Changes in microstructure and mechanical properties of P91 weld metal during creep

Zhang, Yan (2009) Changes in microstructure and mechanical properties of P91 weld metal during creep. PhD thesis, University of Nottingham.

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Abstract

Creep failure of the weld structure in P91 steel components in high temperature power plant applications is often a key factor limiting the lifetime of the components. Whilst creep failure in weld heat-affected zone (HAZ) regions has been studied widely, the creep properties of the weld metal itself have been less well documented. In this work, the creep response of P91 weld metal in isolation was investigated in terms of microstructural evolution and mechanical properties.

The microstructural examination of P91 multi-pass weld metal revealed a typical weld metal structure including columnar regions and refined regions. The columnar region exhibited high hardness whilst the refined region exhibited lower hardness. The anisotropic creep behaviour of P91 weld metal was observed in creep tests of both longitudinal and transverse specimens at 650ºC and various stress levels. This behaviour can be correlated with the microstructural anisotropy observed, where longitudinal specimens with banded columnar regions and refined regions parallel to the stress axis had longer creep life than transverse specimens with overlapped typical-shape beads. Longitudinal weld specimens showed higher strain to failure than transverse specimens.

The microstructural investigation of creep tested P91 weld metal revealed two primary modes of creep fractures. In addition to creep fractures along columnar grain boundaries (typical of weld metal creep failure), creep fractures were also found along creep-weak white-bands which had formed at the inter-bead boundaries. The white-band regions consisted of material where the M23C6 carbides had dissolved during creep testing; the loss of carbides had allowed recrystallisation of the martensitic structure to ferrite and consequently this material was much softer than the bulk weld metal. The element mapping over the weld metal by laser-induced breakdown spectroscopy (LIBS) demonstrated that there was significant inhomogeneity in the distribution of certain elements, most significantly, chromium, manganese and molybdenum. This inhomogeneity resulted in strong activity gradients in carbon (even though the carbon concentration was homogeneous following welding) resulting in carbon loss from the alloy-depleted regions, the associated dissolution of carbides and the recrystallisation that accompanied this, and thus the poor mechanical properties which resulted in creep failure.

The inhomogeneity in the distribution of certain alloying elements can be partially attributed to the solute partition of alloying elements during weld solidification which has been confirmed with examination of simulation P91 TIG welds. However, the homogeneity of weld metal in this case required mixing of a base steel (the core rod in the weld consumable) and particles of various ferro-alloys (delivered into the weld pool from the flux). It is argued that poor mixing in the stagnant layer (unmixed zone) at the solid-liquid interface during weld solidification also makes a significant contribution to the formation of alloy-depleted regions.

The formation of white-bands has been modelled using Thermo-Calc based on the understanding of the formation mechanism involving solute partition and subsequent carbon diffusion out of the alloy-depleted region. A good correlation to experimental results has been shown in the prediction of limiting carbon concentration and M23C6 carbide content in white-bands. In addition, it was also suggested that depletion of carbides and carbon are strongly linked and that depletion of alloying elements only above a critical value will result in total carbide loss and thus recrystallisation into a white-band.

Item Type:Thesis (PhD)
Supervisors:Shipway, P.H.
Hyde, T.H.
Faculties/Schools:UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
ID Code:1419
Deposited By:Dr YAN ZHANG
Deposited On:22 Sep 2010 18:00
Last Modified:22 Sep 2010 18:00

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