Phase transformations in welded supermartensitic stainless steels
Supermartensitic stainless steels have recently been introduced in the oil and gas industries to substitute more expensive duplex stainless steels for onshore and offshore tubing applications. Although easily joined by arc welding processes, the service life of the supermartensitic welded joint in corrosive environments relies to a large extent on the behaviour of the heat-affected zone (HAZ). The microstructure of the HAZ in these new materials has, until now, received little attention. The work presented in this thesis is concerned with the experimental study of the microstructure developing in the HAZ of a range of supermartensitic stainless steels, with particular attention to the retention of the potentially detrimental δ- ferrite phase, and the development of a model to facilitate the choice of a suitable post-weld heat-treatment temperature. The microstructural examination of a variety of welds revealed the presence of retained δ-ferrite in dual-phase and grain-coarsened HAZ regions. Under normal welding conditions, δ-ferrite retention was more pronounced in dual-phase HAZ and in molybdenum containing alloys. However, in multipass welds, δ-ferrite distribution was non-uniform as a result of reheating effects. A number of methods of estimating δ-ferrite retention have been employed. The most corrosion-resistant alloys were successfully predicted to be more prone to δ- ferrite retention. Charpy impact toughness tests performed on simulated heat-affected zones revealed a weak effect of δ-ferrite on the HAZ ductile to brittle transition temperature. In fact, due to its larger grain size, the coarse-grained HAZ determined the impact properties of the tested HAZ. Finally, a neural network in a Bayesian framework has been created to estimate the temperature of the onset of austenite formation as a function of the employed heating rate. The model helps the determination of the appropriate tempering temperature of welded joints to avoid partial austenitisation and can be applied to martensitic stainless steels and low-alloy steels.