17-7ph extension Spring, Heat setting process?

[Bfloreshyetech]

Hi,
My name is Brandon Flores; I am doing research on the heat setting process for small extension springs, in aviation applications. The material is 17-7Ph. So far I have not been able to find much online. I am under the Impression that the spring calls out for 400 Degrees (F) for 5 hours, for the heat setting process. A few question I have is; why the exact temperature and time? Is this an Industry standard? If so, where do I find the specs?

Thank you and I appreciate the assistance!

 

[israelkk]

Spring setting (hot or cold) is relevant for compression spring where there is no way to load the spring beyond the solid height. Extension spring has no physical limit to extension. Therefore, setting to a certain load is very risky if there is no absolute way to assure that the spring will not extend beyond the setting extension during work, installation, handling, etc.

Add to this that extension springs are likely designed with an initial load. Therefore, setting operation will change or cancel the initial load of the spring. Controlling the remaining initial load after setting is not possible as far as I know.

Can you post the exact requirements as appear in the spring drawing?

 

[Bfloreshyetech]

The Exact requirement on the Dwg. is 400*F for 5 hours, at a specific length. (The length is being withheld until final review).

For use in the Fuel Control management systems.

I guess to re-word what I am trying to ask:

"The outstanding issue/question we have is whether there is a correlation between temperature, time and stretch for the heat set process that is dependent with the spring size; or is it more dependent on the application of the spring?" That is our main question, but personally - I - am trying to understand the heat set process for springs in all aspects as well. I took a materials engineering course and Thermodynamics when i was in School, but I don't ever remember "Heat set". I know its to relieve the first stages of stress and effects the relaxation of the spring, But how does one come up with a certain temperature and time.

a lot of things online give me definitions of what heat set is, but nothing is telling me how it comes up with numbers. And actually, when it does give a number, typically saying "Heat treatment" its calling for near 1000*F for like 90 minutes... Nothing near what my spec is calling for.

 

[israelkk]

To my opinion this type of "setting" is to remove most of the spring relaxation due to fixed deflection of the spring. It is not intend to receive larger loads from same wire as in compression springs. It seems to be an accelerated relaxation process to simulate long period at lower temperatures. I am not familiar with direct correlation of high temperature, higher stress, and shorter time to simulate long period relaxation at room/low temperatures. I guess someone took the time to do a long period test for months and tried to reach same relaxation at the specified tempersture.

 

[Bfloreshyetech]

So there isn't any set formula to solve a heat set? Just a trial an Error to find best optimization?

 

[3DDave]

In most of material science, if there is a formula, it was created to match the results of a lot of tests, tests which may not represent the case anyone else has. At best, even if there is a formula, it provides a rough starting point. To get a better answer almost always requires tests, especially when the item is in some custom condition.

All is not lost however, as there are companies that specialize in springs and if you can tell them what result you want, it is likely they have done enough tests on their own products to get you quite close to that result.

When you looked at heat treatment - these are temps and times that are, from tests, known to cause a crystal shape transition or allow the alloying elements to migrate uniformly through the bulk of the metal. The time is typically a minimum for bringing up to temp to make sure the entire part is heated. Usually there is a thickness value as well to account for conductivity. There are some inroads in quantum simulation for atomic interaction, but that's recent. For the most part it is experience based.

To get to heat setting in particular, the process seems to be based on the following: When metal crystallizes during cooling there are some places in the metal that have higher stresses because crystal formation is somewhat random. It's a little like musical chairs with atoms. Some don't quite fit the crystal pattern. What heat setting does is jiggle the atoms that are nearly able to fit into a crystal and its neighbors enough they adjust their position to relieve the stress. This will eventually happen anyway and because those little tense areas are relieved, the spring overall relaxes. With the low temp treatment, it just does so before delivery so the user can be sure the spring will not do it after it is installed. Instead the spring will continue on just like new, for the normal life of the spring. The temp is low enough that it doesn't jiggle the crystal completely out of shape or melt the whole part.

 

[TVP]

Brandon,

I suggest you read the following article, as it is a good starting point with quite a bit of useful information:

http://www.smihq.org/public/publications/07_47_04/_hot_setting.html

I agree with israelkk that in your particular application, heat setting likely is for reducing stress relaxation for a fixed deflection. And no, there is no specific formula when it comes to heat setting. You are trying to understand a complex problem that involves a number of different phenomena that can be partially/completely described by differential equations. Trial-and-error experimentation and experience are the tools used for this type of problem solving.

 

[Bfloreshyetech]

Guys, I cant tell you how much I have appreciated the advice and information! It has honestly answered a lot of my questions, And also caused a few more to arise, but its finally allowed me to ask the "right" questions.
Thank you all! I will give a follow up on how things go once I get to the next stage!