Method of Machining Steel, Malleable or Nodular Cast Iron

In the continuing research for better methods of machining metals with lower costs, particularly by use of ceramics, there has been some lack of understanding of the interdependence of thermal, physical and mechanical properties of materials needed to machine a specific metal.

Ceramic materials used to date as tool bits for cutting by the prior art have fallen into two classes: (a) Al.sub.2 O.sub.3 based ceramics that contain 10-20% of a densifying agent such as TiN, TiO.sub.2, CaO, MgO or Cr.sub.2 O.sub.3, and (b) Si.sub.3 N.sub.4 based ceramics that contain a minor amount (2-20%) of a densifying agent such as MgO or Y.sub.2 O.sub.3. Almost all of today's available commercial ceramic cutting tools are of the class (a) type. Al.sub.2 O.sub.3 based commercial cutting tools vary widely in their tool life and fail by thermal cracking or chipping attributed to inherent brittleness. The quick cure by the prior art to extend tool life has been to concentrate on physical or mechanical properties by making more rigid the holding of the tool while machining, eliminating vibrations in the machining setup, and by improving tool geometry. The poor thermal shock resistance of Al.sub.2 O.sub.3 based ceramics was ignored. Despite a variable tool life, Al.sub.2 O.sub.3 based ceramics have shown excellent chemical stability and thereby superior resistance to cratering resulting from chemical reactivity with the hot chip being removed.

Certain Si.sub.3 N.sub.4 based ceramics possess sufficient thermal shock resistance to dramatically improve tool life when machining metals that do not present a chemical reactivity problem, such as grey cast iron. However, when used to machine steel or nodular iron, the Si.sub.3 N.sub.4 based material exhibit poor resistance to wear or surface abrasion at elevated temperatures such as 1000.degree.-1200.degree. C. and exhibit poor resistance to oxidation. The tool material wears out readily as evidenced by severe crater wear. Similar cratering or wear patterns are experienced on machining steel or nodular iron, regardless of the densifying additives employed. This suggests that such poor wear resistance when cutting steel or nodular iron is an inherent defect of Si.sub.3 N.sub.4. It is believed that the existence of an extremely hot stringer or chip of stock metal, which is unsevered but shear cut by the tool, comes back into contact with the tool bit surface creating the cratering wear. In machining grey cast iron, there is very little extended chip formation that can come back or remain in contact with the tool so the circumstance for promoting cratering is avoided.

What is needed is an improved method of machining steel, nodular cast iron, or malleable cast iron, which metals each present a chemical reactivity problem. The method should utilize a ceramic material that is chemically stable, has a high thermal shock factor, and is reasonably strong at elevated tool bit temperatures.

The invention is a method of removing stock from a metal part comprised of steel, malleable cast iron or nodular cast iron. In the machining of such metal parts, a continuous chip is generated by the shear deformation of the metal. The hot chip is not easily broken and continues to slide on the cutting tool surface. While deploying a cutting tool formed of a substantially fully dense hot pressed ceramic material consisting essentially of a generally uniform mixture of 50-70% by weight Al.sub.2 O.sub.3 and the remainder Si.sub.3 N.sub.4, a relative motion is established between the tool and the workpiece similar to those obtained in milling, turning and boring operations. The tool is then advanced into the surface of the part at a predetermined depth of cut and feed rate which results in the removal of stock from the part during periods of the relative movement. In this method it is preferred that the removal of stock take place at relatively high speeds (1000-6000 sfm) and that the mass removal rate of metal be characterized by a depth of cut of 0.04-0.20 inches and a feed rate of 0.004 -0.02 inches per revolution. The prolonged life of the tool in this method is particularly evident when machining for at least ten minutes. The tool will experience no indication of thermal cracking and wear on the cutting face will be limited to less than 0.025 inches.

It has also been proposed that it could be advantageous to replace an equivalent portion of Al.sub.2 O.sub.3 in the tool with up to 15% of either ZrO.sub.2, HfO.sub.2 or Y.sub.2 O.sub.3.

