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Welcome to the latest edition of our Engineers Guide To Spring Technology where we focus on heat treatment with renown spring expert, Mark Hayes B.Sc., C.Eng., MIMMM.

Over the years Mark has become a prominent figure in the Spring Industry. Mark was employed by the Institute of Spring Technology (IST) for 28 years and has amassed a wealth of experience both academically and practically.
Focus On Heat Treatment

When our engineers write the process route for almost all the springs they manufacture, heat treatment is included, generally straight after spring forming. You may be surprised that this process is required because the wire used for coiling is already spring hard, so why the heat treatment?

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The answer to this question is in triplicate. Our engineers know that the heat treatment brings three separate benefits for their customer's products:

1 The wire they use has a high tensile strength, but it is the elastic limit that is essential for all springs, and the heat treatment, carried out at a temperature that will not reduce the tensile strength of the wire, will increase the elastic limit. This applies to a greater or lesser extent for all grades of spring wire.

2 When the wire is cold formed, residual tensile stresses are certain to arise at the inside of every curved surface, and these are adverse to compression or extension spring performance. The outside surface will exhibit residual compressive stresses and these are beneficial, but can only be exploited for wind-up torsion springs and some wireforms. Heat treatment will reduce the magnitude of these residual stresses, but will not eliminate them. Indeed, this heat treatment is often called stress relieving, and for SiCr raw material has to be carried out very soon after coiling. If SiCr springs are not stress relief heat treated soon after coiling, they may crack – that gives an idea about how important these residual stresses are.

3 If springs are not heat treated after coiling, then their dimensions will not be stable - their outside diameters will drift slowly. The dimensional change is not great, but it is measureable. Our customers like to receive springs with stable dimensions that will not alter during their service life, and once heat treated in manufacture, they will only suffer dimensional change if overloaded or are subject to significant relaxation. The heat treatment is carried out at a relatively low temperature, but it is always higher than the spring's maximum service temperature.

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Process Control
It is clear from the above description that the heat treatment process brings many benefits and so it might be imagined that accurate specification of the process and really good process control are essential.

We advise our customers that there is no need to specify heat treatment on their drawings or blueprints - it is a process that will be carried out unless exceptional circumstances apply - usually springs that are used once only, and for which dimensional and load stability is unimportant!

The temperature at which this heat treatment process is carried out varies from material to material, but for most the allowable temperature range is very wide. For instance, for drawn carbon steel including music wire, any temperature between 200 and 350°C will ensure all three of the above benefits will be achieved. What is more the time required for the process could be as little as one minute, or as long as an hour or two. In general, heat treatment temperatures tend to be higher for compression springs, and lower for extension and torsion springs, but optimisation beyond these guidelines is seldom required, except for springs designed to their limit of performance. The effect of process temperature is quantified in a recent research paper published at an international spring conference in Tokyo in Nov. 2015 (1). We have a copy of this paper, and use the results, together with other historical data, to optimise heat treatment parameters for those rare circumstances when this is necessary. An example graph from this paper is shown as figure 1.

Figure 1 Elastic limit is significantly increased by heat treating carbon steel at 200°C minimum Avanex EGTST Verification
It is quite difficult to verify that the heat treatment process has been carried out. We generally use heat treatment in an on-line oven situated next to the coiler so that it is impossible to omit this important process. However, if it is required to verify that the process was undertaken, then the best means is to re-heat treated example springs, measuring the outside diameters before and after. If the outside diameter changes (carbon steel getting smaller, stainless steel larger), then the spring had not been heat-treated at the test temperature previously. Metallurgically the heat treatment is often impossible to verify, and measuring residual stress (very expensive) is only possible at wire sizes of 3mm or greater.

Clearly, heat treatment after coiling is essential for good spring performance and dimensional and load stability.
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