The main problems and countermeasures of production of spring steel alloys for spring manufacturers
The influence of non-metallic inclusions, surface defects and decarburization on the quality of spring steel products was analyzed. The metallurgical quality of spring steel, chemical composition and process optimization, multi-alloying and rare earth treatment, and development of new steel types were discussed. To improve the performance of alloy spring steel, a new way to improve the overall performance of alloy spring steel was discussed.
Spring steel is widely used in industrial production of steel, according to chemical composition can be divided into 3 categories, namely, carbon spring steel, special purpose high alloy spring steel and universal alloy spring steel. Among them, the universal alloy spring steel (hereinafter referred to as alloy spring steel) has the advantages of excellent performance, strong adaptability, low price, etc. Therefore, it is widely used and has a large amount, and is the most commonly used and most important spring material. However, due to the characteristics of the alloy elements of the alloy spring steel, many problems have been brought about for actual production and use.
I. The main problems of alloy spring steel
Non-metallic inclusions, surface defects and decarburized layers in the alloy spring steel are the main factors affecting the service life of the spring. The data shows that the valve springs account for 40% of the failure caused by non-metallic inclusions under the surface layer; the surface defects and the failure caused by the decarburization layer account for 30%.
Surface quality problems are mainly divided into three categories: First, significant rolling defects, folding and ear defects, and some scratches and skins, mainly due to outdated rolling equipment, finishing facilities behind and hole design adjustment is not in place. In addition, the surface of the blank is not properly ground, resulting in sharp corners and pit scratches. After rolling, it also forms folding defects. Second, it is a surface crack. It is longitudinally continuous or discontinuously distributed on the steel surface, mainly due to the residual material of the blank. The cracks and subcutaneous defects, rolling stress and improper cooling will also produce surface cracks; Third, the surface scratches and warpage, which is related to improper working conditions and operation, in the packaging, transportation will also produce scratches. Their existence is necessarily the origin of failure of the material, which easily leads to the fracture of the material. However, people generally do not pay much attention to defects such as small pits, scratches, chapped skins, and pits. Some of their existence, although permitted by standards, will not become the main cause of failure, but their existence is certainly in the region. It is the weak part of the material. When the overall plasticity of the material is not good, they will also become the breakthrough of cracking. Because small defects have been destroyed or sampled at the time of failure did not test the specific location, this factor is often overlooked in failure analysis. The surface quality problems accounted for 31% of the total number of 64 complaints from a special steel mill.
2. Non-metallic inclusions
The non-metallic inclusions in the 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, and on the other hand, the large size of brittle inclusions and spherical deformations, which are restricted by the microstructure and properties of the steel substrate, and weakly bonded to the matrix. Inclusions are the most dangerous. Moreover, the higher the strength level of the steel, the more harmful the harmful effects of inclusions on the fatigue limit.
Decarburization is a common surface defect in spring steels and has a significant effect on the performance of springs. The so-called decarburization refers to the spring steel in the heating process or heat treatment, the steel surface in the furnace atmosphere under the action of all or part of the decarbonization, resulting in the carbon content of the steel surface than the internal reduction phenomenon. The decarburization of 0.1 mm on the surface of the spring steel significantly reduces its fatigue limit . Moreover, as the depth of the decarburized layer on the steel surface increases, the fatigue life decreases significantly. In particular, the presence of ferrite in the decarburized layer on the steel surface can reduce the fatigue limit by 50%. Due to the decarburization, the surface hardness of the spring decreases, and cracks are easily generated under the action of the alternating stress, so that premature spring fatigue failure occurs. In addition, different coefficients of expansion during quenching in different parts of the surface layer cause stress concentration, resulting in micro cracks in the transition zone between the full decarburized layer and the partially decarburized layer. These visible or invisible microcracks become stress concentration areas. And as the origin of the continued development of the crack, it causes failure or breakage of the spring.