How Hard is High Speed Steel?
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High speed steel (HSS) is a family of high-performance tool steels renowned for their exceptional hardness, wear resistance, and ability to retain these properties at high temperatures. But exactly how hard is it? Typically, HSS achieves a Rockwell “C” hardness of 63–65, which is significantly harder than most standard steels. This high level of hardness allows HSS to effectively cut a wide variety of materials, including other metals, at high speeds. It’s important to note that hardness is not the only critical property of HSS; its ability to maintain this hardness at elevated temperatures (known as red-hardness) is what truly sets it apart. The hardness translates to a Vickers hardness (HV) of up to 750 HV, making it adequate for machining most common metals. This combination of high hardness and temperature resistance is why HSS is such a popular choice for cutting tools.
Understanding the Properties of HSS
The exceptional hardness of HSS is achieved through a carefully formulated alloy composition. These alloys typically include elements such as molybdenum, tungsten, chromium, vanadium, and sometimes cobalt. These elements work together to:
- Enhance Hardness: The primary function of these alloying elements is to create a hard, wear-resistant matrix.
- Improve Red-Hardness: They raise the temperature at which the steel begins to lose its hardness, allowing HSS tools to maintain their cutting edges even under intense heat generated by high-speed machining.
- Increase Wear Resistance: The presence of elements like tungsten and vanadium contributes significantly to the steel’s resistance to abrasion and wear.
- Provide Strength: Although often associated with hardness, these alloys contribute to the overall strength and capabilities of HSS.
The most widely used grade of HSS is M2, which is known for its balance of toughness, abrasion resistance, and hot hardness. This grade often replaces older grades because of improved properties and is the standard choice for many general-purpose cutting applications.
Hardness Compared to Other Steels
Compared to common carbon steels and tool steels, HSS is substantially harder and more capable in high-temperature conditions. While carbon steels may be easier to produce, they lack the high-temperature hardness retention that HSS possesses. Tool steels, while designed for tooling purposes, generally are not as hard and heat-resistant as HSS.
HSS offers superior strength and hardness due to its unique alloy composition, which includes a minimum of 7% combined tungsten and molybdenum, along with at least 0.6% carbon. These alloying elements contribute to the superior performance and makes HSS suitable for high-speed applications.
While carbide is significantly harder, HSS can offer a sharper edge since the material tends to burr rather than crumble when honed. HSS also tends to be more cost-effective than carbide.
Limitations of High Speed Steel
Despite its hardness, HSS has limitations:
- Brittleness: HSS is known for being quite brittle, which means it has a tendency to chip or fracture under impact or bending forces.
- Low Impact Toughness: This characteristic makes it susceptible to damage when subjected to shocks or sudden loads.
- Low Flexural Strength: HSS is not very flexible and can break under bending stress.
- Low Vibration Resistance: Vibrations during machining can cause the tool to wear more quickly or even break.
These limitations must be taken into consideration when choosing HSS for a specific application. While it is excellent for high-speed cutting, it may not be the optimal choice for applications that involve high impact or significant bending stress.
Frequently Asked Questions (FAQs)
Here are 15 frequently asked questions that delve deeper into the topic of high speed steel hardness:
1. Can HSS cut hardened steel?
No, generally HSS is not recommended for cutting hardened steel. While HSS is hard, it lacks the hardness and strength necessary to cut through materials that have already undergone hardening processes. Using HSS in this manner is likely to result in the tool chipping or breaking.
2. Is HSS harder than tool steel?
Yes, high-speed steels (HSS) are generally harder than most standard tool steels. They are specifically engineered to endure higher temperatures and maintain their cutting capability at high speeds.
3. Is HSS harder than steel?
Yes, HSS is significantly harder than standard carbon and most alloy steels. The addition of tungsten, molybdenum, chromium, and vanadium gives it this hardness and high temperature performance.
4. Is HSS stronger than carbide?
No, carbide is a much stronger, more durable, and effective material for extremely tough cutting applications. HSS, while hard and cost-effective, is not as strong or wear-resistant as carbide.
5. Why is it called high speed steel?
The name “high speed steel” comes from its ability to maintain its hardness and cutting ability at elevated temperatures generated during high-speed cutting operations, not possible with standard steels.
6. What is the yield strength of HSS?
The yield strength of HSS typically ranges from 180–550 MPa. It is important to also note ductility as a limiting factor for HSS.
7. Is HSS easy to cut?
HSS is not easy to cut using standard methods due to its high hardness. It is often shaped through grinding or other specialized methods after being hardened.
8. Is high-speed steel brittle?
Yes, one of the main drawbacks of HSS is its high brittleness, leading to limitations in bending and impact resistance.
9. Is HSS expensive?
Yes, HSS is usually more expensive than other steels due to the inclusion of costly elements like tungsten, molybdenum, and vanadium, which contribute to its properties.
10. Is HSS sharper than carbide?
While carbide is harder, HSS can produce a sharper edge because its material tends to burr rather than crumble when sharpened, allowing for a finer cutting edge in certain applications.
11. Can HSS be bent?
Yes, HSS can be bent, but it’s best done in its annealed state before heat treatment. Bending it when it has achieved its final hardness can cause it to fracture.
12. Can you weld high speed steel?
Yes, HSS can be welded, but it is prone to cracking in the weld joint due to the formation of carbides and other metallurgical changes during the welding process.
13. Can you soften high speed steel?
Yes, HSS can be softened through a process called annealing. However, this must be done with care to avoid damaging its metallurgical properties.
14. What grade of high-speed steel is the industry standard?
M2 is widely considered the industry standard grade of high speed steel, known for its excellent balance of toughness, abrasion resistance, and hot hardness.
15. What is stronger than high speed steel?
Cobalt alloys are generally stronger and more heat-resistant than HSS, making them suitable for more demanding heavy-duty machining and drilling tasks.
Conclusion
High speed steel is an incredibly useful material because of its high hardness and ability to maintain that hardness at high temperatures. Its unique alloying elements contribute to properties that make it ideal for high-speed cutting tools. However, it is not without its limitations, most notably its brittleness and poor impact strength. Understanding these properties and limitations is essential when choosing HSS for a particular application, ensuring you use it effectively and obtain optimal performance in your cutting and machining operations.