The diffusivity of interstitial solutes is orders of magnitudes higher than that of substitutional solutes for most industrially relevant temperature ranges. Due to this gap in diffusivity some transformations take place with negligible partitioning of substitutional elements. Transformations which occur with partitioning of substitutional elements are believed to be slow compared to transformations with negligible or no partitioning. Thermodynamic calculations for this boundary between fast unpartitioned ferrite growth and slow partitioned growth (NPLE/PLE boundary), phase diagrams and diffusion-controlled transformation simulations with local equilibrium and paraequilibrium (PE) conditions were carried out. The calculated data are compared to dilatometry measurements for the austenite to ferrite transformation of manufactured model alloys based on the ternery system Fe-C-Mn and the corresponding quartery system expanded with Cu. Copper is chosen because of its appearance as tramp element in the EAF-route for steel making. The observed growth behavior of ferrite is analyzed and discussed using calculated data for different interface conditions.