Effect of Silicon on Precipitates of High-Silicon Austenitic Stainless Steel

Abstract

The effect of silicon contents (4wt%~8wt%) on microstructure of high-silicon austenitic stainless steel ZeCor was investigated by XRD, TEM and indentation deformation. Results show that increasing Si content leads to the phase constitute change of ZeCor alloy: the microstructure is single-phase austenite (γ phase) in ZeCor-4wt%Si alloy, γ phase with a small quantity of σ-phase in ZeCor-6wt%Si alloy, and as for the ZeCor-8wt%Si alloy, the main precipitations are Cr₃Ni₅Si₂ phase and a bit σ-phases. In addition, the Cr₃Ni₅Si₂ phase has a higher silicon and nickel content than the σ-phase. The Cr₃Ni₅Si₂ phase with a micro-hardness HV as high as 7840 MPa is a typical hard and brittle phase, and the precipitation of such phase can greatly increase the micro-hardness of the γ matrix in the ZeCor-8wt%Si alloy. The strengthening mechanism of γ matrix in ZeCor alloy is as follows: the solid solution strengthening is the main strengthening mechanism in ZeCor-6wt%Si alloy, while the solid solution strengthening of Si and the precipitation strengthening of Cr₃Ni₅Si₂ greatly increase the micro-hardness of the γ matrix in ZeCor-8wt%Si alloy, and the Cr₃Ni₅Si₂ phases have a great effect.

Keywords

high-silicon austenitic stainless steel; silicon content; σ-phase; micro-hardness tester; Cr₃Ni₅Si₂ phase

Science Press

Sulfuric acid is the most widely produced chemical in the world today, with extraordinary range of modern uses in chemical, agricultural, military and medical fields^[1,2]. Sulfuric acid shows different properties at different concentrations and temperatures^[3]. At elevated temperatures, high concentration sulfuric acid is highly corrosive, so the sulfuric acid plant corrosion is the main reason for the leakage of equip-ment in the sulfuric acid manufacturing process. Therefore special attention must be given to the key parts in the production of sulfuric acid^[4]. In the past, many studies have shown that silicon can improve the corrosion resistance of stainless steel^[5-12]. For example, in Saramet stainless steel and Sandvik SX stainless steel^[13,14], which are widely used in chemical production, the mass fraction of Si reaches 5wt%~6wt%, and its corrosion rate in concentrated sulfuric acid with a concen-tration greater than 98% at 130 °C is only 0.1 mm/a^[15-23]. In the 1970s, the American Lewis Company developed a new nickel-based Lewmet alloy with a Si content reaching 6wt% and the corrosion rate in 120 °C concentrated sulfuric acid is less than 0.1 mm/a. After age-hardening, the hardness (HRC) of the alloy can rise to around 50 and it has been successfully used to manufacture high-temperature concentrated sulfuric acid pumps. American Monsanto company introduced a high-silicon stainless steel ZeCor. Compared with SX, the content of Cr and Ni in this alloy is reduced while the content of Si is increased. The corrosion rate of this steel is less than 0.0254 mm/a in 93%~99% hot sulfuric acid, so it is generally employed in high-temperature concentrated sulfuric acid production. In China, the research and development of high-silicon austenitic stainless steel for high-temperature concentrated sulfuric acid began in the late 1980s. Several high-silicon stainless steels were studied, such as FS-1, C₂ (00Cr17Ni15Si4Nb), C₄ (00Cr14Ni14Si4) and XDS, but most of them are cast steel and concentrated in 4wt%~5wt% silicon steel^[24-27].

Beside the beneficial effects of Si addition on the corrosion resistance, some problems are also introduced, which will influence part of the mechanical properties of stainless steel^[9,18,28]. For example, σ-phase, χ-phase, M₆C carbide and Cr₃Si can be easily found in the steels when silicon contents are high^[29,30]. Most of these phases are hard and brittle, and have negative effect on the mechanical property of the stainless steels^[31-34]: the appearance of these precipitations will inevitably lead to the formation of Si-poor and Cr-poor areas in the surrounding area, which is also extremely detrimental to the corrosion resistance of the material. In addition, the Si atoms solubilizes in the matrix, causing the lattice distortion, and the degree of lattice distortion will increase with the increase of the Si content, so the hardness of the matrix will increase, and Si will segregate around the dislocation to form Cottrell atmosphere, which also hinders the movement of the dislocation ^[35]. Therefore, understanding the effect of Si on high-silicon austenitic stainless steel is vital to obtain good corrosion resistance and mechanical properties.

