Abstract
Low-temperature active-screen plasma nitriding (ASPN) was applied in this study to improve the bending rigidity and corrosion resistance of a small-diameter thin pipe composed of austenitic stainless steel (SUS 304). The inner and outer diameters of the pipe were ϕ0.3 and ϕ0.4 mm, respectively, and the pipe length was 50 mm. The jig temperature was measured using a thermocouple and was adopted as the nitriding temperature because measuring the temperature of a small-diameter pipe is difficult. The nitriding temperature was varied from 578 to 638 K to investigate the effect of temperature on the nitriding layer and mechanical property. The nitriding layer thickness increased with an increase in nitriding temperature, reaching 15 μm at 638 K. The existence of expanded austenite (S phase) in this nitriding layer was revealed using the X-ray diffraction pattern. Moreover, the surface hardness increased with the nitriding temperature and took a maximum value of 1100 HV above 598 K. The bending load increased with an increase in the nitriding temperature in relation to the thicker nitriding layer and increased surface hardness. The nitrided samples did not corrode near the center, and corrosion was noted only near the tip at high nitriding temperatures of 618 and 638 K in a salt spray test. These results indicated that the bending rigidity of the small-diameter thin pipe composed of austenitic stainless steel was successfully improved using low-temperature ASPN while ensuring corrosion resistance.
Highlights
Austenitic stainless steels are used in a wide range of items such as household products, construction materials and automobile parts as well as in applications related to power generation and chemical and food industries owing to its high functionality, excellent corrosion resistance, ductility and toughness
It is widely accepted that nitriding treatment is useful for austenitic stainless steel because a nitriding layer with high hardness is formed by the diffusion of nitrogen from the surface
The hardness increased with the nitriding temperature and was saturated at 1100 HV owing surface hardness was higher in the nitrided samples than in the untreated sample
Summary
Austenitic stainless steels are used in a wide range of items such as household products, construction materials and automobile parts as well as in applications related to power generation and chemical and food industries owing to its high functionality, excellent corrosion resistance, ductility and toughness. It is widely accepted that nitriding treatment is useful for austenitic stainless steel because a nitriding layer with high hardness is formed by the diffusion of nitrogen from the surface This treatment can significantly reduce the corrosion resistance. N diffused from the surface and Cr contained in the steel combine to precipitate CrN, which reduces the amount of Cr in the solid solution This prevents the passivation film from forming on the surface of the steel, which reduces the corrosion resistance. Low-temperature ASPN is applied to improve the bending rigidity and corrosion resistance of an austenitic stainless-steel pipe with a small diameter. The nitrided sample properties of layer thickness, layer structure, surface hardness, bending load and corrosion resistance are evaluated by varying the changing nitriding temperature
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