It is the opinion of some spring manufacturers that the quality levels achievable for springs are limited by the quality of the wire despite being informed that the quality of spring wire has been improving everywhere in the world over the last thirty years.
IST stand by their contention that the quality of spring wire is improving, but spring wire is far from perfect, and there have been two quality complaints that have been addressed recently. The first concerns the presence of surface defects in stainless steel spring wire, and the second relates to the quality of music wire when the rod raw material has not been lead patented. In both instances the wire supplier commented that the spring maker's improved inspection facilities were revealing faults that have always been present. Companies that are ISO 9001 accredited are obliged to show evidence of pursuing continuous improvement - this applies to wire drawers and spring makers alike, and improved inspection equipment is one way of driving quality standards up.
Stainless Steel Spring Wire Quality
A spring manufacturer was having problems with failures during his customer's life test of the final assembled product. The problem was attributed to wire defects, which metallographic examination revealed to be of a depth of 1.4% of the wire diameter. The spring maker wanted to know whether there was a simpler way to measure such defects accurately, but was disappointed to be advised that metallography was still the best method. This then prompted the question as to whether such a defect would be considered "harmful to use". That answer was easy as the springs were failing the life test and the life prediction from IST's spring design software indicated they should not. The question then arose as to "why is the maximum defect level permitted for dynamic springs not clearly written into the supply standards"? The maximum defect permitted in dynamic quality (not valve spring quality) music wire and silicon chromium steel wire is stated to be 1.0% of wire diameter in most international standards, but defect limits are not set for stainless steel. IST is of the opinion that the maximum defect level should be the same for all wire qualities intended for a dynamic application. They recommend that spring manufacturers add a clause to their stainless steel wire purchases requiring a maximum defect level of 1.0% of wire diameter when they know the wire will be used for springs with a dynamic application. Indeed, at larger wire diameters (greater than 5.0 mm) there is also merit in specifying a maximum defect size of 50 microns - a size that may be reliably found by eddy current testing which the only applicable non-destructive test method for spring wire. For example, if making springs in 10 mm wire for dynamic application, a defect approaching 100 microns or 0.1 mm deep would be damaging to the performance. Shot peening springs of this size would give rise to a residual compressive stress of about 150 microns deep - not much deeper than the defect itself - so there would still be a significant stress raising effect.
Music Wire Quality
Music wire is generally drawn from wire that has been lead patented during wire manufacture and at sizes smaller than about 2 mm, the lead patent process route is the only one possible, albeit that some manufacturers use a fluidised bed instead of liquid lead, for fear that one day the use of liquid lead may be banned on health and safety grounds. IST believes that the use of molten lead in the wire industry is safe, but is not confident that health and safety legislators will listen to the compelling arguments for its continued use. However, that is another story.
Larger sizes of music wire are sometimes drawn directly from Stelmor cooled (air patented rod). The interlamellar spacing of the pearlite will be larger in air patented material in comparison to lead patented, and it is this parameter that determines the ductility of the wire during drawing. As-patented the wire is ductile, but at every wire drawing die the wire becomes stronger and less ductile. Eventually the wire ductility will be reduced to such a low level that there is a risk of internal cracks forming along the centre line of the wire. Such defects are not acceptable in spring wire as the wire will be prone to fracture during spring coiling, or during service. It is IST's opinion that the production route adopted by the wire drawer should be left to their discretion, but if patented wire is drawn too far it will form cracks like the ones shown below.
Over-drawn music wire with chevron shaped cracks along the centre line.
The moral of this is that wire quality may limit a spring manufacturer's quality, but such problems are rare and, with continuous improvement, should become rarer. It is hoped that by publishing details of these quality problems, IST are playing their part in driving quality standards up.