However, little literatures can be found referring to this aspect. In this research, different silicon contents were designed in ZeCor alloy, the microstructure, composition, morphology and micro-hardness of the precipitates and microstructure evolution of ZeCor alloys were studied, and the influence of silicon content on the precipitation mechanism of different types of precipitates was discussed.

1 Experiment

ZeCor alloy with varying Si contents (4wt%, 6wt% and 8wt%) were melted in a vacuum induction melting (VIM) furnace, and the mass of the smelted ingot was 25 kg. The diameter of the cast ingot was 120 mm. The measured chemical composition for each alloy is listed in Table 1.

Table 1  Measured chemical composition of ZeCor alloy with different Si contents (wt%)

Alloy	C	Mn	Si	Cu	Cr	Ni	Mo	P	S	Fe
ZeCor-4wt%Si	0.026	1.95	4.01	1.02	13.8	16.33	1.03	<0.005	<0.003	Bal.
ZeCor-6wt%Si	0.027	1.96	5.93	1.02	13.83	16.19	1.06	<0.005	<0.003	Bal.
ZeCor-8wt%Si	0.028	1.97	8.18	1.01	13.82	16.16	1.05	<0.005	<0.003	Bal.

Cylindrical samples with a diameter of 10 mm and a length of 10 mm were sectioned from the center of the ingots by wire electrical discharge, the cross-sections were subsequently ground by manual grinder for polishing, and the surfaces of these samples were corroded for several seconds with the mixed liquor of 30 mL glycerinum+20 mL HF+10 mL HNO₃. The microstructure of austenitic stainless steels with different Si contents was characterized by optical microscope (OM) and MERLIN Compact scanning electron microscopy (SEM). The precipitates in the ZeCor-Si steels were identified by X-ray diffraction (XRD), which was carried out using Cu Kα radiation in a Rigaku D/max 2500PC X-ray diffractometer; 2θ angles in the range of 10°~90° were scanned with the step speed of 1°/min. The sheets with 0.5 mm in thickness were cut and sanded to a thickness of 0.05 mm using sandpapers, then the thin zone was obtained using a double jet electrolytic thinner to get the TEM samples, and the TEM observations were carried out on a JEM-2100F operating at 200 kV. After that, scanning transmission electron microscope (STEM) imaging, energy dispersive spectrometer (EDS) and selected electron diffraction (SAED) were used to further analyze the precipitates in the ZeCor alloy with different Si contents. The micro-hardness was measured by FM-700e micro-hardness tester under a load of 100 g for 15 s.

2 Results and Discussion

2.1 Microstructure and precipitation phase characteriza-tion

Fig.1 shows the cast microstructure of ZeCor alloy. From Fig.1a and 1d, it can be observed that only γ phase exists and its grain boundaries are clearly viewed in ZeCor-4wt%Si al-loy. However, a large number of black precipitates (marked by I) with fine strips or dots along the grain boundary are observed in ZeCor-6wt%Si alloy (Fig.1e), and these phases account for 7.65% of the total area (Fig.1e and Table 2). However, from Fig.1c and 1f, it can be seen that a lot of gray bulk-like of precipitates (marked by II) appear and these precipitates account for 57.07% of the total area, which greatly reduces the γ matrix and the proportion of the black precipitates decreases a little bit in the ZeCor-8wt%Si alloy (Table 2).

Fig.1  OM microstructures of austenitic stainless steels with different Si contents: (a, d) 4wt%, (b, e) 6wt%, and (c, f) 8wt%

Table 2  Proportion of different precipitations marked in Fig.1 in ZeCor alloy (%)

Alloy	Precipitate I	Precipitate II
ZeCor-6wt%Si	7.65	-
ZeCor-8wt%Si	6.12	57.07

Fig.2 is SEM microstructures of ZeCor alloy. In ZeCor-4wt%Si alloy, the grain boundaries of γ matrix are clearly identified and no precipitated phase is observed in ZeCor-6wt%Si alloy. The black precipitates I in OM are white in SEM micrograph and formed at the austenite grain boundaries. When the Si content is 8wt%, γ matrix is obviously reduced, and gray bulk-like precipitates II in OM show black in SEM micrograph. The black precipitate II mainly precipitates in the matrix, and the white precipitate I is found between the γ phases and the precipitate II.

