Traditional stainless steel is generally produced by casting or forging technology, but stainless steel parts produced by casting are difficult to cut, and have poor dimensional accuracy, rough surface, certain restrictions on shape, easy to produce element segregation, shrinkage holes, sand holes and other deficiencies, and stainless steel produced by forging also has problems such as poor plasticity, difficult deformation, low material utilization and high cost, so they have many technical difficulties in manufacturing mechanical parts.
Since the 1970s, the rapid development of the atomization method to produce pre-alloyed powders has made it possible to prepare high-performance powder metallurgy stainless steel [3]. Powder metallurgy stainless steel has good mechanical, physical and chemical properties. Compared with stainless steel produced by traditional casting and forging technology, it has great advantages in production and application due to its low sintering temperature, close to net shape, high dimensional accuracy, high material utilization rate, good and uniform organizational structure, etc. It has been widely used in machinery, chemical industry, shipbuilding, automobile, instrumentation and other industries.
However, stainless steel powder is a pre-alloyed powder with high alloy content, which has high hardness and poor compressibility. Therefore, stainless steel produced by traditional molding process has pores inside and its density is often low. Its corrosion resistance and mechanical properties are generally lower than those of the corresponding dense stainless steel, which greatly limits the application of powder metallurgy stainless steel. Studies have shown that the density of powder metallurgy stainless steel has a great influence on its corrosion resistance and mechanical properties. Almost all properties of stainless steel improve with increasing density. Therefore, how to prepare powder metallurgy stainless steel with high density, good mechanical properties and corrosion resistance at low cost has always been a major issue faced by powder metallurgy workers.
The basic preparation process of powder metallurgy stainless steel is: original powder → mixing → forming → sintering → post-processing → stainless steel products. Among them, forming and sintering are the two most important links in the process of preparing stainless steel based on powder metallurgy technology. They are introduced below.
1.3.1 Forming of stainless steel powder
Forming is the process of compacting the powder into a block with a certain shape, size, porosity and strength in a steel die. Stainless steel belongs to high alloy steel. Its powder particles are hard and have poor compressibility. Therefore, higher pressure is required during forming. Its pressing pressure is generally around 500~800MPa. In the pressing process of the sample, in order to improve the pressing performance, lubricants are often added to the stainless steel pre-alloyed powder. Its main function is to reduce the friction between the powders and between the powders and the die wall during the pressing process, reduce the demolding pressure and increase the density of the pressed green sheet. Commonly used lubricants include stearic acid, zinc stearate, lithium stearate and paraffin. Room temperature compression molding has high requirements for presses and molds due to the high pressing pressure. The sintered products have severe elastic aftereffects and low density, and can only produce parts with simple shapes. In order to meet the higher requirements for the density and mechanical properties of powder metallurgy materials in industrial applications, a series of high-densification forming processes have been developed, such as warm compaction, metal injection molding and gel injection molding. They can significantly improve the density of stainless steel, and greatly improve its mechanical properties and corrosion resistance, thereby greatly promoting the development of stainless steel powder forming technology. They are introduced below.
(1) Warm Compaction Warm compaction is to mix powder and special lubricant and heat them to a certain temperature, and then press them in a heated mold. It is highly valued because it can produce powder metallurgy parts with high density and good performance. Compared with cold compaction, warm compaction can obtain higher compact density and strength with lower pressing pressure, which can increase by 0.15~0.3g/cm3 and 50~100% respectively, and can reduce elastic aftereffect. Studies have shown that the reason why the warm pressing process can increase the density of the green compact is that, firstly, the warm pressing reduces the work hardening rate of the powder, thereby enhancing the plastic deformation of the powder and thus increasing the green compact density; secondly, due to the effect of the lubricant, when the melting point of the lubricant is high, it is semi-solid at the warm pressing temperature, and its liquid phase component will flow from the particle boundary into the pores, thereby increasing the contact of the particles, and when the melting point of the lubricant is low, it will completely melt and flow out of the compact, playing the role of lubricating the mold wall, thereby reducing the demolding pressure.
In the warm pressing process of 420 stainless steel powder, when the powder temperature is 90°C and the mold temperature is 120°C, when the pressing pressure is 784MPa, the compact density is increased by 0.2g/cm3 compared with cold pressing. Ke Yuanyuan et al. used the warm pressing process to press 316L stainless steel powder at a pressing pressure of 700 MPa and a temperature of 110°C. Compared with cold pressing, the density of the green compact increased by 0.19 g/cm3. After vacuum sintering at 1150°C, the density of the sintered body also increased by 0.19 g/cm3. The tensile strength of the green compact and sintered body increased by 50% and 34 MPa, respectively. However, due to the high density of stainless steel parts formed by warm pressing, it may cause the polymer in the closed pores to be difficult to volatilize after cracking, which will cause defects during pre-sintering. Therefore, the removal of polymer during pre-sintering is a key issue in warm pressing.
