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Optimizing vacuum heat treatment of spring steel

Spring is an important part in various machinery and instruments. According to its use occasion and structural shape, it can be divided into two categories: leaf spring and coil spring. The type of steel used to make 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 on vehicles connect the wheels and the frame. They not only bear the huge weight and load of the carriage, but also bear the impact and vibration caused by uneven ground. Spring can also store energy to enable machine parts to complete pre-specified actions, such as valve springs, plunger springs on high-pressure hydraulic pumps, nozzle springs, etc. Because springs use their elastic deformation to absorb or release energy, spring steel should have a high elastic limit σe (which can also be considered as yield strength σ_0.2); springs generally work under alternating stress, so spring steel It should have high fatigue strength; spring steel should also have good process performance, have certain plasticity to facilitate forming, have low overheat sensitivity, and be difficult to decarburize, etc. In addition, some springs that work under high temperature and corrosive conditions should also have good heat resistance and corrosion resistance.

Chemical composition characteristics of spring steel

According to 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% to 0.9%, and the carbon content of alloy spring steel is 0.5% to 0.7%. If the carbon content is too low, it cannot meet the high yield strength requirements; if the carbon content is too high, the steel will be very brittle.

②Add alloying elements mainly Si and Mn. Si and Mn are alloying elements often used in spring steel 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 can be used under the same conditions. It has higher hardness and strength at tempering temperature. Among them, Si has the greatest effect, but when the Si content is high, it increases the tendency of C to graphitize, and it is easy to decarburize when heated; Mn tends to overheat the steel.

③Add 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 springs for important purposes have high elastic limits and yield limits.

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

Typical spring steel and applications

(1)Carbon spring steel

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

(2)Alloy spring steel

Alloy spring steel is usually divided into two categories according to the type and amount of alloy elements. One type is spring steel alloyed with alloying elements Si and Mn. The representative steel types 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-section sizes. The composite alloying of Si and Mn has better performance than Mn alone. This type of steel is mainly used for leaf springs and coil springs on automobiles, tractors and locomotives. The other type is spring steel containing alloy elements such as Cr, W, and V. The representative steel type is 50CrVA. The combined 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 can withstand heavy loads at 350 to 400°C, such as valve springs and valve springs of high-speed diesel engines. 60CrMnBA steel has good hardenability, and the critical diameter for hardenability in oil can reach 100~150mm. Suitable for making extra large springs, such as stacked leaf springs for bulldozers, large coil springs and large torsion springs on ships.

Vacuum heat treatment characteristics 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 of steel with certain properties through cold deformation or heat treatment, and then cold-formed into springs of a certain shape. Such as high-strength steel wire (piano wire), hard steel wire, stainless steel wire, etc. that are first cold-deformed; vacuum oil quenched and tempered steel wire that is first heat-treated, etc. Cold-formed springs must be subjected to low-temperature vacuum tempering at 200 to 400°C after cold forming.

The heat treatment of hot formed spring steel is vacuum quenching and medium temperature tempering. The vacuum quenching temperature is generally 830 to 870°C. If the temperature is too high, grain growth and decarburization may easily occur. After quenching and heating, cool in oil at 50~80℃. The vacuum tempering temperature is generally 420~520℃ to obtain tempered troostite. The hardness after tempering is about 39~52HRC, the hardness after vacuum tempering of coil springs is generally 45~50HRC, and the hardness of automobile leaf springs after 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 relatively high. During the heating process of vacuum heat treatment, 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, springs are usually shot peened to strengthen the surface and generate residual compressive stress on the surface to improve fatigue strength.