Studies on Lenticular Martensite and Its Temper Structures in AISI440C High Carbon Steel
Date Issued
2012
Date
2012
Author(s)
Chu, Chia-Wei
Abstract
In this work , AISI 440C stainless steel has been investigated. It was homogenized at 1145˚C for 3 days, with subsequent quenching to room temperature. The Ms temperature of the homogenized specimen were measured to -38.7˚C via a DSC tester. In order to obtain the lenticular martensites, the specimens were then subzero cooled by liquid nitrogen (-196˚C).The substructure of subzero lenticular martensites and the following tempered structure have been investigated by OM, SEM and TEM. The results indicate that the substructures of lenticular martensites contained three regions: the midrib, extended twinned region and untwined region. The midrib is composed of high density of transformation{112} twins. The extended twinned region emanates laterally from the midrib region and give a lens-like feature. The untwinned region can be seen to have some dislocations. It should be note that the lenticular martensites transformation combine both twinning and slip modes；the midrib and extended twin keep the lattice-invariant deformation mode of twinning, whereas the untwinned region keep another deformation mode, slip.
Additionally, the result of tempering experiments at 650˚C indicate that the different type of carbides precipitate from martensite matrix and had specific orientation relationships with the tempered matrix. M3C type carbide was dominant after tempering at 650˚C for 30 minutes but M23C6 type carbides was frequent after tempering for 60 and 90 minutes. Analysis the stereographic projection from diffraction patterns, it revealed that the Bagaryatsky OR was found between M3C carbides and ferrite, and Kurdjumov-Sachs OR was found between M23C6 carbide and ferrite.
Another object in this research was to realize the effect of the decomposition of retained austenite, a single tempering and multi-tempering cycles had been investigated by dilatrometric experiments. The resulting dilatrometric curves showed the Ms temperature rises with increasing the repeated cycling numbers, and according to the corresponding optical metallographs of each tempering cycle, it shows that the retained austenite can be transfered into new born martensites due to the higher Ms temperature, during the multi-tempering cycles. As a result, the multi-tempering method would be much efficient to decomposite the retained austenite.
Subjects
AISI 440C stainless steel
Ms temperature
lenticular martensites
the midrib region
alloy carbide
multi-tempering cycles
Type
thesis
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