. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CASTING Q&A
Q A
Beware of Inadvertent Heat Treating
Our metalcaster bakes our
356-aluminum castings, which do not require heat
treatment, in the oven for three hours at 900F (482C) to remove sand cores. Will this affect the castings’ properties?
Sand cores are used in sand casting and semi- permanent mold casting to create hollow interior passageways. The cores
can be diffi cult to remove, particularly from complex castings. Metalcasters employ a variety of means to remove the cores, including baking them out at high temperatures. The American Foundry Society’s Aluminum Casting Technology rec- ommends a temperature of 1,000F (538C) for 12 hours for T6 solution
heat treatment and 650F (343C) for two hours for annealing. At 900F for three hours, the bake-out process will have some annealing effect on a metal casting’s properties. When the castings are air cooled, they will become annealed, which will change the fi nal properties. Most likely, the yield strength will be reduced, and the percent elongation will increase. The service requirements of the fi nal component will determine if this change is acceptable. If the compo- nent does not require heat treatment and the mechanical properties of the as-cast condition (F temper) are satisfactory, these reduced properties may not be signifi cant. However, the practice raises the fol- lowing concerns: Furnace Temperature Accuracy—How
accurate is the temperature of your metalcaster’s core bake-out oven, and how is it measured? What is the thermal profi le in the oven? Some are simple, direct-fi red gas ovens without the forced air circulation common in
heat treatment furnaces. In this type of furnace, there may be large temperature variances within the furnace, and it might not be safe to use a temperature as high as 900F. If the temperature measurement is inaccurate, some parts could reach temperatures above the eutectic, where some incipient melting might occur. Distortion—If the part temperature
reaches 900F, there could be distortion during cooling. The metalcaster should check the part for dimensional accuracy before shipping it. Alternative Processes—Many new
sand binders are designed to break down at the lower temperatures of aluminum casting and provide easier removal. Switching to this technology or making other process alterations could eliminate your metalcaster’s need for the core bake-out oven.
METAL
Recommendations are the opinion of the American Foundry Society Tech- nical Department. If you need assis- tance with a metalcasting issue, email
aspada@afsinc.org.
Cores in complex castings, like the one visible in this large engine block casting, can require special processing to be removed. 48 METAL CASTING DESIGN & PURCHASING JULY/AUGUST 2011
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