What are super stainless steel and nickel-based alloys? Where are they used?

What are super stainless steel and nickel-based alloys? Where are they used?

Super stainless steel and nickel-based alloy are a special kind of stainless steel. First of all, they are different from ordinary stainless steel in chemical composition. They refer to a high-alloy stainless steel containing high nickel, high chromium, and high molybdenum.

According to the microstructural characteristics of stainless steel materials, super stainless steel is divided into several types: super ferritic stainless steel, super austenitic stainless steel, super martensitic stainless steel and super duplex stainless steel.

Super austenitic stainless steel

On the basis of ordinary austenitic stainless steel, by improving the purity of the alloy, increasing the number of beneficial elements, reducing the C content, preventing the precipitation of Cr23C6 causing intergranular corrosion, we can obtain good mechanical properties, process performance and local corrosion resistance, and replace Ti stabilized stainless steel.

Super ferritic stainless steel

It inherits the characteristics of ordinary ferritic stainless steel such as high strength, good oxidation resistance, and excellent stress corrosion resistance, while improving the limitations of ferritic stainless steel such as ductility-brittleness transition, sensitivity to intergranular corrosion, and low toughness in the welded state. . Using refining technology to reduce the C and N content and add stabilizing and weld metal toughening elements, super ferritic stainless steel with high Cr and Mo and ultra-low C and N can be obtained, making ferritic stainless steel more resistant to corrosion and corrosion resistance. Applications such as pitting and crevice corrosion of chlorides have entered a new stage.

Super Duplex Stainless Steel

This type of steel was developed in the late 1980s. The main grades include SAF2507, UR52N, Zeron100, etc. It is characterized by low C content, high Mo and high N content, and the ferrite phase content in the steel accounts for 40% to 45%. , has excellent corrosion resistance.

Super martensitic stainless steel

It is a hardenable stainless steel with high hardness, strength and wear resistance, but poor toughness and weldability. Ordinary martensitic stainless steel lacks sufficient ductility and is very sensitive to stress during deformation, making cold working difficult. By reducing the carbon content and increasing the nickel content, super martensitic stainless steel can be obtained. In recent years, various countries have invested heavily in the development of low-carbon and low-nitrogen super martensitic steels, and have developed a number of super martensitic steels for different uses. Super martensitic steel has been widely used in oil and gas extraction, storage and transportation equipment, hydropower, chemical industry and high-temperature pulp production equipment.

Functional stainless steel

As market demand changes, various stainless steels with special uses and special functions continue to appear. For example, the new medical Ni-free austenitic stainless steel material is mainly Cr-Ni austenitic stainless steel, which has good biocompatibility and contains 13% to 15% Ni. Ni is an allergenic factor and has teratogenic, carcinogenic and other hazards to organisms. If Ni-containing implanted stainless steel is used in the body for a long time, it will gradually be destroyed and release Ni ions. When Ni ions are enriched in tissues near implantation in the human body, it can induce toxic effects and cause adverse reactions such as cell destruction and inflammation. The Cr-Mn-N medical Ni-free austenitic stainless steel developed by the Institute of Metal Research, Chinese Academy of Sciences has undergone biocompatibility tests and has better performance than the Cr-Ni austenitic stainless steel currently used clinically. Another example is antibacterial stainless steel. As people's living standards improve, people pay more and more attention to their environment and their own health, which promotes the research and development of antibacterial materials. Since 1980, developed countries represented by Japan have begun to research and apply antibacterial materials in household appliances, food packaging, daily necessities, bathing equipment, etc. Nisshin Steel Co., Ltd. and Kawasaki Steel Company have respectively developed Cu-containing and Ag-containing antibacterial stainless steel. Cu-containing antibacterial stainless steel adds 0.5% to 1.0% Cu to stainless steel and adopts special heat treatment to make the stainless steel uniform from the surface to the inside. Disperse ε-Cu precipitates and play an antibacterial effect. This Cu-containing antibacterial stainless steel is suitable for use in series products such as high-end kitchen appliances and other products that require high processability and antibacterial properties. Ag-added antibacterial stainless steel has a high antibacterial effect against Escherichia coli and Staphylococcus aureus. This material can always maintain a good antibacterial effect especially when it is processed and ground or the surface is worn.

