Date

2018

Document Type

Thesis

Degree

Master of Science

Department

Materials Science and Engineering

First Adviser

DuPont, John N.

Abstract

Dissimilar metal welds (DMWs) between Grade 91 and nickel-based alloys often fail along the fusion line well before their expected service life. Failure is attributed to various microstructural features including the formation of a carbide free ferrite band and a row of type I carbides, but the evolution of these features during welding, post weld heat treatment (PWHT), and service is not fully understood. In this study, single pass bead on plate welds were prepared between Grade 91 base metal and Inco A, 617, 625, 182, and EPRI P87 filler metals. These DMWs were characterized in the as-welded, PWHT and aged conditions using a variety of electron microscopy techniques. Thermodynamic and kinetic models were used to simulate carbon diffusion across the fusion line. After welding, all DMW samples exhibited a martensitic microstructure along the fusion line and the magnitude of concentration gradient varied at different locations in the partially mixed zone (PMZ). After a 760°C PWHT for one hour, DMWs involving Inco A, 625 182 and EPRI P87 exhibited a band of martensite along the face centered cubic (FCC)/body centered cubic (BCC) interface. However, the DMW involving IN617 exhibited a row of type I carbides and a carbide-free ferrite band in locations where the concentration gradient in the PMZ was steep. After accelerated aging at 625°C, all of the DMW systems exhibited a carbide-free ferrite band and type I carbides after aging for 6000 hours. The carbide-free ferrite band and type I carbides were always associated with steep concentration gradients across the PMZ. The formation mechanism of the type I carbides and carbide-free ferrite band is driven by the diffusion of carbon from the base metal to the fusion zone down the steep carbon chemical potential gradient.

Share

COinS