual
stresses, in addition to the synergistic effects of residual
stresses and microstructures on the da/dN of laser- treated
4130 specimens, had been discussed thoroughly in this
investigation.
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