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Factors affecting the application of high tool life
The rapid development of the manufacturing industry heavily relies on a robust cutting tool industry. Dongfeng Motor Co., Ltd., a company specializing in edge measuring tools, has over 40 years of experience in tool production. The materials used include carbon tool steel, alloy tool steel, and high-speed steel. Heat treatment processes involve isothermal salt bath quenching, vacuum heat treatment, and induction heating with surface PVD coating. Equipment includes salt bath furnaces, vacuum furnaces, high-frequency induction devices, and PVD coating systems. Salt bath furnaces are commonly used for processing large quantities of high-speed steel tools, while vacuum furnaces are reserved for small-batch tools like rolling wheels due to their limitations.
Through extensive production experience, we have identified key factors affecting tool life: material quality (carbide segregation, large carbides), hardness, toughness, heat treatment parameters (quenching temperature, grain size, tempering, and superheat level). Even a slight deviation in quenching temperature can lead to significant quality issues.
High levels of carbide segregation in high-speed steel reduce plasticity, weaken the matrix, and increase the risk of cracking during forging. Tools may chip or wear more quickly under use. During heat treatment, carbide-rich areas are prone to cracking along the segregation bands. Additionally, increased retained austenite in these regions can cause partial tempering, and large carbides can create stress concentration points, leading to cracks under external forces.
Low hardness and toughness result in poor tool life, but excessive hardness may also be problematic. While high hardness improves wear resistance, it reduces toughness, making tools more prone to chipping and failure. Therefore, a balance between hardness and toughness is essential.
Heat treatment must ensure that carbides dissolve into the matrix as much as possible, avoiding precipitation to maximize secondary hardening after tempering. As a manufacturer, strict control over incoming high-speed steel raw materials is crucial. Forging should be used to break down and evenly distribute carbides, addressing issues at the source. My factory has faced challenges in this area, as discussed in articles published in Metal Processing (Hot Processing) in July 2011 and May 2012, which analyzed carbide-related failures in tools.
Figures 1, 2, and 3 illustrate carbide non-uniformity and accumulation in failed tools, highlighting the importance of proper material distribution. Heat treatment should be tailored to each tool type—some require controlled overheating, while others must avoid it. For example, taps should prioritize toughness over hot hardness, with careful adjustment of quenching temperature and finer grain control. Large-diameter materials, often used in complex tools, require reduced quenching temperatures and times to prevent thermal cracking and manage retained austenite.
Measuring hardness without examining metallographic structure is risky. Mr. Zhao Buqing, a veteran in tool heat treatment, emphasized that “hardness is only a surface phenomenon; metallographic organization is the essence.†This principle remains central to our work.
Tool life also depends on parameter design. A large helix angle in spiral groove taps can improve sharpness and reduce cutting forces, but it may weaken the cutting edge, leading to chip clogging and breakage. Proper design enhances cutting edge wear resistance.
Achieving high tool life is a comprehensive effort. Beyond good design and material selection, heat treatment, sharpening quality, and tool management play critical roles. Ensuring tools are repaired before reaching critical wear, maintaining coating performance, and accounting for equipment vibration and rigidity all contribute to longer tool life.