How do ST threading taps achieve a balance between high hardness and toughness to cope with interrupted cutting conditions?
Publish Time: 2026-02-03
Threading often presents challenges with difficult-to-machine workpieces such as high-strength alloys, hardened steels, or composite materials. Especially under interrupted cutting conditions—such as encountering keyways, holes, or discontinuous surfaces during tapping—tools are subjected to severe impact loads and alternating stresses, easily leading to chipping, breakage, or premature wear. ST threading taps, using high-quality cobalt-containing high-speed steel M35 as the base material, achieve a delicate balance between high hardness and high toughness, becoming a reliable tool for such demanding conditions. This performance advantage is not accidental, but stems from the synergistic optimization of material metallurgical properties, microstructure control, and structural design.
1. M35 Cobalt-Containing High-Speed Steel: Dual Enhancement of Red Hardness and Toughness
While ordinary high-speed steel possesses good wear resistance, it is prone to softening and cracking under high temperatures or impacts. M35, by adding 5% cobalt, significantly enhances the material's red hardness and secondary hardening effect. Cobalt promotes the uniform and dispersed precipitation of carbides, refines grain size, and enhances the bonding force between the matrix and carbides. This allows ST threading taps to maintain a high hardness of over 65 HRC even at cutting temperatures as high as 600℃, effectively resisting edge wear. More importantly, the addition of cobalt does not sacrifice toughness—on the contrary, it optimizes the plastic deformation capacity of the martensitic matrix, enabling the material to absorb energy through micro-region plasticity when subjected to instantaneous impact, rather than directly cracking. This "combination of rigidity and flexibility" is key to coping with repeated impacts in interrupted cutting.
2. Heat Treatment Process: Precise Control of Microstructure to Release Material Potential
The potential of M35 material needs to be fully released through precise heat treatment. ST threading taps typically undergo multi-stage quenching and multiple high-temperature tempering processes to achieve a full transformation of retained austenite to martensite and precipitate fine and dispersed MC-type carbides. This process not only maximizes hardness but also eliminates internal stress, improving overall fatigue resistance. Especially in the cutting edge region, a gradient structure of "hard surface + tough core" can be formed by controlling the tempering parameters: the high hardness of the surface ensures wear resistance, while the core retains a certain degree of toughness to buffer impact. This precise control of the microstructure makes the tap less prone to chipping during intermittent cutting, and also less prone to sudden breakage due to cumulative damage.
3. Geometry and Coating Synergy: Comprehensive Strengthening from Material to System
In addition to the material itself, the geometric design of ST taps further enhances their impact resistance. For example, using a reinforced core diameter and optimizing the rake and clearance angle ratio can improve the rigidity of the tool body while ensuring cutting sharpness; the chip flutes are polished to reduce chip removal resistance and minimize torque surges caused by chip blockage. Furthermore, an ultra-hard nanocomposite coating such as TiAlN is often applied to the surface, with a thickness of only 2–4 micrometers, which significantly reduces the coefficient of friction, isolates heat conduction, and prevents adhesive wear. The coating has a good match with the thermal expansion coefficient of the M35 substrate, making it less prone to peeling off during thermal cycling, forming a composite protective system of "hard shell and tough core".
In summary, the key to ST threading taps' remarkable balance between high hardness and high toughness lies in the superior metallurgical foundation of M35 cobalt-containing high-speed steel, coupled with precise heat treatment and a systematic structural design. It is not only a "hard" tool, but also a "smart" tool—knowing how to adapt to impact, withstand high temperatures, and consistently output high-precision threads even under the harsh conditions of intermittent cutting. For this reason, ST taps have become an indispensable precision "thread engraver" in the high-end manufacturing field.
