Arakritik sıcaklıklardan östemperlenmiş ferritik küresel grafitli dökme demirde yeni ferrit oluşumu

Bu çalışmada, ferritik küresel grafitli dökme demirlerde, arakritik sıcaklıklardan östemperleme sırasında oluşan mikroyapılar incelenmiştir. Bu amaçla ferritik küresel grafitli dökme demir, farklı arakritik sıcaklıklarda tavlanmış (kismi östenitlenmiş) ve ardından 365 °C’deki tuz banyosunda farklı sürelerde östemperlenmiştir. Farklı arakritik sıcaklıklardan östemperlemenin ardından, östemperleme süresine bağlı olarak östenitten dönüşüm ürünlerinin miktarlarındaki farklılık, mikroyapı değişim grafikleri ile kantitatif olarak gösterilmiştir. Metalografik ölçümler, özellikle belirli bir arakritik tavlama sıcaklığında oluşan orijinal östenitten östemperleme sırasında dönüşen fazların toplam hacim oranının yaklaşık olarak bütün östemperleme süreleri için orijinal östenitin hacim oranına eşit olduğunu göstermiştir. Arakritik tavlama sıcaklıklarından östemperlenmiş numunelerin her birinde ötektoid öncesi ferrit ve beynitik ferrite ilaveten yeni ferrit (epitaksiyel ferrit) açığa çıkmıştır. Artan arakritik sıcaklığı ile yeni ferrit hacim oranıda artmıştır. Ancak, yeni ferritin arakritik östenitleme sıcaklığındaki orijinal östenitten dönüşüm yüzdesi, azalan arakritik östenitleme sıcaklığı ile artmıştır. Östemperleme süresindeki artış yeni ferrit, beynitik ferrit ve yüksek karbonlu östenitin martensitle yer değişimine neden olmuştur. Numunelerin bazıları karşılaştırma amacıyla 900°C’ den geleneksel olarak östemperlenmiştir. Geleneksel östemperlenmiş numunelerde yeni ferritin varlığına rastlanmamıştır.

The new ferrite formation in ferritic ductile iron austempered from intercritical temperature ranges

In the present study, during austempering, microstructure formations in the ferritic ductile irons austempered from intercritical temperature ranges have been investigated. For this purpose a ferritic ductile iron was intercritically austenitised (partially austenitised) at intercritical temperature ranges and then austempered in a salt bath held at austempering temperature of 365°C for various times. Following austempering from various intercritical annealing temperatures, microstructure variation graphics were created to illustrate the transformation of products quantitatively as a function of austempering time. Metallographic measurements showed that during austempering, the total volume fraction of the transformed phases from the original austenite formed at intercritical austenitization temperature is approximately equal to the volume fraction of original austenite for all austempering times. Within each of the austempered specimen from intercritical temperature ranges, in addition to proeutectoid and bainitic ferrite, the new ferrite (epitaxial ferrite) introduced into the structure. The new ferrite volume fraction increased with increasing intercritical austenitizing temperature but transforming percentage of new ferrite from the original austenite present at intercritical austenitizing temperature increased with decreasing intercritical austenitizing temperature. Increasing the austempering time causes new ferrite, bainitic ferrite and high C austenite to displace martensite. Some specimens were also conventionally austempered from 900°C for comparison. The new ferrite was absent in the conventionally austempered samples.

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