Investigation of abnormal effect of dual-phase ferritic-bainitic microstructures on modification of mechanical properties of MO40 low alloy steel in comparison with full bainitic conditions

Document Type : Research Paper

Authors

• Abbas Yazdizade Khalili ¹
• sadegh ghasemi banadkouki ²
• Hamid Reza Karimi Zarchi ³
• masoud mosallaee pour ⁴

¹ Yazd University

² department of mining and metallurgical engineering, yazd university, yazd, iran

³ Mining Technologies Research Center, Department of Mining and Metallurgical Engineering, Yazd University, University Blvd, Safayieh, Yazd, PO Box: 98195 – 741, Iran

⁴ yazd univesity

Abstract

The purpose of this research is to investigate microstructural features and mechanical properties of MO40 low alloy steel under dual-phase ferritic-bainitic in comparison with those of full bainitic conditions. To do so, various heat treatment cycles including Austenitizing the samples at 860°C for 50 minutes, step quenching in primary 620°C salt bath for different duration times of 1 to 10 minutes, quenching in secondary 350°C salt bath for 30 minutes and then water quenching to room temperature in order to develop dual-phase ferritic-bainitic microstructures in the samples. The isothermal heat treatment in primary 620°C salt bath for various duration times was used in order to develop different volume fractions of ferritic phase in the microstructures, then the secondary 350°C salt bath was used for subsequent transformation of remaining prior austenite to bainite. Tensile test, micro and macro hardness testers, optical metallography and FE-SEM equipped with EDS were used for investigation of variations of mechanical properties and microstructural features. The experimental results show that there is a non-linear and abnormal relationship between the mechanical properties of dual-phase ferritic-bainitic samples and variations of ferrite and bainite volume fractions. The mechanical properties of dual-phase ferritic-bainitic samples involving 8-12 vol% grain boundary ferrite in the vicinity of bainite is considerably higher than other dual phase ferritic-bainitic samples and even full bainitic ones. This improvement in mechanical properties of dual-phase ferritic-bainitic samples involving 8-12 vol% ferrite is originating from interaction of bainite crystals on grain boundary ferrite.

Keywords

• MO40 low alloy steels
• Step quenching heat treatment
• Dual phase ferritic-bainitic microstructures
• Interaction between bainite crystals and ferrite phase
• Abnormal mechanical properties

20.1001.1.22285946.1398.10.37.10.9

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