Microstructural evolution in 316LN austenitic stainless steel during solidification process under different cooling rates
J Mater Sci
Microstructural evolution in 316LN austenitic stainless steel during solidification process under different cooling rates
Congfeng Wu 0 1 2
Shilei Li 0 1 2
Changhua Zhang 0 1 2
Xitao Wang 0 1 2
0 & Xitao Wang
1 Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing , Beijing 100083 , People's Republic of China
2 State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing , Beijing 100083 , People's Republic of China
The solidification sequence and microstructure evolution during solidification process of two 316LN stainless steels with different compositions under different cooling rates were in situ observed with confocal scanning laser microscope. The results show that 316LN solidifies with primary austenite or primary d ferrite when the cooling rate is small in the range of conventional casting process, depending on the value of Creq/Nieq which are calculated by Hammar and Svensson equations. As the cooling rate increases in the range of 0-100 C s-1, the solidification sequences do not change, but both the dendrite arm spacing and the mean free path between d ferrite decrease. In addition, concomitant with the variations of chemical composition in d ferrite and austenite are the shape transformation of interdendritic d ferrite from islandlike to lacy-like and the coarsening of dendrite d ferrite with cooling rate increasing. The mechanism of threephase reaction in 316LN with different compositions, i.e., eutectic reaction or peritectic reaction, was analyzed. The bigger diffusivities of Cr and Ni in primary d ferrite than that in primary austenite and the positions of alloys in phase diagram were thought to be the main reasons for the difference in type of the reaction.
Introduction
The solidification microstructure of austenitic stainless steel
has always been the interest of researches in academia and
industry because it determines the castability, weldability,
hot workability, mechanical properties, and corrosion
resistance [
1–4
]. In austenitic stainless steels, a three-phase
reaction region (L ? d ? c), which can be either eutectic or
peritectic, exists for compositions of over 15 wt% Cr and
10 wt% Ni according to the Fe–Cr–Ni ternary phase diagram
[
5, 6
]. Therefore, the solidification microstructure, which
mainly depends on both composition and cooling rate, is
complex as a result of the complicated three-phase reaction
[
5, 7
]. Suutala [8] investigated the solidification conditions
on solidifying sequence of a range of AISI 300 series steels
by autogenous gas tungsten arc (GTA) welding and
concluded that the composition was of primary importance
while the cooling rate was only of secondary importance.
However, a given austenitic stainless steel with composition
passing through the Cr-rich part of the three-phase region can
solidify with primary d ferrite or primary c phase under
different cooling rates [
8–10
]. The different solidifying
sequences would result in change of elements redistribution
path, and thus may alter the type of three-phase reaction and
the solidification microstructure. Ma et al. [
11
] and Fu et al.
[
12, 13
] investigated the detailed microstructural evolution
process in directional solidified 304 austenitic stainless steel
under estimated cooling rates of 3.3, 1, and 4 C s-1,
respectively. They concluded that eutectic reaction
(L ? c ? d) which resulted in the formation of coupled
structure occurred among the dendrite arms after primary d
ferrite precipitated from liquid. Liang et al. [14] observed a
different phenomenon in 301 austenitic stainless steel at
cooling rates of 4–25 C s-1 under non-directional
solidification condition using differential thermal analysis (DTA).
They found that peritectic reaction and eutectic reaction
coexisted in the microstructure of the sample cooled at
25 C s-1. In addition, many other researchers [
15–17
]
investigated the solidification microstructure of various
austenitic stainless steels by sorts of welding methods which
have much higher cooling rate than casting and directional
solidification. However, the three-phase reaction mechanism
is still unclear and it was not directly observed in the
abovementioned literatures owing to the limitations of test method.
Confocal scanning laser microscope (CSLM) enables the
in situ observation of phase transformation at high
temperature, as shown in references [
18–20
]. Huang et al. [
19
] and
McDonald et al. [
20
] observed the d/c interface and
microstructure evolution during the peritectic reaction at a
cooling rate of 0.05 C s-1 and constant undercooling degree,
respectively. Nevertheless, the effect of larger cooling rate,
which may be confronted in many types of conventional casting
processes, on the microstructure evolution has not been studied,
nor has been the in situ observation.
AISI 316LN steel, a type of nitrogen-alloyed ultralow
carbon (\0.02 wt%) stainless (...truncated)