Stainless Steel Wire Ropes

One question we are sometimes asked at Boat Fittings is “Is stainless steel magnetic?”  More particularly we are asked the question “Is 316 stainless steel magnetic?” 

Sometimes we find that people are asking this question as a kind of proof or test of “Is what I bought really 316 stainless as stated?”, probably having heard somewhere that marine-grade stainless is non-magnetic.  There are other reasons as well for looking into the magnetism of stainless steel, especially in the field of laboratory equipment or medical equipment design where magnetism (or lack of it) can be crucial to the correct function of the equipment.

A mine of information can be found on stainless steel at the British Stainless Steel Association, and some of the information in this article has been distilled from their website.

It is often said the “stainless steel is non-magnetic” but actually this is not quite true, and the real situation is more complicated.  Steels in general and stainless steels are characterised by whether their internal structures are austenitic or martensitic.  Steels which are strongly austenitic are relatively non-magnetic whereas steels which are martensitic are strongly magnetic.  Stainless steels, and more particularly 304 and 316 types of stainless steel are mostly austenitic, when means that their levels of magnetism are low.  A material which is completely non-magnetic is said to have a magnetic permeability of 1.  Materials which are magnetic have permeabilities of greater than 1.

Composition effects on magnetism of 316 stainless

Grades 304 and 316 have 'balanced' compositions to enable them to be readily weldable. This is achieved by ensuring that in their normal annealed (softened) condition, they contain a few percent of delta ferrite. This results in small levels of magnetism.

Effect of welding on stainless steel magnetism

During the welding of these steels, structural changes occur. Some of the austenite in the parent material can transform to ferrite at high temperatures and this partially remains after cooling. Also welding filler rods and wires usually have deliberately high ferrite levels of 5% or sometimes 10%, to minimise the risk of hot cracking during welding, therefore increasing levels of magnetism.  Consequently the permeability of the metal in the weld and the surrounding heat affected zone can be significantly higher than in the original parent material.

In general, castings have compositions with a bias towards ferrite compared to wrought grades and consequently will be more magnetic.

Effect of Cold Work and Temperature on Martensite Formation

Mechanical forming processes (cold-working) are also known to affect the micro-strucutre of stainless steels, and therefore cause an increase in the levels of magnetism.  For example steel pressings, like the process used to form a flat sheet of stainless steel into a kitchen sink, cause mechanical straining of the steel and re-structuring from the original austenitic state, with a partial transformation to martensite, leading to increased levels of magnetism.

Stainless Steel And Magnetism In Practical Terms

In practical terms the answer to the question “Is stainless steel magnetic?” is yes it it slightly, but much less than carbon steels.  This means that if you take a magnet to a stainless steel component (whether it be A2 or A4, ie 304 or 316) you may expect to detect some magnetic pull.  This means that the ‘magnet test’ cannot be relied on to distinguish the grade of stainless that the item is made of.

During the swaging process, the swage fitting is squeezed down in diameter to squeeze onto the rope.  This process has the effect of 'necking' down a portion of the swage fitting, and also there will be a seam line where the two swage dies meet.  Some examples of swage fittings before and after swaging are shown below:

The strength of the finished swaged connection results from the effective reduction in diameter of the swaged region of the fitting.  When the wire rope is first pushed into the swage fitting there is quite a close fit.  When the swaging has been completed the reduction in diameter of the fitting causes and effective interfrence fit, giving great resistance to pull-out.

Wire rope swaging, when done properly, is a very strong and reliable process - as well as giving a neat appearance.

Our steel wire rope swaging is done with hydraulic swaging equipment, which rolls the swage fitting through a set of dies, giving a repeatable result time after time.

When you use our wire rope assembly configurator, after choosing the type and diameter of stainless steel wire rope, you will then be prompted to choose the type of fitting for each end of the wire rope assembly.  Once you have chose the fitting types, our wire rope swage configurator will show, with a red-line at each end fitting, how the length is determined.

For wire rope assemblies up to 2 metres in length, the overall length accuracy that we can produce will be within 5mm of nominal and generally nearer +/- 2mm.

For wire rope assemblies over 2 metres and up to 4 metres in length our length accuracy will be within 10mm of nominal and generally nearer +/-5mm

If you have a special need for greater accuracies please do get in touch with us.

Stainless steel wire rope comes in a variety of constructions, suited to different needs.  Currently we sell 1x19 and 7x19 constructions.  The 1x19 means 19 single strands of wire are wound to make the wire rope.  The 7x19 means that 7 bundles each containing 19 strands are wound to make the steel wire rope.

The 1x19 stainless steel wire rope construction gives the stiffest configuraiton, in the hand it feels significantly stiffer, and is not well suited to applications where it needs to run round a pulley block, or where the end needs to be bent around for clamping.  The 1x19 has a 'brighter' appearance and is well suited to marine rigging with ultimatley a better resistance to staining that the 7x19 construction.

The 7x19 wire rope construction is much more flexible than the 1x19 and can easily be bent into loops and tight radii, and is more appropriate for running around pulley sheaves.  The 7x19 wire rope is not well suited to marine rigging, where the more corrosion resistant 1x19 construction is recommended.

When done properly, wire rope swaging is very strong indeed.  Our hydraulically applied roller swaging fittings (all fittings except our crimped fittings) produce the greatest strengths, and are approximately the same as the strength of the wire rope itself.

If you have a particular strength requirement for your swaged wire rope assembly then please get in touch with us to discuss your requirements.

Our wire rope assemblies are not intended or qualified for either lifting applications or personal safety applications.

At Boat Fittings we send out orders every working day, and our general cut-off time for same day despatch is 2pm.  For wire rope assemblies please allow an additional 24 hours for despatch.  

In the event that your stainless steel wire rope assembly will not be available for despatch within 48 hours of ordering we will get in touch with you to make you aware, giving you the option to cancel your order.

The cost of our custom stainless steel wire rope assemblies is made up of the cost of the wire rope itself (depending on diameter and length), and the cost of the end fittings.  We also add a small cost to perform the swaging operation.

As you build your custom steel wire rope assembly using our wire rope configurator tool, as soon as you have selected with type and diameter of wire rope, the cost will automatically be updated.  Each time you add or change and end fitting the cost will automatically be updated.  Each time you update the length of the bespoke stainelss steel wire rope assembly the cost will be updated automatically.

Our custom steel wire rope assemblies qualify for free delivery (based on order value) in line with our normal free-delivey threshold on the boatfitttings website at the time of ordering.