Microstructural evolution in 316LN austenitic stainless steel during solidification process under different cooling rates
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 δ 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 δ ferrite decrease. In addition, concomitant with the variations of chemical composition in δ ferrite and austenite are the shape transformation of interdendritic δ ferrite from island-like to lacy-like and the coarsening of dendrite δ ferrite with cooling rate increasing. The mechanism of three-phase reaction in 316LN with different compositions, i.e., eutectic reaction or peritectic reaction, was analyzed. The bigger diffusivities of Cr and Ni in primary δ 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.
KeywordsFerrite Austenite Cool Rate Austenitic Stainless Steel Eutectic Reaction
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, 2, 3, 4]. In austenitic stainless steels, a three-phase reaction region (L + δ + γ), 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  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 δ ferrite or primary γ phase under different cooling rates [8, 9, 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.  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 → γ + δ) which resulted in the formation of coupled structure occurred among the dendrite arms after primary δ ferrite precipitated from liquid. Liang et al.  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, 16, 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 above-mentioned 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, 19, 20]. Huang et al.  and McDonald et al.  observed the δ/γ 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 steel, is used as the material of main pipelines in AP1000 pressurized water reactor (PWR) which is about to commercially serve in China. The pipes are manufactured by integral hot forging and the ingots for hot forging are obtained by mold casting or electro slag remelting (ESR) which has typical cooling rates of 0–100 °C s−1 . Therefore, in this regard, the studies of phase transformation in 316LN during solidification under different cooling rates would be of interest.
In this paper, two 316LN stainless steels with different compositions were used to investigate their solidification sequence and microstructure evolution. The three-phase reaction was in situ observed in CSLM under different cooling rates and the variation of chemical composition in different phases was discussed.
Chemical compositions of AISI 316LN stainless steel
After the samples were cooled down to room temperature, they were prepared by conventional process for metallographic observation. The etchant solution contains 0.5 g K2S2O5, 20.0 g NH4FHF, and 100 ml distilled water (Beraha’s etchant modified by Lichtenegger ), whose pH value was maintained at about 2.5 by addition of NH4OH or HNO3. Electron probe microanalyzer (EPMA) (JEOL JXA-8230, Japan) was employed to characterize the variation of chemical composition. The volume fractions of primary δ ferrite and primary austenite during in situ observation were measured by image analysis using software of Image Tool Version 3.0.
As-cast microstructure of mold casting ingots
Solidified microstructures of 316LN under different cooling rates
For the alloys 316LN-1 and 316LN-2 studied here, the ratios of Creq to Nieq equal 1.47 and 1.55, respectively. Hence, alloy 316LN-1 falls into AF mode while alloy 316LN-2 follows FA mode. This indicates that austenite is the primary phase in 316LN-1 and the precipitation of δ ferrite occurs first in 316LN-2, then three-phase reaction (L + δ + γ) comes about at the terminal stage of solidification in both alloys. Subsequently, solid-state transformation of δ → γ continues below solidus lines [12, 25]. Therefore, the results predicted by Hammar and Svensson equations work fairly concordant with the solidified microstructure of mold casting in Figs. 3 and 6.
Solidification process of 316LN
During mold casting process, alloy 316LN-1 follows the solidifying path of AF solidification mode, while 316LN-2 solidifies in FA mode. Both the results are consistent with the prediction of Hammar and Svensson equations.
As the cooling rate increases in the range of 0–100 °C s−1, the solidification modes of both alloys do not change on the whole. However, the ferrite in 316LN-1 changes from island-like to lacy-like and it becomes coarser in 316LN-2 with cooling rate increasing. In alloy 316LN-1, the contents of Cr and Mo decrease slightly in both δ ferrite and γ phase while Ni increases in γ phase and decreases in δ ferrite in a small scale with the increasing of cooling rate. On the other hand, Cr and Mo increase in δ ferrite and decrease in austenite, while Ni has a contrary variation trend in alloy 316LN-2.
Eutectic reaction occurs in the three-phase region of 316LN-1, while peritectic reaction occurs in 316LN-2 when it is cooled at 2 °C s−1. The bigger diffusivities of Cr and Ni in primary δ ferrite than that in primary austenite, as well as the positions of alloys in phase diagram were thought to be the main reasons accounting for the difference in the type of three-phase reaction. Peritectic reaction transforms into eutectic reaction in 316LN-2 when it is cooled at 10 and 100 °C s−1.
The authors would like to thank the National High-Tech Research and Development Program of China (863 Program) for the financial support through Grant No. 2012AA03A507. We also acknowledge Dr. Yu Liu and Dr. Jianhua Wu in the Institute of New Materials in Shandong Academy of Science for their help in carrying out CSLM experiment.
- 21.Katayama S, Matsunawa A (1984) Solidification microstructure of laser welded stainless steels. Proc. ICALEO, pp 60–67Google Scholar
- 24.Hammar O, Svensson U (1979) Solidification and casting of metals. The Metals Society, LondonGoogle Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.