The Analysis of Lenticular Martensite Submicron Structure in AISI 440C Alloying Steel
Date Issued
2015
Date
2015
Author(s)
Chang, Ya-Ling
Abstract
This work mainly focuses on the investigation of the microstructure and transformation of lenticular martensite in AISI 440C stainless steel. The research can be divided into two parts. The goal of the first is to analyze the transformation and morphology of lenticular martensite; the goal of the second is to observe the evolution of carbide formation in lenticular martensite during tempering treatment. In the first part, by controlling the homogenization temperature, an appropriate lenticular martensite structure was obtained, facilitating the following observation. The influences of the alloying element content on Ms temperature and the transformed martensite morphology were analyzed with electrical microscopy. Due to the high alloying element content of AISI 440C stainless steel, even after homogenization treatment, the M7C3 carbides are still distributed in the austenite matrix and therefore become the nucleation sites of lenticular martensite. During the martensite transformation process, the initial nucleation product is plate martensite, which then grows into a lenticular martensite structure. Lenticular martensite contains three regions: a midrib (which can be seen as the former plate martensite structure), a twinned region, and an untwinned region. These three regions differ not only in their morphologies but also in their misorientation distribution. Transmission election microscopy (TEM) and electron backscatter diffraction (EBSD) techniques were applied to explore the misorientation in these regions and the crystal orientation relationship between austenite and martensite. In our observations, a single lenticular martensite grain shows an elliptical morphology. Furthermore, a specimen polished with the focus ion beam (FIB) method provides 3-D images information and confirms that lenticular martensite is shaped like a thick shell. The crystallography of lenticular martensite formed in coarse austenite grains was investigated using electron backscattered diffraction (EBSD). Although the spread in diffracted intensity within pole figures was significant, due to the orientation gradient within lenticular martensite, the trend of pole figures indicated that the lenticular martensite approximately adopted a Kurdjumov-Sachs (K-S) orientation relationship with respect to the austenite matrix. The orientation relationships of variant pairings (zigzag, spear, and kink types) have been analyzed. The aim of the second part of this research is to discuss the effects of tempering treatment on lenticular martensite. After tempering at 400ºC, there exist two different kinds of cementite: the ""needle"" type, which does not transform into M7C3 even after a long tempering treatment, and the ""rice"" type, which first nucleates at twins in the midrib region and dislocations in the untwinned region, continues to grow during tempering treatment, and finally becomes a parallelogram-shaped M7C3 carbide. The discrepancies between the two types of cementites in shape and crystallographic relationship can be attributed to the nucleation and growing mechanisms. With electrical microscopy analysis, the displacive mechanism and diffusion mechanism are applied to elucidate the formation of cementites in lenticular martensite.
Subjects
lenticular martensite
variant pair
tempering
cementite
nucleation mechanisms
Type
thesis
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