Fig.2  SEM microstructures of ZeCor-4wt%Si (a), ZeCor-6wt%Si (b), and ZeCor-8wt%Si (c)

These precipitates in the ZeCor alloy were analyzed by XRD. From Fig.3, it can be observed that precipitate I may be σ-phase, and precipitate II may be Cr₃Ni₅Si₂ phase. The micro-structure of ZeCor alloys changes with increasing the Si contents: γ matrix (4wt%Si) →σ-phase and γ matrix (6wt%Si) →Cr₃Ni₅Si₂ phase, σ-phase and γ matrix (8wt% Si).

Fig.3  XRD patterns of as-cast ZeCor alloy with different Si contents

In order to further determine the types of precipitation phases in Fig.2b and 2c, TEM was employed to identify the precipitates in ZeCor-6wt%Si and ZeCor-8wt%Si alloys. Ac-cording to the SAED patterns in Fig.4a and 4b, the precipitate I formed along grain boundary (Fig.2b) can be identified as the σ-phases, the precipitate II is identified as the Cr₃Ni₅Si₂ phases (Fig.4f). So, considering that the first emerged phase is σ-phase in ZeCor-6wt%Si alloy, it can be inferred that the in-crease of Si content will first promote σ-phase precipitation. While in ZeCor-8wt%Si alloy, with higher Si content, Cr₃Ni₅Si₂ phase subsequently precipitates, and meanwhile the amount of σ-phase changes a little with the increase of Si content.

Fig.4  TEM images (a~c) and SAED patterns (d~f) of the precipitates in austenitic stainless steels with different Si contents: (a, d) σ-phase in ZeCor-6wt%Si, (b, e) σ-phases in ZeCor-8wt%Si, and (c, f) Cr₃Ni₅Si₂ phase in ZeCor-8wt%Si; tetragonal structures of σ-phase (g) and Cr₃Ni₅Si₂ phase (h)

Fig.5 are microstructures and EDS results of ZeCor alloy with different Si contents. Table 3 is the element contents of the precipitation phases and the matrix counted by EDS. According to Fig.5 and Table 3, it can be seen that the structure is composed of a single-phase γ matrix without other phases in ZeCor-4wt%Si alloy, in other words, the Si atoms are solid-dissolved in the γ matrix. The σ-phase (Fig.5b and 5c) enriched with Si, Cr, Mo alloying elements can be observed in both ZeCor-6wt%Si alloy and ZeCor-8wt%Si alloy, while the Si content in γ matrix is slightly less than 6wt% in ZeCor-6wt%Si alloy (Table 3). As for ZeCor-8wt%Si alloy, the Cr₃Ni₅Si₂ phase is enriched in Si, Cr, Ni elements compared with the σ-phase and γ matrix (Fig.5c). This is because precipitated Cr₃Ni₅Si₂ phase consumes large amounts of Cr, Mo in the matrix, which results in lower Cr, Mo content in σ-phase. In addition, it can be found from Table 3 that the contents of Si in the matrix of both the ZeCor-6wt%Si alloy and the ZeCor-8wt%Si alloy are about 5wt%, indicating that no matter how high the Si content is, the maximum amount of Si in the matrix is almost unchanged^[36]. When more than 6wt% of silicon is added to the steel, excess silicon atoms in matrix can be expelled by forming precipitates, such as Cr₃Ni₅Si₂ phase and σ-phases. Studies have also reported that in addition to being solid-dissolved in the γ matrix when silicon is added to stainless steel, part of silicon will exist in the matrix in the form of silicides and silicates^[21,37-41].

Fig.5  Microstructures and EDS results of ZeCor-4wt%Si (a), ZeCor-6wt%Si (b), and ZeCor-8wt%Si (c)

Table 3  Element contents of matrix and precipitated phases in austenitic stainless steels with different Si contents (wt%)

Element	ZeCor- 4wt%Si	ZeCor- 6wt%Si		ZeCor-8%Si
	γ	γ	σ-phase	γ	σ-phase	Cr3Ni5Si2
Si	3.49	5.11	8.51	5.43	8.29	10.03
Cr	14.06	13.72	19.03	12.27	15.28	17.17
Ni	15.10	17.42	12.66	16.33	14.15	17.04
Mo	0.82	1.62	5.51	1.13	1.89	2.27
Fe	Bal.	Bal.	Bal.	Bal.	Bal.	Bal.

