High nitrogen stainless steel has excellent mechanical properties and corrosion resistance. The preparation method of high nitrogen stainless steel is mainly smelting method and powder metallurgy method, which powder metallurgy method of high nitrogen stainless steel including: high pressure melting – gas atomization method, this method has the disadvantage of high pressure equipment, equipment investment, complex operation; The disadvantage of solid powder nitriding method is that the oxygen content of powder increases (150-500)×10-4% after nitriding. Stainless steel powder containing nitrogen was prepared by mechanical alloying method in University of Science and Technology Beijing. After 15h ball milling, the content of nitrogen in the powder was 0.37%(mass fraction).
Fully dense high nitrogen stainless steel can be obtained by hot isostatic forming technique, which can be used by U.S. crucible company to make stainless steel products with high strength and corrosion resistance. In THE North SEA FIELD PROJECT, THE Swedish P/M COMPANY USES HIP TECHNOLOGY TO produce a variety of undersea and surface platform components made of powder stainless steel. Specific components are all kinds of flanges, joints, valve bodies, pipelines, etc., the production of the three pipe each piece of weight up to 155kg, valve weight up to 2T. The delivery time is shortened from 8 weeks to 4 weeks, the manufacturing cost of the whole finished product is reduced by 20%, the service life is longer, and the reliability is higher. There are also several companies in the United States that use the hot isostatic pressing process to produce high-performance petrochemical and gas pipeline components on a large scale. It can be seen that the preparation of high nitrogen stainless steel products by hot isostatic pressing technology has broad prospects.
In this paper, a new preparation process of high nitrogen stainless steel powder, namely, high nitrogen nickel-free stainless steel powder and high nitrogen super austenitic stainless steel powder, was explored.
Nitrogen containing stainless steel powder was prepared by atmospheric melting and nitrogen atomization with nitrogen atomization pressure (20 ~ 50)×105Pa. Due to the use of vacuum smelting and high-pressure nitrogen atomization, the powder has low oxygen content and good fluidity. The powder morphology is mainly spherical, some are irregular, and a small amount of small particles are attached to the surface of large particles. In order to avoid nitrogen spillover, two measures can be taken in smelting: one is by pressurized smelting, the other is by rapid condensation. By chemical analysis, the nitrogen content reached 0.736%. XRD analysis shows that the microstructure of high nitrogen steel powder is single-phase austenite with fine grains. If ferrite is present, all ferrite can be transformed into austenite after hot isostatic pressing.
It can be seen that it is feasible to prepare high nitrogen nickel-free stainless steel and super austenitic high nitrogen stainless steel by atmospheric pressure melting and high pressure nitrogen atomization.