Plazma Nitrürlenmiş Soğuk İş Takım Çeliklerinin Sürtünme-Aşınma Karakteristikleri

D2, D6 ve Calmax soğuk iş takım çelikleri vakum altında sertleştirildi, taşlandı ve 75% N2 + 25% H2 atmosferinde 450 °C sıcaklıkta ve 2 mbar basınç altında 12 saat süre ile plazma nitrürlendi. Numuneler yüzey pürüzlülüğü, mikroyapı, mikrosertlik ve aşınma-sürtünme testleri ile karakterize edildi. Aşınma-sürtünme karakteristiklerinin belirlenmesinde WC-Co bilyasına sahip ball-on-disk sürtünme-aşınma sistemi kullanıldı. Plasma nitrürleme numunelerin yüzey pürüzlülüğünü arttırmıştır. Ancak, bu etki temel malzemenin daha yüksek yüzey pürüzlülüğü ile azalmıştır. Plazma nitrürlenmiş numunelerin yüzeylerinde Fe4N (γ'), Fe3N (ε) ve CrN fazları elde edilmiştir. Plazma nitürlenmiş D2 ve D6 çeliklerinde daha yüksek yüzey sertlikleri elde edilmiştir. Ball-on-disk sürtünme-aşınma sisteminde numunelerde önemli bir kütle kaybı meydana gelmemiştir. Bu yüzden, numunelerdeki aşınma taramalı elektron mikroskobu (SEM) ile karakterize edilmiştir. Plazma nitrürleme çeliklerin aşınma dayanımını iyileştirmesinin yanında sürtünme katsayısını da düşürmüştür. Plazma nitürlenmiş D2 çeliği en yüksek aşınma dayanımına sahip olurken, plazma nitrülenmiş Calmax çeliği ise en düşük aşınma dayanımı sergilemiştir. Ancak, plazma nitrürlenmiş Calmax çeliği diğer plazma nitrülenmiş çeliklere nazaran nispeten daha düşük sürtünme katsayısına sahip olmuştur. Genel olarak, aşınma ürünü kırılgan nitrür parçacıkları sürtünme katsayısını belirleyici rol oynamıştır.

Anahtar Kelimeler:

Soğuk iş takım çelikleri, Plazma nitrürleme, Mikrosertlik, Sürtünme katsayısı, Aşınma yüzeyi karakterizasyonu

FRICTION-WEAR CHARACTERISTICS OF PLASMA NITRIDED COLD WORK TOOL STEELS

D2, D6 and Calmax cold work tool steels were heat treated in vacuum hardening, made grinding and plasma nitrided in 75% N2 + 25% H2 atmosphere at 450 °C for 12 h at 2 mbar. Characterization of samples has been carried out by means of surface roughness, microstructure, microhardness and wear-friction. Wear-friction characteristics of samples have been investigated using a ball-on-disc tribosystem with a WC-Co ball. Plasma nitriding increased the surface roughness of samples. However, this effect was decreased with higher surface roughness of base metal. Fe4N (γ'), Fe3N (ε) and CrN phases were obtained on the surfaces of samples. Higher surface hardness was obtained in the plasma nitrided D2 and D6 steels. No significant mass loss was observed in the ball on-disc tribosystem. So, the wear of samples was characterized with the worn surfaces using a scanning electron microscope (SEM). Plasma nitriding improved the wear resistance and decreased the friction coefficient of steels. Plasma nitrided D2 steel showed the highest wear resistance, whereas plasma nitrided Calmax steel exhibited the lowest wear resistance. However, plasma nitrided Calmax tool steel had relatively lower friction coefficient than the other plasma nitrided steels. In general, brittle layer fractures have determined the friction coefficient.

Keywords:

Cold work tool steels, Plasma nitriding, Microhardness, Friction coefficient, Wear surface characterization,

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