Papers by Keyword: Expanded Austenite

Abstract: Stainless steel is widely used where corrosion resistance is importance. Stainless steel has its origin excellent corrosion resistance due to the nature of presence Cr as alloying element to form stable passive layer that protects the steel. Due to its inherent austenitic structure, this material has relatively low hardness as well as poor wear resistance which hinders a wider applicability of the material and may cause problems in existing applications. The gaseous thermochemical treatments to improve surface properties of material are typically carried out in carbon and/or nitrogen bearing gases and usually associated with temperature above 500 °C.

Abstract: The present paper addresses the influence of chemical induced stresses on diffusion in interstitial systems. This is exemplified by simulations of carbon diffusion in austenite at high temperatures and it is shown that old well established literature data is flawed by the occurrence of composition induced stress. For the technological relevant system of expanded austenite the diffusion can be dramatically affected by composition induced stress.

Abstract: This paper presents the results of investigations on gas nitrided austenitic stainless steel. The treatment was conducted at various temperatures (400-515°C), gas compositions of atmospheres used (20-100% NH3) and times (0.5-12h). The layers were investigated by X-ray diffraction, Light and Electron Microscopy and Glow Discharge Optical Spectrometry. The kinetics of layer growth has been analysed in terms of the process parameters and compared with the data presented for plasma treated steel. The specific nitrogen profiles of nitrided layers are discussed in the context of the layers’ microstructure and phase composition.

Abstract: This paper describes the results of four thermochemical surface treatments of austenitic stainless steels carried out at 450oC in a fluidised bed furnace and they are nitriding, carburizing and the newly developed hybrid process involving the simultaneous and sequential incorporation of nitrogen and carbon to form a dual layer structure in order to achieve much enhanced surface hardness and wear resistance without compromising the corrosion resistance of the steel. In all these treatments there formed alloyed layers with a common feature of being precipitation-free and supersaturated with nitrogen, or carbon or both in the austenite lattice which is known as S Phase or expanded austenite. However the layer thickness was not uniform in any of these treatments and an effective layer was produced after 8h treatment duration. The nitriding treatment produced thicker and harder layer compared to other treatments; the maximum hardness was over 1500 Hv for nitriding and the minimum hardness of 500 Hv for carburizing treatment. The nitriding treatment sample gave high wear resistance which corresponded to high hardness values.

Abstract: The effect of thermal annealing temperature on the microstructure and phase composition of nitrided layer formed on AISI 304 stainless steel by plasma nitriding were investigated. The phase composition and structure of the nitrided have been analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD analysis of nitrided samples at 420oC showed the presence of γN phase in the nitrided layer. The temperature at which the nitrogen expanded austenite started to decompose was about 450oC. Above this critical temperature the γN phase transformed into a mixture of CrN and α-phase. The surface hardness of nitrided layer also slightly changed with annealing temperature.

Abstract: The novel low temperature plasma alloying technique that simultaneously introduces both nitrogen and carbon into the surface of austenitic stainless steel has been used in the past to create a hybrid N-C S-Phase. This S-Phase layer boasts of high hardness and wear resistance without any detriment to corrosion resistance. In this study, the afore mentioned hybrid N-C S-Phase was successfully implemented in the surface of two medical grade austenitic stainless steels: ASTM F138 and F1586. At an optimum process temperature of 430°C a very hard, 20μm precipitate-free S-Phase layer was created. Anodic Polarization tests in Ringer’s solution showed that the corrosion resistance of this layer was similar to that of the untreated alloys. Both dry-wear and corrosion-wear (Ringer’s) behaviour of the surface treated alloys showed an improvement of more than 350% and 40% respectively when compared to the untreated material.

Abstract: The present paper reviews the scientific development of our understanding of S-Phase. It is now known that S-Phase formation is an example of para equilibrium phenomena. A necessary but not sufficient condition for S-Phase formation is the presence of an fcc structure at least in part with structure in the starting alloy. An essential requirement is for a nitride forming element to be present particularly Cr. After surface engineering with carbon, nitrogen or carbon and nitrogen to generate supersaturated solid solutions, the various tribological, corrosion, mechanical and microstructural studies are reviewed for the various alloy systems. The current industrial status of S-Phase technology on an international basis is examined and the potential for its acceptance in china is discussed.

Abstract: The applicability of the Boltzmann-Matano method for evaluation of a diffusion coefficient and its concentration dependency by line profile analysis is tested on three different (model) systems. All systems involve interstitial diffusion. It is shown that the occurrence of trapping corrupts the applicability of the Boltzmann-Matano method.