2006 Volume 92 Issue 7 Pages 448-456
In high temperature heat treatments like carburizing treatment, 4A or 5A elements that make nitride or carbide are doped to suppress abnormal grain growth. Nb, which is one of 4A elements, is known as one of the most effective elements to suppress abnormal grain growth during carburizing and added in commercial steels. Therefore the effects of Nb addition on abnormal grain growth in carburizing has been studied by many researchers, but there are few reports about the influence of Nb(CN) precipitation conditions before carburizing.
In this study, Nb(CN) precipitation conditions were changed into solid solution, fine precipitations and coarse ones by pre-heat treatment, in order to investigate the effects of Nb(CN) precipitation conditions on abnormal grain growth behavior during a heat treatment at high temperature quasi-carburizing. The results are as follows:
(1) Abnormal grain growth was suppressed in a sample containing solid solute Nb before heat treatment quasi-carburizing more than ones in which all Nb was precipitated as carbide.
(2) If the size of Nb(CN) precipitates was too fine or too coarse, abnormal grain growth was stimulated. There is a suitable size of Nb(CN) precipitation to suppress abnormal grain growth. It is considered that factors to cause abnormal grain growth were not only amount and size of precipitation and austenite grain size, which were parameters in the theory of Hillert and Gladman on abnormal grain growth, but also Ostwald riping rate of precipitations.