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18CrNiMo7-6 çeliğine uygulanan farklı yüzey sertleştirme yöntemlerinin tribolojik davranışlarının incelenmesi

Investigation of the tribological behavior of different surface hardening methods applied to 18CrNiMo7-6 steel

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  1. Tez No: 1018245
  2. Yazar: MUZAFFER KAŞİFOĞLU
  3. Danışmanlar: PROF. DR. TUĞRUL ÇETİNKAYA
  4. Tez Türü: Yüksek Lisans
  5. Konular: Metalurji Mühendisliği, Metallurgical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2026
  8. Dil: Türkçe
  9. Üniversite: Sakarya Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: Belirtilmemiş.

Özet

Bu çalışmada 18CrNiMo7-6 sementasyon çeliğine yüzey sertleştirme yöntemlerinden olan karbürleme, karbonitrürleme ve nitrürleme gibi üç farklı sertleştirme prosesi uygulanacaktır. Her bir proses kendi optimum sıcaklıkları altında yapılmış olup; karbürleme 2 saat, karbonitrürleme 2 saat ve nitrürleme işlemi 6 saat olarak uygulanmıştır. Nitrürleme prosesinin 6 saat olarak seçilme sebebi diğer işlemlere göre daha düşük sıcaklıklar altında gerçekleşmesidir. Deneysel çalışmalar için 18CrNiMo7-6 çeliğinden ⌀50 × 100mm numuneler hazırlanmıştır. Hazırlanan numuneler küçük eş parçalara kesilerek zımparalama işlemlerine tabi tutulmuştur. Sonrasında yüzeydeki çiziklerin giderilmesi için elmas suyu solüsyonu kullanılarak parlatılmıştır. Ayrıca optik mikroskop incelemelerinde temiz bir görüntü elde etmek için nital dağlama yapılmıştır. Isıl işlem ve deneysel testler için numune hazırlığı tamamlanan çelik parçaların optik mikroskop altında mikroyapı analizleri yapılmıştır. Sertlik ölçme cihazı ile numunelerin yüzey sertlikleri Rockwell cinsinden karbürleme için 55 HRC, karbonitrürleme için 57 HRC ve nitrürleme işleminde 59,7 HRC olarak ölçülmüştür. Çekirdek sertlik değerleri Vickers cinsinden karbürleme için 453HV, karbonitrürleme için 438HV ve nitrürleme yapılan numunenin çekirdek sertliği 222 HV olarak ölçülmüştür. Mikrosertlik testlerine tabi tutulan numunelerin difüzyon derinliğine göre sertlik değerleri ölçülmüştür. Taramalı elektron mikroskobu (SEM) analizi ile farklı büyütmeler altında görüntüler elde edilmiştir. Üç nokta tayin edilerek yüzey, difüzyon geçişi ve çekirdek bölgelerinde EDS analizleri yapılmıştır. Numunelerin aşınma testleri CSM Instruments cihazında ileri geri salınım (Reciprocaating) hareketi yapan aşınma cihazında gerçekleştirilmiştir. Testler oda sıcaklığında 5 N yük altında, 500 m mesafede 10 cm/sn kayma hızında gerçekleştirilmiştir. Her numune için test süresi 1 saat sürmüş olup, iki kez tekrarlanmıştır. Aşındırıcı uç olarak 10 mm çapında Alümina bilya kullanılmıştır. Aşınma testi sonucunda oluşan yüzeyler SEM ve EDS analizleriyle aşınma türleri incelenmiştir. Sürtünme katsayısı 0,50-0,55 aralığında ve en düşük sürtünme katsayısına sahip olan numunenin nitrürlemeye ait olduğu belirlenmiştir. Karbürlenmiş numunenin sürtünme katsayısı 0,79-0,83 aralığında ve karbonitrürlenmiş numunenin 0,81-0,84 aralığında olduğu belirlenmiştir. Bu verilere göre sürtünme katsayısıları en yüksek olan numuneler karbürlenmiş ve karbonitrürlenmiş parçalara ait olup yakın değerlere sahiptir. Aşınma oranı sonuçları ise karbürlemenin 5,95 × 10-6 mm3/Nm, karbonitrürlemenin 5,82 × 10-6 mm3/Nm ve nitrürlemenin 5,8 × 10-6 mm3/Nm olarak belirlenmiştir.

Özet (Çeviri)

