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What are the characteristics of vacuum heat treatment of spring steel

Springs are important parts in various machines and instruments. According to their use occasions and structural shapes, they can be divided into two categories: leaf springs and coil springs. The steel used to manufacture spring parts is called spring steel.

Performance requirements of spring steel

In various mechanical equipment, the main function of springs is to absorb impact energy and reduce mechanical vibration and impact. For example, leaf springs used in vehicles connect wheels and frames. They not only bear the huge weight and load of the carriage, but also bear the impact and vibration caused by uneven ground. Springs can also store energy so that machine parts can complete pre-specified actions, such as valve springs, plunger springs on high-pressure hydraulic pumps, and nozzle springs. Because springs use their elastic deformation to absorb or release energy, spring steel should have a high elastic limit σ. (It can also be considered as yield strength σ_0.2); springs generally work under alternating stress, so spring steel should have high fatigue strength; spring steel should also have good process performance, have a certain plasticity to facilitate forming, low overheat sensitivity, and not easy to decarburize. In addition, some springs working under high temperature and corrosive conditions should also have good heat resistance and corrosion resistance.

Chemical composition characteristics of spring steel

According to the performance requirements, the chemical composition of spring steel has the following characteristics.

① Medium and high carbon to ensure high elastic limit and fatigue limit. The carbon content of carbon spring steel is generally 0.8%~0.9%, and the carbon content of alloy spring steel is 0.5%~0.7%. If the carbon content is too low, the high yield strength requirement cannot be met; if the carbon content is too high, the steel will be very brittle.

② Add alloy elements mainly composed of Si and Mn. Si and Mn are alloy elements frequently used in spring steel. The purpose is to improve hardenability, strengthen ferrite (because Si and Mn have the best solid solution strengthening effect), and improve the tempering stability of steel, so that it has higher hardness and strength at the same tempering temperature. Among them, Si has the greatest effect, but when the Si content is high, the tendency of C graphitization is increased, and it is easy to decarburize when heated; Mn is easy to overheat the steel.

③ Add Cr, W, V, and Nb to overcome the shortcomings of silicon-manganese spring steel. Because Cr, W, V, and Nb are carbide-forming elements, they can prevent overheating (refining grains) and decarburization, thereby ensuring that important springs have high elastic limit and yield limit.

In addition, since the purity of spring steel has a great influence on fatigue strength, spring steel is high-quality steel (wp≤0.04%, ws≤0.04%) or high-quality steel (wp0.035%, ws≤0.035%).

Typical spring steel and application

(1) Carbon spring steel

Carbon spring steel has poor hardenability. When the diameter is greater than 12~15mm, it cannot be hardened in oil. Therefore, carbon spring steel is only used to manufacture springs with smaller diameters and less important. It is mostly manufactured by cold forming.

(2) Alloy spring steel

Alloy spring steel is usually divided into two categories according to the type and amount of alloying elements. One type is spring steel alloyed with alloying elements Si and Mn. Representative steel grades are 65Mn and 60Si2Mn. Their hardenability is significantly higher than that of carbon spring steel and can be used to manufacture springs with larger cross-sectional dimensions. The composite alloying of Si and Mn has better performance than that of Mn alone. This type of steel is mainly used for leaf springs and coil springs on automobiles, tractors and locomotives. Another type is spring steel containing alloying elements such as Cr, W, and V, and the representative steel is 50CrVA. The composite addition of Cr and V not only improves the hardenability of spring steel, but also has higher high-temperature strength, toughness and better heat treatment process performance. Therefore, this type of steel can be used to manufacture large springs that withstand heavy loads at 350~400℃, such as valve springs and valve springs for high-speed diesel engines. 60CrMnBA steel has good hardenability, and the critical diameter of hardenability in oil can reach 100~150mm. It is suitable for making super-large springs, such as laminated leaf springs for bulldozers, large coil springs on ships, and large torsion springs.

Characteristics of vacuum heat treatment of spring steel

Springs can generally be divided into two categories: cold-formed springs and hot-formed springs. Among them, cold-formed springs are made by cold deformation or vacuum heat treatment to make the steel have certain properties, and then cold-formed into springs of certain shapes. For example, high-strength steel wire (piano wire), hard steel wire, stainless steel wire, etc. that are first cold-formed; vacuum oil quenching and tempering steel wire that is first vacuum heat treated. Cold-formed springs must be vacuum tempered at 200~400℃ after cold forming.

The vacuum heat treatment of hot-formed spring steel is vacuum quenching and vacuum tempering. The vacuum quenching temperature is generally 830~870℃. If the temperature is too high, grain growth and decarburization are likely to occur. After vacuum quenching heating, cool in 50~80℃ oil. The vacuum tempering temperature is generally 420~520℃; tempered troostite is obtained. The hardness after vacuum tempering is about 39~52HRC, the hardness of spiral springs after vacuum tempering is generally 45~50HRC, and the hardness of automotive leaf springs after vacuum tempering is generally 40~47HRC.

Since the stress state of the spring during service is relatively complex, especially the effects of bending and torsion stress, the surface state of the spring is required to be high. During the vacuum heat treatment heating process, the furnace gas must be strictly controlled and the heating time must be shortened as much as possible to prevent and minimize surface oxidation and decarburization. After vacuum heat treatment, the spring is usually shot peened to strengthen the surface and generate residual compressive stress on the surface to improve fatigue strength.