Volume 1, Issue 1
Influence of Hydrogen, Carbon Dioxide, and Alloy Content on Pore Formation in the White Layer of Alloyed Steels
Gaseous nitriding and nitrocarburizing of steel have found new applications in the area of ultra-precision machining of steel with mono-crystalline diamond tools. Steel is normally not diamond machinable because serious chemical reactive wear of diamond tool occurs. The wear can be reduced significantly if the machining is carried out in the white layer of nitrided or nitrocarburized steel, since the chemical reactivity of the white layer is lower compared to the steel substrate. For this application a thick white layer with low porosity is desirable. To produce such white layers the nitriding and nitrocarburizing processes should be specifically adapted. In the present work the influence of hydrogen and carbon dioxide in the process atmosphere on pore formation in the white layer during nitriding and nitrocarburizing was investigated experimentally. The experiments of gaseous nitriding and nitrocarburizing were carried out with varied process atmosphere on alloy steel grades. The generated surface layers were characterized by glow discharge optical emission spectroscopy (GDOS) and by optical microscopy. The results showed that low partial pressure of hydrogen in the nitriding atmosphere and strong nitride-forming elements in the steel are positive in suppressing pore formation, while carbon dioxide promoted pore formation in nitrocarburized white layer.