Östemperleme Sıcaklığının Karbür İçermeyen Beynitik Çelik Dökümün Aşınma Davranışı Üzerine Etkisi

Abstract

The objective of this study is to elucidate how the microstructural features and wear performance of carbide-free bainitic cast steel (CFB cast steel) are influenced by the applied austempering temperature. In this scope, the specimens were first subjected to an austenitizing treatment at 900°C for 60 min. Subsequently, for the austempering stage, the samples were held isothermally for 90 min in molten salt baths prepared at four different temperatures: 275°C, 300 °C, 325°C, and 350 °C. Microstructural characterization was carried out using optical microscopy. The volume fraction of retained austenite in the material was determined by X-ray diffraction (XRD). To evaluate the wear behavior, wear tests were performed in accordance with the procedures specified in ASTM G133-22, and the volume loss on the worn surfaces after testing was measured with high precision using an optical profilometer. The results obtained have shown that the austempering temperature plays a determinative role in the microstructure and mechanical properties. The finest microstructure was achieved in the specimen austempered at 275°C for 90 min, whereas the coarsest microstructure was obtained after treatment at 350°C. These microstructural differences were also reflected in the hardness values. The highest hardness (54.9 HRC) was measured in the sample treated at 275 °C, while the lowest hardness (48.2 HRC) was recorded in the specimen austempered at 350 °C. The retained austenite volume fraction exhibited a directly proportional variation with austempering temperature; the lowest fraction (17.5%) was found in the samples treated at 275 °C, and the highest fraction (24.2%) was measured in those processed at 350 °C. The wear test results revealed that the heat treatment significantly improved the wear resistance. The as-cast showed the poorest performance, with a volume loss of 0.3304 mm³ and a wear rate of 1.32 × 10⁻⁴ mm³/N·m. Among the austempered samples, the best wear resistance was observed in the specimen treated at 275 °C. This specimen exhibited the lowest wear rate of 6.68 × 10⁻⁶ mm³/N·m, the minimum volume loss of 0.0167 mm³, and the lowest average coefficient of friction (μ = 0.684). In contrast, the sample austempered at 350 °C displayed the highest wear values, with a wear rate of 1.62 × 10⁻⁵ mm³/N·m, a volume loss of 0.0406 mm³, and an average coefficient of friction of μ = 0.709.

Keywords

• Carbide-free bainitic cast steel
• austempering temperature
• wear behavior
• retained austenite.

Östemperleme Sıcaklığının Karbür İçermeyen Beynitik Çelik Dökümün Aşınma Davranışı Üzerine Etkisi

Öz

Bu araştırmanın amacı, karbürsüz beynitik çelik döküm (KİB-ÇD) malzemenin mikroyapısal özellikleri ve aşınma performansının, uygulanan östemperleme sıcaklığından ne şekilde etkilendiğini ortaya koymaktır. Bu kapsamda, numuneler öncelikle 900°C sıcaklıkta 60 dakika süreyle östenitleme işlemine tabi tutulmuştur. Ardından, östemperleme aşaması için 275°C, 300°C, 325°C ve 350°C olmak üzere dört farklı sıcaklıkta hazırlanan tuz banyolarında, her bir numune 90 dakika boyunca izotermal olarak bekletilmiştir. Mikroyapısal karakterizasyon, optik mikroskop kullanılarak gerçekleştirilmiştir. Malzeme yapısındaki kalıntı östenit fazının hacimsel oranı ise X-ışını kırınımı (XRD) tekniği ile tespit edilmiştir. Aşınma davranışının belirlenmesi amacıyla, ASTM G133-22 standardında belirtilen prosedürlere uygun olarak aşınma testleri yapılmış ve test sonrası yüzeylerde meydana gelen hacim kayıpları optik profilometre ile hassas bir şekilde ölçülmüştür. Elde edilen bulgular, östemperleme sıcaklığının mikroyapı ve mekanik özellikler üzerinde belirleyici bir rol oynadığını göstermiştir. En ince taneli mikroyapı, 275°C'de gerçekleştirilen 90 dakikalık östemperleme işlemi sonucunda elde edilirken, en kaba taneli yapıya 350°C'de yapılan işlemle ulaşılmıştır. Bu mikroyapısal farklılıklar, malzemenin sertlik değerlerine de yansımıştır. En yüksek sertlik değeri (54,9 HRC) 275°C'de işlem gören numunede ölçülürken, en düşük sertlik (48,2 HRC) 350°C'de östemperlenen numunede kaydedilmiştir. Kalıntı östenit hacim oranı da östemperleme sıcaklığı ile doğru orantılı bir değişim sergilemiştir; en düşük oran (%17,5) 275°C'de, en yüksek oran (%24,2) ise 350°C'de işlem görmüş numunelerde tespit edilmiştir. Aşınma testi sonuçları, ısıl işlemin aşınma direncini önemli ölçüde iyileştirdiğini ortaya koymuştur. Döküm halindeki numune, 0,3304 mm³'lük hacim kaybı ve 1,32x10⁻⁴ mm³/N.m'lik aşınma hızı ile en zayıf performansı göstermiştir. Östemperlenmiş numuneler arasında ise en üstün aşınma direnci, 275°C'de işlem gören numunede gözlemlenmiştir. Bu numune, 6,68x10⁻⁶ mm³/N.m ile en düşük aşınma hızına, 0,0167 mm³ ile en az hacim kaybına ve μ=0,684 ile en düşük ortalama sürtünme katsayısına sahip olmuştur. Buna karşılık, 350°C'de östemperlenen numune, 1,62x10⁻⁵ mm³/N.m'lik aşınma hızı, 0,0406 mm³'lük hacim kaybı ve μ=0,709'luk sürtünme katsayısı ile en yüksek aşınma değerlerini sergilemiştir.

Anahtar Kelimeler

• Karbür içermeyen beynitik çelik döküm
• östemperleme sıcaklığı
• aşınma davranışı
• kalıntı östenit.

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