Chalk, Kieran
(2013)
Weld consumables and PWHT for P92 power plant steel.
PhD thesis, University of Nottingham.
Abstract
P92 steel is a high-alloy steel that has been specifically designed for operating at high temperatures (600°C - 650°C) and has found wide use in the power generation industry, particularly since 2005. For the successful installation and use of this advanced steel, all aspects of its behaviour, in terms of both metallurgy and in-service behaviour, must be investigated. Investigating all the relevant material aspects is beyond the scope of a single PhD, and so the Supergen consortium funds a number of projects working on different material aspects. The purpose of this thesis is to investigate, and seek a greater understanding of, the behaviour of welds in P92 steel so that their in-service behaviour may be better understood particularly the response of the material to post-weld heat treatments (PWHT), the optimum weld consumable composition and the microstructural development during creep-rupture. This understanding has been achieved through a combination of microstructural characterization, thermodynamic modelling and mechanical testing.
Specifications for weld metals define a range of compositions; thermodynamic modelling has enabled a better understanding of how the composition affects the final microstructure of P92 weld metal (given that this work is based upon thermodynamic predictions, the understanding developed here is applicable to both parent and weld metal). Precipitation strengthening is important to the creep resistance of P92 and the modelling has revealed how precipitate levels vary based on composition. Using this knowledge, quality checks on P92 used by industry can better ensure the fitness for service of a material if an accurate composition is known; furthermore, this understanding will enable manufacturers to further tailor compositions to produce the strongest possible material.
Following welding with P92 fillers, post-weld heat treatment is carried out, and there is a desire to perform this heat treatment close to the A1 temperature of the materials involved. As such, it is important to accurately know the A1 temperature of the materials being heat treated. A combination of thermodynamic modelling, experimental thermal analysis and microstructural characterization was used to investigate the key transformation of ferrite to austenite. This investigation focused on the effect of composition on the transformation temperature, A1, and the rate at which austenite could form during PWHT. An equation to predict the Ae1 temperature of P92 is produced and validated. The knowledge of how composition affects the A1 temperature is useful for both welds and parent material, enabling the design and selection of P92 material that will not undesirably transform during heat treatments. It is proposed that the equation for Ae1 allows the determination of maximum safe heat treatment temperatures and will reduce the likelihood of poor quality material entering service. Experimental work has demonstrated that during PWHT (or parent material tempering), equilibrium conditions are approached, confirming that Ae1 should be used to determine maximum heat treatment temperatures instead of the AC1 temperatures which are currently employed.
Creep testing of three different weld consumables was carried out to determine which had the best properties for use in service, and to understand the microstructural features which controlled creep behaviour of these weld metals. Creep testing of weld metal has indentified that δ-ferrite causes early failure as the resulting precipitate-free zones (PFZs) are creep weak. The presence of localized δ-ferrite is caused by an inhomogeneous distribution of ferrite stabilizers, particularly tungsten within the weld metal, resulting in greater stability of δ-ferrite and its retention in the weld. Using this knowledge, alloy specifications of weld consumables and corresponding welding procedures can be improved to ensure a homogeneous distribution of elements so that localized weaknesses in a weld can be avoided. There is tentative evidence that tungsten plays an important role in the creep ductility of P92 and that variations in tungsten and silicon could lead to an optimization of creep strength.
The outcomes of this thesis facilitate a better understanding of P92 parent metal and welds and provide results that are immediately applicable and useful to the power generation industry.
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