1.1 Introduction
Many types of tool materials, ranging from high carbon steel to ceramics and diamonds,are used as cutting tools in today's metalworking industry. It is important to be aware that differences do exist among tool materials, what these differences are, and the correct application for each type of material.The various tool manufacturers assign many names and numbers to their products.While many of these names and numbers may appear to be similar, the applications of these tool materials may be entirely different. In most cases the tool manufacturers will provide tools made of the proper material for each given application. n some particular applications, a premium or higher priced material will be justified. This does not mean that the most expensive tool is always the best tool. Cutting tool users cannot afford to ignore the constant changes and advancements that are being made in the field of tool material technology. When a tool change is needed or anticipated, a performance comparison should be made before selecting the tool for the job. The optimum tool is not necessarily the least expensive or the most expensive, and it is not always the same tool that was used for the job last time.The best tool is the one that has been carefully chosen to get the job done quickly, efficiently and economically.
Author's NoteI wish to express my sincere appreciation to Prentice Hall and to Stephen Helba in particular, for giving me permission to use some of the information, graphs and photos recently published in Applied Manufacturing Process Planning authored byDonald H. Nelson and George Schneider, Jr.
The author also wishes to thank over 40 companies who have provided technical information and photo exhibits ... their contributions have made this reference text possible. And finally, I would like to express my appreciation to Tooling & Production's Stan Modic and Joe McKenna for giving me the opportunity to make this information available to the general public.George Schneider, Jr.
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Figure 1.1. (A) Hardness of various cutting-tool materials as a function of temperature. (B)
Ranges of properties of various groups of materials. |
A cutting tool must have the following characteristics in order to produce good quality and economical parts: Hardness:Hardness and strength of the cutting tool must be maintained atelevated temperatures also called Hot Hardness Toughness:Toughness of cutting tools is needed so that tools don't chip or fracture,especially during interruptedcutting operations.Wear Resistance: Wear resistancemeans the attainment of acceptable tool life before tools need to be replaced. The materials from which cutting tools are made are all characteristically hard and strong. There is a wide range of tool materials available for machining operations, and the general classification and use of these materials are of interest here.
1.2 Tool Steels and Cast Alloys
Plain carbon tool steel is the oldest of the tool materials dating back hundreds of years. In simple terms it is a high carbon steel (steel which contains about 1.05% carbon). This high carbon content
allows the steel to be hardened, offering greater resistance to abrasive wear. Plain high carbon steel served its purpose well for many years. However, because it is quickly over tempered (softened) at relatively low cutting temperatures, (300 to 500 degrees F), it is now rarely used as cutting tool material except in files, saw blades, chisels, etc.The use of plain high carbon steel is limited to low heat applications. High Speed Tool Steel: The need for tool materials which could withstand increased cutting speeds and tempera- tures, led to the development of high speed tool steels (HSS). The major difference between high speed tool steel and plain high carbon steel is the addition of alloying elements to harden and strengthen the steel and make it more resistant to heat (hot hardness). Some of the most commonly used alloying elements are: manganese, chromium, tungsten, vanadium, molybdenum, cobalt, and niobium (columbium). While each of these elements will add certain specific desirable characteristics, it can be generally stated that they add deep hardening capability, high hot hardness, resistance to abrasive wear, and strength, to high speed tool steel. These characteristics allow relatively higher machining speeds and improved performance over plain high carbon steel.
The most common high speed steels.used primarily as cutting tools are divided.into the M and T series. The M series.represents tool steels of the molybdenum.type and the T series represents.those of the tungsten type. Although.there seems to be a great deal of similarity.among these high speed steels,each one serves a specific purpose and.offers significant benefits in its special.application.An important point to remember is.that none of the alloying elements for.either series of high speed tool steels is in abundant supply and the cost of these elements is skyrocketing. In addition,U.S. manufacturers must rely on foreign countries for supply of these very important elements Some of the high speed steels are now available in a powdered metal. (PM) form. The difference between powdered and conventional metals is in the method by which they are made.The majority of conventional high speed steel is poured into an ingot and then, either hot or cold, worked to the desired shape. Powdered metal is exactly as its name indicates. Basically the same elements that are used in conventional high speed steel are prepared in a very fine powdered form. These.
powdered elements are carefully blended together, pressed into a die under extremely high pressure, and then sintered in an atmospherically controlled furnace. The PM method of manufacturing cutting tools is explained in Section 1.3.1 Manufacture of Carbide Products.
Many surface treatments have been developed in an attempt to extend tool life, reduce power consumption, and to control other factors which affect operating conditions and costs. Some of these treatments have been used for many years and have proven to have some value. For example, the black oxide coatings which commonly appear on drills and taps are of value as a deterrent to build-up on the tool. The black oxide is basically a 'dirty' surface which discourages the build-up of work material One of the more recent developments in coatings for high speed steel is titanium nitride by the physical vapor deposition (PVD) method. Titanium nitride is deposited on the tool surface in one of several different types of furnace at relatively low temperature, which does not significantly affect the heat treatment (hardness) of the tool being coated This coating is known to extend the life of a cutting tool significantly or to allow the tool to be used at higher operating speeds. Tool life can be extended by as much as three times, or operating speeds can be increased up to fifty percent.
