Behaviour of C-Steel wire ropes at low temperatures. 

A series of low temperature destruction tests by Bridon®1 on galvanised and bright, 6x36 IWRC, RHO, 1770 grade, 16mm ropes exhibited consistent increases in breaking strength, typically between 5.5% to 5.9% at -50°C. This testing data showed no evidence of unusual fracture characteristics. This finding is similar to tests on multi-strand ropes reported by Buschmann show increased breaking load when testing ropes at temperatures between -90° and -120°C. 

Behaviour of single steel wires at low temperatures. 

There is only limited published physical property data to reliably quantify the behavior of individual carbon-steel wires at low temperatures. However, the data consistently shows the following differences at -60°C compared to properties measured at +20°C:
a. an increase breaking strength
b. a drop in relative ductility
c. the ductile-brittle transition temperature increases with carbon content
d. cold-drawn steel wires have a fine grain size which reduces the ductile-brittle transition at sub-zero temperatures.

Research conducted by Boleantu and Babeu on reverse bends and torsion tests on individual carbon-steel wires did not show a brittle transition at test temperatures of -60°C. However, it is reasonable to assume that when a wire is elastically or plastically deformed in any mechanical test the heat generated will increase the temperature of the test sample, as it would when working in a rope.

In plain carbon steels, the notch sensitivity increases at reduced temperatures4. While notch sensitivity and low impact resistance can be a problem in steel plates at low temperatures to date no correlation has been found with strands formed from steel wires in a rope.

Notch sensitivity is a characteristic that is not usually associated with wire and wire ropes for the following reasons:
• no sharp notches are present on the wire surface after the manufacturing process
• the rigidity of each steel unit is small
• because each wire is a single element the failure of a wire does not create a stress concentration in the adjacent wires giving no propagation of the fracture.

Consequently, it is important to prevent mechanical handling and service damage to the wire surface which can act as notches e.g. through de-icing.

Dynamic Loading. 

Attempts at British Ropes (Bridon®) to complete bend fatigue tests on wire ropes at temperatures below 0°C have been unsuccessful because it was not possible to maintain testing temperatures. This was attributed to the friction between the rope and the capstans and internal friction in the wire rope combining to heat the test sample and raise its temperature. Another important observation from the dynamic tests was the behaviour of the lubricant.

Lubricants and Polymers

It is essential to use suitable rope lubricant compatible with sub-zero applications. Wire rope lubricants can become ineffective at low temperatures due to flaking or cracking resulting in the rope operating similar to a dry rope. This can reduce the fatigue life significantly. Bridon® can lubricate ropes with Brilube® 8 or Brilube® 16, which operate up to -50°C. Bridon® also offers a novel Bristar® polymer core, called Bristar® Polar, which is designed to support the rope at -50°C. 

Considerations for operating wire ropes at low temperatures. 

Factors that should be recognized when operating the ropes at low temperatures include:
a. shock loading and removal of ice by mechanical methods must be avoided to minimize notch sensitivity
b. special consideration should be given to the choice of lubricant, and periodic re-application of a service dressing
c. approach choice of polymer core
d. care in the choice of termination method relating to the risk of severe shock loading on an anchorage point

Conclusions.

Based on Bridon® test reports describing measurements of rope strength at low temperatures, and information from open-literature, the available evidence supports the conclusion the Bridon® steel wire rope is well suited to operating under sub-zero conditions, assuming that ropes are properly inspected, maintained and handled. Evidence suggests that strength of Bridon® wire rope will not deteriorate at operating temperatures of -40°C. While consideration must be made when operating the ropes to prevent shock loading and mechanical damage to the steel wires, no evidence of a brittle transition has been located for wire ropes at sub-zero temperatures. To extend durability in fatigue, it is essential that the correct lubricant is3 applied and maintained to avoid flaking during sub-zero service.

Further guidance is available at contact@bridon-bekaert.com

References
1. M.T. Burtoft, British Rope Technical Report R.24/79426, Rope Breaking Strength at Low
Temperatures, October 1979.
2. K. Buschmann “Ultra Deep Temperature use of Wire Rope”, Crane and Rigging Conference,
September 2012, Edmonton, Canada.
3. L. Boleantu and T. Babeu Considerations on the Low Temperature Strength of Wire Ropes,
Polytechnic Institute Traian Vuia, Timisoara, Romania.
4. Metals Handbook Volume 10, Failure Analysis and Prevention, 8th Edition, American Society for
Metals, Metal Park, Ohio, 1975.