Spring failure analysis and prevention techniques
Spring is an indispensable part of general machinery, which plays a role in buffer balance, energy storage, automatic control, return positioning, safety insurance and so on. The spring often fails due to various reasons during use, resulting in mechanical failure. Therefore, it is necessary to discuss the causes of spring failure and the prevention of application.
The main factors that lead to the failure of the spring are material defects, processing and manufacturing defects, improper heat treatment, improper surface treatment, and working environment factors. Spring surface defects, including collision marks, fretting wear, pits, etc., cause the largest proportion of spring failure, accounting for 50%. In addition, cracks occupy 20%; Inclusion and porosity 13%; Decarbonization, heat treatment and surface strengthening account for about 3%, respectively.
The failure of the spring can be caused by one reason or by the combined action of several reasons. Therefore, the failure analysis of the spring must first investigate and analyse the failure phenomenon of the example, find out its failure mode, and then find out the causes of its failure to put forward improvement measures.
Spring failure caused by spring raw materials:
(1) Due to the different smelting methods of steel, there will be different degrees of inclusions in the steel that cause early fatigue failure of the spring, excessive inclusions or large size, poor uniformity will affect the mechanical properties of the material, and it is easy to early fatigue failure. Example: A torsion bar spring used for vehicle suspension broke when the new car was out of the warehouse. The analysis shows that the fracture originated from a coarse, brittle inclusion on the subsurface of the spring.
Preventive measures:
The spring material must have excellent gold treatment quality, such as strict control of chemical composition, high purity, and low inclusion content, and also requires the uniformity and stability of material composition and organisation. In order to reduce the harmful gases and impurity elements in steel and improve the purity of steel, refining technology such as vacuum refining and electro-slag remelting should be adopted.
(2) Defects that may be caused by the rolling process: residual pipe shrinkage and central cracks; Folding defect; Linear defects, scratches;
Surface corrosion pit: Overburnt, orange peel surface, hemp pit; These can cause the spring to fail.
Preventive measures:
Steel mills should try to avoid and eliminate the defects generated in the rolling process; spring plants should strengthen the quality inspection of spring raw materials and try to use good surface quality materials.
Spring failure caused by manufacturing process:
Surface defects of cold-formed helical springs may occur due to poor equipment or improper adjustment during spring coiling.
For example, when the automatic spring coiling machine cuts the spring, the cutting knife may insert the inner surface of the steel wire near the spring ring. The fatigue life of the spring is greatly reduced because the surface of the spring has orange peel defects due to the high temperature of hot forming.
Or, when hot forming, because the heating temperature is too low, the plasticity of the steel is not enough, and the surface stress of the spring exceeds the strength limit of the material during the hot forming process, cracks will occur.
Preventive measures:
In the manufacturing process, it is also necessary to strengthen the quality inspection of the spring surface to avoid surface defects as far as possible.
Spring failure caused by heat treatment process defects:
The uneven surface and centre temperature distribution of the spring during heating or cooling will cause thermal stress, and the process of phase change will cause organisational stress, whose total value exceeds the strength limit of the material, resulting in cracking.
This defect is more common in the larger size of the spring quenched in water; the crack can not be repaired after it can only be scrapped. In addition, the defects of raw materials, such as residual shrinkage holes in steel, white spots, cold processing tool marks, scratches and folding in the process of cold drawing and hot rolling, will cause stress concentration and cracking during quenching.
A company spring due to the initial surface of the short and shallow folding crack, during the quenching heat treatment, the crack along the radial expansion to 3.9mm, in the fatigue test, it first expanded until the critical size and caused the instantaneous break of the spring. Abnormal structure caused by improper heat treatment, such as coarse quenched martensite;
Preeutectoid ferrite or free ferrite; Carbide segregation; Heat treatment deformation of spring; Surface oxidation and decarburisation will cause spring failure.
Preventive measures:
In addition to strictly controlling the heating temperature and holding time, it is very important to control the atmosphere in the furnace and regularly analyse the composition of heating gas to ensure the normal heat supply. In order to reduce deformation and eliminate quenching cracking, in addition to excessively large thermoforming springs, the general thermoforming springs are cooled in oil.