A previously developed heat transfer model is used to study the influence of welding parameters on weld defect development in thermite welds. Weld defect formation maps are constructed from a series of heat transfer simulations. For the current normal rail thermite welding conditions, it is found that shrinkage cavity formation can be avoided but cold-lap and centre-line defects are likely to occur. This study also shows that increasing the preheating time or the liquid temperature can suppress the development of cold-lap and centre-line defects but can lead to a slightly higher microporosity content in the railhead as well. Alternatively, it is found that instead of increasing the preheating time or the liquid temperature, the same effect can be achieved by increasing the 25 mm weld gap (standard) to a slightly larger value.