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

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

References

1. [1] Topbaş, M.A., “Çelik ve Isıl İşlem El Kitabı”, Ekim Ofset, İstanbul, (1998).
2. [2] Karagöz, İ., “Analyse of the conditions to increase the deepness of hard surface of heat treatment and diffusion on carburized steel”, Masters Thesis, Marmara University Institute of Pure and Applied Sciences, (2007).
3. [3] Karagöz, İ., Kurt, H.İ., Samur, R., “The effect of carburization time on the hardness and microstructure at heat treatment carburized steels”, 3rd International Energy&Engineering Congress, Gaziantep, 771-777, (2018).
4. [4] Levy A.V., Yan J., “San-water slurry erosion of carburized AISI 8620 steel”, Wear, 1985, 101: 117-126.
5. [5] Asi O., Can A.Ç., Pineault J., Belassel M., “The relationship between case depth and bending fatigue strength of gas carburized SAE 8620 steel”, Surface&Coatings Technology, 2007, 201: 5979-5987.
6. [6] Genel K., Demirkol M., “Effect of case depth on fatigue performance of AISI 8620 carburized steel”, International Journal of Fatigue, 1999, 21: 207-212.
7. [7] Erdoğan M., Tekeli S., “The effect of martensite particle size on tensile fracture of surface-carburised AISI 8620 steel with dual phase core microstructure”, Materials and Design, 2002, 23: 597-604.
8. [8] Shen Y., Moghadam S.M., Sadeghi F., Paulson K., Trice R.W., “Effect of retained austenite-compressive residual stresses on rolling contact fatigue life of carburized AISI 8620 steel”, International Journal of Fatigue, 2015, 75: 135-144.

9. [9] Roy S., Sundararajan S., “The effect of heat treatment routes on the retained austenite and tribomechanical properties of carburized AISI 8620 steel”, Surface&Coatings Technology, 2016, 308: 236-243.
10. [10] Roy S., Zhao J., Shrotriya, P., Sundararajan, S., “Effect of laser treatment parameters on surface modification and tribological behavior of AISI 8620 steel”, Tribology International, 2017, 112: 94-102.
11. [11] Asi O., Can A.Ç., Pineault J., Blassel M., “The effect of high temperature gas carburizing on bending fatigue strength of SAE 8620 steel”, Materials and Design, 2009, 30: 1792-1797.
12. [12] Yeğen İ., Usta M., “The effect of salt bath cementation on mechanical behavior of hot-rolled and cold drawn SAE 8620 and 16MnCr5 steels”, Vacum, 2010, 85: 390-396.
13. [13] Çelik A., Efeoğlu İ., Sakar G., “Microstructure and structural behavior of ion-nitrided AISI 8620 steel”, Materials Characterization, 2001, 46: 39-44.
14. [14] Izciler M., Tabur M., “Abrasive wear behavior of different case depth gas carburized AISI 8620 gear steel”, Wear, 2006, 206:90-98.
15. [15] Boyle E., Northwood D.O., Bowers R., Sun X., Bauerle P., “The effects of initial microstructure and heat treatment on the core mechanical properties of carburized automotive steels”, Materials Forum, 2008, 32: 44-54.
16. [16] Lu J.Z., Zhong J.W., Luo K.Y., Zhang L., Dai F.Z., Chen K.M., Wang Q.W., Zhong J.S., Zhang Y.K., “Micro-structural strengthening mechanism of multiple laser shock processing impacts on AISI 8620 steel”, Materials Science and Engineering A, 2011, 528: 6128-6133.
17. [17] Sharma S.K., Rizvi S.A., Kori R.P., “Optimization of Process Parameters in Turning of AISI 8620 Steel Using Taguchi and Grey Taguchi Analysis”, Journal of Engineering Research and Applications, 2014, 4(3): 51-57.