The evolution of austenite grain sizes is investigated by two in-situ techniques; laser ultrasonics for metallurgy (LUMet) and high temperature confocal scanning laser microscopy (HT-LSCM). The real-time evolution of the mean grain size is monitored during the heat treatment by means of LUMet, whereas the evolution of the surface grain structure is captured from in-situ micrographs obtained by the HT-LSCM technique. The mean grain sizes obtained from LUMet agree reasonably well with those determined from HT-LSCM measurements indicating that grain growth at the sample surface is representative for grain growth in the bulk. The evolution of the grain size distribution during heat treatment is obtained by HT-LSCM measurements. As LUMet only allows to determine mean grain sizes, the microstructural information obtained from HT-LSCM measurements complements the LUMet results. In addition, austenite reconstructions from electron backscatter diffraction (EBSD) were conducted and confirmed the results of the in-situ measurements.