Journal
JOURNAL OF IRON AND STEEL RESEARCH INTERNATIONAL
Volume 28, Issue 4, Pages 488-495Publisher
SPRINGER
DOI: 10.1007/s42243-020-00521-x
Keywords
Case hardening steel; High-temperature carburizing; Grain size; Heat treatment distortion; Microstructure
Categories
Funding
- Ministry of Science and Technology (MOST) of China [2016YFB0300102]
- CITIC Metals
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By adding a small amount of niobium element in carburizing steel, a uniform and fine prior austenite grain size can be obtained, resulting in a deep hardened layer and hindered austenite grain growth. This leads to lower heat treatment distortion and improved performance of the steel.
In order to research the effect of microalloying Nb, the microstructure and heat treatment distortion of case carburizing steels with and without Nb addition are compared. Results show that a uniform and fine prior austenite grain size was obtained for the steel with the addition of 0.03 wt.% Nb even after being carburized at 980 degrees C for 37 h, resulting in a very deep hardened layer of about 4 mm. Nb is an effective microalloying element to hinder austenite grain growth of gears during high-temperature carburizing. Theoretical calculation and experimental observation of NbC precipitates indicate that fine NbC precipitates have not evidently dissolved at 980 degrees C, and thus they can act as grain refiners due to pinning effect. Heat treatment distortion of Nb-added steel is much lower than that of the steel without Nb addition. It may be contributed to its fine and uniform grain size, which presumably influences stress during martensitic transformation.
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