The metalworking industries, and many others, couldn’t operate without tool steels. This branch of alloy steels serves to cut and grind and bend and deep draw metals and materials from one form or shape to another. They are indispensable.
The basic requirements for a tool steel are hardness, thus wear resistance, fatigue strength, toughness, or shock resistance. Plus good distribution of carbides and an absence of decarburization. This calls for extreme care throughout the making, shaping, and treating of the steels, and of the tools made from them.
There are a good number of classifications of tool steel, namely water hardening, oil hardening, air hardening, shock resisting, cold work tool steels, hot work tool steels, and high speed tool steels – where increasing “red hardness” has been developed over the decades by adjustments to steel chemistry.
Water hardening tool steels are typically high-carbon steels – around 1% C – that possess a hard case and a tough core. They are suitable for such applications as cold heading dies. The steels are sensitive to cracking in heat treatment when there is a section small enough to harden all the way through adjacent to one that hardens only partially, in which case a move up to oil or air hardening steels is in order. Water hardening steels are the least expensive option.
We then move to higher alloyed steels, with greater hardenability, to be used for numerous applications, where all the mechanical properties that might be obtained from such steels may be used to maximum effect.
Shock-resisting tool steels have around 0.5% carbon, with principal alloying elements being silicon, chromium, tungsten, and sometimes molybdenum or nickel.
The steels have high strength and moderate wear resistance and are used where repetitive high-impact loading is developed. Types with high tungsten may be used for hot shearing or heading where heat resistance is important.
A high carbon, high chromium, type D2 tool steel with 1.5% carbon, 12% chromium and 1% molybdenum is used as a cold work steel and is commonly used for shear blades. Deep hardening is the norm with attendant wear resistance. The steel, and others in the class with minor tungsten and molybdenum additions may also be used for blanking and forming dies.
There are many so-called hot work tool steels, some with chromium as the major alloying element, some tungsten, some molybdenum. Then we move to high speed tool steels, designed and developed for “red hardness” and wear resistance. There are two major classifications, one based on tungsten, one on molybdenum. Tungsten high speed steels are used almost entirely for such cutting tools as tool bits, drills, reamers, taps, broaches, milling cutters and hobs. The steels also find use in dies and punches and for high-load, high-temperature structural components such as aircraft bearings and pump parts. These steels, highly alloyed, show good hardenability, and the formation of molybdenum, tungsten and vanadium carbides.
There are numerous combinations of steels, uses, part designs, tool fabrication methods, heat treatment and machining. As such, there are many stages that warrant extra care to ensure that the tool comes through its processing in the best possible condition. First among these might be the tool design, where adjacent heavy and light sections could spell trouble. During liquid quenching, the light section will cool rapidly and harden before the heavier section, setting up quenching stresses and subsequent cracks. The answer to this might be an air-hardening steel. And avoid sharp corners. And don’t overload your tools; try to find the best combination of tool properties that will do the job.
The steel itself may contain surface or internal defects, and any such defects will be cause for rejection. Care must be taken during preliminary steel processing, and the solidity of the steel should be checked by ultrasonic testing at appropriate stages. Where deemed necessary, carbide distribution should be checked.
Most tool failures are the result of faulty heat treatment. It may be too drastic a quench such as oil or water instead of air. Tools should be preheated before being heated to the quenching temperature, and all liquid-quenched tools should be quenched until a temperature of 150 to 200 F has been reached. Air hardening steels should be cooled to 150 F. Tools should be tempered immediately after quenching.
Tool steels shouldn’t be overheated. This causes grain coarsening and subsequent embrittlement. Double tempering is a wise precaution when treating tools made from high carbon, high-chromium steels and high-speed steels, where phase transformation tends to be sluggish. The choice of a good heat treater is primordial, as is their counsel on tool design.
Final grinding of tools may be adversely affected by grinding with a dull wheel, a wheel of too fine a grit size for the job, or ineffective coolant use.
The tool steel scenario is a vast subject. What has gone before in this article is a broad outline that illustrates the more important aspects of the precautions to be taken when handling the many grades of tool steel. It goes without saying that the primary production stages, melting and forging, of these steels must be performed with the utmost care. We are dealing here with very expensive material, whose rejection due to faulty processing becomes more expensive yet.
Information on tool steels may be found in many sources, some of it erroneous. AMFG can point you to reliable sources on all stages of tool steel and tool production.
