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Metal injection molding (MIM) technology embracing developing opportunity in high nitrogen stainless steel

Categories: ZCMIM NewsStars: 3StarsVisit: -Release time: 2019-10-09 09:01:00
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In recent years, with the continuous development of manufacturing process and technology, people have realized that nitrogen has great advantages in stabilizing austenite in steel, and can retain the excellent characteristics of austenite such as non-magnetism. For stainless steel products, the same is true. Furthermore, with the continuous development and application of 3D printing technology, Metal injection molding (MIM) high-nitrogen stainless steel is gaining more and more advantages in the electronics industry.

High nitrogen stainless steel was born to replace nickel

Stainless steel is one of the greatest inventions in the development history of materials. Due to its excellent corrosion resistance, stainless steel is widely used in various harsh industrial environments. It be used to make a variety of consumer goods components or final products (such as tableware), and can maintain a long time silver metal luster, loved by consumers.

In the early stage of stainless steel development, the research on nitrogenous stainless steel has not attracted much attention. Firstly, it is difficult to add gaseous nitrogen into liquid steel due to the limitation of production process. Second, whether nitrogen would cause brittleness of stainless steel was controversial at that time. It was not until 1912 that the significant influence of nitrogen on steel mechanical properties and the stability of austenite was first documented. Then, in 1926, a similar effect of nitrogen on chromium and ferrochrome alloys was reported. Since the 1930s, studies on adding nitrogen to fe-cr alloy to improve its strength have been documented. During world war ii, due to the shortage of nickel resources, the possibility of stabilizing austenite with nickel nitrogen has become a hot topic. In addition to the known effect of nitrogen on stainless steel structure and strength, the corrosion resistance of nitrogen on stainless steel was first discovered.

In the development history of high nitrogen steel, there are two factors that promote people's thinking about the significance of nitrogen as an alloying element in stainless steel. The second is to produce high strength austenitic stainless steel. When AOD furnace (argon oxygen decarburization) has realized the possibility of nitrogen as an alloying element, the nitrogen alloying of stainless steel is rapidly promoted. Especially in austenitic stainless steel, by adjusting nitrogen and manganese content instead of nickel, it is possible to produce high-quality and cheap high nitrogen stainless steel, or even reduce the content of nickel to less than 0.1%.

Austenitic stainless steel is one of the most important engineering materials, because of its strong corrosion resistance, high ductility, non-magnetic, has a wide range of applications in industry. Traditional austenitic stainless steel contains a lot of nickel. Although the existence of nickel stabilizes the austenite structure in steel, there are some difficult problems. Nickel, for example, has a higher cost; material strength and hardness cannot be improved effectively because of the substitution of solid atoms in austenite. Poor biological compatibility, easy to cause human allergic reaction, limited its application in the field of consumer electronics and biological medicine.

To solve these problems, nitrogen was introduced into austenitic stainless steel instead of nickel, and high nitrogen stainless steel was born. Compared with traditional austenitic stainless steel, high nitrogen stainless steel has relative advantages. For example, nitrogen is much more stable to austenite than nickel. A small amount of nitrogen can effectively stabilize the austenite structure in stainless steel, reducing ferrite and martensite formation during processing, thus preserving high corrosion resistance and non-magnetic properties of austenitic stainless steel. As an interstitial solid solution element, nitrogen can effectively improve the hardness and strength of austenite while maintaining good ductility of the material. Nitrogen can reduce nickel release, improve the biological compatibility of material, and improve the resistance of austenitic stainless steel to pitting and cracking corrosion.

Therefore, high nitrogen austenitic stainless steel has become a research hotspot in recent years, and its application in industry is increasing.

High nitrogen stainless steel is manufactured with MIM technology

The early development of high nitrogen austenitic stainless steel is mostly based on casting technology. Due to the low solubility of nitrogen in liquid iron, a higher nitrogen partial pressure is needed to dissolve enough nitrogen in the molten steel. However, this method requires expensive high-temperature and high-pressure equipment, and has certain risks, so it is blocked in the industrial promotion.

In contrast, the solid solubility of nitrogen in austenite is much higher than that in liquid iron, so the stainless steel powder can penetrate more nitrogen in solid state at low pressure. This makes powder metallurgy process more economical and efficient for high nitrogen austenitic stainless steel. In addition, powder metallurgy process can also achieve near-net forming products, reduce follow-up processing, and obtain more uniform structure and properties than casting.

MIM technology is a new near net forming technology introduced by injection molding in the field of powder metallurgy. In process of metal injection molding, firstly select qualified metal powder and polymer binder, and then mixed and extruded under appropriate technological conditions to produce even granular feeding materials. Secondly, through injection molding, the feeding material is injected into mold cavity in molten state to form a blank. Finally, remove binder from green part by degreasing process, and then obtain densified metal products by sintering. After sintering, the theoretical density of finished product can reach 96%~98%, and mechanical properties are close to forging material.

MIM technology has the advantage of being able to mass produce complex and sophisticated metal parts at very low cost. At present, the most common high nitrogen nickel free stainless steel manufactured by MIM technology in industry is PANACEA. Its chemical composition (mass fraction) is: carbon ≤0.2%, nitrogen ≥0.65%, chromium 16.5%~17.5%, nickel ≤0.1%, molybdenum 3.0%~3.5%, manganese 10%~12%, silicon ≤0.1% and residual iron. Nitrogen content of original powder was no more than 0.3%, the nitrogen content could be increased to more than 0.65% by sintering process, and finally obtain high nitrogen and nickel free austenitic stainless steel with good performance. Although this stainless steel has excellent performance, there are still technical barriers to mass production. For example, the control of nitrogen content in this material is involved in the understanding of thermodynamics and kinetics of nitriding process. The existence of nitrogen in stainless steel is related to heat treatment. Because sintering furnaces used by different manufacturers are different, the optimized sintering conditions need to be fully verified in the early stage of production. All these factors increase the difficulty of material's production.

High nitrogen nickel free stainless steel made with MIM technology has higher strength and hardness than traditional austenitic stainless steel, excellent corrosion resistance, and no magnetism. It is an excellent material for making structural parts of electronic products. Huawei began using the material to make camera holder for its flagship phone in late 2017, after two generations of handsets. Currently, four camera mounts have been mass-produced, each with several million units shipped, which is a classic example of injection-molded high nitrogen nickel free stainless steel. With the promotion of huawei, more and more mobile phone structural components will choose this high nitrogen nickel free austenitic stainless steel material. It is believed that in near future, the application of MIM technology manufacturing high nitrogen nickel free stainless steel will embrace more development opportunities.

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