Effect of Intercritical Austenitization Temperatures in Normalizing Heat Treatment on Microstructure and Mechanical Properties of Ductile Iron

Abstract

In this investigation, the effect of normalizing heat treatments from annealing at intercritical austenitization temperatures on the microstructural evolution and mechanical properties of ductile iron was examined. Tensile specimens were prepared by turning from Y block castings made from unalloyed ductile iron. The specimens were air-cooled from different intercritical austenitization temperatures at (770°C to 820°C) for 60 minutes. Microstructural characterization was performed by optical microscope and scanning electron microscope (SEM). Mechanical properties were determined by tensile and hardness tests. Ductile iron containing different pearlite volume fractions (8 % to 75 %) can be produced by normalizing heat treatment from intercritical austenitization temperatures in ductile iron with a certain chemical composition. The pearlite volume fraction is significantly increased with increasing intercritical austenitization temperatures. Moreover, the hardness, yield, and tensile strength are significantly increased with increasing pearlite volume fraction. However, total elongation and reduction in area decreased with decreasing volume fraction of proeutectoid ferrite. With increasing volume fraction of pearlite from 8 % to 75 %; the hardness increased from 181 BHN to 259 BHN; yield strength increased from 348 MPa to 470 MPa; and the tensile strength significantly increased from 569 MPa to 790 MPa, while the total % elongation decreased from 12.6 % to 8.3 %. The fractographic analysis showed that the number of dimples zones decreased with increasing volume fraction of pearlite.

Keywords

• Normalizing heat treatments
• Ductile cast iron
• Intercritical austenitization
• Mechanical properties
• Pearlite volume fraction

NORMALİZE ISIL İŞLEMİNDE ARAKRİTİK ÖSTENİTLEME SICAKLIĞININ KÜRESEL GRAFİTLİ DÖKME DEMİRİN MİKROYAPI VE MEKANİK ÖZELLİKLERİ ÜZERİNE ETKİSİ

Abstract

Bu araştırmada, arakritik östenitleme sıcaklıklarında tavlama sonrası, normalize ısıl işlemlerinin alaşımsız küresel grafitli dökme demirin (KGDD) mikroyapısal değişimine ve mekanik özelliklerine etkisi incelenmiştir. Y blok olarak dökülen alaşımsız KGDD’den çekme numuneleri tornalamayla hazırlanmıştır. Çekme numuneleri farklı arakritik östenitleme sıcaklıklarında (770°C ila 820°C) 60 dk östenitleme sonrası havada soğutulmuştur. Optik mikroskop ve tarama elektron mikroskobuyla (SEM) mikroyapılar karakterize edilmiştir. Çekme ve sertlik testleriyle mekanik özellikler belirlenmiştir. Belirli bir kimyasal bileşimdeki KGDD’de arakritik östenitleme sıcaklıklarından normalize ısıl işlemiyle, % 8 ila % 75 perlit hacim oranlarında KGDD üretilebilmektedir. Arakritik östenitleme sıcaklığının artışıyla perlit hacim oranının belirgin bir şekilde arttığı belirlenmiştir. Ayrıca perlit hacim oranının artışıyla sertlik, akma ve çekme dayanımında belirgin artış tespit edilmiştir. Buna karşın toplam uzama ve kesit alanı daralmasının azalan ötektoid öncesi ferrit hacim oranıyla azaldığı belirlenmiştir. Perlit hacim oranının % 8’den % 75 artmasıyla; sertlik 181 BHN’den 259 BHN’ye; akma dayanımı 348 MPa’dan 470 MPa’ya; çekme dayanımı 569 MPa’dan 790 MPa’ya belirgin bir şekilde artarken toplam % uzama % 12,6’dan % 8,3’e azalmaktadır. Artan perlit hacim oranıyla kırık yüzeylerdeki çukurcukların azaldığı gözlenmiştir.

Keywords

• Normalize ısıl işlemi
• Küresel grafitli dökme demir
• Arakritik östenitleme
• Mekanik özellikler
• Perlit miktarı

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