1. Working conditions of measuring tools and performance requirements of steel used for measuring tools
Measuring tools are tools used to measure the size of workpieces, such as calipers, block gauges, plug gauges and micrometers. Since the measuring tool is often friction and collision of the workpiece during use, and the measuring tool itself must have very high dimensional accuracy and constancy. Therefore, the following properties are required:
(1) High hardness and high wear resistance. This ensures that it will not be quickly worn out during long-term use and lose its accuracy.
(2) High dimensional stability. To ensure that the measuring tool keeps its shape and size constant during use and storage.
(3) Sufficient toughness. To ensure that the measuring tool will not be damaged due to accidental factors-collision.
(4) Corrosion resistance in special environments.
2. Steel for commonly used measuring tools
According to the types of measuring tools and accuracy requirements, different steel grades can be selected for measuring tools:
(1) Measuring tools with simple shapes and low precision requirements. Optional carbon tool steel, such as T10A, TllA. T12A. Due to the low hardenability of carbon tool steel, water quenching for large-sized measuring tools will cause greater deformation. Therefore, this type of steel can only manufacture measuring tools such as calipers, templates, gauges, etc., with small dimensions, simple shapes, and low precision requirements.
(2) High-precision measuring tools (such as block gauges and plug gauges) usually use high-carbon low-alloy tool steels. Such as Cr2, CrMn, CrWMn, and bearing steel GCr15. Because this type of steel is made of high-carbon steel with Cr , Mn, W and other alloying elements, so it can improve hardenability, reduce quenching deformation, and improve the wear resistance and dimensional stability of steel.
(3) For measuring tools that are simple in shape, low in accuracy, and susceptible to impact in the rain, such as simple flat templates, calipers, rulers and large measuring tools, carburized steel 15.20, 15Cr, 20Cr, etc. can be used. But the measuring tool must be used after carburizing, quenching and low temperature tempering. After the above treatment, the surface has high hardness, high wear resistance, and the core maintains sufficient toughness. Medium carbon steel 50, 55, 60, 65 can also be used to make measuring tools. But it must be quenched and tempered, and then used after high-frequency quenching and tempering. The accuracy of the measuring tools can also be guaranteed.
(4) Measuring tools working under corrosive conditions can be made of stainless steel 4Cr13, 9Cr18. After quenching and tempering, the hardness can reach HRC56-58, while ensuring that the measuring tools have good corrosion resistance and sufficient wear resistance .
If measuring tools require particularly high wear resistance and dimensional stability, you can choose nitriding steel 38CrMoAl or cold work die steel Cr12MoV.
After quenching and tempering, 3CrMoAl steel is finished and shaped, and then chlorinated. Finally, it needs to be ground. Cr12MoV steel undergoes surface nitriding or carbon and nitrogen co-nitriding after quenching and tempering or quenching and tempering. After the two kinds of steel are heat treated, the measuring tools can have high wear resistance, high corrosion resistance and high dimensional stability.
3. Heat treatment of measuring tool steel
The main feature of the heat treatment of measuring tool steel is to take various measures as far as possible to keep the dimensions of the measuring tool stable during long-term use while maintaining high hardness and high wear resistance. The phenomenon that the size of the measuring tool changes with time during use is called the aging effect of the measuring tool. This is because: ① The hypereutectoid steel used for manufacturing measuring tools contains a certain amount of retained austenite after quenching, and the retained austenite becomes martensite to cause volume expansion. ②Martensite continues to decompose during use, and the decrease in squareness causes volume shrinkage. ③The existence and redistribution of residual internal stress causes the elastic deformation to partially transform into plastic deformation and cause dimensional changes. Therefore, in the heat treatment of measuring tools, the following heat treatment measures should be adopted for the above reasons:
(1) Quenching and tempering treatment. Its purpose is to obtain tempered sorbite structure to reduce quenching deformation and improve the smoothness of machining.
(2) Quenching and low temperature tempering. The gauge steel is hypereutectoid steel. Usually adopt incomplete quenching and low temperature tempering under the premise of ensuring the hardness now, try to reduce the quenching temperature and preheat to reduce the temperature difference and quenching stress during heating and cooling. The quenching method of the measuring tool is oil cooling (20～30℃). It is not suitable to use graded quenching and austempering. It should be considered only under special circumstances. Generally, low-temperature tempering is used, the tempering temperature is 150-160℃, and the tempering time should not be less than 4-5h.
(3) Cold treatment. High-precision measuring tools must be cold treated after quenching to reduce the amount of retained austenite, thereby increasing dimensional stability. The cold treatment temperature is generally 70-80°C and is carried out immediately after quenching and cooling to room temperature, so as to prevent the retained austenite from aging and stabilizing.
(4) Aging treatment. In order to further improve the dimensional stability, after quenching and tempering, the aging treatment is carried out at 120-150°C for 24 to 36 h, which can eliminate the residual internal stress and greatly increase the dimensional stability without reducing its hardness. In short. The heat treatment of measuring tool steel is in addition to the normal heat treatment of a section of hypereutectoid steel (incomplete quenching and low temperature tempering). Three additional heat treatment processes are required. That is, quenching and tempering treatment and normal quenching treatment are performed before quenching. Between cold treatment and aging treatment after normal heat treatment.