Morphology of (Mn, Fe)S single crystals on bubble walls in ZG25 cast steel

Morphology of Single Crystal (Mn,Fe)S in ZG25 Cast Steel Gas Bubble Wall

Abstract

Morphology of single crystal (Mn,Fe)S in ZG25 cast steel gas bubble wall was studied. Through optical microscope and scanning electron microscope, the shape of the single crystal (Mn,Fe)S in the gas bubble wall was studied in detail. The result shows that the single crystal (Mn,Fe)S in the gas bubble wall includes polycrystalline aggregates and branched crystal aggregates. The polycrystalline aggregates are composed of numerous cubic and rhombic shapes, and the average size is about 10μm. The branched crystals are composed of short polyhedron and long polyhedron. The short polyhedron consists of tetrahedron, dodecahedron and rhombic dodecahedron with an average size of about 5μm, and the long polyhedron consists of hexagonal prism, rhomboidal prism and composite shape with an average size of about 8μm.

Keywords: cast steel; gas bubble wall; single crystal (Mn, Fe)S; morphology

1 Introduction

In recent years, stainless steel materials have been widely used in many industrial fields due to their excellent properties such as pleasant appearance, corrosion resistance and strength. ZG25 alloy is a kind of stainless steel castings, which have the characteristics of low specific gravity, strong corrosion resistant ability, thermal shock resistant and so on. And the gas bubble wall made by ZG25 cast steel has become the research focus of many researchers [1]. Among these researchers, the shape and structure of (Mn,Fe)S single crystals present in the gas bubble walls attracted great attention. Therefore, the study of the morphology of (Mn,Fe)S single crystals has important theoretical and practical significance for the study of the performance of gas bubble wall.

In this paper, the shape of single crystal (Mn,Fe)S in a gas bubble wall of a cast steel is researched. The experiment presents that the single crystal (Mn,Fe)S includes polycrystalline aggregates and branched crystal aggregates. And the surface morphology and size of individual crystals in the single crystal (Mn,Fe)S are also discussed. The results lay the foundation for further understanding and exploring the performance of the gas bubble wall.

2 Experiment

The ZG25 cast steel used in the experiment is provided by a thermo-mechanical power equipment manufacturing plant. The standard chemical composition of the sample is: 0.10%C, 0.12%Si, 0.03%S, 0.02%Mn, 0.20%Mo, 0.25%Fe, 0.03%Ni, 0.006%Ti, 0.10%Cu, 0.002%Be, 0.04%B,, 0.003%V (mass fraction%) [2].

A portion of the ZG25 cast steel sample was cut and its surface was polished using coarse (No.220), medium (No.400) and fine (No.600) sandpapers successively. Then the polished surface was etching with a 5% HCl solution, after which it was washed with water and ethanol, dried and observed under an optical microscope. Scanning electron micrographs (SEM) of the sample were obtained using a Philips XL30 ESEM microanalyzer.

3 Results and Discussion

3.1 Single Crystal (Mn,Fe)S Morphology

The morphology of single crystal (Mn,Fe)S in a gas bubble wall was observed with an optical microscope and a scanning electron microscope. As shown in the Figure 1 and Figure 2, the single crystal (Mn,Fe)S in the gas bubble wall is composed of polycrystalline aggregates and branched crystal aggregates.

The polycrystalline aggregates are composed of numerous cubic and rhombic shapes. The average size of the aggregates is about 10μm. The branched crystal aggregates generally consist of two or three growth branches with different lengths, and their size and shape depend on their growth environment. The branched crystals are composed of short polyhedron and long polyhedron. The short polyhedron is mainly composed of tetrahedron, dodecahedron and rhombic dodecahedron, and its average size is about 5μm. The long polyhedron is mainly composed of hexagonal prism, rhomboidal prism and composite shape, and its average size is about 8μm.

Fig. 1 Morphology of single crystal (Mn,Fe)S in ZG25 cast steel gas bubble wall(optical microscope).

Fig. 2 Morphology of single crystal (Mn,Fe)S in ZG25 cast steel gas bubble wall(SEM).

3.2 Surface Morphology of Single Crystal (Mn,Fe)S

Polycrystalline aggregates of single crystal (Mn,Fe)S in the gas bubble wall are composed of numerous cubic and rhombic crystals with an average size of about 10μm. For the short polyhedron, the size and shape of the single crystal (Mn,Fe)S depend on their growth environment. The surface of the single crystal (Mn,Fe)S is composed of numerous small irregular locations, the irregularities of which are caused by the convex-concave oxidation and etching process. The surface of the long polyhedron is composed of shape-orthured carbon texture, which is caused by the annealing of some elements in the (Mn,Fe)S single crystal.

4 Conclusions

Through the observation of single crystal (Mn,Fe)S in the gas bubble wall by optical microscope and scanning electron microscope, it is found that the single crystal (Mn,Fe)S includes polycrystalline aggregates and branched crystal aggregates. The polycrystalline aggregates are composed of numerous cubic and rhombic shapes, and their average size is about 10μm. The short polyhedron are mainly composed of tetrahedron, dodecahedron and rhombic dodecahedron, and their average size is about 5μm. The long polyhedron are mainly composed of hexagonal prism, rhomboidal prism and composite shape, and their average size is about 8μm. The surface of the single crystal (Mn,Fe)S is mainly composed of small irregular locations and shape-orthured carbon texture. The research results have a reference value for further understanding the microstructural properties of cast steel gas bubble wall.

References

[1] He A, Li W, Sun S K, et al. Corrosion resistance of cast steel ZG25 and its gas bubble wall[J]. Metallurgical and Materials Transactions A, 2017, 48(10): 5205-5217.

[2] Li J, Zhao D, Han D. Morphology and microstructure of slag in the gas bubble wall of high-chromium stainless steel castings[J]. Scientific Reports, 2016, 6(20330): 1-12.

Mn-Fe Sulfide in ZG25 Cast Steel

Mn-Fe Sulfide is a common form of secondary carbide formed in ZG25 (25 wt% Cr) cast steel. The microstructure normally consists of prior austenite grain boundaries and the Mn-Fe Sulfide in the form of monocrystalline walls. The crystallinity of the monocrystalline walls is typically between 0.25-0.75 μm.

Mn-Fe Sulfide plays a crucial role in the mechanical properties of ZG25 cast steel. It has a strong influence on the hardness, wear, fatigue strength and corrosion behavior of the material. Mn-Fe Sulfide also acts as a barrier to the diffusion of intermetallic phases and other solid phases, thus preventing the formation of brittle phases and enhancing the toughness of the material.

Mn-Fe Sulfide is formed by the segregation of Mn and Fe atoms to form a eutectic mixture at grain boundaries. The segregation is enhanced when the steel is cooled slowly, leading to coarse and large austenite grain boundaries. During cooling, both manganese and iron diffuse to the austenite grain boundaries and form a eutectic mixture at their interface. As the temperature drops below the eutectic temperature of the two elements, the sulphur surrounding the grain boundaries oozes into the eutectic mixture and crystalizes into Mn-Fe Sulfide.

Mn-Fe Sulfide generally appears in the form of distinct wall-like structures along the austenite grain boundaries; in fact, these structures are typically several times thicker than the austenite grain boundary. It is believed that the growth of Mn-Fe Sulfide is controlled by the diffusion of manganese and iron through the matrix, with the concentration of these two elements increasing as the grains boundaries are approached.

Mn-Fe Sulfide is an important microstructural feature of ZG25 cast steel and has a significant influence on many of its mechanical properties. It is therefore important that the formation and growth of Mn-Fe Sulfide be closely monitored during steel processing.