A Guide to Pre-Hardened Steel (NAK) CNC Machining
Last updated : 2026/8/13
Key Points
Pre-hardened steel (such as NAK) is a type of steel developed for ease of machining, as it has already undergone heat treatment and does not require quenching, tempering, or precision grinding.
Some grades, such as HPM38 and G-STAR, can have their hardness increased after machining through quenching and tempering.
Overview
Pre-hardened steel is a type of steel that has been heat-treated in advance to achieve the desired level of hardness. Instead of requiring additional heat treatment after machining, it’s ready to use as-is. Steel manufacturers sell dedicated grades (such as NAK, HPM, etc.), and there are many pre-hardened steels available for a wide range of applications.
Most pre-hardened steels are set to a hardness of about HRC45 or below—hard enough for durability, yet still easy to machine without additional treatment. Because they are already heat-treated, there is no need for additional processes such as quenching, tempering, or precision grinding, reducing both lead time and machining costs.
The key advantages of pre-hardened steel are its machinability and the fact that it is already heat-treated. Grades such as HPM38 and G-STAR show minimal dimensional change, and after machining, their hardness can be further increased to HRC50 or higher through quenching and tempering, even for high-precision parts.
However, age-hardened steels like NAK55 and NAK80 achieve hardness differently. Their hardness cannot be raised through quenching and tempering, but surface hardness can be enhanced with nitriding after machining.
What is NAK?
NAK steel is a steel for plastic molds, manufactured by Daido Steel Co., Ltd. It is made by adding nickel, aluminum, and copper to iron. It requires no heat treatment, has high hardness, and offers excellent machinability.
NAK materials include NAK55, which offers excellent machinability, and NAK80, which provides a high-gloss finish.
Summary of Commonly Used Types of Pre-Hardened Steel and their Characteristics
Name | Hardness at Delivery | Characteristics |
|---|---|---|
NAK55 | HRC37～43 | Polishability (mirror finish), texturing ability, weldability, machinability |
NAK80 | HRC37～43 | Polishability (mirror finish), wear resistance, machinability, weldability, texturing ability |
HPM1 | HRC37～41 | Machinability |
HPM38 | HRC29～33 | Polishability (mirror finish), corrosion resistance, low dimensional change during heat treatment, additional heat treatment possible |
PX5 | HRC30～33 | Polishability (mirror finish), texturing ability, machinability, EDM machinability |
G-STAR | HRC33～37 | Corrosion resistance, additional heat treatment possible |
S-STAR | HRC31～34 | Polishability (mirror finish), corrosion resistance, low dimensional change during heat treatment, texturing ability, EDM machinability |
STAVAX(H) | HRC27～35 | Polishability (mirror finish), corrosion resistance, wear resistance, machinability |
GO40F | HRC36～40 | Low machining distortion, machinability |
DH2F | HRC37～41 | Resistance to weld erosion (heat damage), machinability |
Applications of Pre-Hardened Steel
Pre-hardened steel is mainly used for forging dies and plastic molding dies, and can also be used as a substitute for SC-series steels in cases such as:
When medium hardness is sufficient.
When skipping quenching, tempering, and subsequent precision grinding to reduce costs and save time.
When manufacturing precision parts where distortion or dimensional change from heat treatment must be minimized.
Pre-Hardened Steel CNC Machining at Yumoto Electric
Yumoto Electric delivers high-quality components fast, worldwide. We provide precision machining for a wide range of metals using milling, turning, and specialized metal processing. We have experience with over 200 materials including aluminum alloys, stainless steel, molybdenum, and titanium.
If you’re unsure about material selection or machining methods, we offer complimentary consultations to help optimize your design and production. For any questions, please feel free to contact us.
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