Case Hardening in Wood – Tension Set

I’ve recently gone through a stretch of unstable lumber from various different sources. All the lumber reacted the same way when cut. Right before finishing the cut with the blade the lumber would *POP* releasing tension and cupping the wood instantly. I thought it would be a good learning opportunity to see what was going on.

My research indicated case hardening or tension set. I thought it was interesting so I made this video. If you have more to share on the topic, have a correction to something I said, or simply want to tell me that the Detroit Lions will never win a Super Bowl then feel free to leave a comment. Have a good day.

Test for case hardening in lumber: https://www.popularwoodworking.com/woodworking-blogs/how-to-test-for-case-hardened-lumber/

Post I was reading in the video: https://www.woodweb.com/knowledge_base/Case_Hardening_Explained.html

11 COMMENTS

  1. That is interesting. I wonder how “deep” the casehardening penetrates the wood. Maybe some of these pieces could have been salvaged by removing the bulk of material from the exterior surfaces. This would limit the yield from the lumber and you likely wouldn’t get to incorporate any nice book matched grain patterns but at least the lumber wouldn’t be scrapped completely.

  2. Good one…my take from this is there is no way to detect this prior to cutting into the wood, or maybe this is a question. Moisture content from the end grain…outer grain to inner grain? Interesting subject, have never heard of it, experienced a little of it, I think

  3. I think if you use more lumber from the same lot, you should probably try your hand plane prior to running it through your planer. Case hardened wood is detected by hand tool users. Last month I milled two ash logs and intended to air dry for a couple months to get the moisture content down. We had a heatwave and I put the load in the kiln with only fans running for several weeks. Did not want the sun and high heat to dry the exterior faster than the core. This is my understanding of case hardening. Of course there is also a natural tension in the tree.

  4. Curiosity, if you have a piece of this that has yet to be cut, try planing both sides prior to resawing and see if it still happens or if by planing you release the casehardened sheath.

  5. Thanks, Jay. After far too many decades of putting “good” lumber into the woodstove because of these discoveries after sawing, I appreciate you sharing this for future projects. Thanks for including the links (as always).

  6. Based on an explanation of case hardening and then slicing through the thicker board and exposing a non case hardened surface that caused the cupping, would it be correct to assume that if both sides of the thicker case hardened pieces were planed before resawing, this would have relieved the tension on the outside and thus created more equal tension between the outside and the inside? Based on the size of the chunk of wood Jay was working with, planing both sides of the slabs might not have been an option.

  7. In your video, you quoted Professor Gene Wengert and wondered what “PL” meant.

    This is his description of “PL”:

    https://www.woodweb.com/knowledge_base/Tension_Set_and_Lumber_Conditioning.html
    “From Professor Gene Wengert, forum technical advisor:
    You do indeed have it correct. The PL is the point at which semi-permanent deformation occurs. So, with a rubber band, you can stretch it out and then let go and it will return to the original size. This is the elastic range. You can do it again and again, and nothing permanent. (Actually, over time – years – some permanent deformation can occur, called creep, but we are considering short term now.)

    Now when you go above the PL with the stress, the rubber band gets stretched out and this will be permanent… Let go and the band is a bit larger. In other words, the PL is the stress level at which point you go from elastic to plastic behavior. “

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