An image showing steel bars with Griggs steel color codes

Choosing the Right High-Speed Tool Steel Grade

High-speed steel is a specialty tool steel for cutting tools, and is frequently used in turning, milling, drilling, and tapping applications. It’s often referred to as HSS, which sometimes gets it confused with ‘high-strength steel.’ High-speed steel and high-strength steel aren’t the same, although high-speed steels are generally strong.

Where HSS differs from other tool steels is red hardness and resistance to wear, which allows cutting edges to remain sharp and continue working in high-speed applications.

In the U.S., steel specifications, in terms of composition and performance, are defined by ASTM International. Their A600 specification lists 26 compositions, all with slightly different properties. This blog offers guidance on how to choose the right one for your application.

Common HSS Grades

HSS splits into two types: 1) Those based around molybdenum (the M grades), and 2) Those based around tungsten (the T grades). Powder metal (PM) grades are also available.

Most application requirements are met by the following nine grades:

  • M2: Tough, good wear resistance, and good red hardness. Other names are: HS 6-5-2, 1.3343, S600 Steel, E M2, REX M2®, SKH 51, P6M5, and 6-6.
  • M3: Good wear resistance and commonly used in form tools. Also known as: REX M3-1®, PM M3, and Corsair.
  • M4: Excellent abrasion resistance. Other names are: HS 6-5-4, E M4, SKH 54, and Stark.
  • M7: Widely used for drills. Known as 1.3348, HS 2-9-2, E M7, REX M7®, SKH 57, and Tatmo-VN.
  • M42: Allows higher cutting speeds. Also known as: 1.3247, HS 2-10-1-8, S500, E M42, REX M42®, and Dynamax.
  • PM M4: Excellent impact toughness and bend strength. Other names are: CPM REX M4 steel®, ASP 2004, and S690.
  • PM T15: Provides long life. Known as: 1.3202, ASP 2015, CPM T15®, SKH 10, HS 12-0-5-5, and S399.
  • PM A11: Good wear resistance: Other names are: CPM® 10V® and AISI A11.
  • PM M48: Offers higher cutting speeds. Known as: 1.3207, CPM REX 76®, ASP 2048, HS 10-5-3-9, and SKH 57.

Factors to Consider When Selecting Your Grade

Required Properties

Cutting tools require toughness, wear resistance, and red hardness. Toughness is especially important for applications with interrupted cuts or where ceramic or sand particles might be present. Conversely, for softer materials like aluminum and brass, red hardness may be a higher priority than toughness.

Wear resistance is always important, but its priority depends on the material being cut. Some non-metallic materials, such as fiberglass, are very abrasive, but so is cast iron. Other alloys generate less wear at the cutting interface.

M2 is often selected for general-purpose applications as it provides a combination of toughness and wear resistance. M4 is often used for high-speed machining and finishing, while M42 might be picked for interrupted cuts.

Heat Treatment Sensitivity

HSS undergoes an austenitizing process to raise hardness. In this process, the material is heated to a temperature that causes austenite to form, and then it is quenched to create martensite. It may then be tempered to remove retained austenite and add toughness while slightly reducing hardness.

Some HSS grades need precise control during heat treatments to achieve the desired results. M42 and T15, in particular, have very narrow heat treatment windows as a result of higher cobalt levels.

At the other end of the sensitivity spectrum are the PM grades of HSS. Their very uniform structure reduces the risk of uneven response during heat treatment. More specifically, PM A11 behaves very predictably.

Machinability

High-speed steel alloys tend to form carbides that provide wear resistance and hardness. However, these also make them difficult to machine. Those alloys with higher levels of vanadium, cobalt, and tungsten are more challenging. PM grades also have lower machinability than the conventional tool steel grades.

More specifically, T15 and M42 both have very poor machinability but can usually be cut and ground. Particularly for PM T15 and PM M4, EDM is often the best machining method.

HSS grades with the best machinability are M1 and M2. PM A11 is also known for its excellent grinding performance.

Desired Performance

When choosing an HSS grade, always consider how it will be used. Cutting speed, feed rates, depth of cut, and the material properties determine cutting forces, temperatures, and wear rates. In addition, part geometry, especially features that will cause interrupted cuts, also plays a role.

To provide some examples, M2 is often used in light cuts, while M42 works well with interrupted cuts, and PM A11 and M4 are good choices when high wear rates are expected.

Still Need Help? Contact Griggs Steel

Griggs Steel has been a leading U.S.-based supplier of high-speed steel for over seven decades. Since our founding, we’ve only supplied HSS tool steel. When you work with us, know that you’ll always be connected to an expert who understands steel and that you’re partnering with a supplier that can provide custom steel recommendations and handle recurring orders. We can even provide steel certificates if needed.

If you need help finding the right HSS grade for your application, contact us today to speak with one of our experts.