High-speed steel cutting tools generally do not undergo tensile strength testing, but mainly metallographic and hardness testing.
After correct heat treatment, the Rockwell hardness of tungsten-based and molybdenum-based high-speed steel can reach over 63, and that of cobalt-based high-speed steel can reach over 65. The acid-leached low-magnification structure of steel must not have visible shrinkage cavities or skinning. The center is loose, and generally the looseness should be less than level 1.
The content of metallographic examination mainly includes three items: decarburization layer, microstructure and carbide unevenness.
1. High-speed steel should not have obvious decarburization. The microstructure must not contain fishbone eutectic ledeburite.
2. Carbide unevenness in high-speed steel has the greatest impact on quality. Currently, metallurgical and mechanical departments attach great importance to the level of carbide unevenness. Different level requirements for carbide unevenness can be put forward according to different uses of steel. Normally, it should be less than level 3.
3. High-speed steel is used to make cutting tools. In addition to its high hardness, high wear resistance and sufficient toughness, another important factor is its red hardness. (Red hardness refers to the ability of the cutting edge to resist softening in a red hot state during high-speed cutting.)
One way to measure the red hardness is to first heat the steel to 580~650℃, keep it warm for 1 hour, and then cool it. Repeat this four times and then measure the hardness value. The quenching temperature of high-speed steel is generally close to the melting point of the steel. For example, the quenching temperature of tungsten series high-speed steel is 1210~1240℃, and the high-molybdenum series high-speed steel is 1180~1210℃. After quenching, it generally needs to be tempered three times between 540~560℃. Increasing the quenching temperature can increase the red hardness of steel. In order to improve the service life of high-speed steel cutting tools, the surface can be strengthened, such as low-temperature cyanidation, nitriding, sulfur-nitrocarburizing, etc.
