Hardening Soft Copper Tubing Project pictures and narrative

2/14/23 ~ by Griz (aka Steve Stovner) ~
I have a large roll (100 pounds) of 3/8" soft copper tubing I bought at a farm auction many years ago for $25. But the project I had in mind (see here), required hard copper tubing. Google search yielded 2 methods for work hardening soft copper tubing, Vibration and Stretching. Somebody said putting soft copper in the bed of a pickup and driving around would shortly harden the copper. So, vibration seemed the most logical and easiest to implement. I bought 4 small vibrators (12vdc motors with offset shaft weights). I cut a 10-foot piece off the roll, straightened it (roughly) and checked the hardness by laying it flat on the table with 2ft cantilevered over the edge, made initial measurement from floor, hung one pound weight from end and measured deflection, removed weight and measured set (permanent no-load deflection from initial state). I repeated the test several times along the 10 ft length and got very consistent readings. I hung the tube from the overhead, attached a vibrator about 2 feet from the lower end and turned it on. I was pleased to see the standing waves (about 1" tall) along whole tube. After 4 hours of vibration, I repeated the one-pound cantilever test with no detectable change in hardness. I mounted all 4 vibrators (at 2-foot intervals and different orientations) on the tube and ran it 72 hours. This yielded very little change in harness. See results below. I then stretched a new piece of soft tube by 8% (see pics below) and remeasured.

Note: the 1/16" set after 8% stretch was not cumulative. In other words, subsequent weight application and removal did not add more detectable set to the initial 1/16" set.

I assume more stretch would yield harder copper, but 8% was hard enough for my project. And a great benefit of this method is a dead straight section of tubing to work with.

7/24 ~ by Griz (aka Steve Stovner) ~
Proof is in the pudding. After 16 months in the wild (40 knot winds, rain, snow, 4°F to 98°F) with 1 pound load at 29", there is no sign of droop. However, 2 pound load at 33" was too much and very quickly it began to sag and continued to get worse over time.