The Influence of Silicon Contents on Interphase Precipitation Carbides in Low-Carbon-Low-Alloyed Steels
Date Issued
2010
Date
2010
Author(s)
Chen, Po-Yu
Abstract
Silicon is one of the most common alloy elements in steels. Its major effect on steel phase transformation is to stabilize ferrite phase and broaden ferrite phase region. According to this feature, Si additions might influence the precipitation during austenite decomposition. The purpose of this research is to fully understand how Si performs in the interphase precipitation mechanism.
The research includes two experiments: isothermal heat treatments and continuous cooling heat treatments. All heat treatments are conducted by dilatometer. The samples are first heated to austenizeand then isothermally heated or slow cooling from the temperature range which nearing TA1. The results are observed by OM and TEM, and analyzed with the condition of ferrite phase and interphase precipitation distribution.
In isothermal heat treatments, with the information collected by dilatometer, the TTT diagram can be drawn. Si additions effectively accelerate ferrite transformation. Under OM obsevation, Si also inhibits bainite transformation. The ferrite phase transformation can proceed in lower temperature. However, interphase precipitation distribution seems not to be apparently influenced by Si additions. The precipitation sheet spacing and particle size mainly concerned transformation temperature. The lower temperature, the denser and smaller precipitation can be observed. Hardness test results reveal that ferrite can be strengthened by solid solution hardening of Si and precipitation hardening. The former effect provides the same hardness range in different temperature range and the latter one cause the lower transformation temperature the harder the ferrite matrix.
In continuous cooling heat treatments, the OM observation also indicates that the Si addition effectively stabilize ferrite transformation. The lower starting temperature would have higher hardness and Si could maintain the ferrite matrix and prevent from forming bainite phase.
Judging from the above, Si additions mainly contribute two efforts: solid solution hardening and stabilization ferrite phase. The heat treatments can be conducted in lower temperature to get denser interphase precipitation distribution in ferrite matrix which can be effectively strengthened by Si solid solution.
Subjects
HSLA steels
Interphase precipitation
Nano-sized carbide
Si
Ti
V
TEM
Isothermal heat treatments
Continuous heat treatments
TTT diagram
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