Process and heat treatment of spring steel

The spring mainly works 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 process performance, 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 spring

Most of the springs formed by this method are hot-forming and heat treatment combined, while most of the 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.

Heat treatment process of cold formed spring

For springs made of cold-rolled steel plates, steel strips or cold-drawn steel wires, the materials have been strengthened due to cold plastic deformation and have reached the required performance of the spring. Therefore, after the spring is formed, it is only necessary to perform stress relief treatment within a temperature range of about 250°C for about 30 minutes to eliminate the internal 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 atmosphere, so special spring steel and appropriate heat treatment specifications must be selected.

Common defects and preventive measures during spring quenching

1. Decarbonization (reducing service life)

Preventive measures:

(1) Use salt bath furnace or controlled atmosphere heating furnace for heating.

(2) Adopt rapid heating process.

2. After quenching, the hardness is insufficient, the number of non-martensite is large, and ferrite appears in the core (residual deformation occurs, reducing service life)

Preventive measures:

(1) Choose 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) Increase the quenching heating temperature appropriately.

3. Overheating (increased brittleness)

Preventive measures:

(1) Strictly control the forming and quenching heating temperature.

(2) Strengthen the metallographic inspection during quenching.

4. Cracking (increased brittleness, severely reduced service life)

Preventive measures:

(1) Control the quenching heating temperature.

(2) When it is cooled to 250-300℃ 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 20℃ 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.