Relationship between temperature and hardness in spring heat treatment

The spring is a special part whose stress should be below the elastic limit of the steel. Therefore, the steel is required to have a high elastic limit. However, as the strength and hardness of steel increase, the sensitivity to stress concentration and the constant load increase, the tendency of spontaneous fracture increases. Therefore, the spring should not be heat treated to Rockwell hardness above HRC48, generally tempered to HRC48 is suitable. The hardness is too low, the spring returning ability will be reduced, or even fail, losing the function that the spring should have.

Aviation springs, whether in the structure of aircraft, or in devices and systems, are mostly small parts, but once fractures fail, they often cause the loss of function of a certain device or a certain system, and even lead to serious consequences. Therefore, aviation The use of springs is safe and reliable as the first requirement. There are many reasons for the failure of the spring, but there are two kinds of internal factors and external factors: the internal factors are mainly the quality of the spring material and the correctness of the heat treatment. The quality of the material directly affects the life of the spring. In order to reduce the impurity content of the steel and improve the purity of the steel, the important springs are often electroslag remelted steel, and even double electroslag remelted steel.

When heat treatment, the quenching and tempering temperature should be properly controlled to obtain the desired microstructure and hardness. In order to prevent surface decarburization during quenching heating, it should be heated by a salt bath furnace or even vacuum. The external factors are mainly the spring wire and the surface quality of the strip. The micro cracks on the surface of the wire, especially the surface corrosion pit, will become the source of the crack. Under the changing working stress, the spring will break due to corrosion fatigue. For springs made of carbon steel that is not resistant to corrosion, or springs made of alloy steel, although surface anti-corrosion measures can be taken, the protective layer is also hard to be protected from damage, and the damage will accelerate corrosion.

Type 18-8 austenitic stainless steel and semi-austenitic precipitation hardened stainless steel are the most commonly used stainless spring steels. The former relies on a high degree of cold hardening to obtain the necessary strength and elasticity, while the latter relies on both cold work hardening and age hardening to obtain the necessary strength and elasticity. The cold drawn steel wire and cold rolled steel strip of these steels are directly used to wrap the spring, and then tempered by 400-500e, or aging, which can obtain stable elasticity, greatly simplifying the manufacturing process. These steel-made springs not only have good corrosion resistance, they do not require surface protection in the atmosphere, and can operate at higher temperatures.

Plastic additives are the compounds which must be added to improve its processing performance or improve resin performance when molding. Thus, plastic additives play the particularly important position in the plastic molding process.

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