THERMOMECHANICAL PROCESSING OF ALLOY 625 AND ITS MECHANICAL PROPERTIES UNDER STATIC AND DYNAMIC LOADING

Abstract

Ni-based superalloy 625 is a concentrated solid solution alloy that provides elevated temperature strength in combination with excellent oxidation and corrosion resistance. Due to these exceptional properties, it finds an extensive range of applications in several industries like aerospace, chemical, nuclear, marine, and power plant industries. Out of these, its application in building advanced ultra-supercritical (AUSC) power plants, especially in India is gaining lots of recent interest. As the cast form of alloy 625 consists of massive solute segregation with second-phase particles distributed in the inter-dendritic regions, it results in inferior mechanical properties under static and dynamic loading conditions. In this regard, to produce a homogeneous fine-grained structure (FGS) with superior properties, the present research work aims to investigate the hot and warm deformation characteristics of alloy 625 under various loading conditions. Initially, the hot deformation behavior of alloy 625 is investigated in a wide temperature and strain rate range (T: 1000-1200°C, έ: 0.01-10s-1) by performing single-hit uniaxial compression and multi-hit plane strain compression experiments in Gleeble-3800 simulator. The initial microstructure used for the hot deformation study was a homogenized large-grained structure (LGS), >1mm. The hot compression flow stress curves recorded at various processing conditions reveal a single broad peak with no sign of steady state up to the true strain of 0.7, implicating slow recrystallization rates. This is attributed to the sluggish high-angle boundary mobility in alloy 625 due to the solute drag effects. Moreover, at constant deformation temperature, the kinetics of dynamic recrystallization (DRX) deteriorates with increasing έ up to 1s-1 and then again shows an unusual improvement at 10s-1. The drastic change in kinetics is associated with the change in recrystallization mechanism from a high-angle boundary migration dominated at a lower strain rate (έ = 0.01s-1) to a twin-assisted DRX nucleation dominated at intermediate strain rates (έ =0.1-1s-1), and finally to a GND (geometrically necessary dislocation)-assisted DRX nucleation at higher strain rate (έ =10s-1). These findings suggest that multiple DRX mechanisms are operative in alloy 625, wherein the dominant mechanism at a given processing condition defines the final microstructure. Furthermore, a processing map based on DMM is constructed at ε = 0.7, highlighting the deformation at 1150°C and 0.01s-1 as the most efficient (54%) safe workable zone leading to develop a relatively coarse-grained structure.