THE SURFACE CONDITION OF WIRE: ANOMALIES THAT CAN DAMAGE SPRINGS
Let’s discuss factors related to material manufacturing processes that can lead to spring failure. During wire production, defects can arise that affect the wire’s quality, specifically those related the surface condition of wire.
Surface condition of wire
Most spring failures due to fatigue are caused by defects in this area. These defects can arise during the billet-to-rod (LINK WIKIPEDIA) or rod-to-wire drawing process.
The main surface defects are as follows:
These are the zones where solidification fronts meet in the billet. These can be prevented by using magnetic fields that rotate the liquid fraction in the billet, making solidification uniform.
These defects are created by play between the drawing dies during billet-to-rod drawing. They are responsible for surface cracks.
This is caused by localized heating due to lubrication defects between the wire and the drawing die surface.
Corrosion (generalized, intergranular, pitting)
This compromises surface integrity, can cause cracks, and initiate fractures.
Scratches, flakes, folds
Scratches occur during wire drawing due to lubrication problems or advancement speed.
Flakes appear as scales on the wire’s surface, which can be caused by corrosion or uneven cooling.
Folds occur when metal is overlapped at cold temperatures, creating non-bonded layers. These occur when there are rolling edges on the wire, which are portions of material created during extrusion when the two semi-shells of the rolling dies are not perfectly adherent to each other and material infiltrates the space that forms between the surfaces. This type of defect falls under the roundness defect classification, which transforms into a fold when this extra material is folded and compressed onto the main surface during wire drawing.
These harden the material and form martensite, whose effects have already been explained.
These may be present on the wire due to grinding problems on the billet.
A crack is a fissure that compromises the structural integrity of the material. Cracks can be found both inside the material and on its surface. Surface cracks are certainly the most dangerous for fatigue resistance.
Cracks can form for various reasons, among which the following are the most well-known and recurring.
Due to improper cooling during quenching, the volume change between austenite and martensite can create cracks.
Mechanical processing that creates mechanical stress on the material
In this case, surface imperfections are crack initiation zones.
Due to excessive wire drawing or overheating of the wire during wire drawing.
- Caused by folds, as seen in a previous point.
- Caused by seams.
- Caused by corrosion and stress corrosion cracking.
In this article and in the previous one, we attempted to create an exhaustive list of the defects present in the raw materials, analyzing the causes and phases of occurrence in the production process from melting to finished product.
He graduated in electronic engineering from the Milan Polytechnic in 1992. Since 2000, he has been working at Mollificio Valli as technical sales manager.
Over the years, he has acquired extensive experience in the calculation and technical aspects of spring production.
He has always been passionate about mathematics and statistics, and has had the opportunity to apply his knowledge in statistical control techniques, metrological aspects and in general in the practical field of problem solving and continuous improvement.