IST regularly receives questions regarding property changes which occur in spring materials during simple stress relieving treatments. For example, a recent IST training course attendee wanted to know why stainless steel spring wire behaved differently than music wire. He knew that the outside diameter of music wire springs decreased when heat treated, while the outside diameter increased when stainless steel springs are heat treated. He had also been told to use the same value of the torsional modulus (G) for music wire both before and after heat treatment, but to use a different value of G for stainless steel after heat treatment.
To understand the mechanism causing the increase in outside diameter for stainless steel springs on stress relieving, the metallurgy of the wire drawing process needs to be considered. When stainless steel spring wire is drawn, the microstructure of the rod stock material is austenite, but in every die a little of the austenite is mechanically transformed to martensite as a consequence of the cold reduction. After several reductions in area, the wire will acquire the tensile strength required for springs, and the microstructure will be a mixture of austenite and martensite (it is the latter constituent in the microstructure that makes stainless steel spring wire slightly magnetic).
The spring manufacturer then coils the wire into springs, and thereby imparts a residual stress at the inside surface of the coil. When the spring is heat treated, some more of the austenite will transform to martensite, and the transformation will occur most at the position where the residual stress is at a maximum. A volume expansion is associated with this transformation, and the overall outcome is that the spring diameter becomes larger.
It might be expected that thus microstructural change would be visible on optical metallography but that is not IST’s experience. Although the percentage of austenite and martensite could be determined by X-ray methods, the percentages of each that would equate to satisfactory/unsatisfactory spring performance is not known, so there is no point in using these expensive test methods.
It is also the change in the percentage of martensite in the microstructure that causes the moduli (E and G) of stainless steel wire to increase after heat treatment, as the stiffness of martensite is greater than that of austenite. This explains why 316 stainless steel spring wire has a lower modulus than 302, which itself is lower than 631 (17/7PH) – the last having the greatest percentage martensite – but all three grades have a microstructure which is referred to as “austenitic.” It should also be noted that, if the springmaker makes springs with a small coiling ratio (index), there will be more residual stress and consequently more transformation during heat treatment. The small index spring will have a higher modulus than similar springs, made from the same wire, with larger index.
There is very good guidance about the expected change in the G modulus included in the appendix to the European specification for stainless steel spring wire, EN 10270-3. The values published in this specification are approximate, and depend upon a number of factors. However, IST strongly recommends use of the values in this standard as the best available, and good enough for spring design purposes.
In another training course several delegates were convinced that both their music wire and silicon chromium alloy wire springs would have different microstructures before and after heat treatment, and they wanted to know how this change could be recognized. Their company had the facilities to examine microstructures and one of their automotive customers requested use of the CQI-9 quality procedure. This procedure covers stress relieving of springs, and demands that the metallographic structure produced be examined. However, these delegates were surprised to hear that optical metallography would show the microstructure of music wire, silicon chromium and 302 stainless steel to be exactly the same both before and after heat treatment. For this simple reason, it is IST’s opinion that the CQI-9 quality procedure should not be applied to stress relieving. The procedure is appropriate for spring manufacturers who carry out hardening and tempering or austempering heat treatments, where a significant change in microstructure occurs.
Mark Hayes is technical advisor to the Institute of Spring Technology (IST) in Sheffield, England. He is also the principal trainer for the spring training courses that the Institute offers globally. Readers are encouraged to contact IST with comments about this cautionary tale, and with subjects that they would like to be addressed in future tales, by email email@example.com.