The myth of the chrome-silicon gun spring ‘not taking a set.’
It is NOT true that chrome silicon magazine springs are more resistant to taking a set than all stainless steel or music wire. I wish I could say that I am surprised this false marketing claim took hold, but sadly in the past I believed it as well. That is, until I researched it.
Springs have a characteristic called the proportional limit. When you compress a spring, you add stress. When the spring deforms, that is strain. Normally there is a proportional ratio between stress and strain. If you continue to deform the spring past a certain point, this ratio is no longer proportional and you have reached the limit for taking a permanent deformation (set). For three types of springs commonly used in guns, Music Wire (ASTM A228), Chrome-Silicon Valve Spring Quality (ASTM A877), and 17-7 PH stainless - the stress limit is 45% of the minimum tensile strength on the material certification.
For music wire, the range of the tensile strength property is 230-399 KSI. For chrome silicon, it is 235-300 KSI. For 17-7 PH, it is 235-335 KSI.
http://www.tribology-abc.com/calculators/properties_of_common_spring_materials.pdf
One cannot make a general statement about any of these materials without knowing the exact tensile strength of the specific material the spring maker used. You can see they have overlapping strengths, so without further information, they can be considered about the same.
One brand may make a better choice on post-winding stress relieving, or some post-processing such as shot peening, but one thing is clear – the BEST music wire is stronger than the BEST chrome-silicon. This is because with the higher tensile strength, 45% of that will be a higher stress value that one can impart before there is disproportionate strain resulting in a permanent set. Music wire wins the ‘resistance from set’ argument.
So why does chrome silicon exist? Music wire is limited to 250 degrees F. In a car engine, the temperature exceeds that. Chrome silicon wire is used for valve springs for this reason. It is more resistant to taking a set AT TEMPS ABOVE 250 degrees. Needless to say, firearm magazine springs do not reach this temperature and recoil springs likely never will either (AR extractor springs may). And if you want to go above the 475 degree F limit of chrome silicon, there are stainless alloys. They cost more, but have similarly high proportional limits as chrome silicon.
There is also an issue of fatigue strength. This is the ability to resist damage that occurs from cycle loading. Cycles are often measured in thousands or millions, and are not that important for magazine springs. They are important for recoil springs. Does chrome silicon wire outperform music wire for fatigue strength? No, it is worse. Music wire has a cleaner surface – and surface defects can reduce fatigue strength.
http://tinyurl.com/c9hdqu
“Music Wire:
Due to superior surface quality, these can withstand higher stresses under repeated loading than any other spring material.”
http://www.centuryspring.com/pdfs/techfaqs.pdf
“14. What are the best materials for fatigue applications?
The two most popular materials for fatigue applications today are Music Wire (ASTM A228) and Chrome-Silicon Valve Spring Quality (ASTM A877). At wire sizes below approximately 0.080" (2.0 mm), Music Wire offers higher tensile strength; however, Music Wire’s maximum service temperature is less than that of Chrome-Silicon.
… processing of music wire is done in a manner to provide a finished surface with smaller allowed defects than hard drawn wire. Since surface defects are one of the most common initiation sites for fatigue cracks in springs, smaller surface defects (and their corresponding reduction in stress concentration) enable music wire to be used in high cycle fatigue applications.”
The way one designs a spring to not take a set is to physically not allow it to deform in a way where the stress exceeds the proportional limit. This may mean designing a pistol magazine to only take 12 rounds rather than 13, or add more coils to the spring, or use thicker wire, or use flat wire, or increase the OD. A properly designed system will allow for a fully loaded magazine to sit for over 100 years, or for an action to remain locked open without the recoil spring taking a set. Are some platform magazines improperly designed? Most certainly, but no one has seems to identify which ones.
What about tests that prove chrome silicon gun springs take less of a set than music wire or stainless steel? I have seen a few tests and they prove no such thing. They are often done with different wire diameters, free lengths, number of coils, shot peening, stress relieving, and heat treatment. It is impossible to know if each spring was properly made. One does not need to do a test to know which alloy has a higher proportional limit as that is well known and defined in engineering texts. However, one can do tests to show if a specific brand spring is better than another specific brand spring but it would NOT be just because they replaced stainless or music wire with chrome silicon. So search for a quality spring, but don’t go by alloy alone.
So what is the ultimate magazine-spring material? Since all three have similar resistance to taking a set given the same wireform, one needs to look at cost, corrosion resistance, and temperature resistance. Temp resistance is not a factor and fatigue life is usually not either. Corrosion resistance is a strong factor. Cost is a factor for large production but not individual enthusiasts. 17-7 PH seems to win.
What is the ultimate recoil spring? Again all three have three have similar resistance to taking a set given the same wireform, In some applications, the 250 degree F limit of music wire is not an issue (handguns). In some (belt feds), it may be. If a pistol design allows for enough room to have a wireform where the ratio of stress to tensile strength stays low, then one may go with 17-4 PH to pick up corrosion resistance. If the design, as is true with a 1911, is one of high stress and yet temp is not a factor, then music wire seems to be best (I believe this is what Wolff uses). If the application is such where temps may get over 250 degrees F (either through combustion or simply through spring cycling) and there are high stress and fatigue considerations, such as an AR15 extractor spring, then chrome silicon would seem best. And if the temp gets really hot, such as in a gas block part, then 17-7 would be good.
For any spring, proper heat treat / stress relief and shot peening will increase the limits.
When you see a gun spring company claim chrome silicon is 1000 times more resistant to taking a set than music wire or stainless (yes I have seen this claim), you should ask them to prove it. They can’t, because it is not true. No non-gun based spring company seems to make that claim.
