Relationship between the yield strength and surface finish steel chains

[vukk]

Hi,
Do you have please any explanation for this phenomenon?

Our problem relates to steel chains ( lifting or anchor chains -the type of chain makes no sense in this case).

Many times we experimentally verify that when we applied to the surface of the chain links any color coating or zinc coating, we reduced the tensile strength and elongation of this chain. Tensile strength was reduced by 70 to 100 MPa, elongation up to about 7%, while the chain is always cracking at the same location.

During the tensile test, the individual areas of chain link are subjected to tension, compression, bending and shear. I understand that the application of paint or coating on the surface of chain increases the lubricity of the surface -so reduce the friction at the interface of two adjacent links. But this is no answer - I can not explain this phenomenon from a theoretical (mechanics) point of view.

 

[vukk]

Sorry: Correct name of this tread is: Relationship between the tensile strength and surface finish steel chains

 

[Nereth1]

Hydrogen embrittlement with some coatings on high strength steels? Although I don't know that lifting chains are made of such steel.

A layer of paint in some cases can reduce the strength of the material it is painted onif the paint is less flexible than the material under it - the paint cracks and that crack gives a stress raiser to the part underneath.

Other than that there is the typical stress raiser stuff, but I would have thought that most coatings would have a better surface finish than the lifting chain it is applied to, i.e. zinc coatings tend to be pretty smooth...

Can you recreate the results using a standard tensile test specimen of the same material? Other materials? Other coatings?

 

[ornerynorsk]

What is the time factor involved? Have you tested samples immediately after plating, or have they been in service x years or months before such testing? Are they being baked after plating? Are the platers using any high temp process to degrease before plating and is the bath chemistry where it should be? What is the alloy of the chain? What is meant by "color coating?

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[CoryPad]

What is the alloy and mechanical processing and heat treatment for the chain?

Is the zinc coating hot-dip galvanizing? That requires elevated temperatures that can reduce the strength of high-strength steel.