Phase Transformations of Low-Alloy Steel DIN 1.5025 under Isothermal Holding in the Bainitic Region and the Formation of Microcomposite Microstructures

Subject Areas : journal of New Materials

Shima Pashangeh ¹ , Seyed Sadegh Ghasemi Banadkouki ² , Mohammad Hadi Mohammad Hadi ³

1 - Assistant prof. of Materials Engineering, Department of Materials Engineering, Yasouj University, Yasouj, Iran
2 - Associate prof., Department of Mining and Metallurgical Engineering, Yazd University, Yazd, Iran
3 - aAssistant prof. of Electrical Engineering, Department of Electrical Engineering, Yasouj University, Yasouj, Iran

Received: 2025-03-02 Accepted : 2025-03-02 Published : 2025-03-02

Keywords: Heat treatment, Microcomposite structures, Multi-phase steels, Bainite, Hardness testing,

Abstract :

In this study, a medium-carbon steel sheet sample (wt.%C: 0.529) with high silicon content (wt.%Si: 1.67) was selected. The process involved applying heat treatment cycles with isothermal holding in the bainitic region, which included austenitizing at 900°C for 5 minutes, followed by rapid transfer to a molten salt bath in the temperature range of 400-500°C. The samples were held for varying times, from 5 seconds to 1 hour, and then quenched rapidly in water. The goal of the heat treatment process was to investigate phase transformations in microcomposite structures. In the next step, initial microstructural investigations and phase transformations under different heat treatment conditions were conducted using laser and scanning electron microscopes, along with dilatometry tests. Subsequently, the mechanical behavior of the heat-treated samples under different conditions was evaluated using macrohardness tests. The results indicated that achieving microcomposite structures was possible using these heat treatment conditions. Additionally, increasing the holding temperature in the bainitic region accelerated the kinetics of bainite formation. Prolonged holding times at these temperatures led to a tempering process, as confirmed by dilatometry tests. Hardness tests also revealed that the hardness increased with decreasing isothermal holding temperatures in the range of 400-500°C. Specifically, the maximum hardness values at 500°C, 450°C, and 400°C were measured at 341, 370, and 442 HV30kg, respectively.

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