In response to questions from two spring manufacturers, I have been asked to discuss problems of formability of stainless steel spring wires and ductility testing in this Cautionary Tale.
In Cautionary Tale VIII, Figure 1 (repeated here, right) showed that the plastic deformation range (ductility) for hard-drawn spring wires (carbon or stainless steel) is much reduced after stress relief heat treatment and, for this reason, all severe forming operations are undertaken by the spring manufacturer before stress relief. However, the ductility of spring wires can be described in various ways, and Figure 1 describes tensile ductility, which is very similar for both carbon and stainless steel. Another measure of ductility is the wrap test, applied to wires in the size range 0.3 – 3.0 mm, in which wire is wrapped eight times around itself. Again, the ductility requirements are the same for carbon and stainless steel spring wire. It might reasonably be expected, therefore, that the torsional ductility of both wires would be similar, and sometimes it is, but the point of this Cautionary Tale is that sometimes it is not.
Torsional ductility is usually measured by a twist-to-failure test, such as that specified in ISO 7800. For carbon steel and music wire at 2.0 mm (0.078 in.) 22 twists minimum is required for a gauge length of wire 100x its diameter (i.e. 200 mm for 2.0 mm wire). Carbon steel always passes this test, and results of 30 – 35 twists are typical for this size of music wire – that is to say that a length of wire 100 times its diameter can be twisted at least 22 times through 360 degrees before it fractures. Furthermore, when it does fracture, there will be no splitting of the wire, and the fracture will be at 90 degrees to the axis. Oil-tempered wire exhibits very good torsional ductility, albeit not quite so good as hard-drawn carbon steel wire.
The torsion test is always missing from international specifications for stainless steel spring wire. The reason is that some wires behave in a similar way to carbon steel, but other wires do no twist evenly and they start twisting locally, as shown in Figure 2, bottom. Recent tests at the Institute of Spring Technology (IST) showed that local twisting occurs in 4.00 mm and 0.50 mm wires. The fractures were ductile and perpendicular to the wire axis, as they would be if the wire had twisted evenly, but failure occurred at two to 10 twists, compared with 20+ twists if the wire had twisted uniformly along its length. Usually in this column I try to give explanations but, in this instance, I would be grateful if any reader could provide an explanation of why stainless steel spring wire starts twisting locally, as this is not understood at IST.
The practical importance of this observation to spring manufacturers is that they have to be careful when forming stainless steel springs to avoid this local torsioning. Lifting end hooks on extension springs is one operation in which great care is required with stainless steel. In forming end hooks, or making any other sharp bends that are in a different direction to previous wire forming processes, the wire should be plastically deformed in bending or twisted as uniformly as possible. If local torsioning occurs and the spring manufacturer doesn’t notice, his customer certainly will when the spring fails on assembly.
In order to try and learn more about this subject, IST has tried stress relieving wires to see how this affects the number of twists to failure. As expected, carbon steel gave just less than half the number of twists when stress relieved at 250 degrees C (380 degrees F), and examination of the wires after testing showed that there was additional twisting at the point of failure. Stainless steel wires that gave only three twists did not improve if they were stress relieved at 450 degrees C (840 degrees F). However, stainless steel wires that gave more than 20 twists before stress relief only gave three twists after. So it is clear that torsional ductility is reduced by stress relief heat treatment, but it is still not clear why stainless steel shows local torsioning to a much more marked extent than carbon steel does.
Mark Hayes is the Senior Metallurgist at the Institute of Spring Technology (IST) in Sheffeld, England. Hayes manages IST’s European Research Projects, the spring failure analysis service, and all metallurgical aspects of advice and training courses given by the Institute. Readers are encouraged to contact him with comments about this column, and with subjects that they would like to be addressed in future installments, by phone at (011) 44 114 252 7984 (direct dial), fax at (011) 44 114 2527997 or e-mail at Mark's Email.