Determining Inoculants' Effects on Gray Iron Machinability.

In an effort to improve machine tool life, the effects of three inoculants on the machinability of a gray iron plate casting were studied.

Many efforts have been made to advance the "art" of machining, but machinability, as a material property, remains difficult to define in gray and ductile irons. This is especially true in operations, like drilling, which involves more than one mechanism of metal removal and more than one type of "chip." In past machinability trials, consistent results have been an issue because some of the machined chips have been tested as microscopic particles and others as continuous ribbons.

This article explores the effect of inoculants on the machinability of gray iron. A plate casting was drilled using a CNC machine to determine if the type and amount of various inoculants had a measurable effect on machinability and tool wear.

Iron Microstructure Development

Minimizing iron carbide formation in iron (to create an easier-to-machine condition) demands that graphite form during eutectic solidification. Inoculants provide substrates upon which graphite can begin to grow. The same additions are effective in both gray and ductile irons, although the more effective substrates for heterogeneous graphite nucleation include salt-like carbides [calcium (Ca), aluminum (Al) and silicon (Si) carbides], graphite particles, sulfides that may form during cerium (Ce) and magnesium (Mg) treatments, and boron nitride. The localized, high-Si areas resulting from additions of Si-rich metals also aid graphite nucleation.

Graphite growth also requires sufficient time for carbon (C) to diffuse from the austenite over distances of 2.5-25 microns and attach to existing graphite. The conditions that make it difficult for the austenite to decompose into ferrite and graphite include:

* cooling rates that do not provide sufficient time for C diffusion;

* elements that retard C diffusion or retard attachment of the C to pre-existing graphite;

* elements that reduce the free-energy difference between graphite and iron carbide.

Factors in Tool Wear

Factors that can influence tool life and machinability include metallurgical conditions, such as graphite size and distribution, composition, ferrite/ pearlite ratio, cooling rate from the eutectic through the eutectoid temperatures, and the presence of either endogenous or exogenous inclusions (Fig. 1.)

Plastic deformation produced by the advancing machining tool generates heat, and the metal being removed also impinges on the rake face of the tool, producing frictional heat. Under some circumstances, the heat and abrasion cause craters to develop on the tool rake face.

In addition, some of the C dissolved in austenite during eutectic solidification must diffuse from the austenite and migrate to graphite flakes or nodules. Because elements that inhibit this C diffusion produce austenite supersaturated with C, high cooling rates from the eutectic to the eutectoid temperature may not provide enough time for the C to diffuse. The supersaturated austenite then decomposes in the eutectoid range to produce abrasive microcarbides in the iron matrix. Massive carbides formed during solidification are hard and can degrade the machining characteristics by chipping or breaking tool tips.

The volume and distribution of the graphite also may affect the friction characteristics of the iron in contact with the rake and flank faces of the cutting tool. The friction characteristics affect the amount of heat, and higher tool temperatures generally cause faster wear. The graphite distribution also affects the mechanical strain at the tool tip and the size of the chip formed.

In addition, molding and metal handling practices can introduce oxides into the metal that abrade, wear and chip cutting tools. Sand grains picked up from the mold degrade tools due to their abrasiveness.

Casting Procedure

Class 40 gray iron plate castings (Fig. 2) were produced in a commercial foundry. The metal for the gray iron heats was melted in an induction furnace and tapped into 1000-lb pouring ladles. Preweighed inoculating additions were made as the metal was tapped into the pouring ladles.

The ladles were inoculated with 0.2% additions of 50% ferrosilicon (FeSi) containing strontium (Sr), 75% FeSi containing Ca and Al and 40% FeSi containing 9.1% Ce. The compositions of these inoculants are given in Table 1. The Sr inoculant had a mesh size of 3/8 x 32; the 75% FeSi had a mesh size of 3/8 x 30; and the Ce-bearing inoculant had a mesh size of 5 x 30.

Three sets of castings were poured in nobake sand molds, and each mold contained six plate castings. Four molds were poured from each ladle to produce 24 plates, and two ladles were poured with each inoculant to produce a total of 48 plates for each inoculated condition. The test castings were cooled to below 500F (260C) in the mold before shakeout.

A fourth set of castings was poured in green sand molds after inoculating with an addition of 0.2% Sr-bearing 50% FeSi. These castings also were below the critical temperature at the time the molds were opened and the castings removed.

