During a recent IST training course in China, a delegate asked many questions about the changing color of 302 stainless steel spring wire during spring production at his factory. This prompted another delegate to ask about the colors arising during the manufacture of springs made from music wire. This is not the first time that such questions have been asked, so it seems to be a good subject for a cautionary tale. The first point to note is that the color changes in both materials have the same root cause, but the colors are not real – they are merely a trick of light.
Whenever steel is exposed to temperatures above ambient, the oxygen in air reacts with the steel surface and an oxide will grow. Steel will always have oxide on its surface because it will form naturally at ambient temperatures, but that oxide is much too thin (measured in nanometers) to be seen.
Indeed, the existence of this oxide is seldom acknowledged for carbon steel in most texts, but rest assured it is there. Stainless Steel is assumed to have a chromium oxide on its surface, and this explains why it does not go rusty. The surface of both carbon steel and stainless steel oxidizes in air to form an oxide that confers corrosion resistance, and this brings the first cautionary aspect to this tale. Oxidation and corrosion are separate phenomena, but they are frequently confused. A quick internet search for “oxidation of steel” will bring up many articles on corrosion, a few articles on high temperature oxidation, but almost none on ambient temperature oxidation. The main oxides of iron are Fe2O3 or FeO, which are said to be respectively red or black in color and this is true when the oxide is micrometers thick. However, the oxide which forms on a bare steel surface in air is nanometers thick and is, to all intents and purposes, colorless and translucent. This very thin oxide confers corrosion resistance and, as springmakers already know, steel springs do not generally go rusty red in their factory despite having no protection from oil at various stages of manufacture. We will talk more about this later. The formulas for rust (Fe2O3-nH2O and FeO(OH)-Fe(OH)3) contain H because rust occurs due to the reaction of oxygen and water with a spring surface.
The nanometer thick oxide formed on bare steel surfaces is colorless and so cannot be seen on carbon or stainless steel. However, most carbon steel springs are heat treated in air at 390-660°F (200-350°C) after coiling, and this will cause the oxide to grow thicker. At 435°F (225°C) the oxide is thick enough to refract the yellow part of incoming white light, so the springs appear to turn yellow. However, carbon spring steel wire often has a phosphate coating with some soap and, as the surface is quite black prior to heat treatment, the yellow color is not usually apparent unless the wire surface is particularly bright. At 525°F (275°C) the oxide is thick enough to refract another part of incoming white light, so the springs appear to be blue in the original black wire surface permits. So, the heat treatment colors are a trick of light as the translucent oxide refracts incoming light rather than actually coloring the steel surface.
The oxide colors are more readily seen after spring end grinding. A yellow color indicates that the grinding did not heat up the ground end excessively, and the same can be said if the end coil goes light blue. However, the grinding may have been abusive if the color was dark blue or black; these colors can be used as a simple but effective quality control check for the grinding process.
If carbon or silicon chromium spring steel is shot peened, it is common practice to apply a low temperature heat treatment (or warm prestress) afterwards in order to recover the relaxation resistance, which is adversely affected by shot peening. The shot peening removes any oxide from previous processes and leaves the spring surface bright but vulnerable to corrosion. The oxide will grow back at ambient temperature but, in conditions of 100% humidity, rust may form prior to the oxide. In tropical or monsoon-affected countries, springs can be seen to go rusty red soon after they emerge from the peener. As a result, it is good practice to get freshly peened springs into an oven as soon as possible, and make that corrosion resistant oxide grow. Carbon or silicon chromium springs will go an agreeable yellow color if the heat treatment is carried out at 435°F (225°C) – a temperature commonly used for this process.
Up to now it has been stated that the colors on carbon and low alloy steel and on stainless steel have the same root cause. This is correct, but the temperature required to cause 302 stainless steel to go a light yellow color is much higher than that required for carbon steel. The yellow color is just visible if the heat treatment temperature after coiling is 660°F (350°C), and spring manufacturers often limit the stress relieving to this temperature so as to avoid the darker yellows that would appear at 750-840°F (400-450°C). However, many customers frequently say that they don’t want to receive yellow springs – and this is despite the fact that heat treating stainless steel compression springs at higher temperatures than 660°F (350°C) will improve their relaxation and fatigue performance. If good performance is required and an absence of the yellow color, then springmakers should employ a pickle and passivate process after heat treatment. The first (pickle) removes the oxide which is causing the yellow color, while the second (passivate) puts in place a very thin, adherent oxide that is too thin to give the yellow color but has excellent corrosion resistance.
The moral of this cautionary tale is that the thin oxide on spring steel is beneficial and the apparent stainless steel heat treatment colours formed can be used as a simple quality control tool in production.
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.