This study presents a comprehensive comparative investigation of three thermochemical surface hardening treatments carburizing, carbonitriding and nitriding applied to 18CrNiMo7-6 case hardening steel. The primary objective was to evaluate the influence of these treatments on microstructural evolution, hardness distribution, elemental diffusion characteristics and tribological performance. Surface engineering processes such as carburizing, carbonitriding and nitriding are extensively employed in mechanical components subjected to severe contact stresses, including gears and transmission elements, where high surface hardness combined with a tough core is required. Therefore, understanding the differences in performance induced by these treatments is of considerable industrial and scientific importance. The treatments were performed under their respective optimized conditions. Carburizing and carbonitriding processes were conducted for 2 hours, while nitriding was applied for 6 hours. The extended duration selected for nitriding was associated with its relatively lower processing temperature compared to the other two thermochemical methods, thereby necessitating a longer diffusion period to achieve a sufficient nitrogen-enriched surface layer. Cylindrical specimens with dimensions of Ø50 x 100 mm were machined from 18CrNiMo7-6 steel and subsequently sectioned into equal segments for experimental analysis. Prior to microstructural examination, the specimens underwent standard metallographic preparation procedures. Grinding was performed progressively using silicon carbide abrasive papers to eliminate surface irregularities and achieve a flat geometry. This was followed by polishing with a diamond suspension to remove fine scratches and residual deformation layers generated during grinding. Chemical etching using a nital solution was then carried out in order to reveal microstructural features clearly under optical microscopy. Optical microscopy analyses were conducted to charecterize the microstructual features of the hardened layers, including case depth formation, diffusion zones and core microstructure. Distinct morphological differences were observed among the treated samples, reflecting the diffusion mechanisms and phase transformations specific to each thermochemical process. Ther carburized and carbonitrided specimens exhibited carbon-entiched surface layers resulting from high-temperature austenitization followed by quenching, whereas the nitrided specimen showed the formation of a nitrogen-based compound layer and a diffusion zone without undergoing bulk phase transformation. Surface hardness measurement were performed using the Rockwell hardness (HRC) method. The nitrided sample exhibited the highest surface hardness value of 59.7 HRC. The carbonitrided and carburized samples showed surface hardness values of 57 HRC and 22 HRC, respectively. These results indicate that nitriding effectively enhances surface hardness due to the formation of hard nitride phases within the compound layer. Core hardness measurements were carried out using the Vickers hardness (HV) method to evaluate the mechanical properties of the substrate region. The carburized specimen (2h) exhibited a core hardness of 453 HV, while the carbonitrided specimen (2h) showed 438 HV. In contrast, the nitrided specimen (6h) demonstrated a significantly lower core hardness value of 222 HV. This difference can be attrisbuted to the absence of high-temperature austenitization and quenching steps in the nitriding process, which preserves the original core microsturcture without significant martensitic transformation. In order to asses hardness gradients throughout the hardened layers, microhardness measurements were performed as a function of diffusion depth. The resulting hardness profiles provided detailed insight into the tickness and effectiveness of the modified surface regions. The carburized and carbonitrided samples exhibited gradual hardness decreases from the surface toward the core, consistent with carbon diffusion-controlled case formation. The nitrided sample showed a steep hardness gradient associated with the compound layer and diffusion zone structure. Scanning Electron Microscopy (SEM) was employed to further examine microstructural characteristics and surface morpohologies at higher magnifications. SEM analyses revealed the distinct layered structures produced by each treatment. Additionally, Energy Dispersive X-Ray Spectrocopy (EDS) was conducted at three representative regions surface, diffusion transition zone and core to determine elemental distribution patterns. The EDS results confirmed carbon enrichment in carburized and carbonitrided samples and nitrogen processes. Tribological behavior was evaluated using a reciprocating wear testing device (CSM Instruments) under dry sliding conditions. Wear tests were performed at room temperature under a constant normal load of 5 N, with a sliding speed of 10 cm/s and a total sliding distance of 500 m. Each experiment lasted 1 hour and was repeated twice to ensure resproducibility. A 10 mm diameter alumina (Al2O3) ball was used as the counterface material to maintain consistent contact conditions. The friction coefficient measurements demonstrated significant differences among the treated specimens. The nitrided sample exhibited the lowest coefficient of friction, ranging between 0.50 and 0.55. In coantrast, the carburized and carbonitrided samples showed higher and relatively similar friction coefficient values, ranging between 0.79-.083 and 0.81-0.84, respectively. The reduced friction behavior of the nitrided specimen can be attributed to the presence of a hard and stable compound layer that minimizes adhesive interactions during sliding contact. Wear rates were calculated based on volumetric material loss and normalized with respect to applied load and sliding distance. The carburized sample exhibited a wear rate of 5.95 × 10-6 mm3/Nm, while the carbonitrided and nitrided samples showed wear rates of 5.82 × 10-6 mm3/Nm and 5.80 × 10-6 mm3/Nm respectively. Although the wear rate differences were relatively small, the nitrided specimen demonstrated the most favorable overall tribological performance when considering both friction coefficient and wear resistance together. Post-wear surface examinations conducted by SEM and EDS analyses revealed that abrasive wear was the dominant wear mechanism for all treated samples. However, varying degrees of adhesive wear and localized plastic deformation were also observed depending on the surface condition and hardness level of the treated layers. The nitrided surface exhibited comparatively shallower wear grooves and more controlled damage morphology, wehereas the carburized and carbonitrided samples showed more pronounced sliding marks. In conclusion, the comparative evaluation of carburizing, carbonitriding and nitriding treatments applied to 18CrNiMo4-6 steel demonstrated that each process significantly alters surface characteristics and tribological performance. Among the investigated treatments, nitriding provided the highest surface hardness, the lowest friction coefficient and competitive wear resistance under the selected testing conditions. These findings contribute to a better understanding of the relationship between thermochemical surface modification, microstructural development, hardness distribution, and wear behavior in case hardening steels, offering valuable insights for industrial applications requiring enhanced surface durability.

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