Metallurgical Investigation of Failed Automotive Helical Suspension Spring

LPD Lab Services frequently carries out failure investigations and consultancy on metal parts. Knowing why a component failed gives a starting point from where counter measure can be devised and implemented to prevent a repeat of the same failure.

The following is a summary of a metallurgical failure examination conducted by the laboratory’s experienced metallurgist performed on a portion of a fractured automotive suspension spring.

Visual and Low Magnification Optical Microscopy Examination
The spring had failed at a location near the end where the entire protective coating was lost and the metal had sustained a significant degree of corrosion. Optical microscopy of the fractographic features on the fracture face revealed that the fracture initiated at the external surface on the centre line radius, which is a normal fracture initiation point for helical springs.

Scanning Electron Microscopy and Energy Dispersive X-Ray Analysis
By using the high magnification of scanning electron microscopy, it was observed that cracking had propagated all along the grain boundaries. This intergranular failure mode was not a normal, because springs usually fail by fatigue cracking or transgranular cracking. EDX analysis of the corrosion products revealed the presence of sulphur.

Compositional Analysis by Inductively Coupled Plasma – Optical Emission Spectroscopy
The spring material was analysed by ICP-OES and the composition indicated it to be a conventional spring steel material such AISI/SAE 5160.

Metallography and Hardness Testing
Using inhouse metallography capabilities, a section taken through the fracture face was metallographically prepared and examined. Numerous cracks akin to hydrogen induced cracking were observed at the fracture face, suggesting that corrosion had played a major role in the fracture. The microstructure consisted of fine tempered martensite with a Vickers hardness of 554HV, indicating that the spring had been correctly heat treated to achieve the desired mechanical properties.

Metallurgical Failure Analysis Findings, Consultancy and Product Improvement
The coil spring failed by a mechanism known as corrosion hydrogen embrittlement and hydrogen induced cracking. During corrosion, hydrogen is developed at the cathodic areas. The hydrogen atoms normally combine to form hydrogen gas molecules which depart from the steel surface, but this cannot happen in the presence of sulphides and the small hydrogen atoms then diffuse into the steel and embrittle it. The primary cause of the failure was a breach of the corrosion protective layer. Applying a significantly more durable and wear resistant protective coating to the spring would have prevented it from corroding and suffering the same hydrogen related embrittlement.

 Example Metallurgical Examination of a Failed Automotive Helical Suspension Spring report