I watched the vid last night. Very interesting. I have to believe it doesn't last very long or has some other reason it doesn't work out because otherwise we would be doing that way more often and buying new springs less often. If I knew I could take apart my spring packs and hit them with a hammer and put it all back together and the sagging would be gone for the foreseeable future, I would have done it a long time ago and never bought new packs. Im guessing a heat-treat is the missing puzzle piece but I wouldn't bet money on it.
When I watched the video, you can see small hammer marks (about the size of a quarter) as they go up one side and down the other. I would think a bigger head struck in the middle would be better. But those poor folks make do with what they have and that is usually very little.
OK, when making a spring you start with alloy steel. But the old method, like when our XJs were made, the metal is heated cherry red in a forge, the eyes and tapers are formed. Then the arch is added on the second go around. Then after forming, the steel is quenched in oil, which makes it brittle. Then they are reheated to 400° and cooled gradually. Then the spring is bendable. It is all about crystal structure of the steel. Hitting it with a hammer and putting the steel under pressure accomplishes the same thing, but it doesn't go as deep or last as long
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