In situ detection and characterisation of phase transformations in weld metals

The work presented in this thesis is concerned with the in situ detection of phase transformations in weld metals. In order to obtain favourable mechanical properties of a material it is useful to map its phase transformation behaviour. Cooling rate and chemical composition are factors of great influence to the transformation behaviour. For a given composition, a continuous cooling transformation (CCT) diagram can be used to illustrate this behaviour for a number of cooling rates. Usually these diagrams are constructed from data obtained through dilatometry, which is an expensive and, in the case of welding, not always accurate method. The aim of this work was to develop a new methodology and a set of tools for the construction of weld metal CCT-diagrams. The methodology is based on analysis of weld thermal histories acquired from temperature measurements in the fusion zone. The temperature was measured with thermocouples, logged with LabView and analysed numerically. The microstructure was characterised with light optical microscopy and field emission gun scanning electron microscopy. Hardness testing was performed using the Vickers technique. A description of the methodology is given and the results of an analysis of two low alloy weld metals are reported for the purpose of demonstration. Through the demonstration it becomes clear that the two central parts of the methodology, microstructural characterisation and thermal analysis, in combination offers the information needed to produce continuous cooling transformation diagrams.Validerat; 20101217 (root