Aging for improved machinability: waiting a few days after an iron part is cast has been shown to make machining operations easier.

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Cast gray iron exhibits an increase in mechanical properties after aging at room temperature due to the precipitation of iron-nitrides. According to the results of a recent study, aging also could have a beneficial effect on the material's machinability.

In the study, a computer controlled cutting tool force measurement system for facing cuts was used for quantitative evaluation of this potential effect. Tests were performed in a laboratory setting, as well as on industrial cast brake disks produced at a metalcasting facility according to typical operations. Results from the tests showed significant decreases in machining cutting forces as a result of natural aging. Further research was conducted in which tool forces, tool wear and machined-surface roughness were measured after machining industrial cast brake disks. Results from the tests showed the machined tool force is significantly less on aged brake disks, and a model describing how aging has an effect on machinability was developed.

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According to these results, tooling cost savings can be achieved if cast iron is machined on the optimum day of machinability, which was suggested to be five days of aging, although tool wear still can be reduced after this day. Metalcasters can collaborate with their casting purchasers and machine shops to determine the cost benefit of aging iron castings for improved machinability and to accommodate the extra time and floor space needed for the aging.

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Measuring Tool Wear

Age strengthening does not occur for all cast gray iron alloys and melts. Aging is related to the nitrogen content of the iron. High ratios of nitrogen to nitride forming elements provide favorable conditions for iron aging. In order to promote aging, the casting facility will minimize nitride-forming elements, such as titanium and aluminum, and elevate nitrogen content, without exceeding the solubility limit with respect to gas defect formation.

Metalcasters and their customers will have to weigh the amount of savings incurred through reduced tooling wear and machining times against the effect that nitrogen additions to the melt and reduction of nitride-forming elements may have on the desired properties of the material. Similarly, too much nitrogen could lead to potential gas porosity.

In the first round of experiments, both laboratory and industrial castings were produced in class 35 gray iron with low titanium and high nitrogen content. The laboratory castings, produced in nobake molds with ceramic filters, were machined shortly after production and after several days to allow natural aging of the iron. For the industrial trial, the iron aging effect was confirmed by testing mechanical properties of multiple samples poured at the same time as the castings, which were 10-in. diameter passenger car brake disks.

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Two sets of machining tests for tool force analysis were performed, and three machining parameters were varied, including depth of cut, linear surface speed and feed. The cast brake disks underwent normal machining procedures at the metalcasting facility. On day one, 50 castings were machined using one set of inserts. On day 10, 50 more castings were machined using another set of inserts. In addition, a 200 casting machining run was conducted on day 10 with another set of inserts.

Two data sets were collected during machining of the industrial castings. The first set represented measurement of "tilt," which is a cross-sectional taper of the brake disk flange from increasing cutting force caused by extensive tool wear. The second data set analyzed represented tool wear, which was measured using image analysis (Fig. 1).

Decreasing carbon equivalent caused increases in hardness in both the aged and unaged laboratory castings. All aged castings had higher values of hardness than the unaged castings. Measurement of tool force during six sequential facing cuts using constant machining paramenters showed the tool force increased as a result of tool wear, and in all cases, the value of cutting force was large for castings in the unaged condition (Fig. 2).

The statistical results from mechanical testing of the industrial castings in the unaged and aged conditions are given in Table 1. The data showed that aging resulted in a statistically significant increase in mechanical properties. Further, comparing the tilt data showed that machining the 50 unaged castings required two tool position changes, while no tool position changes were required during the machining of the 50 castings aged 10 days (Fig. 3). Tool wear on the top view area, side view area and side view depth, depicted in Fig. 1, also was significantly greater in the unaged castings compared to the aged castings (Fig. 4).

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Machinability's Curve

Following the findings in the first experiment, a second study was conducted to investigate how tool forces can serve as an indicator of tool wear rate before and after aging. The results narrowed down the day at which an aged casting exhibits optimum surface finish, which can be used as a gauge of machinability.

