Compared to foreign tools, domestic cutting tools still face a significant gap in terms of cutting life. Some people believe that the issue lies in the strength of the tool materials, thinking they are not as good as those from abroad. As a result, high-performance high-speed steels such as M42 and M35 are often imported, yet even with these materials, the quality of the final tools is still not on par with foreign counterparts. Experts suggest that the problem may lie in the heat treatment process, which is not as refined as it should be. Here are some practical tips for improving tool life.
1. Understand the Role of Carbides
Heat treatment of high-speed steel primarily involves the transformation and distribution of carbides. Carbides act like a double-edged sword—when properly treated, they can significantly enhance hardness, wear resistance, and hot hardness. However, if not handled correctly, they can lead to overheating or over-burning, which negatively affects the tool's lifespan. To extend the life of high-speed steel tools, it’s essential to deeply understand the nature of carbides in the material and how they form and change during processing. This knowledge helps guide development toward better performance and longer tool life.
One of the most straightforward ways to evaluate this is by checking the uniformity of carbide distribution. By using pressure processing techniques, we can improve the morphology and distribution of carbides. During quenching, it's important to focus on carbide solubility and monitor the changes in tempered carbides.
2. Pay Attention to Carbide Solubility When Checking Grain Size
Grain size has long been used as an important indicator for assessing the quality of quenched high-speed steel. In some cases, it's even considered the sole measure of hardness. But how much does grain size really affect the life of high-speed steel tools? Opinions differ between domestic and foreign manufacturers.
In the former Soviet Union, tests on f8mm P6M5 (M2) straight drills showed that the optimal service life was achieved when the grain size was 9.5, while too fine or too coarse grains reduced the bit’s life. Similar findings have emerged from national brand initiatives like “Chuangyou†and “Creating a National Brand Drill Bit.†Two key points should be added: sufficient carbide dissolution and a post-heat-treatment hardness above 65HRC.
Different types of cutting tools require different levels of quenching austenite grain size and carbide solubility. For example, gear hobs typically require a grain size of 9, turning tools 8, and machine taps up to 10.5. The ideal condition is full dissolution of both grain and carbide structures.
While grain size reflects the heating temperature during quenching, carbide solubility is more critical for tools requiring high thermohardness. Therefore, when examining the metallurgical phase before the furnace, both factors must be taken into account.
3. Focus on Hardness, but Not Just Any Hardness
Many papers emphasize the idea that "hardness is superficial, while microstructure is the essence." While achieving a hardness of 64HRC or higher is desirable for high-speed steel tools, creating the ideal microstructure is far more challenging. For example, super-hard high-speed steels like M42 don't need to be as hard as possible; instead, they require a suitable range. Too much hardness can actually shorten the tool's life.
High-speed steel tools have never been known for their extreme hardness. Each steel grade has an optimal hardness value under specific working conditions. If it's too high or too low, it won't contribute to a longer life. Based on experience, HSS cutters should reach 65–66HRC, HSS-E around 66–67HRC, and HSS-L should maintain 65–66HRC due to its lower alloy content. In practice, tools with longer life tend to have higher hardness, but high hardness alone doesn’t guarantee a long life. It’s crucial to balance hardness with toughness.
4. Surface Strengthening Techniques
Some argue that there's little room for improvement in the heat treatment of high-speed steel tools, suggesting that the focus should be on surface treatments. While this is partially true, it’s not the whole story.
Surface strengthening technologies, such as steam treatment, oxynitriding, TiN coatings, TiAlN coatings, and multi-layer composite coatings, are rapidly advancing and offer real benefits for extending tool life.
However, two key issues must be addressed: pre-treatment must ensure the tool is not too soft or overheated, as no amount of surface treatment will compensate for poor base material. Also, surface strengthening should be targeted and purposeful, ultimately aimed at improving the tool’s overall performance and durability.
5. Ensure Consistent Tool Life
Market feedback shows that a high-quality tool product not only has a long average life but also offers stable performance and consistent life across units. Unfortunately, many Chinese tool manufacturers struggle with this. While individual tools may perform well, the variability in life between pieces is often too high, leading to a drop in the average performance of the batch.
Several domestic factories have conducted similar tests: compared to single pieces, domestic tools are not necessarily worse than foreign ones. However, when testing 10 pieces together, it’s common to find 1 or 2 that fall short.
The stability of tool life is a reflection of a factory’s technical and management capabilities. Achieving consistently high tool life is a complex, system-wide effort that starts from raw material inspection, machining, heat treatment, to final packaging. Strict procedures, detailed work instructions, comprehensive testing methods, and skilled staff are all essential.
The adoption of advanced heating equipment, such as precision temperature control and vacuum furnaces, has greatly improved the consistency and quality of tool products. Additionally, tools like the German Zeiss metallurgical microscope help ensure quality stability.
In conclusion, improving China’s tool life requires collective effort—from steel mills to mechanical processing, but the most critical factor is the commitment of heat treatment professionals. With attention to every step of the process, we can create high-life tools and contribute to China’s goal of becoming a global manufacturing powerhouse.
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