The characteristics of spring steel-springs mainly work under dynamic load, that is, under the conditions of impact and vibration, or under the action of alternating stress, using elastic deformation to absorb impact energy and play a buffering role.
Because springs are often subjected to vibration and work under the action of shared stress for a long time, the main fatigue failure is, so the spring steel must have high elastic limit and high fatigue limit. In addition, it should have sufficient toughness and plasticity to prevent sudden brittle fracture under impact force.
In terms of manufacturability, spring steel should have good hardenability and low overheating and decarburization sensitivity. Reducing the surface roughness of the spring can increase the fatigue life.
In order to obtain the required performance, spring steel must have a high carbon content. The carbon content of carbon spring steel is between 0.6-0.9%. Due to the poor hardenability of carbon spring steel, it is only used to make springs with a cross-sectional dimension not exceeding 10-15mm. For springs with larger cross-sectional dimensions, alloy spring steel must be used. The carbon content of alloy spring steel is between 0.45-0.75%, and the added alloying elements are Mn, Si, W, V, Mo, etc. Their main function is to improve hardenability and tempering stability, strengthen ferrite and refine grains, and effectively improve the mechanical properties of spring steel. Among them, Cr, W, and Mo can also increase the high temperature strength of steel.
Springs formed under hot conditions (diameter or thickness generally above 10mm)
Springs formed in a cold state (diameter or thickness generally less than 10mm)
Heat treatment process of hot-formed springs-most springs formed by this method are combined with hot-forming and heat treatment, while most coil springs are heat-treated after hot forming. The heat treatment method of this kind of spring steel is quenching + intermediate temperature tempering, and the structure after heat treatment is tempered troostite. This kind of organization has high elastic limit and yield limit, and has certain toughness.
The heat treatment process of cold-formed springs–for springs made of cold-rolled steel plates, steel strips or cold-drawn steel wires, the material is strengthened due to cold plastic deformation, and has reached the required performance of the spring. Therefore, after the spring is formed, it is only necessary to perform stress relief treatment in the range of about 250C and heat preservation for about 30 minutes to eliminate the door stress of the cold formed spring and to finalize the shape of the spring.
Heat treatment of heat-resistant spring steel-The valve springs of internal combustion engines work at higher temperatures, and some still have corrosive atmospheres. Therefore, special spring steels and appropriate heat treatment specifications must be selected.
Common defects and preventive measures during spring quenching
(1) Decarburization (reducing service life)-1. Use a salt bath furnace or a heating furnace for heating. 2. Using rapid heating process.
(2) After quenching, the hardness is insufficient, the number of non-martensite is large, and ferrite appears in the core (total residual deformation, reducing service life)-1. Use materials with better hardenability. 2. Improve the cooling capacity of quenching coolant. 3. The temperature of the spring entering the coolant should be controlled above Ar3. 4. Appropriately increase the quenching heating temperature.
(3) Overheating (increased brittleness)-1. Strictly control the heating temperature of forming and quenching. 2. Strengthen the metallographic inspection during quenching.
(4) Cracking (increased brittleness, severely reduced service life)-1. Control the quenching heating temperature. 2. When it is cooled to 250-300C during quenching, take out air cooling. 3. Tempering in time
Measures to improve spring quality
(1) Thermomechanical heat treatment–combining steel deformation strengthening and heat treatment strengthening to further improve the strength and toughness of steel. Thermomechanical heat treatment is divided into high, medium and low temperature. High temperature thermomechanical heat treatment is quenched immediately after deformation occurs in a stable austenite state, and can also be combined with forging or hot rolling, that is, quenched immediately after hot forming. Thermomechanical treatment has been used in the production of automobile leaf springs. (60Si2Mn)
(2) Austempering of springs-Austempering can be used for springs with small diameters or sufficient permeability. It can not only reduce deformation, but also improve strength and toughness. It is best to perform tempering again after austempering. The elastic limit can be increased, and the tempering temperature is the same as the austempering temperature.
(3) Spring relaxation treatment-the spring works under the action of external force for a long time, and the result of stress relaxation will produce a small amount of permanent (plastic) deformation, especially the spring that works at high temperature, the stress relaxation phenomenon is more serious at high temperature. Reduce the accuracy of the spring, which is not allowed for general precision springs. Therefore, this kind of spring should be relaxed after quenching and tempering-pre-load the spring so that its deformation exceeds the deformation that may occur when the spring is working. Then heat it at 20C higher than the working temperature for 8-24h.