In the experiment, 10-in. diameter passenger car brake disks were cast from class 35 gray iron in green sand molds. Ladles were inoculated with 4.4 lbs. of casting grade FeSi with a 0.66-lb. nitrided FeMn addition to increase the nitrogen content for greater aging. Five disks were machined shortly after production, five other disks were machined after five days of room temperature aging and another five disks were machined after 50 days of aging. The iron aging effect was confirmed by the ultimate tensile strength change compared to ASTM B-bars poured alongside the castings.

Tests on the tensile bars showed a 14.3% increase in strength after 50 days of room temperature aging. The strongest unaged bar possessed less strength than the weakest aged bar.

Tool force measurements on the 15 brake disks were used to gauge the effect aging has on tool wear. Statistical comparison of the aging effect on normal forces for the first machining cut indicated that day five tool forces were less than production day forces, although cuts following the initial first cut did not show significant differences in tool force. Average forces for all cuts on the brake disks aged for 50 days were less than those on the unaged disks.

Measurements of tool wear on the face (top) of the machining tools revealed that castings machined in the aged condition tended to cause less tool wear than those machined ,in the unaged condition. The results also suggested that machining the disks aged for five days resulted in the least tool wear measured in the experiment, although previously published work suggests a continuous increase in tool life.

Surface roughness after machining was found to be greater on the castings aged 50 days than on the castings machined on the day they were cast. Castings aged five days were found to be smoother than castings machined on the first day, as well as castings aged 50 days (Fig. 5). This suggests that the chips formed during machining separated from the casting most easily after five days of aging.

Pinpointing Optimum Machinability

The premise that disks aged for five days had the lowest normal force supported other data trends observed during research in optical tool wear measurements, profiler tool wear measurements and machined surface roughness measurements. This corresponded with a general consensus among engineers at automotive casting facilities that machining should be performed on gray iron castings after five days of aging. Some industry representatives also have reported that optimum machinability has been achieved with room temperature aging for periods both longer and shorter than five days. Previous work has shown that aging can be accelerated by increasing temperature or delayed by increasing manganese content. The point of optimum machinability of aged gray cast iron, therefore, may be shifted forward or backward by hours or even days.

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Scientific inquirty explains why machining gray iron castings when partly aged is easier than machining unaged or fully aged castings. Kinetics studies show a dip in strength during the aging process when a small percentage of strength gained by aging is lost. In Fig. 6, a graph of the aging behavior of gray cast iron shows a dip in strength at about 300 hours. The precipitate phase or morphology in cast iron that occurs before the dip may provide the optimum balance between increased strength to fracture resulting from aging and decreased energy absorption in plastic deformation.

Metalcasting facilities with sufficient free nitrogen in ferrite solid solution to provide noticeable age strengthening should be able to utilize natural aging to improve machinability. Machining on the day of optimum machinability will reduce tool wear and ultimately tooling costs. Ongoing work and comparison to unpublished research indicates that aging may be of particular benefit to machinability for castings produced in facilities With variable surface microstructure control. Decreases in machinability associated with certain amounts of free ferrite and degenerative graphite in the casting skin can be reduced by aging.

For More Information

"Age Strengthening of Gray Cast Iron: Nitrogen Effects and Machinability," J. Edington, W. Nicola, V. Richards, 2002 AFS Transactions (02-120).

"Effects of Room Temperature Aging on Ductile Iron," V. R/chards, D. Van Aken, O. Mereau, W. N/cola, 2004 AFS Transactions (04-038).

Jared Teague, Von Richards, Simon Lekakh and Kent Peaslee, Missouri University of Science and Technology, Rolla, Missouri 

Jared Teague is a PhD candidate at Missouri Univ. of Science and Technology, Rolla, Mo. Von Richards is associate professor of metallurgical engineering, Simon Lekakh is a research engineer and Kent Peaslee is an associate professor at the university.

Table 1. Mechanical Statistical Results of
Industrial Cast Iron in Unaged and Aged Conditions

Statistical
parameter    Day 1 UTS (MPa)  Day 10 UTS (MPa)  Day 1 HBW  Day 10 HBW

Average      259.4            264.6             230.2      234.2
Standard
  deviation  5.1              4.6               3.2        1
P-Value      0.093            0.093             0.055      0.055