| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 94 === The purpose of this experiment is to determine the optimal heat-treatment condition, for three spring steels containing different level of vanadium, which are 9254(Fe-1.41Si-0.62Cr-0.54C(wt%)),9254LV(Fe-1.46Si-0.61Cr-0.08V-0.53C)and 9254V (Fe-1.39Si-0.60Cr-0.16V-0.53C).
After various heat-treatments and mechanical tests, the
microstructures were analyzed by optical microscopy(OM),
transmission electron microscopy(TEM)and scanning electron
microscopy(SEM).The correlation between mechanical
properties and its microstructures was established.
Test results show that the optimal heat-treatment condition of these spring steels is austenitizing at 850℃for thirty minutes,oil-quench and then tempering at 400℃ for one hour. Its hard-ness can between HRC 50~52,the optimal hardness for use in spring steels. At this treating condition, the master tempering parameter M=T(12+logt) for 9254,9254LV and 9254V are8200~8600,8300~8500 and 8100~8300 respectively.
When the specimen was tempered at 350℃, tempered
martensite embrittlement (TME) occured due to the decom-
position of interlath retained austenite into cementite.
Vanadium is a powerful grain-refiner as well as an effective softening-resister as the specimens were quenched from 850℃ and tempered at 500~650℃.
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