PSI - Issue 68

Rahul Singh et al. / Procedia Structural Integrity 68 (2025) 715–721 Rahul Singh et.al / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction Nano-structuring of pearlite has emerged as an effective approach to improve several material properties, including tensile strength, shear strength Khiratkar et al. (2021), plane strain fracture toughness Mishra and Singh (2017), resistance to subcritical crack growth under cyclic loading Mishra et al. (2019), and wear resistance Mishra et al. (2018). The enhancement in strength results from the increased resistance to dislocation motion as the interlamellar spacing becomes finer. Meanwhile, the improvement in K IC can be attributed to the more homogeneous slip and fracture processes controlled by crack propagation, as opposed to the shear-cracking and nucleation-controlled fracture typically observed in coarser pearlite structures by Kavishe and Baker (1986); Mishra et al. (2020). However, the impact toughness of nano-pearlitic steels has a more complex dependence on the various pearlitic micro-structural scales. The energy absorbed during impact testing at room temperature (approximately 25°C) was found to be influenced by several microstructural factors, including the prior austenite grain size (PAGS), the size of pearlitic nodules, the dimensions of pearlitic colonies, and to a lesser extent, the interlamellar spacing (λ), Garbarz and Pickering (1988); Hyzak and Bernstein (1976); Kavishe and Baker (1986); Lewandowski and Thompson (1986); Taleff et al. (2002). However, another study investigated the behavior of nano-pearlitic steels (0.72 C,0.63 Si, 1.44 Cr, 2.63 Co, 1.56 Al, 0.15 Mn, 0.25 Mo, 0.012 S, 0.009 P and balance Fe (Weight %) using standard Charpy impact tests with interlamellar spacings ranging from 79 to 120 nm. At room temperature (25°C), the absorbed impact energy remained unaffected by variations in interlamellar spacing. However, at elevated temperatures, the impact energy and ductile nature of the fracture increased with finer pearlite interlamellar spacing. These findings suggest that while interlamellar spacing plays a crucial role in enhancing impact toughness at higher temperatures, the nodule size is the primary factor governing toughness at lower temperatures, Khiratkar et al. (2019). The current study investigates the effect of the change of composition on absorbed impact energy for Charpy samples tested at room temperature. High Mn content as well as relatively low C has been chosen for this steel and this has resulted in high impact toughness. 2. Materials and methods The steel used in this study has the following composition (in weight %): 0.7 C, 0.99 Si, 0.36 Cr, 3.11 Co, 2.13 Al, 1.81 Mn, 0.3 Mo, <0.010 S, <0.007 P, with the remainder being Fe. This composition was confirmed to be near eutectoid using the Thermo-Calc 2023(a) TCFE8-Steels/Fe-Alloys v8.1 database. The addition of cobalt and aluminum was intended to promote more nucleation sites by increasing the free energy for pearlitic transformation [1]. The steel was melted in an open-air induction furnace, cast into a slab measuring 120 mm × 50 mm × 30 mm in a metallic mold, and subsequently homogenized at 1000°C for 15 hours in a muffle furnace. The slab was divided into two halves, each measuring 60 mm × 50 mm × 30 mm, and hot-rolled at 1000°C to reduce the thickness to 13 mm. After hot rolling, the slab was austenitized at 1000°C for one hour, followed by an isothermal transformation at 550°C for 270 minutes to achieve a fully pearlitic structure. The material transformed at 550°C is referred to as NP550. Tensile tests were performed on miniature tensile samples machined from the NP550 slab, each having a gauge length of 5 mm, using an Instron Universal Testing Machine (UTM) to evaluate their mechanical properties. To ensure the accuracy and reliability of the results, a minimum of two samples were tested. The tests were conducted at a strain rate of 10 -3 s -1 , corresponding to a test rate of 0.3 mm/min. For impact testing, Charpy impact specimens were machined from the NP550 slab, each with dimensions of 55mm × 10 mm × 10 mm and a V-notch, in accordance with the ASTM E23-16(b) standard. The Charpy impact tests were performed at 25°C, for five specimens. Additionally, small cubes machined from the NP550 slab were ground, polished, and etched in a 2% Nital solution to reveal the microstructure. The microstructural analysis was conducted using a JEOL-IT800 dual vacuum high-resolution scanning electron microscope (HR-SEM). A 3 mm sample was extracted from the NP550 cube of 200 μm thickness, and further thinning was achieved through a twin jet polishing machine using an electrolyte composed of 95% methanol and 5% perchloric acid, operating at a voltage of 15V. The electrolyte temperature was maintained at -25°C