Springs of Steel

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06 Sep 2019

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The steel used for suspension springs is of a grade designed for quenching and tempering to achieve the tensile strength required for an automotive component.

The quality of the surface of the material has a great impact on the performance of the spring. Cracks, flakes and other surface defects significantly reduce spring performance, lowering the spring fatigue strength which leads to premature breakage. The quality inside the material also influences its performance: decarburisation, macro and micro non-metallic inclusions being major factors. The type, size, distribution and quantity of such non-metallic inclusions can influence the fatigue strength and lead to premature breakage.

Wear and Corrosion

Suspension systems are designed with the spring fully exposed to the road and this is a very harsh environment in which to operate. The spring is continually under attack from water, stones and salt during the winter months, and without adequate protection, the steel would begin to rust very quickly.

Corrosion is the most common cause of broken springs. There are various methods to protect the steel. Aftermarket products commonly use a one-layer wet paint or coating which is sufficient unless the paint layer gets scratched. Then, because of the crevice effect under the paint surface, creep corrosion spreads rapidly. Defects created by corrosion on the steel surface heavily increase the risk of spring breakage.

Defects in the layer of lacquer cannot be avoided, so another form of protection between the paint and the steel should be used. Car manufacturers normally opt for either an epoxy powder or a zinc phosphate method. Either technique effectively protects the steel against corrosion even if defects occur in the paint layer. To get the right protection effect, a multi-stage chemical process is needed.

The process NAPA employs includes: activating, epoxy powder coating, passivating and finally painting. NAPA also use a multi-stage surface treatment line, incorporating shot blasting, an epoxy powder coating process and finally the ink-jet printing of the part number and production batch number onto each spring.