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Wear-Resistant Peripheral Coatings
Nitriding and Nitrocarburizing

Nitriding / nitrocarburizing is a process used mainly for enhancing the performance of piston rings in gasoline engines. The nitrided case with the resulting significant increase in hardness has the effect above all of reducing abrasive and adhesive wear and improving scuff behaviour.

Nitriding / nitrocarburizing of piston rings is a thermal treatment in which nitrogen and carbon are absorbed at a temperature ranging between 450 - 585°C to form a multi-phase nitrided layer. This hardened layer is created by phase transformation of the near-surface regions of the material so that a so-called "case" is produced.

The choice of piston ring materials for nitriding is instrumental in determining the formation of the case and hence its characteristics. For use as nitrided compression rings and oil control rings (steel rails, 2-piece oil rings) high-chromium alloyed steels (GOE 65C, GOE 66) are mainly chosen. The nitriding produces a so-called "diffusion layer" which has a surface hardness > 900 HV 0,1 and a nitrided case thickness of NHT 700 HV0.5 = 0.05 to 0.07 mm.
Nitriding / nitrocarburizing is also carried out on cast iron piston rings. In such instances the nitrided case is characterized by the formation of what is termed a "white layer" consisting of iron nitrides and carbonitrides, with a diffusion layer beneath. With its high hardness of 600 - 800 HV 0.05 the 2 - 10 µm thick white layer acts as the wear-resisting layer. The diffusion layer has the effect of improving ring performance, especially in the case of heat-treated cast irons, due to the significant hardness increase.


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