3. Heat treatment type of spring
Spring materials are steel and non-ferrous metals, most of which are steel. The raw materials include bar, wire, plate and strip; some are supplied in the state of annealing, quenching and tempering or bainite isothermal quenching after hot rolling, cold rolling and cold drawing, and the final heat treatment is carried out after the spring is made.
3. 1 Preparation heat treatment
3.1.1 annealing
In order to facilitate further cold drawing or cutting, softening annealing (spheroidizing annealing) is necessary for various round steel and hard steel wire used in hot forming spring, so as to reduce material hardness, improve its cutting performance and improve its cold plastic deformation ability. The annealing of spring steel should be heated in the furnace with protective atmosphere to prevent serious oxidation and decarburization on the surface. The annealed microstructure should be spheroidal pearlite. The hardness is about 180 HBW.
3.1.2 normalizing
For some springs, such as automobile torsion bar spring, it is easy to cause uneven structure and performance and large residual internal stress during hot working. Normalizing can make the spring steel structure uniform and fine, eliminate the internal stress, and is easy to process.
3. 2 final heat treatment
3.2.1 quenching
During quenching and heating of spring, special attention should be paid to prevent surface decarburization, overheating and over burning, grain boundary oxidation and other defects. The spring must be heated in the furnace with protective atmosphere or coated with protective coating in the furnace, and then cooled with water or oil according to the steel type.
For example, in order to prevent hydrogen embrittlement, 65Mn is made of 65 ᦇ and 70 ᦇ steel. The technical requirements are 43-47hrc. The equipment sy-805-4 muffle free continuous mesh belt furnace production line is used. The heating temperature is 820 ℃± 5 ℃, the rotating speed of mesh belt is 35min, the carbon potential is controlled at 0.62% – 0.65%; the tempering temperature is 410 ℃± 10 ℃, the water is used for discharging, and the hardness value is 43-45hrc. Operation precautions: when charging furnace, it is necessary to ensure that the charging is uniform, the height is 3-4 gasket thickness, do not stack too thick or scattered, resulting in uneven heating or overheating phenomenon, quench into the rapid quenching oil, do not start the mixer, so as to avoid excessive deformation. After tempering, it is cooled by water quickly to prevent the second kind of temper brittleness.
Martensite step quenching or bainite austempering has been widely used in production. It is also possible to keep a part of austenite during bainite austempering for a short time without reaching the end line of bainite transformation, and then continue to cool to room temperature to transform this part of austenite into martensite, which has both bainite and austenite in structure and high strength and good performance Toughness is a method of strengthening and toughening. Only the spring with small cross-section can use this quenching process.
3.2.2 tempering
After quenching, the spring should be tempered in time, and the residence time after quenching should not be more than 4 hours, so as to prevent quenching cracking, improve plasticity and toughness, and reduce the internal stress of quenching. The second kind of spring steel with temper brittleness should be cooled quickly after tempering.
3.2.3 low temperature annealing (stress relief tempering)
Non ferrous metal wires such as piano steel wire, cold drawn hard steel wire, stainless spring steel wire or phosphor bronze are usually cold formed spring materials; quenching tempering steel wire, thermomechanical treatment steel wire and heat treatment steel strip are also cold forming spring materials. All kinds of springs made of these materials have high internal stress, which must be annealed at low temperature (stress relief tempering) to remove the internal stress. The heating temperature of low temperature annealing (stress relief tempering) is generally below the recrystallization temperature. After holding for a proper time, the furnace is cooled by air. This method can not only partly eliminate the internal stress in the spring, stabilize the shape and size, but also effectively improve the mechanical properties, such as hardness (2 ~ 3HRC), tensile strength, yield ratio, elastic limit, fatigue resistance and stress relaxation resistance.
3.2.4 precipitation hardening
The precipitation hardening process includes heating the alloy above the transformation point to obtain the supersaturated solid solution of some element, and then quenching the solution heat treatment, heating the spring and its steel wire to a certain appropriate temperature above the alloy solubility curve, holding for a proper time, so as to precipitate uniformly distributed fine hard phase particles in the supersaturated solid solution. This heat treatment method can make the spring material reach the required strength and toughness elasticity. The precipitation hardening materials used in spring manufacturing are 07cr17ni7al (0cr17ni7al), be Cu alloy, Fe Ni based high elastic alloy, ni73cr15fe7nbti (lnconell X-750) and lnconell-718, etc.
3.2.5 thermomechanical treatment
The high temperature thermomechanical treatment of 60Si2Mn and 55simnb leaf springs can improve the strength, toughness and fatigue life of leaf springs, improve the labor productivity of leaf springs, and reduce the labor intensity and cost. The processing and manufacturing of automobile leaf spring is to bend the leaf spring with pressure bending machine in the heating state, quenching in the bending state, quenching in oil, and tempering at medium temperature.