Before carrying out an illustrative mode in accordance with this invention of continuous or interrupted machine cutting of solid steel, malleable cast iron or nodular cast iron stock by milling, turning or boring, the tool must be constituted of a particular mixture of Al.sub.2 O.sub.3 and Si.sub.3 N.sub.4. The ceramic cutting tool is preferably prepared by mixing Al.sub.2 O.sub.3 and Si.sub.3 N.sub.4 powders (the Al.sub.2 O.sub.3 powder constituting 50-70% by weight of the mixture) and then hot pressing the mixture at a sufficient pressure and temperature for a period of time to achieve substantially full density.

The Al.sub.2 O.sub.3 Si.sub.3 N.sub.4 powders are selected to have a chemical content containing less than one percent cation impurities, the most frequently occurring impurities being Ca, Fe, Al and O.sub.2. Individually, it is preferred that the elements be limited as follows: less than 0.5% Fe, less than 0.01% Ca, less than 0.4% Al, and less than 2.0% O.sub.2. The preferred average particle size diameter for the alumina powder is about 1-2 microns. The Si.sub.3 N.sub.4 powder should advantageously have at least 85% alpha phase Si.sub.3 N.sub.4, which is converted to the beta phase during heating and hot pressing. The selected average particle size diameter for the Si.sub.3 N.sub.4 powder is preferably 2.0-2.5 microns. The amount of Al.sub.2 O.sub.3 powder needed for a specific application is measured and weighed, the amount being in the range of 50-70% by weight of the intended mixture, and preferably 65-70% when the only other additive is Si.sub.3 N.sub.4. The Si.sub.3 N.sub.4 powder is also measured and weighed to provide the remainder of the mixture, preferably being 30-35%. The two measured powder quantities are placed together in a milling device and milled for a period of time to provide uniform blending and an average particle diameter of about 1.0-1.4 microns.

For carrying out hot pressing, it is preferable to use an assembly having a die body, a base, a sleeve punch unit and a susceptor, all constituted of graphite. The graphite forms an inert environment about the powder mixture during pressing and thus a flowing, gaseous, protective atmosphere (such as N.sub.2) is not mandatory but desirable. Prior to pressing, the graphite die parts are coated with a BN film to a thickness of about 0.002 inches. The pressing sequence is initiated by applying a nominal pressure of about 400-600 psi to the mixture which has been placed in the assembly. After the powder charge has been suitably precompacted, the pressure is increased to the desired level of about 500 psi (operably between 3800-6500 psi) and heating is initiated. The pressurized powder is heated to the range of 1500.degree.-1600.degree. C. for a period of time (typically less than 30 minutes) to achieve full density. During this heating and pressing sequence, alpha phase silicon nitride is converted to beta phase silicon nitride.

When full density is achieved, the pressure and heat is shut off and the pressed body is preferably cooled to room temperature at a rate of about 100.degree. C./min. The resultant hot pressed body is then shaped as a cutting tool with desired cutting surfaces according to standardized configurations in the industry. The shaped tool will be a mixture, not a single phase, of the added ingredients. It will be a marriage of separate ingredients that retain their physical identities and thereby contribute their separate physical properties. The shaped tool bit will exhibit a combination of at least two principal physical characteristics, a thermal shock resistance which is much higher than commercial Al.sub.2 O.sub.3 based ceramics and chemical inertness which is similar to commerical Al.sub.2 O.sub.3 based ceramics. This unique combination enables the resulting material to be used as a cutting tool material on steel nodular cast iron and malleable cast iron at relatively high speeds. The material retains a high modulus of rupture strength at high temperatures, typically at least 45,000 psi at 1200.degree. C. The density of the hot pressed ceramic will be about 3.54 grams/cm.sup.3 and the hardness will be about 90 Rockwell 45-N.

A preferred method for carrying out the invention is as follows:

(1) While deploying a cutting tool formed of a substantially fully dense hot pressed ceramic material consisting essentially of a generally uniform mixture of 50-70% by weight Al.sub.2 O.sub.3 and the remainder Si.sub.3 N.sub.4, establishing a relative motion between the tool and a metal part comprised of steel, malleable cast iron or nodular cast iron. This relative motion is similar to those obtained in turning, milling or boring operations. Typically, the rotation is sufficiently fast to provide a relative speed of 1000-6000 sfm.