All test castings were stress-relieved for 1 hr at 1050F (566C), which is the normal production practice in this foundry. Chill bars were poured with each experimental condition. No significant chill was present in any sample. Ladle temperatures were measured on every tap before pouring, and the pouring temperatures were 2700-2720F (1482-1493C).

Samples for microstructural examination and physical property determinations also were removed from the test plates. Cylindrical specimens were examined for optical microscopy, matrix and surface microhardness, eutectic cell count, Rockwell hardness, density, chemical composition and compression properties.

Machinability Evaluation

The castings were drilled on a computer numeric control (CNC) milling machine, and the tool wear was measured throughout the experiment. There were recessed panes on the bottom of the plate to prevent the drill from penetrating the CNC bed. Drills were periodically removed from the CNC, and the amount of drill wear was measured as progressively more holes were drilled.

All drills were from the same production lot, heat-treated at the same time and ground on the same drill-producing machine. Each 0.25-in.-diameter M7 high-speed steel drill was examined under a low-power microscope for uniformity of the cutting edge before being used. The presence of a chipped cutting edge or other unusual feature caused the drill to be discarded.

Drillability experiments were conducted using at least four drill speeds with each material, and either two or three drills were used at each speed selected. All experiments were performed at the same feed per revolution (0.009 in./revolution), and 120 holes were drilled in each plate.

Prior to the drill wear experiments, all plates were center-drilled to a depth of 0.02 in. at each target location to minimize drill bending caused by asperities on the plate surface. The chisel edge was guided by the pilot hole, but the pilot hole was so shallow that the cutting edge encountered the as-cast surface.

Drillability experiments were conducted as follows:

* drills were examined under an optical comparator to ensure there were no chipped edges;

* drills were positioned in the tool holder so the cutting edge was parallel to the x-axis of the optical comparator;

* coordinates were recorded so that the same position could be found in successive measurements (these coordinates established a standard reference point);

* progressive tool wear measurements were made as holes were drilled in the plates, by digitizing the wear area on one cutting edge. Tool wear measurements were performed after drilling the first and second hole and then at various intervals, depending on the rate of tool wear;

* drills and chips from the experiment were archived for future examination.

After the flank wear pattern from the chisel edge to the lip of the drill wear was digitized, recorded and stored on a computer connected to the optical comparator (Fig. 3), the average wear values were used to prepare tool wear curves that illustrate the progressive tool wear with the number of drilled holes in each material at a given speed.

Observed Results

Tool wear data was obtained from each material and drill and was summarized as part of each wear curve (Fig. 4). The data included the number of holes drilled before tool "squeal" (if holes were drilled after this point, the tool would weld to the plate), the number of holes drilled to produce either 6.5 mils (0.0065 in.) of average flank wear, the least squares linear slope of the drill wear curve in the linear wear region, and the average volume of metal removed per drill before failure.

In general, the tool wear curves can be divided into three distinct regions: an initial break-in period during which considerable wear occurs as the first few holes are drilled; a steady-state wear region during which the tool flank wears at an approximately linear rate; and a region in which squeal failure occurs.

The linear wear rate is a function of the speed of cutting and the metallurgical characteristics of the material being drilled. The important metallurgical characteristics include pearlite content, pearlite microhardness, microcarbide content of the pearlite, graphite spacing and the modulus, yield strength and the strengths of the iron.

Processing Effects

Processing effects on drill wear are presented in Table 2. The rate of tool wear increased with inoculating additions of 75% foundry grade FeSi, and Ce-bearing FeSi produced higher wear rates, compared to Sr-bearing FeSi.

It was concluded at the 99% confidence level that the tool speed influences tool life, as expected from shop experience. The inoculating alloy was also found to have a statistically significant effect at the 99% confidence level at a tool speed of 98 sfm. This is the only speed below the tool wear transition point occurring at 98 sfm (1500 rpm) where data on all materials could be obtained.

