Vol.22, No.3, 2022, pp. 299–304
UDC:

MICROSTRUCTURE EVOLUTION AND MECHANICAL PROPERTIES DEGRADATION OF HPNB ALLOY AFTER AN ELEVEN-YEAR SERVICE

Milica Timotijević1, Olivera Erić Cekić1,2*, Dragan Rajnović3, Marina Dojčinović4, Petar Janjatović3

1) University of Kragujevac, Faculty of Mechanical and Civil Engineering in Kraljevo, Kraljevo, SERBIA

*email: olivera66eric@gmail.com

2) University of Belgrade, Innovation Centre of the Faculty of Mechanical Engineering, Belgrade, SERBIA

3) University of Novi Sad, Faculty of Technical Science, Novi Sad, SERBIA

4) University of Belgrade, Faculty of Technology and Metallurgy, Belgrade, SERBIA

 

Abstract

In this work, the heat-resistant cast steel HP40Nb alloy, produced in the form of a centrifugally cast tube, which failed after 11.4 years service is investigated. The microstructure is examined using a scanning electron microscope (SEM), while the phases observed are analysed using an energy dispersive X-ray analyser system (EDS). Additionally, fractography is performed on the fractured surface of failed specimens. Mechanical properties of the tube are evaluated by using tensile and Charpy impact testing at room temperature (298 K). Main results indicate that the failed tube microstructure consists of an austenitic matrix and a continuous network of primary eutectic carbides of two types: one rich in Nb (bright particles), and one rich in Cr (dark particles). These carbides are NbC and complex M7C3 (M = Cr, Ni, Fe) type. The brittle intergranular fracture mode is explained by the presence of massive precipitation and coarsening of intergranular carbides at grain boundaries. Lower tensile properties of the in-service exposed tube might be correlated to the morphological, chemical, and distribution changes of precipitates during service at high temperature. Decreased values of hardness can be attributed to increase in grain size, as well as due to the decomposition of Cr-carbide during the heat treatment at temperatures above 1073 K.

Keywords: HP40Nb alloy, microstructure, fracture mode, carbide phases

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