Processing Thickness Window for As-Cast Ausferritic Castings

A process to obtain ausferritic as-cast microstructures through engineered cooling after solidification (without heat treatment) was developed in previous studies for a single alloy and for a specific casting (steering knuckle). However, many automotive castings that are candidates for this technology present geometries with significant thickness variations and consequently with different cooling rates. These differences can complicate or even make impossible the production of fully ausferritic as-cast parts by engineered cooling. The aim of this work is to develop an experimental model that defines the thickness window in which an ausferritic as-cast microstructure can be achieved without the use of conventional austempering heat treatment by chemical composition adjustments. Castings with different thermal moduli between 0.4 and 1.5 cm (0.16 and 0.59 in.), and three chemical compositions (in the range 3.0-5.0% Ni; 0-0.2% Mo and 0.1-1.0% Cu by weight) were produced through engineered cooling after solidification. The mechanical properties of these castings were related to their different thicknesses, thermal moduli and/or cooling rates, ensuring that they can meet the requirements of the ASTM A897/A897M-06 (2011) and UNE-EN 1564-12 standards. A shakeout temperature window has been defined taking into account the different thermal moduli of a given casting. A transformation temperature range has also been defined where fully ausferritic microstructures could be achieved. The Excel spreadsheet model calculates the transformation temperature of the different sections of the casting and establishes if they are within the correct range.