Table 1. Different Aging/Machinability Scenario Observed in Gray Cast Iron Scenario 1
Gray Iron 2 3
Lab (4.1% CE with Ti) Lab (4.3% CE) Lab (3.9% CE)
Industrial Brake Disks Industrial Test Articles (Ferritized) Industrial Test Articles with Elevated Cr and P
Table 2. Chemistry (wt. %) of Test Articles C
Si 3.26 2.03 Mn 0.61
iron showing increased cutting forces after aging had no free ferrite but was entirely pearlitic with cementite/stea- dite phases. Tis differing behavior of aged cast
irons depending upon metal matrix relates to the energy of chip formation. Although gray cast iron is a brittle ma- terial in tension, chips can experience significant plastic deformation because the stress state during machining is dominated by compression and shear. If chip formation is assumed to be a plastic strain to fracture event, then
Aging Effect Confirmation No
Yes Yes Yes Yes
Yes
Phases, % Area
Ferrite 5-15
25-27 5-7
˜1
40-60 < 0.2
Steadite/Carbides – – – –
1.8–2.0 1.8–2.0
Aging Effect on Machinability
No Effect Improved Improved Improved Improved
Increased Tool Forces, Lower Machinability
S 0.08 Cr 0.15 Cu 0.2
changes in fracture toughness would logically affect machining behavior. Fracture work during tensile testing was estimated from the stress-dis- placement curve. In the pearlitic iron, the work of fracture and cutting forces increased after aging. On the contrary, iron ferritized by
heat treatment showed decreased work of fracture and cutting forces due to aging.
Tool Wear and Industrial Machining Measurements
Tool wear is lower when machining Al 0.01 Ti 0.001 N 0.010
gray cast iron aged at room tem- perature because aged iron requires less work input from the machining center to form and break off chips. Te decrease in required work has been demonstrated by tool force measure- ments and by testing amperage drawn while machining unaged and aged iron. Te least power was required to machine castings aged for 3-6 days versus iron aged for 1, 9 and 20 days. At that optimal aging time, machined castings had better surface quality (less roughness), but all aged iron had bet- ter surface finish than unaged iron. Other tests were performed with
industrial face machining of brake discs for a passenger car. Excessive tool wear produced changing tool geometry and increased cutting forces, which promoted elastic deformation of cast- ing with increasing tilt and destroying required tolerance on perpendicularity (“tilt”). Tilt data from the machining of industrial castings were compared in two ways. Te machining of the 50 unaged castings required two tool position changes. Tool position changes were not required during machining of aged castings after 50 or 200 castings, indicating more consistent dimensions and reduced downtime for tool position corrections. Figure 5 gives a compari- son of measured tool wear for different operations. Aging decreased tool wear significantly in most of the operations.
Industrial Recommendation for Improving Cast Iron Machinability by Aging
Tree possible scenarios exist for
changes in machinability of gray iron during natural aging (Table 1). First scenario: Aging does not oc- cur and therefore, has no influence on machinability. Lack of aging effects in
28 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2013
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