Material properties of steel for spring 1

[hamsuplo]

I have tested heating of springs at different temperatures and duration (i only know my steel is steel grade 70)

I have noticed as temperature/duration goes up, the steel color turns from silver -> yellow/brass -> reddish/brown -> purple ->blue -> light blue

I would like to know, for my purposes (a trampoline spring, with maximum load 55kg) what color indicates the best heat treatment and what properties does the color exhibit. I would like high spring strength but also high fatigue life.

Thanks in advance.

 

[hamsuplo]

Additional note:

I'm mainly talking about the ends of the hooks of the springs (the cross sectional ends at the end of spring when its cut during manufacturing), is where the color change occurs. the body of the spring also changes color but slower - for example, by the time the hooks are blue, the body is still only yellowish/brown

 

[redpicker]

You haven't given much information, but, based on what you have given, I think the color you are after is "silver". That is, do not heat them.

The term "Grade 70" does not mean much, by itself, but it could be used to describe material with a minimum UTS of 70 KSI. This is not usually considered a hardenable steel and may not respond predictably to heat treatment. I would need quite a bit more informtion to be more specific, but, in general, you will do more harm than good unless you know what you are doing.

rp

 

[EdStainless]

For high fatigue life in a coil spring you need the applied stress to be a smaller fraction of the tensile strength. The higher the strength the more durable the spring.
However since you have end hooks that you want to bend before they brake you may need to temper at a higher temperature than you would just for the spring itself.
It sounds like you need to build a test fixture to load springs and see how durable they are.

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Plymouth Tube

 

[mrfailure]

The color is not an indication of heat treatment. The color you see is really refraction of a thin oxide film which forms on the surface, and is based on its thickness. Film growth slows and stops when held at a given temperature. I can often estimate maximum temperature the part saw based on color provided the component was at that temperature for enough time. I would not use color to determine best heat treatment - you instead need to go to the literature and/or try measuring temperature, mechanical, and microstructural properties from your own heat treat experiments.