2.2 Effect of silicon content on the micro-hardness of ZeCor alloy

The micro-hardness test was carried out on the samples, as shown in Fig.6. It can be observed that the micro-hardness of the γ matrix gradually increases when the silicon content is rai-sed from 4wt% to 8wt%. Compared with the micro-hardness of γ matrix in ZeCor-4wt%Si alloy, the micro-hardness of ma-trix in ZeCor-6wt%Si alloy is approximately improved by 34.3% (~461 MPa). The micro-hardness of γ matrix in ZeCor-8wt%Si alloy is improved by about 51.0% (~911 MPa) compared with the micro-hardness of γ matrix in ZeCor-6wt%Si alloy. As mentioned before, the Si atoms in γ matrix of ZeCor-8wt%Si alloy is not increased significantly (Fig.6), so the silicon content may not be the main reason for the micro-hardness increment in γ matrix of the ZeCor-8wt%Si alloy.

Fig.6  Vickers hardness and Si content of γ matrix for austenitic stainless steel containing 4wt%, 6wt% and 8wt% Si

Fig.7 shows the slip bands around the micro-hardness trace on the matrix and precipitates in three kinds of alloys after indentation deformations. It can be observed that the deformation zone around the indentation is not hindered when squeezed by an external force in ZeCor-4wt%Si alloy, so it has a wide range of slip lines. While in ZeCor-6wt%Si alloy (Fig.7b), the σ-phases are distributed in γ matrix and act as hard phases^[30,31], which hinder the slip movement in the ma-trix. In ZeCor-8wt%Si alloy, it is obvious that the slip bands in the γ matrix are hindered by the Cr₃Ni₅Si₂ phases, which greatly improves the micro-hardness of the γ matrix (Fig.7c). Furthermore, the micro-hardness of Cr₃Ni₅Si₂ phase is ~7840 MPa, which is 1.58 times larger than that of γ matrix and the cracks form in the Cr₃Ni₅Si₂ phase after indentation deformation (Fig.7d). It can be inferred that the Cr₃Ni₅Si₂ phase is hard and brittle phase, which can hinder the dislocation to enhance the micro-hardness of the γ matrix.

Fig.7  Slip bands around the compression zone on the matrix and precipitates of ZeCor with different silicon contents: micro-hardness inden-tation on γ matrix of ZeCor-4wt%Si (a), ZeCor-6wt%Si (b), ZeCor-8wt%Si (c), and micro-hardness indentation on Cr₃Ni₅Si₂ (d)

What's more, a schematic diagram of the effect of Si con-tents and precipitation phases in γ matrix is shown in Fig.8. Compared with ZeCor-4wt%Si alloy, the Si atoms solubilized in γ matrix of ZeCor-6wt%Si alloy is increased significantly (Fig.8a and 8b), which increases the distortion of γ matrix lattice, so that its micro-hardness is improved (Fig.6). Fig.8d~8f show the strain area A of γ matrix in three studied steels. In the hardness test, the applied force (F) in γ matrix is a constant value under a loading of 100 g. According to the theory of elasticity, the σ can be calculated:

φ=F/A

(1)

Fig.8  Characteristics of solid solution Si atom and slip lines in matrix of ZeCor-4wt%Si (a, d), ZeCor-6wt%Si (b, e), and ZeCor-8wt%Si (c, f)

where φ is inversely proportional to the force area (A). The area A in ZeCor-6wt%Si is bigger than that in ZeCor-8wt%Si, meaning that φ_6%Si is lower than φ_8%Si. These Cr₃Ni₅Si₂ phases are the main factors resulting in different micro-hardness of γ matrix in ZeCor-6wt%Si alloy and ZeCor-8wt%Si alloy.

3 Conclusions

1) The microstructures of ZeCor alloy change with increasing Si contents: γ matrix (4wt% Si)→σ-phase and γ matrix (6wt% Si)→Cr₃Ni₅Si₂ phase, σ-phase and γ matrix (8wt% Si). The increase in Si content can promote Cr₃Ni₅Si₂ phase formation, but the amount of σ-phase changes little.

2) The σ-phase is mainly precipitated along the grain boundary and enriched with Si, Cr and Mo contents, Cr₃Ni₅Si₂ phase is enriched with Si, Cr, Ni and Mo contents, and in ZeCor-8wt%Si alloy, a large number of Cr₃Ni₅Si₂ phase is precipitated in the γ matrix, which results in lower Cr, Mo content in σ-phase.

3) With the addition of silicon, the solid solution of Si atoms in the γ matrix gradually increases, which increases the degree of distortion of γ matrix lattice, and the Cr₃Ni₅Si₂ phase acts as a hard brittle phase to hinder the dislocation slip. The Si content and the Cr₃Ni₅Si₂ phases are the main factors resulting in the highest micro-hardness of γ matrix in ZeCor-8wt%Si alloy.