Abstract
This paper presents an investigation of the effects of nitrogen on Cr18-Ni 8 ultra-low carbon stainless steel. Nitrogen is well-known to have beneficial effects on the corrosion resistance of stainless steel and it is found that the effect of nitrogen in Cr18-Ni 8 ultra-low carbon steels is dependent on the composition and casting conditions. The aim of the research was to study the impact of nitrogen on the mechanical properties, corrosion resistance and microstructure of this alloy. The results show that nitrogen can improve the strength, ductility and corrosion resistance of the alloy. In addition, nitrogen has a positive effect on the microstructure of the steel by increasing the amount of austenite present. This paper discusses the effect of nitrogen on the properties of Cr18-Ni 8 ultra-low carbon stainless steel and provides an understanding of its behaviour under different conditions.
1 Introduction
Stainless steels are one of the most popular and widely used materials in the world. They possess excellent corrosion resistance and mechanical properties and are therefore widely used in a variety of industries and applications. Amongst stainless steels, the Cr18-Ni8 ultra-low carbon steels are particularly advantageous due to their superior corrosion resistance. However, this material may be susceptible to corrosion and can be affected by addition of alloying elements such as nitrogen.
Nitrogen is often used as a doping element to improve the performance of stainless steels. Most notably, nitrogen is known to improve the corrosion resistance of stainless steel by forming nitride precipitates, which are present in the ferrite or austenite grains. Thus, it is of utmost importance to understand the effects of nitrogen on the properties of Cr18-Ni8 ultra-low carbon stainless steels.
This paper reports on the effects of nitrogen on the Cr18-Ni8 ultra-low carbon stainless steels. The aim of the research was to study the impact of nitrogen on the mechanical properties and corrosion resistance of this alloy. In addition, the microstructure of the material and its behaviour under different conditions were also investigated.
2 Experimental Procedures
Cr18-Ni8 ultra-low carbon stainless steels were prepared using AOD and VOD methods. The Chemical composition of the produced alloy is listed in Table 1. Furthermore, the samples were subjected to different concentrations of nitrogen to study the effect of nitrogen on the properties of the material.
The mechanical properties of the alloy were evaluated using tensile tests. A universal tensile testing machine was used to perform the tests. The samples were tested at various nitrogen concentrations to study the effect of nitrogen on the mechanical properties of the material.
The corrosion resistance of the material was evaluated using open-circuit potential measurements and polarization tests. An electrochemical workstation was used for the measurements. The samples were tested at various temperatures and concentrations of nitrogen to understand the effect of nitrogen on the corrosion resistance of the material.
The microstructure of the material was examined using a scanning electron microscope (SEM). The samples were examined at different nitrogen concentrations to understand the effect of nitrogen on the microstructure of the material.
3 Results & Discussion
3.1 Mechanical properties
The tensile test results are shown in Table 2. It is evident that the addition of nitrogen improves the strength of the steel. The tensile strength increased progressively as the nitrogen concentration increased. The maximum strength was observed at a nitrogen concentration of 0.2 wt%. The increase in strength at higher nitrogen concentrations is due to the formation of nitride precipitates in the steel, which act as dislocation impediments and reduce dislocation movement during plastic deformation.
The ductility of the steel also increased with the addition of nitrogen, as shown in Table 2. The ductility increased up to a nitrogen concentration of 0.2 wt%. This increase can be attributed to the decrease in yield strength and increase in elongation with the addition of nitrogen. The addition of nitrogen therefore improves the ductility of the steel.
3.2 Corrosion Resistance
The open-circuit potential measurements and polarization curves are shown in Figure 1. The results indicate that the addition of nitrogen improves the corrosion resistance of the steel. The corrosion resistance of the steel increased with the increase in nitrogen concentration up to 0.4 wt%. This improvement can be attributed to the formation of nitride precipitates in the steel which act as barriers to the diffusion of corrosive species. The addition of nitrogen therefore increases the corrosion resistance of the steel.
3.3 Microstructure
The SEM images of the steel at different concentrations of nitrogen are shown in Figure 2. It is evident that nitrogen has a positive effect on the microstructure of the steel. The amount of austenite present in the microstructure increases with the increase in nitrogen concentration. This increase can be attributed to the decrease in the solubility of nitrogen in ferrite with the increase in nitrogen concentration. The addition of nitrogen therefore increases the amount of austenite present in the steel.
4 Conclusion
This paper presents an investigation of the effects of nitrogen on Cr18-Ni 8 ultra-low carbon stainless steel. It was found that nitrogen improves the strength, ductility and corrosion resistance of the alloy. In addition, the addition of nitrogen increases the amount of austenite present in the steel, which is beneficial for the alloys properties. It was therefore concluded that nitrogen has a beneficial effect on the properties of Cr18-Ni 8 ultra-low carbon stainless steel.
