Hardness change due to carburization time and material thickness during heat treatment of SAE 8620 (21NiCrMo2) plates

Year 2019, Volume: 6 Issue: 3, 748 - 754, 30.09.2019

İdris Karagöz

https://doi.org/10.31202/ecjse.578057

Cited By: 1

Abstract

In this study, SAE 8620 (21NiCrMo2) cementation steel was carburized in a salt bath containing 10% cyanide (Potassium Cyanide KCN). Samples, which were cut into two different sizes as 15x15x100 mm and 20x20x100 mm, were gradually heated from room temperature to the hardening temperature of 930 °C and kept at this temperature for two different holding times as six and seven hours. The temperature was gradually reduced to 860 °C and the samples were kept at this temperature for 40 minutes and then quenched in the oil cooling medium. After quenching, the materials were tempered at 180 °C for two hours. The surfaces of the samples which were sanded according to microstructure examinations and polished with diamond paste were etched with 5% Nital solution. Surface hardness and microhardness of the samples were measured and their microstructures were examined with an optical microscope. The hardness depth was 1.6 mm and the effective hardness depth was 1.2 mm for the materials. It was observed that hardening up to a 0.2 mm depth was at maximum level and hardness values decreased while approaching the core. In the microstructure examinations, it was observed that the martensitic layer was formed on the surface and this layer lost its effect as it penetrated inwards. In the cementation process, it was determined that material thickness and carburization time had an effect on material properties.

Keywords

SAE 8620 (21NiCrMo2); carburizing; heat treatment; hardness; microhardness

SAE 8620 (21NiCrMo2) plakaların ısıl işleminde karbürizasyon süresi ve malzeme kalınlığına bağlı olarak oluşan sertlik değişimi

Abstract

Bu çalışmada; SAE 8620 (21NiCrMo2) sementasyon çeliğine %10 siyanür içeren (Potasyum Siyanür KCN) tuz banyosunda karbürizasyon işlemi uygulanmıştır. 15x15x100 mm ve 20x20x100 mm olarak iki farklı ölçüde kesilen numuneler oda sıcaklığından sertleştirme sıcaklığı olan 930 °C’ ye kadar kademeli şekilde ısıtılmış ve bu sıcaklıkta altı ve yedi saat süreyle iki farklı tutma süresinde bekletilmiştir. Kademeli bir şekilde sıcaklık 860 °C’ ye düşürülmüş ve 40 dakika bu sıcaklıkta bekletilmiş numunelere yağ soğutma ortamında su verilmiştir. Su verme sonrası malzemeler 180 °C’de iki saat süreyle temperlenmiştir. Mikroyapı incelemelerine uygun olarak zımparalanan ve elmas pasta ile parlatması yapılan numunelerin yüzeyleri %5 nital çözeltisi ile dağlanmıştır. Numunelerin yüzey sertlikleri, mikrosertlik ölçümleri yapılmış ve optik mikroskop ile mikroyapıları incelenmiştir. Malzemeler için sertlik derinliği 1,6 mm ve etkili sertlik derinliği ise 1,2 mm olarak tespit edilmiştir. 0,2 mm derinliğe kadar sertleşmenin maksimum seviyede olduğu, çekirdeğe yaklaşıldıkça sertlik değerlerinin düştüğü görülmüştür. Mikro yapı incelemelerinde yüzeyde martenzitik tabakanın oluştuğu ve içeriye doğru penetre edildikçe bu tabakanın etkisini yitirdiği gözlemlenmiştir. Sementasyon işleminde, malzeme kalınlığının ve karbürizasyon süresinin malzeme özellikleri üzerinde etkili olduğu tespit edilmiştir.

Keywords

SAE 8620 (21NiCrMo2); Sementasyon; ısıl işlem; sertlik; mikrosertlik

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