Case Hardening Explained

A semi-technical discussion about how the wood drying process can create stresses between interior and surface wood layers and about how to reduce or prevent that situation. January 19, 2012

I have always been under the impression that once a piece of case hardened wood is in the hands of a woodworker it is not retrievable. I know the test for case hardened wood that any woodworker can do. I also have always thought it was a drying defect, not something a turner, for instance, could do with a piece of dull sandpaper when he burnishes the surface of the wood. Am I wrong?

Forum Responses
(Sawing and Drying Forum)
From contributor E:
I am wondering what the larger question is - is your concern that using dull sandpaper might caseharden a finished turning? If that was possible, what is your concern with regards to the finished piece?

From contributor K:
Once a turner is sanding a piece, I can't think of any problem that would occur after the fact, even if it could be casehardened. I believe what is occurring on the lathe is burnishing, not casehardening.

I've been trying to figure out how to relieve casehardening in my kiln operation. It's a normal step done at the end of drying, so I would think most standard cases of casehardening are just a difference in the jacket wood being dryer than the core, so adding just the right amount of moisture back to the jacket at the end relieves the tensions built up. Honeycombing, however, is actually damaged wood and not recoverable. Not sure if casehardening works itself out over time if lumber is left to sit and stabilize, as it will pick up moisture from the air.

From Professor Gene Wengert, forum technical advisor:
Casehardening, as you appreciate already, has nothing to do with hardness. It results earlier in drying when the shell tries to shrink (because the fibers are drying under 30% MC) but cannot. At the end of drying when the MC is uniform, there will still be this shell that is larger than the core. You cannot caseharden dry wood.

So, if casehardening was caused by restricted shell shrinkage, we can offset this by adding water to the shell to try and get the shell to expend. Of course, the shell is limited in expansion. So the attempted expansion offsets the earlier attempted shrinkage, so we are free of casehardening or drying stress. The key is to add moisture quickly, as adding moisture slowly will only increase the average MC. Heat makes it happen faster. One way to add moisture faster is to cool the lumber a bit before steaming. Casehardening does not go away with time. Note that planing the wood does remove the casehardening stress. However, plane only one side of casehardened lumber and the piece will cup dramatically as the stress in the shell of the un-planed side is still there.

From contributor E:
I have turned 1,000 bowls, almost all of them green. I set them aside to dry, then finish turn them when they are at about 12% MC or so. I finish turn them, then sand them. By the end of that process they are under 10% MC, sometimes around 8% or so. I am struggling to understand how casehardening could occur in the process I use.

From Professor Gene Wengert, forum technical advisor:
Casehardening does occur, but you only see it when you machine the pieces when they are fairly dry. Because you do not do that, casehardening will not affect you. Casehardening is really a drying stress situation. If you saw a bowl in half, you will note that the two pieces will not fit perfectly back together... That is casehardening.

From contributor T:
I disagree and think when you cut the bowl in half, whether green or dry, it's the growing stresses that prevent the rematch, not casehardening, since most bowls are turned green and to a semi-close finish tolerance, then are mostly air dried to MC and finished. This is a slower drying process and the constant slow heating and cooling helps relieve the wood stresses as drying.

Casehardening is the quick drying, as you stated, and the higher heat from speed is causing the issue. I do agree pulling moisture out on the outside quicker than inside causes a stress, but the higher heat and no cooling (relieving stress) stage is skipped with the natural air drying process.

It's like taking a big deep breath and holding... The exterior muscles are tight and not accustomed to the stretch/stress, but exhale and do this in slow increasing steps and the body becomes adjusted to the changes.

Bottom line, in my opinion, is the drying process/air drying used by most bowl turners does not create casehardening, which is caused by a quick moisture change and the exterior and interior of wood drying at an extremely different pace.

From Professor Gene Wengert, forum technical advisor:
I did not say that casehardening is caused by quick drying or by heat. The quickness is how we offset the casehardening (more technically called tension set).

It is true that air-dried lumber has little casehardening, but that is because of the high RH every night. The temperature is not a major factor. It is the humidity mainly. There are a lot of people that learned air drying can go too fast and check and honeycomb the wood. If you get a crack, then we know you also had casehardening, as casehardening occurs prior to cracking (when the PL is exceeded and we are in the plastic range which occurs before the ultimate stress level is exceeded). I tried to avoid a highly technical discussion about stress, set, plastic range, elastic range, PL and so on. Casehardening is well understood and can be technically explained.

It is also true that wood can be dried very slowly and casehardening avoided in that manner, but such slow drying is hard to achieve and would result in mold, mildew, etc. It is true that cutting a bowl in half can also show growth stresses as well as casehardening stresses. However, we are fortunate that most US woods do not have much growth stress. The cutting will also show tension wood and compression wood stresses.