In situ detection and characterisation of phase
transformations in weld metals

Abstract: 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.