ALUMINUM FOR GRAIN-SIZE CONTROL

Recently, difficulties have been experienced by heat treating companies with the aluminium contents of steels1. The aluminium-containing steels may suffer grain refinement with an attendant loss of hardenability, and fail to harden. Basic theory and data concerning aluminium nitride and grain growth have been used to indicate the limiting aluminium and nitrogen contents that would be required in order to permit austenite grain growth to a level of ASTM No. 4, which can be regarded as a 'coarse-grained' steel for hardenability purposes. The borderline between 'coarse' and 'fine' grained steel chemistries can be related to the austenitising parameters (i.e. time and temperature) that are important in controlling the volume fraction and particle size of aluminium nitride. Attention is drawn to the ineffective aluminium present as alumina in aluminium-deoxidised steels.