Magnetic Properties of Austenitic Stainless Steel 316l and 316lvm after High Temperature Gas Nitriding Treatment

Abstract

Biometallic materials for implant devices not only have to good corrosion resistance but also stable nonmagnetic properties. Various method have been developed for enhanced the corrosion resistance i.e low temperature gas nitriding treatments. Unfortunatelly, low temperature gas nitriding produce weakly ferromagnetic due the presence of expanded austenitic phases. Another treatments methods which is capable for improvement the mechanical properties is high temperature gas nitriding. However, the evaluation of magnetic properties of austenitic stainless steel 316L and 316LVM not yet investigation. The evaluation of magnetic properties of austenitis stainless steel 316L and 316LVM after high temperature gas nitriding treatments have been succesfully done. The magnetic properties are evaluated by vibrating sample magnetometre (VSM) test. The magnetic properties such as magnetic remenance, magnetic saturation and magnetic permeability are improved. As treated 316L and 316LVM have more stable non-magnetic properties and they more safe and compatible for MRI test.Keywords: 316L, 316LVM, high temperature gas nitriding, magnetic properties, MRI 1.Introduction Austenitic stainless steel 316L and 316LVM are ones of metallic biomaterial that widely used as implants and medical devices. The corrosion resistance can be used as the parameter of their biocompatibility [1]. Recently, increasing of corrosion resistance is not only concern for improvement of metallic biomaterial but also stable nonmagnetic properties. The development magnetic resonance imaging (MRI) as clinical imaging tools needs metallic biomaterial that has stable non-magnetic properties. The principal issues for MRI safety and compatibility are magnetically induced displacements and torque, RF heating and image artifact [2]. Implanted and medical devices from austenitic stainless steel meet the MRI safety and compatibility in the up to 1.5T systems. However the developments of new 3.0T MR system cause their MR compatibility being decrease [3,4].Various surface treatments such as gas nitriding, ion implantation, and plasma nitriding have been successfully applied to enhance their corrosion resistance. These treatments usually have been conducted at low temperature or below the austenite temperature. Low temperature nitriding produces thin layer of expanded austenite phase which improve their hardness and corrosion resistant. This expanded austenite phase is ferromagnetic [5,6]. Furthermore this phase cause weakly ferromagnetic properties of nitride austenitic stainless steel.In the previous research, HTGN treatment increase the corrosion resistance of 316L and 316LVM [7,8]. However the effect of HTGN treatments on the magnetic behavior of its materials not yet investigation. This papers deal with the effect of HTGN treatment on the magnetic properties of 316L and 316LVM. 2.Materials and methods Specimens were prepared from 316L and 316LVM plate. The chemical compositions of the specimens are shown in table 1. Specimens were rinsed using ultrasonic clea...