MACHINABILITY T
CAST IRON
The effect of aging on material properties determines the optimal machining time. SIMON N. LEKAKH AND VON L. RICHARDS, MISSOURI UNIVERSITY OF SCIENCE AND TECHNOLOGY, ROLLA, MISSOURI
here are various ways to improve casting machinability. Natu- ral aging’s effect on cast iron machinability differs based on its alloying elements, which are nitride-forming (titanium), carbide- forming (chromium) and nitrogen mobility modifiers (manga- nese). Relationships between cast iron aging and casting machin-
ability have been verified in multiple laboratory and industrial tests, with respect to parameters such as cutting force, tool wear, surface quality and dimensional accuracy. A recent confirmation test verified the optimal aging time for a specific composition to improve gray iron machinability.
Cast Iron Natural Aging Understanding how age strengthening affects machinability enables manufac-
turers to schedule the optimal window for machining. Room-temperature aging phenomena has been documented for different types of ferrous alloys, including cast irons and steels. In gray cast iron, tensile strength increased by 5%-15% after 5-30 days of room-temperature aging. Aging studies in quenched iron-based alloys indicated a three-stage precipita-
tion process. In some cases, a dip in strength is observed during the start of the aging process. Elevated temperature aging kinetics in the cast iron revealed typical age strengthening curves obtained at different temperatures (Fig. 1). An Arrhenius plot was constructed using the rate constants versus the reciprocal of the absolute temperature (Fig. 2).
Aging sometimes improves iron casting machinability, depending on its material properties.
Fig. 1. These graphs show strengthening of cast iron during aging (a) at room temperature, (b) at 360F (182C) and (c) at 545F (285C). January 2014 MODERN CASTING | 35
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