Effect of Long-term Thermal Exposure on Microstructure Evolution and Mechanical Properties of Alloy 617 Aditya Narayan Singh1,$, A. Moitra1,$, P. Bhaskar1, G. Sasikala1,$, Arup Dasgupta1, A.K. Bhaduri1 $Homi
Bhabha National Institute, Mumbai, Maharashtra 400 094 India 1Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu 603 102 India Corresponding Author: A. Moitra1, email:
[email protected]
INTRODUCTION
India is developing Advanced Ultra-Supercritical (AUSC) coal-fired power plant with steam temperature up to 750ºC (1023K) and pressure 350-370bar.
RESULTS As-received
SEM
(a)
(b)
(a) Alloy 617: Nickel base Superalloy (Solution Annealed at 1170oC and water quenched
Fibrous dimples
OBJECTIVE
Aged 1000h
(b) (c)
Investigate Ageing induced degradation
(d)
Grain Size distribution using Optical Measurement
Characterize alloy 617 using SEM investigation Intergranular brittle fracture
Identify and evaluate the different intermetallic phases and precipitates evolved using TEM
Aged 5000h
(c) SEM images unveiling the effect of aging duration on Charpy impact test of Alloy 617: (a) as-received (b) 1000h (c) 5000h (d) 10000 h
METHODS
TEM
As-received specimens aged at 750ºC for 1000, 5000, and 10000 hrs
Hardness (VHN)
Charpy Impact energy (J)
Mechanical Properties:
Microstructural Characterization
Optical (Grain Size distribution )
Aged 10000h (d) Opticaletched with an optimized solution mixture of 60% HNO3 and 40% H2O. TEM
SEM
SEM (Fractography Examination) TEM (Phases & precipitates Examination)
CONCLUSION
The grain size changes from asreceived to aged condition but distribution remains fairly stable over the time. The stability of grain size over the aging duration imparts high mechanical strength to the Alloy.
TEM investigation of as-received alloy shows precipitates of Mo2C
Embrittlement of Alloy 617 due to ageing at 750oC has been characterized and correlated to the microstructural evolution. Grain boundary precipitation of M23C6 leads to increased tendency for intergranular brittle fracture. The alloy shows two types of precipitates; one fine and other coarse. The grain size remains fairly stable in the range of ASTM number 5-7. SEM reveals the transition of fracture mode from fibrous ductile for the un-aged material to intergranular for the aged material. The extent of intergranular nature increases with increasing ageing time. TEM micrographs along with SAD pattern shows dispersed γ’ precipitates of size 30-40 nm, rich in Al and Ti along with extensive precipitation of M23C6(a,b) at the grain boundaries.
