PSI - Issue 14

P Ponguru Senthil et al. / Procedia Structural Integrity 14 (2019) 729–737 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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by volume fraction, morphology and stability of carbon enriched retained austenite. Garcia et al (2005) has observed that the ductiliy of nano bainitic steel is governed by the fraction and stability of retained austenite. Avishan (2018) has compared mechanical stability of retained austenite during compressive and tensile loading and found that the retained austenite stability is higher during compressive loading. Avanish Kumar et al (2018) have studied the influence of volume fraction of the retained austenite on the fracture toughness of nano bainitic stees. The retained austenite is present in two forms, blocky retained austenite and filmy retained austenite. The filmy austenite has higher carbon content and higher stability than the blocky austenite (Caballero et al (2009)). Retained austenite transforms to martensite during deformation leading to enhanced strength and ductility called as TRIP effect (Abbasi et al (2016)). Some fraction of bulk retained austenite transforms to martensite upon cooling to room temperature after austempering. Because of poor mechanical stability of blocky retained austenite, it transforms to martensite at early stages of deformation leading to very little benefit from TRIP. Austenite with higher stability will undertake large strain before transforming to martensite leading to effective strain hardening. Hence increasing the fraction of filmy austenite and reducing the bulk retained austenite leads to enhanced mechanical stability and results in better mechanical properties (Avishan et al (2012)). Refinement of Prior Austenite Grain Size (PAGS) results in finer and higher fraction of bainitic sheaves because of enhanced nucleation of bainitic ferrite. It also has been found to decrease bulk retained austenite size and fraction in nano bainitic steels (Peet et al (2017)). PAGS refinement has been achieved by reducing the austenitizing temperature or by addition of microallying elements such as Vanadium and improvement in strength as well as toughness has been reported by Peet et al (2017) and Kritika et al (2018). Improvement in mechanical properties of HSLA steels by addition of Nb as micro alloying element is well known (Speer et al (1989) and medin et al (1999)). Niobium (Nb) forms very fine nano-scale carbides, nitrides or carbo-nitrides which effectively pin the prior austenite grain boundaries and retard grain growth leading to finer PAGS. Niobium carbo-nitride precipitation also occurs during controlled rolling of austenite which retards recrystallization (Speer et al (1987)). Arunim (2017) has reviewed the effect of Nb microalloying on pearlitic and bainitic microstructure and mechanical properties. Nb solubility decreases with increasing carbon and silicon content in steels. Centre line segregation in Nb added high carbon steels leads to the formation of coarse primary niobium carbo-nitrides during solidification (Arunim (2017)). These primary carbo-nitrides does not dissolve completely during reheating and limits the use of Nb as an effective micro alloying in high carbon steels. However, if the primary niobium carbo-nitrides are finer in size, it will not have any detrimental effect on mechanical properties. Controlled addition of Nb in high carbon pearlitic rail steels resulted in improved strength and wear resistance (Arunim (2017)). Studies on the influence of Nb on the transformation kinetics and mechanical properties of carbide free nano bainitic steels are limited. Nb microalloyed bainitic rail steels showed improved strength as well as toughness. However, influence of Nb on high silicon, high carbon nano bainitic steels is not well understood. The present study aims to study the influence of small amount of Nb addition on the microstructure, transformation kinetics and mechanical properties of high silicon carbie free nano bainitic steel. To study the influence of Nb, two melts of steel have been prepared, one without Nb (hereafter referred as B1) and the other with 0.037 wt% Nb addition (hereafter referred as B1Nb). The compositions of the steels are given in table 1. Ingot of each composition weighing 20 Kg has been melted by vacuum induction melting using pure alloying elements. The ingots were homogenized at 1200°C for 48 hours followed by furnace cooling. The ingots were subjected to hot forging to get a 50 mm thick slab followed by hot rolling into 15 mm plates. The Hot forging and hot rolling were carried out in the temperature range of 1050°C to 950°C. After forging and rolling the 2. Experimental procedure