Microstructural stability of a nickel-based alloy overlay on a 2.25Cr1Mo steel substrate

Saghafifar, Hassan (2011) Microstructural stability of a nickel-based alloy overlay on a 2.25Cr1Mo steel substrate. PhD thesis, University of Nottingham.

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Abstract

Ni-based superalloy weld overlays are widely used in electricity generating plants to significantly reduce high temperature corrosion problems of ferritic steel components under service conditions. Welding a nickel alloy similar to IN625 onto the outer bore of a 2.25Cr1Mo steel tube enhances its service life as a superheater tube in the highly corrosive environment of a Waste-to-Energy boiler. For the purposes of studying the effects of high temperature service on the microstructure of this laid tube with a weld overly, a series of thermal exposure tests at 650˚C was performed for different times from 1 day up to 128 days. The microstructural evolution was studied using a combination of analytical techniques along with changes in hardness profile across the interface. Changes in the microstructure were examined using OM, SEM, EPMA, EBSD and FIB-STEM.

Hardness survey results indicate significant changes in the interfacial region during ageing. Formation of a soft zone ~300 μm wide and its subsequent re-hardening was observed in the steel side while the hardness of the bulk steel remained unchanged. Development of a hard band ~30 μm wide adjacent to the interface in the weld overlay region and hardening of the bulk overlay material occurs in the early stages of ageing and remained unchanged with ageing time. Thermodynamic calculations were performed using Thermo-Calc software and TCFE6 and TTNI7 databases to aid interpretation of experimental data.

Microstructural evolution in the steel region is related to the carbide transformation process and carbon migration. In the bulk of the steel tube, the bimodal microstructure is stable and in the tempered martensite/bainite areas, the initial M3C transform to M23C6 through a series of metastable carbides while in the ferrite grains, M2C carbide precipitates and transforms directly to the equilibrium carbide. The main process for supply of carbon atoms is transformation of carbides and the rates of transformation are related to the as-welded microstructure. On the steel side of interfacial region where bainite was formed after welding, the stability of metastable carbides is related to the carbon content. Long term ageing causes Mo replenishment in the coexisting ferrite and fine grains have formed in this region following long term ageing.

Experimental observations confirmed that a network of alloyed M23C6 carbide precipitates was formed at the interface in the steel side which are believed to interrupt the carbon migration across the interface. Moreover, there is a carbide precipitation region within ~100 μm from the interface in the weld overlay. Beyond this region intermetallic phases like Mo-rich μ and Nb-rich δ were formed in the interdendritic regions and along the grain boundaries. Hard band formation is related to precipitation of the σ phase.

Item Type:Thesis (PhD)
Supervisors:McCartney, D.G.
Shipway, P.H.
Faculties/Schools:UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
ID Code:2393
Deposited By:Mr. Hassan Saghafifar
Deposited On:08 May 2012 14:45
Last Modified:08 May 2012 14:45

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