Experimental Conclusions

Based on the results of the wear tests, it was concluded that:

* longer tool lives are obtained at lower tool speeds. Lower tool speeds generate less heat and permit the heat produced in the machining operation to be diffused away from the cutting surface. Lower tool speeds thereby minimize overheating the tool tip;

* class 40 gray iron inoculated with Sr-bearing 50% FeSi and poured in nobake molds exhibited the lowest tool wear rate and longest life. The Sr-bearing irons exhibited slightly lower densities, which suggests this inoculant to be a better graphitizer;

* the same Class 40 iron inoculated with Sr-bearing FeSi and poured in green sand molds produced a slightly higher drill wear rate than the iron poured in the nobake molds, perhaps because of slightly higher cooling rates;

* the number of holes that could be drilled in irons inoculated with either the foundry grade 75% FeSi or the Ce-bearing 40% FeSi was about half that attainable with the Sr-bearing FeSi at speeds below 100 sfm;

* all tool life curves obtained on the Class 40 gray iron converged at a drill speed of about 110 sfm;

* the eutectic cell counts were higher in the irons inoculated with the Sr bearing FeSi, and higher in the iron poured in the green sand mold, compared to the castings poured in nobake molds. The higher cell count in the green sand mold may be a result of a higher cooling rate during solidification;

* the average eutectic cell sizes measured in the gray iron inoculation plates were statistically different at a 99% confidence level. The irons with higher cell counts produced the lowest tool wear rate and best tool life.

This article was adapted from a presentation (99-122) at the 1999 AFS Casting Congress and is available from the AFS Library at 800/537-4237.

                        Compositions of Inoculants
                          Used to Treat Gray Iron
Element Sr 50% Si Ce 40% Si Ca Al 75% Si
        Inoculant Inoculant  Inoculant
Si       46.59%    41.08%      77.01%
Al        0.36%     0.35%       0.8%
Ca       0.014%     0.96%      1.44%
Mn        0.41%     0.33%      0.11%
Sr        0.99%    0.072%      0.093%
P        0.028%     0.44%      0.022%
Ti          -       0.08%      0.051%
Ce          -       9.08%        -
Fe       Balance   Balance    Balance
                Gray Iron Processing Effects on Drill Wear
Surface Ft per Min/
Revolutions per Min  85 sfm/1300 rpm                      98 sfm/1500 rpm
Inoculant, Mold Type       HTS [*]   Slope [**] AFW [***]       HTS [*]
0.2% Sr 50% FeSi in
Nobake Molds               n/a        n/a       n/a             644
0.2% Sr 50% FeSi in
Green Sand Molds           526       0.92       192             400
0.2% 75% FeSi in
Nobake Molds               255       1.36       151             140
0.2% Ce 40% FeSI in
Nobake Molds               202        2.0       61              87
Surface Ft per Min/
Revolutions per Min                       111 sfm/1700 rpm
Inoculant, Mold Type Slope [**] AFW [***]       HTS [*]    Slope [**]
0.2% Sr 50% FeSi in
Nobake Molds         0.76       332             100        1.86
0.2% Sr 50% FeSi in
Green Sand Molds      5.2       208              73        1.03
0.2% 75% FeSi in
Nobake Molds          2.2       162              96         5.8
0.2% Ce 40% FeSI in
Nobake Molds          5.4       76               77         7.9
Surface Ft per Min/
Revolutions per Min            124 sfm/1900 rpm
Inoculant, Mold Type AFW [***]       HTS [*]    Slope [**] AFW [***]
0.2% Sr 50% FeSi in
Nobake Molds         74               36        4.64       56
0.2% Sr 50% FeSi in
Green Sand Molds     66              n/a         n/a       n/a
0.2% 75% FeSi in
Nobake Molds         83               42         3.7       102
0.2% Ce 40% FeSI in
Nobake Molds         55               26         4.0       37
Surface Ft per Min/
Revolutions per Min  144 sfm/2200 rpm
Inoculant, Mold Type       HTS [*]    Slope [**] AFW [***]
0.2% Sr 50% FeSi in
Nobake Molds                15         n/a       63
0.2% Sr 50% FeSi in
Green Sand Molds            13         6.9       47
0.2% 75% FeSi in
Nobake Molds                21         4.9       81
0.2% Ce 40% FeSI in
Nobake Molds                22        10.1       36
(*.)Holes Drilled Before Tool Squeal
(**.)Slope of Wear Curve
(***.)Holes Drilled fo Produce an Average 0.0065 in. Flank Wear
 n/a=not available