(2) The tool is then advanced into the surface of the part at a predetermined depth of cut and feed rate which results in the removal of stock from the part during periods of the relative movement. It is preferred to carry out such advancement with a depth of cut in the rangae of 0.04-0.02 inches and a feed rate of 0.004-0.02 inches/revolution.

14 samples were prepared in accordance with the method described above; a 70-30% ratio of Al.sub.2 O.sub.3 /Si.sub.3 N.sub.4 was employed. The ultimate pressing pressure was 5000 psi and the hot pressing temperature was 1600.degree. C. Some of the samples were employed in the machine cutting of ASA 1045 and 1018 steel under different types of machining modes and other samples were used to machine nodular and malleable cast iron in a turning operation.

The machine cutting conditions and performance data are listed in Table 1, along with comparative performance data on these metals for a commercial hot pressed ceramic Al.sub.2 O.sub.3 based cutting tool (with 10-20% TiC), Si.sub.3 N.sub.4 based tool (2-8% Y.sub.2 O.sub.3 or MgO), and a Si.sub.3 N.sub.4 based tool containing a minor amount of Al.sub.2 O.sub.3. In addition, performance data for a Si.sub.3 N.sub.4 based tool used to machine grey cast iron and for a Al.sub.2 O.sub.3 based commercial tool used to machine aluminum and grey cast iron.

The normal accepted standard for commercial tools for machining steel is that the tool must be able to machine at 800-900 sfm for a period of ten minutes with no greater than 0.025 inches of wear and no thermal cracking. As soon as thermal cracking appears, catastrophic failure is imminent, even if wear is not excessive. It should be noted in Table 1 that Sample 3 was run for a period of 9.5 minutes at a surface cutting speed of 1000 sfm and wear experienced was 0.0137 inches with no thermal cracking. This is about one-half the normal wear experienced with commercial cutting tools having an Al.sub.2 O.sub.3 base. Similar conditions and results were recorded for Samples 4 and 5.

For Sample 11, the tool was used for milling 1018 steel for a period of seven minutes at a cutting speed of 1000 sfm. The indicated wear was 0.002 inches, about one-tenth the expected wear with a commercial Al.sub.2 O.sub.3 based cutting tool. With respect to Samples 13 and 14, each was run for 11 minutes at a cutting speed of 1000 sfm, the indicated wear was 0.005 inches, which is about one-fifth the expected wear with commercial cutting tools.

The actual measured difference in wear between a commercial Al.sub.2 O.sub.3 based cutting tool and a specific sample of this invention is recorded for Samples 1, 9 and 10. The difference in wear between the cutting tools used under identical cutting conditions shows that the commercial tools experienced failure by thermal cracking, chipping or fracture at or before the time during which the sample tool of this invention when used for machining 1045 steel. There was no observed thermal cracking, chipping or fracture failure with the tool manufactured in accordance with the present invention.

Qualitatively, the machining wear of the tool bits used in accordance with the present invention appears to be entirely of an abrasive nature. This may originate as a weakness at the interface between the Al.sub.2 O.sub.3 and the Si.sub.3 N.sub.4 grains, resulting in microerosion at the grain boundaries and removal of the primary grain through abrasion. As an alternative modification to the present invention, it is proposed that machining be carrried out with a tool material that has the bonding interface between Si.sub.3 N.sub.4 and Al.sub.2 O.sub.3 modified by the presence of an oxide selected from the group consisting of stabilized ZrO.sub.2 or HfO.sub.2, each of which should improve the wear resistance of the resulting ceramic. It is suggested that the powder mixture for hot pressing should preferably consist of 55% Al.sub.2 O.sub.3, 30% Si.sub.3 N.sub.4, and up to 15% ZrO.sub.2 which has been stabilized. It is believed that the introduction of stabilized zirconia in the matrix of the resulting ceramic will also improve the strength of the material at higher temperatures.