A widely used base material for line pipes are thermomechanically treated microalloyed steels, e.g. an X-80 steel grade. It offers a combination of required mechanical properties such as high strength, ductility and excellent weldability. These properties are determined by the microstructure of the steels, which result from thermomechanical processing and also evolve during welding. A key parameter during microstructure evolution is the austenite grain size. Classical metallographical methods allow to measure austenite grain sizes only after quenching the samples to room temperature. However, the grain size evolution can be in-situ monitored by novel experimental techniques such as high temperature laser scanning confocal microscopy (HT-LSCM) and laser ultrasonics for metallurgy (LUMet). Both techniques HT-LSCM and LUMet have advantages and drawbacks when investigating grain growth. HT-LSCM is a surface technique, i.e. it is only applicable to microstructures where the microstructural parameters do not alter significantly from surface near regions to the bulk of the material. The HT-LSCM is a powerful technique as the microstructural changes at the surface can be observed directly. With LUMet the attenuation of the ultrasonic signal can be correlated with the average grain sizes in the bulk material during heat treatment, however, information about the grain size distribution is not available. It is the goal of this work to compare and combine the HT-LSCM and LUMet results of grain growth experiments in a selected X-80 steel. Both techniques complement each other and their combination provides in-depth information on austenite grain growth behaviour. The grain size distributions will be compared to theoretical distributions and their evolution. In particular bimodal grain size distributions containing also comparatively large grains can be identified by an appropriate evaluation of the HT-LSCM results.