High Speed ​​Steel Tool Hardness and Heat Treatment
Hardness refers to a material's ability to resist local plastic deformation on its surface. To date, hardness remains the only parameter that can directly reflect the mechanical and technological properties of the surface layer as well as its wear resistance. In this text, we will briefly discuss the hardness and heat treatment of high-speed steel tools.
**Table 1: Recommended Hardness for High-Speed Steel Tools**
| Tool Name | Recommended Hardness (HRC) | Overheating Allowed? |
|----------------------------------------|----------------------------|----------------------|
| Straight Shank Twist Drill | ≤ Ø4mm: 63.5–66 | Not allowed |
| | > Ø4mm: 64–66 | 65–67 |
| Large Blade Cutters | 62–65 | 64–66 |
| Turning Tools | 4–16 mm: 64–66 | 66–67 |
| | >16 mm: 65–67 | 66–68 |
| Gear Hob | 64–66 | 65–67.5 |
| Medium Tooth Saw | Thickness ≤1mm: 62.5–65 | Not allowed |
| | Thickness >1mm: 63.5–66 | 65–67 |
| End Mill | ≤Ø6mm: 63.5–66 | 65–66.5 |
| | >Ø6mm: 64.5–66.5 | 65–67 |
| Three-Blade Cutter | Thickness ≤8mm: 64–66 | 65–67 |
| | Thickness >8mm: 64–66.5 | 65–67 |
| Taps for Grinding Machine | M3–8: 62–65 | Not allowed |
| | M>8: 63–66 | 65–67 |
| Semi-Circular Milling Cutters | 64–66 | 65–67.5 |
| Angle Cutters | 63.5–66 | 64–67 |
| Broaching Tools | 63.5–66 | Not allowed |
| Push Cutters | 64–66 | 65–67.5 |
| Slotting Cutters | 64–66 | 65–67 |
| Planing Cutter | 64–66 | 65–67 |
| Chipping Blade | 62–64 | Not allowed |
| Bevel Gear Milling Cutter | 64–66 | Not allowed |
Generally, the hardness of quenched and tempered high-speed steel (HSS) ranges from 63 to 67 HRC. High-performance HSS (HSS-E) and powder high-speed steel (SPM) can reach up to 66–70 HRC. However, for most cutting tools, there is an optimal hardness range. For example, while M42-type HSS-E can achieve 69–70 HRC, the ideal range for such tools is usually 66–67 HRC.
Hardness and toughness are often in conflict in tool materials. As tools have evolved from carbon tool steel to high-speed steel, hard alloys, ceramics, and cubic boron nitride, their hardness has increased, but their toughness has decreased. It is more challenging to achieve high toughness than high hardness. In high-speed steel cutting tools, only hardness is typically specified. Most HSS tools are required to be between 63–66 HRC, except for drills and center drills, which may require lower hardness. Decades of practical experience have shown that excessive hardness can reduce tool life.
In the early 1960s, the National Conference of Engineers in the Tool Industry decided that HSS tools with hardness above 66.5 HRC would not be shipped. At that time, ultra-hard HSS and powder HSS had not been widely used, so this regulation was significant for the industry. In the 1970s, Japanese scholars proposed controlling the hardness of general tools at 65–66 HRC. According to nationwide tool evaluations, first-class and superior-quality HSS cutters typically have hardness above 65 HRC, indicating that low hardness does not equate to long life.
It’s important to note that even with the same hardness, different materials or heat treatment processes can result in significantly different tool lives. The metallographic structure is more critical than hardness alone. While high hardness may improve wear resistance, it doesn’t guarantee long life. Factors like carbide distribution, grain size, tempering conditions, and overheating levels all play a role.
The quenching temperature is one of the most crucial factors in achieving the desired hardness. Table 2 lists the recommended quenching temperatures for common high-speed steels:
**Table 2: Common Quenching Temperatures for High-Speed Steels**
| Grade | Recommended Quenching Temperature (°C) |
|----------------------------------------|-----------------------------------------|
| W18Cr4V | 1260–1290 |
| W6Mo5Cr4V2 | 1210–1230 |
| W9Mo3Cr4V | 1220–1240 |
| W2Mo9Cr4V | 1180–1200 |
| W7Mo4Cr4V | 1210–1240 |
| W4Mo3Cr4V | 1160–1185 |
| W6Mo5Cr4V2Al | 1200–1215 |
| W2Mo9Cr4VCo8 | 1160–1190 |
| W6Mo5Cr4V2Co5 | 1190–1215 |
| APM23 | 1170–1190 |
| HAP50 | 1180–1220 |
| S390PM | 1190–1230 |
| CPM | 1170–1190 |
| M4 | 1170–1190 |
For large-size drills, turning tools, and hobs, the austenite grain size should be controlled between 9 and 9.5. For other tools, it should be within 10–10.5. Powdered HSS should have a grain size of 10–11. Tempering is generally done at 550–560°C, repeated four times.
In conclusion, hardness is a key factor in determining tool life. While higher hardness improves wear resistance, it reduces toughness. Excessive hardness can lead to chipping and failure during use. Moderate hardness offers better toughness and allows for wear without compromising performance. When the workpiece material is hard, the tool’s hardness should be set to a maximum. Conversely, when the material is softer, the tool can be less hard. Tools with hardness between 63–64 HRC may meet national standards, but their actual service life may still be limited.
Ultimately, finding the right balance between hardness and toughness in high-speed steel tools remains a continuous challenge for toolmakers and heat treatment professionals.
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