Tool Steel Selection
Tools are subjected to extremely high-pressure loads and are commonly applied rapidly. The tools must withstand these loads without breaking and without undergoing excessive wear or deformation. No single tool material will provide the maximum wear resistance, hardness, toughness, and resistance to softening at elevated temperatures. The selection of the proper tool material for a given application is often a trade-off. Sometimes more than one grade will provide sufficient results but another grade may provide an optimal combination of properties.
There are three basic classes of tool steel including cold work, hot work and high speed steel. They are categorized by their primary usage. Typically cold work tool steels are suited to applications not exceeding operating temperatures of 500 degrees F. Hot work tool steels can withstand operating temperature approaching 1200 degrees F. High speed steel are a special class with high levels of tungsten or cobalt designed to withstand extreme intermittent operating temperatures usually created by high speed machining operations.
Within these three basic classes, there are many types of tool steel. Cold work tool steel can be further categorized as water-hardening, oil-hardening, air-hardening, shock-resistant and high carbon high chromium. High-speed steels are categorized as tungsten or molybdenum types, based on the primary type of carbide that they form.
Read Properties of Tool Steels and Alloying Elements to understand the terminology used to select the proper tool steel. The basic constituents of tool steel are iron and carbon alloyed with many combinations of other elements such as manganese, silicon, cobalt and vanadium. Each element adds a special characteristic to the tool steel. The proper combination of alloying elements can provide the optimum tool.
Before selecting a tool steel grade for your tool, some questions will need to be addressed. First, what type of operation will this tool be performing? Will the tool be forming, shearing, blanking, drawing, extruding, rolling or compressing another material? Each one of these operations will require different properties from the tool.
For example a tool used in a rolling operation will require a fair amount of toughness and significant wear resistance. Whereas a tool used to compress powdered metal into a complex shape may require extreme toughness and moderate wear resistance. A blanking tool would require excellent edge retention with good toughness. An end mill cutter will demand high hardness, wear resistance and red hardness.
What hardness will the tool need to possess? A tool with high hardness will have less toughness. If the tool is too soft then it might deform or collapse under pressure. The correct amount of alloying elements must be diffused into the steel to provide the proper hardness without making the tool brittle.
How much toughness will the tool require? Toughness in tool steel generally decreases as the hardness increases. If the tool isn’t tough enough then it may chip or crack. A lack of toughness will almost always result in catastrophic failure of the tool. Complex tooling shapes will require more toughness then a simple shape.
How much wear resistance is needed? Some tools may be in operation for very long periods of time, while others may have short production runs. Wear resistance is increased by increasing the amount or type of carbides embedded in the steel matrix. When the amount of carbides are increased in the tool steel then the toughness is decreased. Changing the type of carbides used can increase the wear resistance with very little detriment to its toughness.
Is there a need for a high degree of red hardness? Red hardness is a special property used in extreme temperature exposure. The operating temperature and exposure time at that temperature will determine the amount of red hardness needed.
Secondary properties will also need to be considered when selecting a tool steel. The steel will need to be machined into a tool, so machinability may be a concern, as will grindability and polishabilty. Of course the cost of the tool steel will be of some concern. There is no need to produce a tool capable of a million cycles when it will only be required to perform one thousand cycles. Availability is another secondary property to consider. Some tool steels are only available in limited sizes or quantities because they may be designed for very specific applications. Choosing the optimum tool steel, only to find that it is not available can be very frustrating.
If you are addressing the needs of a previously failed tool, the same properties will come in play. Why did the tool fail? Did it crack, chip, collapse or wear out? If the tool cracked or chipped, it may require more toughness. If the tool collapsed or plastically deformed, then higher hardness is required. If the surface of the tool eroded or the edge rounded off, then adding more wear resistance would be advisable.
Compromise is necessary in order to achieve the correct balance of all the properties available in tool steel. After addressing the criteria required to select the proper tool steel, you can now start searching for the correct grade. Some advanced planning should provide the tool user with excellent production results. Start Choosing the Correct Tool Steel .
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