(2) Metal Injection Molding (MIM) Metal Injection Molding (MIM) is a powder metallurgy technology that is close to net forming. It is a process of mixing metal powder or pre-alloyed powder with an organic binder in a certain proportion and under certain process conditions to form a uniform viscoelastic body, which is then injection molded by an injection molding machine, and then the binder is removed and finally sintered into a high-performance powder metallurgy product. It is suitable for producing parts with complex shapes, and the dimensional accuracy of the stainless steel parts produced by it can reach ±0.3%~0.5%. At the same time, it can overcome the shortcomings of low density and poor mechanical properties of conventional molded-sintered products. Its sintered density can reach 95%~99.5% of the theoretical density, the tensile strength can reach more than 500MPa, and the elongation can reach more than 45%.
Metal injection molding has requirements for the particle size, particle shape, fluidity, and bulk density of the raw material powder. The original powder is generally prepared by water mist or gas atomization. Among them, water atomized powder has better formability, while gas atomized powder has better sintering properties and its powder is finer. Therefore, a certain amount of gas atomized powder is often added to the water atomized powder to fill it in the gap of the water atomized powder to improve the density of the pressed green body and the sintered body. The main binders for metal injection molding are: paraffin-based, oil-based and polymer-based binders. Stainless steel degreasing mainly includes thermal degreasing, solvent degreasing, siphon degreasing and catalytic degreasing, among which thermal degreasing is the most commonly used method. After injection molding, if the degreasing is not done cleanly, the product will easily produce defects such as bubbling and cracking, and residual impurities such as oxides, carbon and carbides, which will affect its sintering performance. Generally, before 200°C, slowly heating at a rate of less than 1°C/min can remove some low-molecular components such as paraffin. After removing 30%~40% of the binder, quickly heating to 400~500°C at a rate of 5°C/min and keeping warm can decompose the polymer components in the binder, thereby achieving the purpose of completely removing the binder. Sintering is the last process of stainless steel metal injection molding. It has a higher temperature than traditional powder metallurgy products, which is also conducive to improving the density of the sintered body. Sung et al. mixed 17-4PH stainless steel powder and binder in a planetary mixer at a ratio of 60:40 for 1 hour, and then injected them at an injection pressure of 300kg/cm2. After thermal degreasing, they were sintered in a hydrogen atmosphere. When the sintering temperature was increased from 900°C to 1350°C, the relative density of the sample increased from 61% to 99%, and the tensile strength continued to increase as the pores in the sintered body spheroidized, shrank and disappeared. However, due to the high lubricant content in the metal injection molding process, a special degreasing process is required, which greatly increases the cost.
(3) Gel Casting
Stainless steel products prepared by traditional compression molding have poor mechanical, corrosion resistance and appearance properties due to their high porosity, and are limited to the production of parts with simple shapes. Although metal injection molding can achieve net shape of stainless steel parts, it is difficult to achieve the preparation of large-sized and complex-shaped parts. Gel casting is another near-net-size forming technology developed after slip casting and injection molding. It uses polymer chemical monomers to form powder materials by polymerization. That is, after preparing a concentrated suspension with low viscosity and high solid content, various parts with complex shapes can be prepared, thereby obtaining a high-strength and uniform green body.
Using gel casting technology, under the best process conditions, the stainless steel green body formed has excellent performance and uniform structure. The bending strength of the green body can reach 38MPa, the relative density of the sintered body can reach 95%, and the yield strength can reach 160MPa. It can also form a large-sized stainless steel green body with complex shapes. Using gel casting technology, natural agar and polyacrylic acid are used as gel and dispersant respectively. Under the best process conditions, the formed slurry can be sintered to prepare 316L stainless steel products with complex shapes, and the yield strength of the sintered body can reach 138MPa. However, when preparing stainless steel metal powder, due to its large particle diameter and density, it is easy to precipitate in the suspension, which leads to slurry coagulation and dilatant flow, making it difficult to prepare high-concentration suspension slurry. In addition, after the slurry gels into a blank, problems such as warping, deformation, loading and unloading and transportation will occur.
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