Pengaruh penambahan sodium dodecyl benzene sulphonate sebagai surfaktan terhadap konduktivitas termal nanofluida berbasis carbon nanotubes sebagai media quench pada proses perlakuan panas baja S45C = The effect of sodium dodecyl benzene sulphonate addition as surfactant on the thermal conductivity of carbon nanotubes-based nanofluid as quenchant in the heat treatment process of S45C Steel