Fig. 5. a) Expected age strengthening kinetics for tested composition and (b) effect of aging time on cutting force (black markers) and tool wear (red markers) are graphed.
the iron can be caused by elevated ni- tride forming elements (particularly ti- tanium) relative to nitrogen. Additions of nitrogen to iron are possible and can enhance aging. T ermodynamic data can be applied to determine if there is enough “free nitrogen” to age a cast iron. A simplifi ed criterion might be: If %N < (0.15-0.20) %Ti, aging will not occur. Second scenario: If cast iron exhibits aging, this phenomenon can be used for improving casting machin- ability. Aging is accompanied by de- creasing cutting forces and tool wear. T ese irons have enough free nitrogen to promote age strengthening. Decreased cutting forces and
increased mechanical properties were proven in laboratory castings having diff erent carbon equivalents. T ese irons had some free ferrite and no free cementite or steadite. Optimal aging time depends upon particular “free manganese” content and could be evaluated. Decreasing aging time for improving machinability could be done by warm (slightly elevated temperature) aging. T ird scenario: Gray iron has
elevated concentrations of carbide forming elements such as chromium in addition to a large percentage of phosphorus. T ese combinations of chemistry with a particular cooling rate could promote steadite/cementite formation in fully pearlitic matrix. If this iron has negligible free ferrite, aging will increase cutting forces in this iron. Eff ective inoculation and chemistry control will aff ect casting machinability interaction with aging in these cast irons. However, in this scenario, “fresh” castings might prove more machinable.
Confi rmation Test Five AFS 5J, 10-in. diameter test
articles were poured into nobake molds from one 200-lb. induction furnace heat. T e cast iron chemistry is shown in Table 2. Microstructure was mostly pearlitic with approximately 5%-10% ferrite. Measured hardness in the middle section of the test article was 200-210HB in the as-cast condition (unaged). T e as-cast surface layer (1/8 in.) was removed in preliminary ma- chining to avoid the eff ects of cast sur- face structure, mold-metal interaction and geometry variance on test results. Test articles were face CNC machined at day 0, day 5, day 9, day 15 and day 22 with measurement of cutting forces. Eight cuts (30 min. total machining
time) were performed from each disc, using a new tool insert each time. T e thickness of the test article produced eight duplicate cuts and each test was repeated twice. T e test results are shown in Fig. 7. T ese test results were compared to
the predictions according to suggested methodology. Step 1—Evaluation of the possible
age strengthening: Nfree = N-0.20Ti = 0.01-0. 2*0.008 = 0.0084 wt.% or 84 ppm; total %N and %Ti leads one to expect approximately 0.14 wt. % Fe4N. Age strengthening will occur. Step 2—Control microstructure:
In a matrix without free carbide/ steadite having a small amount of free ferrite around fl ake graphite, age strengthening can improve casting machinability according to the second scenario (Table 1). Step 3—Aging time: Full aging time is 15-17 days and prestrengthen- ing time is 7-9 days. T e tool force dropped signifi cantly during the fi rst
fi ve days and was also low at 15 days, roughly corresponding to the ex- pected times for room-temperature age strengthening. T e predictions based on the previ- ous studies were confi rmed. A signifi - cant decrease in cutting force and stan- dard variation were observed after 9-15 days of natural aging, which is between predicted prestrengthening and full aging time. Regarding other machin- ability parameters, tool wear not only depends on the average value of cutting force but also the stability of cutting process, and tool wear continued to decrease up to the full aging time. T ese rules can assist in determin- ing the optimal machinability window for aged cast iron: Estimate free nitrogen based on
total nitrogen and concentration of ti- tanium as %N >0.2 %Ti, but not high enough to form gas porosity in order to have age strengthening. Check microstructure concern-
ing ferrite/pearlite content without steadite/carbides. If no free ferrite is present, particularly with all pearlite and some carbide or steadite, the machinability might be better with fresh castings given a composition that will age strengthen. If free ferrite is present, age strengthening will provide a corresponding improvement in machinability. Estimate room-temperature aging time based on free manganese left after sulfi de formation. Acceleration of aging with a low temperature bake is possible.
This article is based on a technical pa- per (12-026), “Aging and Machinability Interactions in Cast Iron,” presented at the American Foundry Society’s 116th Metalcasting Congress in 2012.
Sept/Oct 2013 | METAL CASTING DESIGN & PURCHASING | 29
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