Strain Aging in wire ropes

[BrianBT]

I have been asked to sit on a commitee that will determined discard criteria for steel wire ropes in the industry I work in. These ropes are typically formed by twisting galvanised compacted multistrand wires of tensile strength 1960N/mm2.

It is posible that we will enter into discussion with regard to a move away from regular periodic destruction testing and over towards NDT and maintenance. Something I am not too happy about because you are then reliant on the competence of the NDT operator

I have been trying to appreciate the relevance of Strain Aging however the information I have recovered I am finding difficult to apply.

Could anybody help me out with this or at least point me in the right direction.

 

[TVP]

I am not sure that I fully understand your request. Is your investigation of strain aging related to the discard/NDT issue? What I can tell you is that high-carbon steels used for spring and rope wire are frequently given strain aging treatments to increase their strength. Strain aging occurs when strained steel is heated in the range of 200-400 C. The strain could be due to wire drawing, coiling, in-service use, etc. Feel free to add more background if you need more specific information.

 

[BrianBT]

PLease find below an extract from a presentation that has been sent to me

A process called “strain ageing”can be triggered in heavily work-hardened carbon steel; it increases yield but significantly reduces ductility.
•The process involves diffusion of carbon atoms into the larger lattice spaces in the crystal structure associated with dislocations. The carbon atoms “pin”the dislocations thus effecting the change in properties.
•The process is thermally activated following an Ahrenius(sic) rate law, the effect accumulating according to time and temperature. It does not seem to be fully understood and there may be some threshold in terms of temperature and degree of work-hardening.
•In a wire rope the local stressing of wires at cross contacts requires sufficient ductility for the wire to develop its full tensile strength (the situation is not the same as in a simple tensile test on wire).
•Therefore the low ductility of aged wire can lead to premature failures and a reduction in rope strength, even though the wire itself may be stronger.*
•Multi-strand ropes are particularly vulnerable due to the inevitable cross-contacts between layers.
•Multi-strand ropes of high tensile grades, galvanised and compacted seem most vulnerable: strength losses can exceed 10%.

 

[BrianBT]

Further to previous, I also noted in a report from the HSE on the deterioration in compacted strand wire ropes.

quote
[evaluation] identified a potential deterioration in the mechanical properties of the rope over time. This deterioration occurred regardless of whether the rope was in service or storage during this time

End quote

The substatial report then goes into much detail with regard to the methodology of the testing and the results obtained. The final conclusion being it was a manufacturing process fault, which was corrected and everything was tickety boo. The rope was manufactured from wire of temsile strength 2160N/mm2 although no significance appears to have been attached to this.

Although in no way inferred in the report I wondered was there a relationship between 'strain aging' and the deterioration they had seen.

 

[EdStainless]

"tickety boo"? I love it.

It has been many years since I made wire rope, but I'll toss my comments in. As I recall when you galvanize high strength wire you have top be careful about the temperature and time. This is up in the stress relief range and you can screw up the properties if you aren't careful.
Unless the wire was not properly stress relieved I can't see any aging effects. You should be able to tell this with new wire by giving it some stress relief/aging treatments and seeing how it responds.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[CoryPad]

Is it possible that your ropes see 200 to 400 [°]C in service? Of course your ropes will receive the strain, but I don't think it is very likely they will see the aging. Without the carbon atom diffusion, you won't see the ductility loss.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[BrianBT]

I am now understanding from your comments that 'Strain Aging' is not a significant defect expected to be seen in wire rope after construction.. Instead it is part of a process of improving the properties of the wire.

I believe the inclusion of Arrhenius Rate Law into the wording of the presentation was misguided as at normal operating temperaures ( Sea/Atmospheric) the processes(reactions?) involved in strain aging would be occurring at such a rate variations in temperatures of a few 10's of degrees would be negligible.

I believe any reference to strain aging in this presentation could lead to an incorrect believe that there is a significant process which degrades rope not in service-that is to say not subject to normal wear and tear, is erroneous and distracting

Following on from this, and I apologise for taking up too much time on this but I would be grateful for your comment on the following

Galvanised Wire rope in service subsea can reach a condition generally termed 'dry'. That is there is insufficient grease diffused into the construction. It is possible that the wire then becomes corroded however it is also the case that the individual wire strands become brittle and liable to fracture. In this condition the wire is not recoverable by pressure greasing and is scrapped.

I had always associated this to the diffusion of halides into the material- this due to a perhaps mistaken believe that some stainless steels were subject to a similar process that led to premature failure when exposed to sea water.

Your coments would be much appreciated- please understand I am demonstrably not a metallurgist.

 

[rayband]

BrianBT,
How about simple hydrogen embrittlement? Galvanized, high tensile, steel wire rope in sea water!! I don't know about in storage, especially if properly lubricated, but hey!, you got a natural battery (Zn + FE) just looking for hydrogen to suck up. If so, a simple bake-out should restore.

rayband

RAYBAND
Rock Island Arsenal
AMSTA-RIA-SEM