During the last years, nano structure Ni- free high nitrogen stainless steels (H) have been extensively investigated due to their excellent properties. Presence of nitrogen in nano structural materials have led to, excellent mechanical properties and high corrosion resistance. Replacement of nickel with nitrogen could eliminate the allergetic effect and make the material suitable as implants, in human body. So far manufacturing these steel have been carried out using different melting methods. Due to limited solubility of nitrogen in liquid phase, researchers have concentrated on solid state methods, like powder metallurgy. The aim of this research work was to synthesis and characterize the properties of nano structureed Ni- free high nitrogen stainless steels of Fe-Cr-Mn-Mo and Fe-Cr-Mn by mechanical alloying under nitrogen atmosphere. In this way, nano structured solid solution H powders (Fe-18Cr-8Mn-4Mo, Fe-18Cr-8Mn-8Mo, Fe-18Cr-8Mn and Fe-18Cr-9Mn) were synthesized and then Fe-Cr-Mn-Mo and Fe-Cr-Mn powders were cold pressed.The cold pressed samples were sintered at 1250°C for 60 minutes and then cooled in a mixture of ice and water. Morphological and structural changes were examined using Scanning Electron Microscopy (SEM). Chemichal composition of products were determined using Elemental Analysis System (EDS) attached to SEM. Nitrogen content was measured using Leco nitrogen analyser. X-Ray Difraction (XRD) was used to determine the phases produced. Hardness and elastic modulus of sintered samples were measured using microhardness and nanoindentation tester. Mechanical alloying results showed that nitrogen solubility in powders (Fe-18Cr-8Mn-4Mo, Fe-18Cr-8Mn-8Mo, Fe-18Cr-8Mn and Fe-18Cr-9Mn) significantly increases by increasesing the milling time as 0.76, 1.1, 0.9 and 0.8 wt%, respectively. Microhardness of powders increased by increasesing the nitrogen solubility. Hardness of bulks was evaluated as 422, 604, 517 and 434 HV, respectively. Also elastic modulus of four solid solution was determined as 272, 390, 344 and 326 GPa, respectively. Density and grain size measurement after cold pressing and sintering showed that the method used to produce H bulk is successful. Wear properties of samples (wear rate, friction coffecient and wear mechanism) were studied using pin on disk machine applying 800 g load. Wear test results showed that, wear resistance is increased and friction coefficient is decresed due to presence of nitrogen in steel. The dominant wear mechanism was also determined to be adhesive. Also cold pressed and sintered samples were characterized by Tafel polarization. Results showed that the current density of Fe-18Cr-8Mn-4Mo sample is lower than other ones. Key Words: High nitrogen stainless steel, Nano structure, Mechanical alloying, Phase transformation, Wear, Nanoindentation, Corrosion.
