Tool steel refers to a variety of carbon and alloy steels that are particularly when used for cutting and drilling. The four major alloying elements that form carbides in tool steel are: tungsten, chromium, vanadium and molybdenum. The two materials from which most cutting tools are made are carbide and high speed steel (HSS).
High Speed Steel is a high carbon tool steel, containing lots of tungsten and cobalt and is rich in molybdenum, tungsten and vanadium. It forms a special class of highly alloyed tool steels, combining properties such as high hot hardness and high wear resistance. These properties are possible to be attained due to a special microstructure, composed of a matrix around 65 HRC even in high-temperature in the case of high-speed cutting.
General carbon tool steel remain very high hardness at room temperature after quenching and low temperature tempering, but when the temperature higher than 200 ℃, was a sharp reduction in hardness, when up to 500 ℃ the hardness was similar to its annealing condition before and completely lost the ability of cutting, this limits the carbon tool steel used for cutting tools. However, due to the exist of its red hardness mentioned above, high-speed steel (representative material M2 steel) makes up for the fatal defects of carbon tool steel. HSS is mainly used to manufacture complex thin blade and impact-resistant metal cutting tools, as well as high-temperature bearing and cold extrusion die, such as turning tool, drill bit, hob, machine saw blade and high demand die etc. It is also used to make small, complicated tools.
A Carbide steel is a compound of carbon with another alloy metallic element. Commonly, referred to tungsten carbide, which is a common example of a metal carbide. Carbide tools enable harder materials to be machined, potentially up to 70+HRC. It has a high red hardness, even at 1000 ℃ it still has a high hardness. Tungsten carbide is extremely hard and abrasion resistance. Most of its major uses including drill bits and cutting tools, sports equipment and the tips of ballpoint pens.
Applications of High Speed Steel
Broadly, high speed steel excels in hardness and abrasion resistance, with different grades trading for toughness, hot hardness or reduced brittleness. As a result, these alloys see the most use in industrial cutting tools—tool bits, milling cutters, saw blades, drills, taps, broaches and more.
Tools made of high speed steel frequently keep a sharp edge for longer than other carbon steels, and the variety of grades and surface treatments available provide options for specialized applications. These products see use anywhere from woodworking to machining high-grade alloys.
Though not traditionally considered to be cutting tools, punches, dies and other components in progressive stamping can also be made from high speed steel. Additionally, the properties of high speed steels, particularly hardness and wear resistance, are desirable for hand tools such as chisels, files, blades for hand planes and kitchen and pocket knives.
Why Carbide Tool
Carbide tipped tools are far more durable than solid carbide tools. Under abnormal conditions the carbide may start to crack due to hard spots in the material being machined, incorrect feed rates or inadequate holding of workpiece. These cracks that shatter solid carbide tools are stopped in carbide tipped tools by the tough hardened tool steel body – usually permitting the tool to complete the production run.
The carbide grade in carbide tipped tools can be selected wholly based on its cutting and wear characteristics rather than compromising for the carbide’s structural strength, as is required for solid carbide tools. Some high cost specialty carbides necessary for aggressive machining of many tough alloys are not even available in solid carbide round form.
Although carbide tipped tools use higher quality more costly carbide, the overall cost is usually substantially less than solid carbide tools because only the thick cutting edge is carbide. The body and shank are made from less costly tough hardened tool steel.
Carbide vs high speed steel: Which is better?
Let’s look at three common machining operations – drilling, tapping and milling – to gain a better understanding of when to use HSS or Carbide tools.
Carbide drills are generally used for high-volume hole production, where the higher tool cost can be justified on a cost-per-part basis. For deep high-volume holes, they are often available with internal coolant ducts, resulting in longer tool life and stable production. Use of through-the-spindle and high-pressure coolant offers excellent chip evacuation, particularly in deeper holes (>3xD), and is the most effective method for cooling the edge in cut.
Carbide drilling is also the fastest way to produce holes in a wide range of metals, due to the higher cutting speeds and feeds possible. However, it’s important to know that in some higher Ni-Cr alloy steels (such as stainless steels) although the hole can be produced with high speeds, the condition of the walls of the hole can quickly work-harden. This can lead to other issues in the machining process, particularly if the hole is to be have an internal thread; the tool life of the tap will be considerably shortened since it will be trying to cut through a hardened skin or surface.
Importantly, Carbide tools can be justified in low-volume production for their higher hardness because they enable harder materials to be machined, potentially upto 70+HRC.
HSS drills have very wide range of uses – from handheld applications to CNC machining in short batch runs – due to their toughness and lower cost. They are ideal for less rigid applications, such as hand-held drilling, stack drilling, and for deep hole drilling where an internal coolant supply is not available.
There are various geometries available for specific material grades, to really cut through the material and leave it in its best annealed condition. Ideal for pre-tap drilling in stainless steel, HSS drills can really benefit the life of the tap when the right geometry is used to produce the hole!
HSS tools are typically the first choice for tapping. They are by far the most common for internal thread production, with many HSS-PM versions available more recently for the various CNC machine tapping applications, different thread types and materials groups.
Given their toughness attributes, HSS tapping tools are also common in the Maintenance-Repair-Operations industries (MRO), with hand taps or straight flute taps the most widely used.
HSS taps are even used in large volume applications as well as the difficult-to-machine material applications, particularly HSS-PM taps. Where they are still the first choice due to the process stability they offer.
Carbide taps are not as popular due to the brittleness of carbide. It tends to chip in most tapping applications, particularly in blind holes. Carbide will fracture in steel applications at full depth, when the tap reverses and breaks the chips that were produced from the down cut in order to back out of the hole. As mentioned earlier HSS has superior toughness over carbide, in the tapping process this really is most important. Due to the nature of tapping being a slow speed-high feed type process, and with the spindle slowing-then reversing at the full thread depth & breaking the chip produced from the down stroke of the machine, it’s this action that the HSS toughness characteristic performs superior than carbide.
Carbide endmills are by far the most popular because they offer the best Metal Removal Rates (MRR). Solid Carbide endmills have become the first choice, given the variable helix designs combined with CAM packages that provide tool paths to suppress chatter from the natural vibration produced in a milling cut. Milling strategies such as trochoidal methods are now quite common.
HSS endmills still have a place, such as for manual milling machines, smaller volumes, less rigid set-ups, and the like. However, their use has declined in recent times due to the many industry advances.