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Machinability of Gray Cast Iron - Experimental Procedures

• Casting production
• Lathe machinability evaluation
• Specimen characterization
• Statistical analysis

Casting production

Three gray irons were produced for the study including a 67 mm diameter cylindrical bar designated G1 and two 77 mm diameter cylindrical bars designated G2 and G3, respectively. The composition of the irons is given in Table 1. To produce the bar stock, molten iron was placed in a holding crucible and pulled horizontally through a water-cooled graphite die located near the crucible base. This process gives a solid outer rim and a molten core as the iron moves through the die leading to a finer graphite and higher ferrite content in the rim than in the center of the bar. The irons were inoculated with proprietary alloy containing manganese and zirconium. The irons were air-cooled, notched and broken off into six and twelve feet lengths prior to shipment to UAB.

Lathe machinability evaluation

Three-quarter inch by three-quarter inch square, un-coated fine-grained carbide (SPG 631) inserts were used to perform the tool life (wear) experiments. The inserts were 0.1900 inches thick with an 18 degree taper. Before being used, each insert was examined for cutting edge uniformity. The presence of a chipped edge or other anomalous feature caused the insert to be discarded. The insert was placed on a gauge block with parallel sides such that three of the corners overhung the gauge block and the fourth corner overhung a center hole on the block. This prevented any edge build-up on a cutting edge from affecting the wear pattern determination.

Table 1: Composition of Experimental Gray Irons (Weight Percent)

The iron was sectioned into seven inch lengths and mounted in the hydraulic chuck in the lathe. Tool wear was initiated using face cuts on the lengths. The flank of the tool showed progressively more wear as more metal was machined. The area and shape of the wear land on each insert edge was recorded periodically by removing the tool from the CNC lathe and digitally recording the wear pattern. The rate of flank wear was related to the machinability of the material by plotting the average flank wear as a function of the volume of material removed.

Each insert was used until it had an average wear of 0.015 inches across the cutting edge or until it had been used to remove at least 70 cubic inches of metal. Care was taken during handling to prevent inserts from chipping or shattering. Gross fractures caused by aggressive cuts did not produce a natural wear pattern and the associated measurements were rejected.

The insert wear rates obtained with each material were determined using the following procedure:

1. The insert was examined under an optical comparator to ensure there were no chipped edges.

2. The insert was positioned on the gauge block so the cutting edge was parallel to the x-axis of the optical comparator. The origin of the cutting edge was considered to be the extreme point on the cutting edge located at the intersection of the horizontal and vertical axis. This location was established by rotating the cross-hair of the comparator Y axis until it was aligned with the clearance plane of the edge. This rotation of the cross-hair was temporary and necessary for establishing the origin. As the wear profile was traced, the cross-hair was rotated back to a normal orientation. This procedure established a reference point and prevented edge wear at the tip of the insert from affecting the measured flank wear area.

3. The coordinates of the datum were recorded so the same position could be found in successive measurements.

4. The amount of tool wear was measured as progressively larger volumes of metal were removed from the experimental castings. The frequency of the measurements depended on the tool wear rate being experienced.

5. The inserts were catalogued and archived for future examination.

Turning experiments were performed using at least four speeds on each material, and triplicate experiments were performed at each speed. All experiments were performed at the same feed per revolution (0.030 inch). The CNC lathe was programmed to keep a constant surface speed as face cuts were made across the castings.

The insert wear land area was digitized from the original edge (which may have been worn off by successive passes) to a length of 0.042 inches (engagement length corresponding to a depth of cut of 0.03 inches and an angle of approach of 45 degrees) using an optical comparator.

A schematic illustration of a worn insert is shown in Figure 2. Flank wear areas and lengths were measured, and the flank area divided by the flank wear length provided the “average” flank wear.

Figure 2: Schematic of Worn Tool

A representative tool wear curve for one of the irons is illustrated in Figure 3. The solid line drawn through each data set represents the best linear fit to the insert wear data, and the dashed lines on either side of the solid line represent one standard deviation in tool wear. Tool wear data obtained under specific conditions is summarized in the boxes on each of the graphs. The data include the volume of metal machined away before the tool reached a specific amount of average wear, usually 6.5 mils, and the least squares linear slope of the wear curve. The data obtained with each insert edge is plotted with a different symbol so the wear on any insert can be followed if desired.

Figure 3: Representative Tool Wear Curve for Gray Iron

Specimen characterization

One inch thick sections were removed from each casting for evaluation of the microstructure and certain physical and chemical properties. Samples for microstructural evaluation were removed at ½ radius to avoid rim effects. The samples were etched with Stead’s Reagent (2 grams CuCl2-2H2O, 8 grams MgCl2-6H2O 4 ml HCl 100 ml grain alcohol) and the eutectic cell size and count were determined manually on 25X magnification optical microscope images. The cell count was determined by counting the number of cells within each image while the cell size was determined using a line intercept count. The volume percent graphite (Vv), ratio of surface area of graphite to volume of material (SV), mean spacing between graphite flakes, and graphite flake thickness were made. Procedures for mounting and examining the specimens used for microstructure have been previously published. (Griffin et.al.)

Scanning electron microscope (SEM) analysis using energy dispersive x-ray spectroscopy (EDS) was used to identify the hard inclusions on nital etched samples. Quantitative measurements of the volume percent and number density of hard inclusions including alloy carbo-nitrides, massive carbides and steadite, and complex carbides were performed on samples that were etched with nital (4% nitric acid in methanol). The measurements were performed on one hundred images collected at 500X magnification on an optical microscope. Measurements of the area of the inclusions were performed using an image analysis system. The area of each alloy carbo-nitride found was estimated by measuring the diameter and calculating a corresponding area. Each steadite and eutectic carbide identified was traced and the area determined with the image analysis system.

Chemical compositions and combined carbon measurements were also performed on the irons. Mechanical properties measured included tensile properties, compression properties, Brinell hardness measurements, and pearlite microhardness. The foundry that produced the irons provided tensile results. Pearlite Vickers microhardness was measured on polished and Nital etched samples using a diamond indenter with a 50g load. Images of the indents were acquired at 500X magnification and measured using an image analysis system. Twenty measurements were made on each specimen but only the top ten microhardness measurements were used for calculating average pearlite microhardness to attempt to avoid contributions from graphite.

Brinell hardness measurements were made with a 10 mm diameter steel ball indenter and 3000 kg load on one inch thick sections that had been ground plane parallel prior to indentation. An automated image analysis system was used to measure the Brinell indentation diameters.

Statistical analysis

Statistical analysis was performed to determine if there were significant differences between measurements on the materials. A confidence level of 95% was the cut-off used for statistical significance. The data was also statistically analyzed for factors that might influence machinability. While the overall machinability data that is presented is an average of the results from three tools, the statistical analysis used the results for each tool. The analyses are based on the volume of material removed at 650 surface feet per minute (sfm). Only linear and exponential fits between the factors and volume of material removed were considered and only variables that had a p statistic at the 95% confidence level (p<0.05) or better were considered to be a non-random event. In addition to the probability that a variable has a significant contribution to the response being measured, the R2 statistic was also considered. The R2 statistic gives a measure of how much of the variation in the dependent variable can be explained by the independent variable. Values of R2 less than about 25% are considered to have a weak effect; values ranging from about 25% to 55% have a moderate effect; and values above 55% are considered to have a strong effect.

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