Powder injection molding process of nickel-free high nitrogen stainless steel

Nickel-free high-nitrogen austenitic stainless steel is a type of stainless steel with high mechanical properties and corrosion resistance. In particular, it does not contain nickel, which can cause severe allergic reactions in some people, and can effectively prevent the “nickel sensitivity” problem. Therefore, there is a broad market space in biomedical materials, which is currently attracting much attention from academic and industrial circles at home and abroad.

Although hot isostatic pressure smelting, pressure induction smelting, pressure plasma smelting, pressurized slag smelting, nitrogen slag smelting and other new technologies and new processes have been adopted, many brands of nitrogen-containing stainless steel materials have been developed. However, to varying degrees, the above-mentioned production technologies have problems such as complicated equipment, dangerous high-pressure gas, difficult process control, and high production cost, and there are certain difficulties in obtaining higher nitrogen content in materials.

The powder metallurgy production of high-nitrogen steel has many advantages, such as no high-pressure smelting facility in production, and low cost; fine-grain strengthening, dispersion strengthening, etc. strengthen the material properties. In addition, experimental facts show that nitrogen is solid in solid austenitic stainless steel. The solubility is greater than the solubility in the liquid state, so the powder metallurgy solid nitriding process is an economically and technically feasible method for producing high nitrogen steel.

Metal injection molding (MIM), as a product of the combination of traditional powder metallurgy and advanced plastic injection molding process, has unique advantages in preparing near-net shape products with three-dimensional complex geometric shapes, uniform structure and high performance. The fine powder particle size is conducive to diffusing nitrogen into the powder particles to increase the nitrogen content, and finally obtain high-nitrogen stainless steel. At present, the research on the production of nickel-free high-nitrogen austenitic stainless steel materials by injection molding technology at home and abroad is still in the preliminary stage.

The experiment uses the 0Cr17Mn11Mo3 stainless steel powder produced by the high-pressure inert gas atomization method. It can be seen that the powder has high sphericity, its average particle size is small (D50=17.4μm), and the bulk density and tap density are high (3.67 and 3.67 and respectively). 4.95g·cm-3), these characteristics of the powder are conducive to improving its fluidity and loading during the injection molding process.

The binder adopts a thermoplastic paraffin-based binder system with good fluidity, and the formula is 65wt%PW+30wt%HDPE+5wt%SA. The stainless steel powder was mixed with an appropriate amount of binder to make a feed. The powder loading was 64%, and then the rheological properties of the feed were tested. The injection process is carried out on the CJ-80E injection molding machine.

The feed is injection molded into a rod blank with a size of 8mm×70mm, and then a pre-sintered blank with a certain strength is obtained after solvent debinding, thermal debinding and pre-sintering at 800°C. After the pre-sintered compacts were sintered in high-purity nitrogen at different pressures at a temperature range of 1200 to 1350°C for 1 to 2.5 hours, the density of the sintered body was measured by Archimedes method (drainage method), and inert gas pulse-infrared -Thermal conductivity method (QB-Q-02-1997) measures the nitrogen content of the sintered body.

The sintered body is machined into a standard tensile bar of 5mm×60mm according to the national standard, and after 1150℃×90min solution treatment and water quenching, its tensile mechanical properties (such as yield and tensile strength) are measured with the Instron universal testing machine And elongation, etc.), and compare the performance with the nitrogen-free samples prepared by the same sintering process in argon atmosphere.

1) 64vol% gas atomized 0Cr17Mn11Mo3 powder mixed with a binder with a composition of 65wt%PW+30wt%HDPE+5wt%SA has good rheological properties; the optimal injection process parameters of the feed are The injection pressure is 60~85MPa, and the corresponding injection temperature is 160~170℃;

Increasing the sintering temperature is beneficial to increase the density of the sintered body, but is not good for obtaining a high nitrogen content. Increasing the pressure of the sintering nitrogen atmosphere can obtain a higher nitrogen content, but is not good for increasing the density of the sintered body. Experiments show that the best sintering process is sintering at 1300°C for 2 hours under a nitrogen pressure of 0.1 MPa. At this time, the relative density of the sintered body can reach 99%, and the nitrogen content can reach 0.78%. After the sintered body is solution treated at 1150℃ and cooled by water quenching, the yield strength, tensile strength, elongation, reduction of area and hardness reach 580MPa, 885MPa, 26.0%, 29.1%, and 222HV10, respectively.