Nitrogen alloyed stainless steel

Adding N as an alloying element to stainless steel can improve austenite stability, balance the proportion of phases in duplex steel, improve the strength and corrosion resistance of steel without affecting the plasticity and toughness of the steel, and can partially replace stainless steel. Ni in. In dual-phase steel, N retards the dispersion and precipitation of intermetallic compounds; in martensitic steel, N and other elements form nitrides distributed on the grain boundaries, which can improve the hardening ability and prevent austenite and ferrite during high-temperature tempering. The growth of bulk grains. The high-N-content austenitic stainless steel developed in recent years, that is, high-strength non-magnetic austenitic stainless steel, has high-temperature strength and will be widely used as low-temperature superconducting materials, high corrosion resistance and non-magnetic materials.

High purity stainless steel

At present, the scrap rate of products in domestic stainless steel plants due to inclusions is as high as more than 20%. Therefore, high-quality purification centered on inclusion control in the stainless steel smelting process has attracted more and more attention. In the stainless steel smelting process, endogenous inclusions are generated during deoxidation, alloying and crystallization of molten steel. Foreign inclusions in finished products are generated during the smelting, casting and transportation of molten steel. In order to obtain high-purity stainless steel, attention should be paid to the raw materials entering the furnace, deoxidizer, deoxidation system, refining and continuous casting process system, etc.

The more famous one is steel containing 6% Mo (254SMo). This type of steel has very good resistance to local corrosion and has good resistance to pitting corrosion (PI≥ 40) and good stress corrosion resistance, it is a substitute material for Ni-based alloys and titanium alloys.

Secondly, in terms of high temperature resistance or corrosion resistance, it has better high temperature or corrosion resistance and is irreplaceable for 304 stainless steel. In addition, from the classification of stainless steel, the metallographic structure of special stainless steel is a stable austenite metallographic structure.

Since this special stainless steel is a high-alloy material, the manufacturing process is quite complex. Generally, people can only rely on traditional processes to manufacture this special stainless steel, such as pouring, forging, rolling, etc.

In many fields, such as:

1. Ocean: Marine structures in the marine environment, seawater desalination, seawater aquaculture, seawater heat exchange, etc.

2. Environmental protection field: flue gas desulfurization equipment for thermal power generation, wastewater treatment, etc.

3. Energy field: atomic power generation, comprehensive utilization of coal, wave power generation, etc.

4. Petrochemical industry: oil refining, chemical equipment, etc.

5. Food field: salt production, soy sauce brewing, etc.

In many of the above fields, ordinary stainless steel 304 is not competent. In these special fields, special stainless steel is indispensable and cannot be replaced. In recent years, with the rapid development of the economy and the continuous improvement of the level of the industrial field, more and more projects require higher-grade stainless steel - special stainless steel (super stainless steel, nickel-based alloys).

Some representative special stainless steels are:

1. Super stainless steel, which means it is a special stainless steel containing about 6% molybdenum. There are more than ten kinds of steel in the world. We also call it 6 molybdenum stainless steel. For example, the main component is; 25Ni-23Cr-5.5Mo-0.2N

2.Incoloy series alloys, such as Incoloy800, the main components are: 32Ni-21Cr-Ti, Al

3.Inconel series alloys, such as Inconel600, the main components are: 73Ni-15Cr-Ti, Al

4. Hastelloy, such as C-276, the main component is: 59Ni-15Cr-16Mo-4W

5. Monel alloy, such as Monel 400, the main component is: 65Ni-32Cu

Based on the above examples, if ordinary stainless steel (such as 304) is selected instead of special stainless steel, ordinary stainless steel (304) is not suitable for such high temperature or highly corrosive environment. The material will corrode immediately, or high temperature will occur. Oxidation. Therefore, in many environments where high temperature resistance and corrosion resistance are required, special stainless steel is the best choice.