High Speed ​​Steel Tool Hardness and Heat Treatment
Hardness is the material's ability to resist localized plastic deformation on its surface. So far, it remains the only parameter that directly reflects the mechanical and technological properties of the surface layer and its wear resistance. When discussing the hardness and heat treatment of high-speed steel tools, we will provide a brief overview.
**Table 1: Recommended Hardness for High-Speed Steel Tools**
Tool Name | Recommended Hardness (HRC) | Overheating Allowed?
---|---|---
Straight Shank Twist Drills ≤ Ø4mm | 63.5–66 | 64–66 | Not allowed
> Ø4mm | 64–66 | 65–67 |
Large Blade Cutters | 62–65 | 64–66 | No
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 | 64–65.5 | No
>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 |
>8mm | 64–66.5 | 65–67 |
Taps for Grinding Machine M3–8 | 62–65 | 64–66 | No
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 | 64–67 | No
Push Cutters | 64–66 | 65–67.5 |
Slotting Cutters | 64–66 | 65–67 |
Planing Cutters | 64–66 | 65–67 |
Rake Blade | 62–64 | 63–65 | No
Bevel Gear Milling Cutter | 64–66 | 65–67 | No
Generally, quenched and tempered high-speed steel (HSS) can reach 63–67 HRC, while high-performance HSS (HSS-E) and powder high-speed steel (SPM) can go up to 66–70 HRC. However, for most cutting tools, there exists an optimal hardness range. Table 1 shows the recommended values for some HSS tools. As seen in the table, for general tools, maintaining hardness between 65–66 HRC is ideal. For example, HSS-E like M42 can achieve 69–70 HRC, but the best performance is typically found in the 66–67 HRC range.
Hardness and toughness are often at odds in cutting tools. As tool materials have evolved from carbon tool steel to high-speed steel, carbide, ceramic, and cubic boron nitride, their hardness has increased, but so has the trade-off with toughness. In this sense, achieving high toughness is more challenging than achieving high hardness. Most high-speed steel tools only specify hardness, with no explicit requirement for toughness. Except for drills and center drills, which require lower hardness (around 63 HRC), other tools are generally between 63–66 HRC.
Historically, excessive hardness was known to reduce tool life. In the early 1960s, the National Conference of Tool Industry Engineers decided not to ship high-speed steel tools with hardness above 66.5 HRC. This rule had significant influence on the industry before the widespread use of ultra-hard and powder high-speed steels. In the 1970s, Japanese researchers suggested that general tools should be kept around 65–66 HRC. According to nationwide tool evaluations, first-class tools typically had hardness over 65 HRC, proving 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 varying tool lifespans. The same material and specifications may show differences due to factors like carbide distribution, grain size, tempering conditions, and overheating levels. Therefore, hardness is just one surface indicator, while microstructure is the real key.
The choice of heat treatment process is crucial in achieving the desired hardness. Among these, the quenching temperature is the most critical factor. Even a small mistake here can lead to quality issues. The quenching temperatures for common high-speed steels are listed in Table 2.
**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
In addition, except for large drills, turning tools, and hobs, the austenite grain size should be controlled between 9 and 9.5. Other tools should be within 10–10.5. Powdered high-speed steel should be between 10–11. Tempering is usually done at 550–560°C, repeated four times.
In conclusion, hardness is a primary factor affecting the life of high-speed steel tools. While higher hardness improves wear resistance, it reduces toughness. Excessive hardness can cause chipping and permanent failure. On the other hand, moderate hardness ensures better toughness, allowing the tool to wear gradually and be resharpened. When cutting harder materials, the tool's hardness should be set as an upper limit, and vice versa.
Tools with hardness between 63–64 HRC may meet national standards, but their actual service life may not be very long. Therefore, finding the right balance between hardness and toughness is a continuous challenge for tool manufacturers and heat treatment professionals.
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