PSI - Issue 23

Petr Král et al. / Procedia Structural Integrity 23 (2019) 287–292

288

Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Ni-based superalloys are widely used at high-temperature applications in aerospace and power engineering industries, Reed (2006) and Tawancy et al. (1984). The creep strength of many Ni-based superalloys depends predominantly on fine and uniformly dispersed γ , -particles which precipitates from supersaturated solid solution during ageing treatment. However, microstructure of some Ni-based alloys can be strengthened only carbides. These alloys are characterized by good combination of creep rupture strength and high temperature oxidation for prolonged exposures. Ni – Cr – W based superalloy with high temperature mechanical strength and long-term creep rupture strength with application temperature of abou t 1000 °C is developed recently, Liu et al. (2009). The alloy derives its strength from solid-solution and carbides, Lee et al. (2015), and can be used in wide variety of component applications such as aerospace and chemical process industries. The influence of carbides on creep behavior is twofold. The primary coarse carbides provide only a minor dispersion strengthening effect and more significant role in strengthening have intragranular secondary carbides. The aim of present work is to characterize creep behavior, microstructure changes and damage processes of Ni-Cr-W based alloy at high temperatures used in the glass industry. 2. Experimental material and procedures Experimental Ni-Cr- W based alloy used in the investigation was cast by a foundry company PBS Velká Bíteš, a.s ., Czech Republic. The chemical composition (in wt.%) was following: Ni – 0.47 C, 29.2 Cr, 2.44 Fe, 7.49 W, 0.05 Mn, 0.33 Co, 1.45 Nb, 1.34 Si, 0.005 P and 0.006 S. The alloy was investigated in the as-cast state. Tensile creep tests were carried out at 1173 - 1373 K under different initial applied stresses. The creep testing was performed in a protective argon atmosphere using cylindrical specimens with the gauge length of 50 mm and the diameter of 5 mm. All creep tests were run up to the final fracture of specimens. During creep exposure the creep elongation was measured, recorded digitally and computer processed. The microstructure investigations were performed by means of scanning (SEM) and transmission electron microscopy (TEM). The TEM studies were carried out on thin foils using a JEOL 2100 F microscope operating at 200 kV. The particles were identified on the basis of their local chemical compositions measured by energy dispersive X-ray spectroscopy (EDS) and diffraction patterns.

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Fig. 1. Microstructure of initial state before creep testing (a) coarse primary carbides M 23 C 6 ; (b) secondary carbides M 23 C 6