Surface hardening treatments such as nitriding can improve the mechanical properties to extend the applicability of austenitic stainless steel (ASS) fine and precise machining. Improving these characteristics without changing the design and material is highly advantageous, particularly for medical and surgical instruments. In particular, an improved bending rigidity of medical injection needles is desirable because a small needle diameter reduces invasiveness. However, no method has yet been reported for improving the bending strength of ASS without reducing its corrosion resistance. Moreover, nitriding has not been applied to thin pipes with a small diameter such as medical injection needles.
In the present study, low-temperature active-screen plasma nitriding (ASPN) and active-screen plasma carburizing (ASPC) are applied to improve the bending rigidity and corrosion resistance of an ASS pipe with a small diameter. The inner and outer diameters of the pipe were 0.3 and 0.4 mm, respectively, and the pipe length was 50 mm. ASPN and ASPC were conducted for 4 h at 578–638 K at 200 Pa.
The nitriding and carburizing layer thicknesses increased monotonically with the processing temperature. These results were consistent with the nitrogen and carbon concentration distributions measured using an electron probe micro analyzer. The existence of expanded austenite (S phase) was revealed using the X-ray diffraction patterns for ASPN and ASPC. Nanoindentation confirmed that the Young's modulus was improved in the nitrided region of ASPN. Bending strength increased with the nitriding temperature because the nitriding layer became thicker and the surface hardness increased. In the corrosion test, the nitrided and carburized samples did not corrode at low temperatures. These results indicated that low-temperature ASPN and ASPC achieved flexural rigidity and corrosion resistance in the small-diameter thin pipe comprising ASS.