High Temperature Gas Nitriding Treatment of Aisi 430 Using Low and High Purity Nitrogen Gas

Abstract

IntroductionOnes of materials which widely use in industry is stainless steel due high resistance to corrosion and appropriate mechanical properties. Stainless steel is ferrous alloys which contains at least 11% chromium. Depending on the phase in the microstructure, stainless steel can be grouped as austenitic, ferritic or martensitic.AISI 430 is the most widely produced ferritic stainless steel. AISI 430 is cheapest and fairly good corrosion resistance in a variety of environments. However in certain uses such as high corrosive environments, the mechanical properties and corrosion resistance are less fulfilling. In order to improve its properties, this stainless steel can be treated by high temperature gas nitriding (HTGN) treatment.HTGN treatment is able to enhance the properties of stainless steel such as strength, hardness, wear resistance and corrosion resistance [1]. These treatments not only improve the properties, but also it can modify the initial phase [2,3]. The research of HTGN treatment for AISI 430 has been conducted by many investigators [4,5]. Depending on the variable process, the micro structure of ferritic stainless steel AISI 430 can be modified into martensitic which hard or austenitic stainless steel which soft and non-magnetic.The basic process of HTGN is by exposing the stainless steel in high purity nitrogen gas atmosphere at temperature 1050 o C ± 50 o C within certain time prior quenching [6]. During the process, nitrogen gas become dissociate into two nitrogen atom prior diffuses into stainless steel.The effects HTGN treatment on the properties of stainless steel depends on the amounts of nitrogen atom which diffuse into stainless steel. The resulting effects depend on the process variable such as chemical composition of stainless steel, temperature process, nitrogen gas pressure and duration of holding time [1,6,7].Generally, the increasing strength obtained after treatment is not higher than increasing of its hardness due to the grain coarsening. Holding time for long duration produce excessive grain size. To overcome grain coarsening, some refinement methods have been developed [8]. The refinement methods promise the prospective application of HTGN treatment in industry.Despite HTGN treatment use high purity nitrogen gas, there are several grade of purity on the market. The highest purity and the most expensive grade is scientific or research grade which has minimum purity 99.9999%. The other grades are ultra high purity (UHP) grade (99.999%), zero grade (99.998%), specialized zero grade (99.998%), oxygen free grade (99.998%) and industrial/welding grade (99.5%). The grade purity of nitrogen gas depends on the percentage of moisture, oxygen, hydrocarbon, and CO x .The most widely available grade is industrial or welding grade. The industrial/welding grade promises in cost production reduction for HTGN treatment. However, the effect of nitrogen gas purity on the effect of HTGN treatment ABSTRACTThe properties of stainless steels can be improved by high temperature gas nitriding (HT...