The test results for the two batches of wire and steel wire met the required standards, with performance metrics showing good consistency and a satisfactory spring effect. However, when evaluating the fatigue life of the suspension springs, significant differences were observed. This suggests that non-metallic inclusions in batch A had a substantial impact on the fatigue life, particularly those inclusions that are not easily deformed. The difference in plasticity between these inclusions and the steel matrix can strongly influence the mechanical behavior of the wire. During deformation, non-metallic inclusions experience less elongation than the surrounding steel, leading to stress concentration at their interface. This stress can initiate cracks or other discontinuities, which propagate under external loading, eventually causing premature failure.
Cone gaps and hot tears are common issues associated with non-deformable inclusions (v = 0). To make these inclusions harmless or even beneficial during the steel processing, it is essential to incorporate them into the plastic flow of the steel through specific thermal processing steps. This ensures that the inclusions undergo sufficient deformation, reducing the risk of crack initiation.
The effect of different types of inclusions on fatigue performance varies significantly. Inclusions with high deformation indices remain well-bonded to the steel matrix across all temperatures, without forming voids or weakening the structure. Oxide inclusions, however, are generally detrimental to fatigue performance. Among them, spherical, non-deformable calcium aluminate and Al₂O₃ inclusions are particularly harmful. These retain their shape during deformation, with pores often forming around calcium aluminate inclusions. Al₂O₃ inclusions, typically angular, act as stress concentrators and are known to be sources of fatigue cracks.
For automobile suspension spring materials, it is crucial to control not only chemical composition, mechanical properties, decarburization, and surface quality, but also the quantity and type of non-metallic inclusions. In particular, total non-metallic inclusions should not exceed 2.5, and B-type inclusions should remain below 0.5. Exceeding these limits can have a negative impact on the fatigue life of the suspension springs. Proper inclusion control is therefore essential for ensuring long-term reliability and performance in critical automotive components.
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