Non-metallic inclusions, surface defects and decarburized layers in alloy spring steel are the main factors affecting the service life of the spring. Data show that the failure of valve springs due to non-metallic inclusions under the surface accounts for 40%; the failures caused by surface defects and decarburized layers account for 30%. A special steel plant has received 64 complaints from users in the past two years. The distribution of direct complaints according to steel grades is shown in Table 1
Table 1 Steel grade and content distribution of user complaints
Number of steel grade complaints
60Si2Mn43 fracture and cracking 39
60Si2Cr8 surface crack 9
50CrV4 surface decarburization 5
65Mn7 split ears 6
55Si2MoV1 Fixed length and ovality 2
55SiMnVB1 carbon segregation 1
Scratches and peeling 2
1.1 Non-metallic inclusions
The non-metallic inclusions in steel are mainly Al2O3 and TiN inclusions produced during the smelting process. Their influence on fatigue performance depends on the type, number, size, shape and distribution of inclusions on the one hand; on the other hand, restricted by the structure and properties of the steel matrix, brittle inclusions with large sizes and spherical shapes with weak bonding force to the matrix do not deform. Inclusions are the most harmful. Moreover, the higher the strength level of steel, the more significant the harmful effect of inclusions on the fatigue limit .
1.2 Surface defects
Surface quality problems are mainly divided into three categories: First, obvious rolling defects, folding and ear defects, and partial scratches and peeling, mainly caused by outdated steel rolling equipment, backward finishing facilities and inadequate adjustment of pass design. In addition, the surface of the blank is improperly ground, resulting in sharp edges and pits and scratches, and folding defects are also formed after rolling; the second is surface cracks, which are longitudinally continuous or intermittently distributed on the steel surface, mainly due to the remaining blanks Surface cracks caused by cracks and subcutaneous defects, rolling stress and improper cooling will also produce surface cracks; third, surface scratches and peeling, which are related to improper tooling conditions and operation, and scratches during packaging and transportation. Their existence must be the origin point of material failure, and it is easy to directly cause material fracture. However, people generally do not pay much attention to defects such as local small pits, scratches, scars, pits, etc. Although their existence is allowed by the standard, they will not become the main cause of failure, but the area where they exist is definitely They are the weak parts of the material, they will also become the breakthrough point for cracking when the overall plasticity of the material is not good. Because small defects have been destroyed at the time of failure or specific parts have not been inspected during sampling, this factor is often ignored in failure analysis. In 64 complaints from a special steel plant, surface quality problems accounted for 31% of the total .
1.3 Decarburization layer
Decarburization is a common surface defect of spring steel, which has a significant impact on the performance of the spring. The so-called decarburization refers to the phenomenon that the steel surface is completely or partially decarburized under the action of the furnace atmosphere during the heating process or heat treatment of the spring steel, which causes the carbon content of the steel surface to decrease compared with the inside. Decarburization of the spring steel surface by 0.1mm will significantly reduce its fatigue limit . Moreover, as the depth of the decarburized layer on the steel surface increases, the fatigue life decreases significantly. In particular, the appearance of ferrite in the decarburized layer of the steel surface can reduce the fatigue limit by 50%. Due to decarburization, the surface hardness of the spring decreases, and cracks are easily generated under the action of alternating stress, which makes the spring fatigue failure prematurely. In addition, different parts of the surface layer have different expansion coefficients during quenching, causing stress concentration, resulting in micro-cracks in the transition zone between the fully decarburized layer and the partially decarburized layer of the part. These visible or invisible micro-cracks become stress concentration areas. And as the origin of the continued development of cracks, causing the failure or fracture of the spring.