Keywords: quenching medium, quenching, UCONA, cooling, temperature, hardening, hardness,

Application of aqueous quenching medium in induction quenching of powder metallurgy parts
　　In order to reduce the cracking of automobile sprockets manufactured by iron-based powder metallurgy during induction quenching, UCONA type water-soluble polymer is used as quenching medium. The results show that the UCONA quenching medium not only does not produce foam during spray cooling, but also can ensure that the quenching medium has a high cooling rate in the temperature range of the lowest stability of the undercooled austenite by adjusting the medium concentration, and can significantly slow down the quenching medium at 200-300 ℃.
　　range of cooling rates. Under the condition that the hardness of the sprocket and the depth of the effective hardening layer meet the technical requirements, the UCONA type water-soluble quenching medium minimizes the stress during induction quenching cooling of the iron-based powder metallurgy sprocket, thus controlling the quenching cracking ratio within 1%.
　　iron-based powder metallurgy uses iron powder as the main raw material, graphite, alloying elements, zinc stearate and engine oil, etc. after mixing, placed in a specified mold cavity, cold pressed with a press, and then sintered at a high temperature of about 1100 ℃ under protective atmosphere to make parts. The microstructure of iron-based powder metallurgy materials is similar to that of steel, but there is a certain amount of pores in the microstructure. The pore of iron-based powder metallurgy parts plays the role of local notch, which will cause stress concentration, increase the stress value of the pore part, and lead to low stress fracture. Due to the existence of pores in iron-based powder metallurgy parts, the thermal conductivity is reduced. Therefore, the quenching heating temperature of iron-based powder metallurgy parts is higher than that of steel parts with the same chemical composition, the heating time is longer than that of steel parts with the same chemical composition, the cooling speed is faster than that of steel parts with the same chemical composition, and the quenching cracking tendency is larger than that of steel parts with the same chemical composition. In the induction heating process, the cracking of iron-based powder metallurgy parts due to uneven heating can be reduced by slow heating or preheating. However, in order to improve the strength and hardness of iron-based powder metallurgy parts, it is necessary to quickly cool the parts heated to the quenching temperature. Rapid cooling can be done with oil, water or a water-soluble quenching medium.
　　Although oil cooling reduces the distortion and cracking of iron-based powder metallurgy parts, the oil fume generated during quenching will pollute the environment and easily cause fire. More importantly, oil can only be immersed but not sprayed during induction quenching, and immersion quenching is difficult to reach the critical cooling speed required for iron-based powder metallurgy parts. Water cooling is safe and economical, but the cooling rate near the martensitic transformation start point is too fast. UCON water-soluble quenching medium has the characteristics of adjustable cooling rate range from water to medium speed quenching oil. Choosing the right concentration can have the advantages of both oil and water, and has good wettability to the surface of heated parts. Considering the poor thermal conductivity of iron-based powder metallurgy parts, UCONE water-soluble quenching medium is only suitable for slow cooling occasions. Therefore, UCONA is preferred as the quenching cooling medium when designing the induction quenching process for automobile sprockets manufactured by iron-based powder metallurgy.

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