201 vs 304 Stainless Steel
The main differences between stainless steel 201 and 304 are as follows:
- Specifications: The commonly used stainless steel sheet is divided into two types, 201 and 304, which are actually different in composition. 304 is of better quality but more expensive, while 201 is of lower quality. 304 is imported stainless steel sheet, and 201 is domestic stainless steel sheet.
- The composition of 201 stainless steel is 17Cr-4.5Ni-6Mn-N, which is a low-nickel steel and a substitute for 301 stainless steel. After cold processing, it has magnetic properties and is used for railway vehicles.
- The composition of 304 stainless steel is 18Cr-9Ni, which is the most widely used stainless steel and heat-resistant steel. It is used in food production equipment, chemical equipment, nuclear energy, and so on.
- The surface of 201 stainless steel is very bright with a dark brightness due to its higher manganese content. It is prone to rusting. In contrast, 304 stainless steel contains more chromium and has a matte surface. It does not rust easily. The most important difference is the difference in corrosion resistance. 201 stainless steel has poor corrosion resistance, so it is much cheaper in price. Additionally, since 201 stainless steel contains less nickel, its price is lower than that of 304 stainless steel. Thus, its corrosion resistance is not as good as that of 304 stainless steel.
- The main difference between 201 stainless steel and 304 stainless steel is the nickel content. Moreover, 304 stainless steel is currently more expensive, but at least it can ensure that it will not rust during use (it can be tested with a chemical solution).
- Stainless steel is not easy to rust because a chromium-rich oxide film forms on the surface of the steel body to protect it. 201 stainless steel belongs to high-manganese stainless steel, which has a higher hardness and lower nickel content than 304 stainless steel.
- The composition is different (mainly differentiated by the carbon, manganese, nickel, and chromium content between 201 stainless steel and 304 stainless steel).
The dual nature of carbon in stainless steel
Carbon is one of the main elements in industrial steel. The properties and structure of steel are largely determined by the form of carbon in it.
In stainless steel, the influence of carbon is extremely significant. The impact of carbon on the structure of stainless steel is mainly manifested in two aspects.
On the one hand, carbon is an element that stabilizes austenite and has a large effect (about 30 times that of nickel). On the other hand, due to the high affinity between carbon and chromium, a series of complex carbides are formed with chromium.
Therefore, from the perspective of strength and corrosion resistance, the role of carbon in stainless steel is contradictory. By understanding this impact law, we can select stainless steel with different contents based on different usage requirements.
The role of nickel in stainless steel only comes into play when it is combined with chromium.
Nickel is an excellent corrosion-resistant material and an important alloying element in steel. Nickel is an element that forms austenite in steel, but in low-carbon nickel steel, the nickel content must reach 24% to obtain a pure austenite structure, and it is only when the nickel content reaches 27% that the corrosion resistance of steel in certain media is significantly improved.
Therefore, nickel cannot constitute stainless steel alone. However, when nickel and chromium coexist in stainless steel, nickel-containing stainless steel has many valuable properties.
Based on the above situation, the role of nickel as an alloying element in stainless steel is to change the structure of high-chromium steel, thereby improving the corrosion resistance and process performance of stainless steel.
Manganese and nitrogen can replace nickel in stainless steel.
The role of manganese in austenitic steel is similar to that of nickel. Manganese does not contribute to the formation of austenite, but rather lowers the critical quenching speed of steel and increases the stability of austenite during cooling. It also suppresses the decomposition of austenite, allowing the austenite formed at high temperatures to be retained at room temperature.
However, manganese has limited effect on improving the corrosion resistance of steel. Even when the manganese content in steel ranges from 0 to 10.4, it does not cause significant changes in the corrosion resistance of steel in air or acid. This is because manganese does not significantly increase the electrode potential of iron-based solid solutions, and the protective effect of the oxide film formed is also low.
Therefore, although austenitic steel can be alloyed with manganese in industry, it cannot be used as stainless steel. The stabilizing effect of manganese on austenite in steel is half that of nickel, and the effect of 2% nitrogen in steel is even greater than that of nickel in stabilizing austenite.
Adding titanium and niobium to stainless steel is to prevent intergranular corrosion.
Molybdenum and copper can improve the corrosion resistance of certain types of stainless steel.
Type 201 has poor acid resistance and is usually used in indoor, dry and ventilated places; while type 304 has good acid resistance and is generally used outdoors or in humid environments.