Metal treatment – Stock – Carburized or nitrided
Reexamination Certificate
2000-09-29
2002-09-24
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Carburized or nitrided
C148S218000, C148S219000, C148S334000
Reexamination Certificate
active
06454880
ABSTRACT:
FIELD OF THE INVENTION
The field of the present invention relates to materials for tooling components, including shot sleeves and/or cold chambers used in nonferrous die casting. More particularly, the present invention relates to tooling components exhibiting oxidation and “washing” resistance, as well as the process for the manufacture thereof.
BACKGROUND OF THE INVENTION
Conventional non-ferrous die casting is the method used for molding aluminum, magnesium and zinc parts. Usually the dies are made of steel, with inlets to receive the material to be formed into a part. The inlet of the steel die is connected with a tubular element typically referred to as the shot sleeve or hot/cold chamber. The hot/cold chamber (hereinafter simply referred to as a shot sleeve) receives the molten metal material which the casting or molded part is to be made from. Typically the shot sleeve chamber is oriented in a horizontal fashion with a hole for receiving the molten metal along its cylindrical side oriented vertically upward. A plunger with a sealing head tip is slidably mounted within the shot sleeve. The plunger creates pressure in the shot sleeve, injecting the molten material into the die cavity. The shot sleeve, plunger and plunger tip are all considered to be perishable tooling components which need to be replaced after appropriate cycles. Additionally, core pins in the mold for forming cavities within the molded article are also considered perishable. Prior to the present invention, most shot sleeves and other perishable tooling were manufactured from H-13 and premium H-13, hot working tool steel per NADCA, “North American Die Cast Association” standard 207-97 (NADCA 207-97 type material). NADCA 207-97 identifies and specifies base material chemistry, micro cleanliness and inclusion ratings and impact and fatigue properties for H-13 raw material.
A chart approximating typical prior alloy constituents for a conventional H-13 steel is provided below, percentages being represented by weight.
Element
Upper Spec Limit %
Lower Spec Limit %
Carbon
.4
.3
Manganese
.4
.3
Silicon
1.2
.8
Chromium
5.5
4.7
Molybdenum
1.75
1.2
Vanadium
1.2
.8
The material is first annealed to increase its formability, and then forged to near net shape or is optionally machined. Then it is heated and quenched and tempered per HRC 44-48 standard, followed by finishing. The finish machine operation typically includes cross matching which is intended to facilitate the movement of carbon based lubricant along the length of the cold chamber.
Prior art methods for making the conventional tooling components are as follows: The prior art tools were first heated in accordance with NADCA recommendations to a predetermined temperature in a near vacuum (approximately −29.8″ of Mercury). An electric furnace allowed the temperature to be elevated while a vacuum was maintained. The furnace was brought to and maintained at a temperature of 1200° F. The furnace was then partially pressurized to 500 microtorr with nitrogen, or any other appropriate inert or substantially inert gas. The temperature was then ramped up to 1550° F. The temperature was then subsequently raised to 1875° F. for a predetermined period of time to form surface and core austenite. The tools were then quenched in an inert or substantially inert gas such as nitrogen which is used to pressurize the chamber to 10 Bar (135 pounds per square inch) of pressure and was circulated through the chamber and past a heat exchanger. The temperature was then brought down in stages in an effort to obtain optimum hardenability and impact or fatigue properties while minimizing product distortion, grain growth and carbide particle size/micro chemistry segregation. The quenching was controlled to insure a minimum cooling rate of fifty degrees Fahrenheit per minute. Other prior quenching methods have included cyanide-treated salt quench, 2 Bar quench, 4 Bar quench and similar variations, with each method having its inherent problems and/or traits which ultimately contribute to reduced tool performance.
Conventional tips/plungers were formed of a beryllium-copper composition due to the thermal properties of that material. As beryllium is a heat resistant constituent added to the non-ferrous copper alloy, the molten process metal did not solidify before it was mechanically pushed into the die cavity. The properties of the tip/plunger were also selected to insure consistent thermal expansion and contraction so that the molten metal bleed by and premature wear through friction are minimized.
FAILURE MODES OF PRIOR ART
Prior art cold chambers and core pins experienced soldering, heat checking, sticking, spalling, galling, erosion and inter-granular failure. Beryllium-copper tips or plungers exhibit spalling, galling and pitting since they are typically much softer than the materials used for the cold chamber. Soldering is a solidification of the molten magnesium, aluminum or zinc on the die cast tooling components. Heat checking means the beginning of or incipient inter-granular failure on the die cast tooling components. Inter-granular failure is the complete failure of the die cast tooling or components and includes cracking, splitting, burst, etc. Spalling, galling or pitting is defined as premature wear due to the sliding wear of any die cast tooling component. Erosion means the wearing away of the base material due to pressure and temperature of the molten die cast metal.
QUALITY AND ENVIRONMENTAL ISSUES OF PRIOR ART
Die casters usually use carbon or molybdenum-based die lubricants to reduce friction between the perishable components that slide together, such as the plunger tip and the shot sleeve. These lubricants become airborne, thus coating surrounding walls and floors of the die casting equipment. In order to decrease operator exposure to the airborne particles, scrubbers and air exchange systems are commonly employed. While scrubbers may reduce the worker's health concerns it does not eliminate all housekeeping and residual exposure concerns. In addition, the lubricant may cause voids or porosity in the finished product. Die cast industry estimates place the associated porosity defects as high as 6 percent of the total product manufactured in die cast machines of non-ferrous metals.
Beryllium-copper components require a clean room environment for their fabrication, complete with operator safety equipment such as breathing apparatus and protective clothing, so that the operator does not come into external or internal contact with the beryllium. It has been determined that exposure to beryllium, even in small amounts of less than 25 ppm (parts per million), can contribute to a spectrum of health problems up to and including terminal illness. In addition, beryllium-copper plungers are much softer than the shot sleeve, so any variation in fit and function results in extensive wear or spalling or galling of the plunger.
Material Selection of Prior Art
Ferrous metal tooling material has been conventionally selected by manufacturers and producers for its machinability, availability, hardenability, cost, formability, quality and suitability for specified design requirements. Tool design variables may include material hardenability, material chemistry, tool cross section, material tensile strength, yield, elongation (ductility) and reduction in area. Some of these variables interact with each other. For example, hardenability is largely a function of the chemistry of the steel, but is also a function of tool cross section. Past practices have identified H-13 as the material of choice for shot sleeves. However, H-13 steel is relatively expensive when compared with low alloy and plain carbon steels. The North American Die Cast Association has determined that a Rockwell hardness of 46-50 is optimum for shot sleeves, since H-13 at that hardness is sufficiently heat resistant and has desirable wear characteristics. It is also known to use H-11 alloys. H-11 steel has amounts of carbon, chromium, and molybdenum similar to those of H-13 steel. Although the aforementioned
Dykema Gossett PLLC
Yee Deborah
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