Comparison of ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel

This paper compares the ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel carried out to improve the surface corrosion resistance. The gas composition for plasma nitriding was 85% N-2-15% H-2 and that for plasma nitrocarburizing was 85% N-2-12% H-2-3% CO2. Both treatments were performed for 5 h, for different process temperatures of 570 and 620 degrees C for ferritic and austenitic plasma treatment, respectively. Optical microscopy, X-ray diffraction and potentiodynamic polarization technique in 3.5% NaCl solution, were used to study the treated surfaces. The results of X-ray analysis revealed that with increasing the treatment temperature from 570 to 620 degrees C for both treatments, the amount of epsilon phase decreased and gamma' phase increased. Nitrocarburizing treatment resulted in formation of a more amount of epsilon phase with respect to nitriding treatment. However, the highest amount of epsilon phase was observed in the ferritic nitrocarburized sample at 570 degrees C. The sample nitrided at 620 degrees C exhibited the thickest layer. The potentiodynamic polarization results revealed that after plasma nitriding and nitrocarburizing at 570 degrees C, corrosion potential increased with respect to the untreated sample due to the noble nitride and carbonitride phases formed on the surface. After increasing the treatment temperature from 570 to 620 degrees C, corrosion potential decreased due to the less epsilon phase development in the compound layer and more porous compound layer formed at 620 degrees C with respect to the treated samples at 570 degrees C. (C) 2010 Elsevier Ltd. All rights reserved.