(4) Low-temperature carbonitriding-the process of combining tempering and low-temperature carbonitriding (soft nitriding) can significantly improve the fatigue life and corrosion resistance of the spring. This process is mostly used for coil springs.
(5) Shot peening-surface defects such as scratches, folds, oxidative decarburization, etc. tend to become stress concentration places and sources of fatigue fracture during spring operation. If a small steel shot is used to spray the surface of the spring at high speed, it will not only improve the surface quality of the spring, increase the surface strength, and put the surface in a state of compressive stress, thereby increasing the fatigue strength and service life of the spring.
(1) Check the surface for defects such as decarburization and cracks before heat treatment. These surface defects will seriously reduce the fatigue limit of the spring.
(2) The quenching heating should pay special attention to prevent overheating and decarburization, do a good job in salt bath deoxidation, control the furnace gas atmosphere, and strictly control the heating temperature and time
(3) In order to reduce the deformation, the method of loading the spring during heating, the form of fixture and the method of quenching during cooling.
(4) Tempering as soon as possible after quenching, and heating as much as possible. Quick cooling after tempering can prevent temper brittleness and cause surface compressive stress, and improve fatigue strength.
Before heat treatment–(1) The rolled surface of steel is often the surface after the spring is made, so there should be no cracks, folds, scars, hair lines, bubbles, interlayers, and pressed oxide scales.
(2) Surface decarburization will significantly reduce the fatigue strength of the spring. The depth of the decarburized layer should be inspected as required.
After heat treatment–(1) There should be no cracks, corrosion pitting and severe quenching deformation when observing the spring watch with the naked eye or low magnification.
(2) The hardness and its uniformity meet the requirements. In mass production, it is allowed to use a file to sample the hardness, but it must be noted that the position of the file mark should not affect the final accuracy of the spring.
(3) The metallographic structure should be troostite or a mixed organization of troostite and sorbite.
(4) After the leaf spring is assembled, the permanent deformation and static deflection test under working load are usually carried out.
Heat treatment specifications and hardness requirements of various spring steels
Steel grade Quenching temperature Quenching medium Hardness requirement Tempering temperature Cooling medium Hardness requirement Application range
65 780-830 Water or oil 400-600 Coil springs and spring washers with wire diameter less than 12-15mm
65Mn 810-830 oil or water >60 370-400 water 42-50 5-10mm leaf spring and 7-10mm coil spring
50CrV 850-870 oil >58 400-55 water 45-50 large and important leaf springs and coil springs
50CrV 850-870 oil >58 370-420 45-52 high temperature spring working below 300C
60Si2MnA 860-880 oil >60 500-520 water HB363-444 plate spring with thickness 8-12mm
Heat treatment specification for material selection and maximum use temperature of heat-resistant spring steel
Steel grade Maximum use temperature Hot forming (degrees) Quenching temperature Tempering temperature
50CrVA 300 880-900 850-870 Oil 430-500
4Cr13 400 850-1050 980-1050 Air cooling 540-560
Cold drawn 18-8 stainless steel wire 400 cold coil stress relief tempering 400C, 15-60min
W18Cr4V 600 1000-1200 1280-1290 Oil or air cooling 700
60Si2MnA 250 880-900 860-880 oil 350
Specification for austempering of common spring steel
Steel number Heating temperature Austempering temperature Isothermal holding time Hardness HRC
T10A 800+-10 250-360 10-30 40-53
65 820+-10 320-340 15-30 46-48
65Mn 820+-10 270 / 320-340 15-30 52-54 / 46-48
50CrVA 850+-10 300 30-45 52
60Si2MnA 87+-10 280 30-45 52
Comparison of fatigue properties of 50CrV steel valve spring after low temperature carbonitriding (soft nitriding) and ordinary heat treatment
Heat treatment process Breaking strain cycles Residual deformation after fracture (mm) Indoor atmospheric corrosion resistance test
Oil quenching at 860C, tempering at 420C for 1h 1.78*10000 2.4 No oil after blackening, rust spots appear after 14 days
860C oil quenching, 480C soft nitriding 3h >=100*10000 0.2 No rust for more than 850 days
860C oil quenching, 510C soft nitriding 3h >=100*10000 0.6 No rust for more than 850 days