Determination of Synergistic Effects of Austempering + Cryogenic Heat Treatments Applied to R260 Rail Steel

Abstract

In this study, commercially obtained R260 grade rail steels were subjected to cryogenic treatment after isothermal holding at different austempering times. The effects of cryogenic treatment applied to the test specimens for five hours at -70 ℃ after austenitisation at 950 ℃ for five minutes, followed by austempering heat treatment at 350 ℃ in a neutral salt bath with different isothermal holding times (30-180 min) on microstructure, mechanics and crystallography were investigated. When the SEM microstructures of the alloy were examined, it was observed that with increasing isothermal retention times, significant subbainite transformation was completed, and hardnesses increased. While the untreated sample had a hardness value of 269 HV1, the hardness was measured as 607 HV1 with an increase of 226% after 180 min austempering and cryogenic processes. As a result of XRD measurements, it was determined that the amount of intermediate carbides and residual austenite decreased with increasing heat treatment time and improved mechanical properties.

Keywords

• R260 steel
• microstructure
• cryogenic treatment
• austempering
• hardness

R260 Ray Çeliğine Uygulanan Östemperleme+Kriyojenik ısıl işlemlerin Sinerjik Etkilerinin Ortaya Çıkarılması

Abstract

Bu çalışmada, ticari olarak temin edilen R260 kalite ray çelikleri, farklı östemperleme sürelerinde izotermal tutma işleminin ardından kriyojenik işleme tabi tutulmuştur. Deney numunelerine 950 ℃'de beş dakika östenitlemenin ardından -70 ℃'de beş saat uygulanan kriyojenik işlemin, ardından farklı izotermal tutma süreleriyle (30-180 dk) nötr tuz banyosunda 350 ℃'de östemperleme ısıl işleminin mikroyapı, mekanik ve kristalografi üzerindeki etkileri incelenmiştir. Alaşımın SEM mikro yapıları incelendiğinde, artan izotermal tutma süreleri ile önemli subbainit dönüşümünün tamamlandığı ve sertliklerin arttığı gözlenmiştir. İşlem görmemiş numunenin sertlik değeri 269 HV1 iken, sertlik 180 dk östemperleme ve kriyojenik işlemler sonrasında %226 artışla 607 HV1 olarak ölçülmüştür. XRD ölçümleri sonucunda, ara karbür ve kalıntı ostenit miktarının artan ısıl işlem süresiyle azaldığı ve mekanik özelliklerin iyileştiği belirlendi.

Keywords

• R260 çeliği
• mikroyapı
• kriyojenik işlem
• östemperleme
• sertlik

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